How to make a cheap isolated dual-battery setup for $50

[dwh]

Kevin and Mitch
Thanks for the reply. My vehicle is a 2003 and has a separate wire from the alternator to turn on the relay. I was concerned about blowing either the regulator or MPPT, thanks for the confidence that that is not an issue.
I have an additional question- I understand you should not disconnect either the solar panel while a load is on it- other than covering the solar panel (impractical) what is a way to properly disconnect the solar from the circuit if you should need to do so. Also , what happens if you do that they advise against it, I did see the note on it destroying switch contacts. Wouldn't a small capacitor across the switch stop that?
This is a very helpful thread. A lot of good information.

Wait till dark.

But it's not really an issue. Most solar charge controllers say to connect the battery first since that's where they get the power to run the brain in the charge controller. They don't get their power from the solar array side.

So just disconnect the battery and the charge controller shuts down, and there is no load on the PV modules (solar panels).

 

[dwh]

Lemme add a bit of real world to this thread. Not long ago, I had to replace the alternator belt on my truck. It was stretched. I knew it was bad because when there was a heavy load on the alternator, it would squeal.

Even with the house battery hammered flat, after I'd start the truck, that belt would only squeal until the house battery came up to a surface charge roughly equal to the bus voltage, then the squeal would go away because there just wasn't all the much current flowing into the house battery and thus not much load on the alternator.

So...any guesses how long it would squeal?











5 - 10 seconds.

 

[Eurovan2003]

Kevin and DWH
I appreciate the info once again, this thread has more info than I have seen elsewhere.
Kevin, I figured I should disconnect the solar panel if I am not using the vehicle for a week or more to avoid overcharging the battery, I do gather the controller will stop charging when the charger reaches full charge, however I wasn't sure if it should still be disconnected.
Along that idea, what are the best settings for the MPPT controller as far as the PV off (13.4v), Load off (11.9v), load on (12.2v) based on the percent battery charges (I showed what I set them at). I figured the alternator usually sits at 13.4 to 13.7v and since a battery is at 60% at 12.24v and 100% at 12.73. Am I simply overthinking this whole thing.
Thanks to your info, I felt confident enough to hook it up and all seems to operate just fine so far.
You have been a great help in further understanding this solar/battery.
Phil

 

[dwh]

The primary purpose of any solar charge controller is to prevent overcharging the battery. You'd have to worry about it if you connected the solar panel directly to the battery with no controller, but with a charge controller you don't have to worry about it.

If you've got a programmable charge controller, program it for the recommended charge profile of whichever battery you have. 9 times out of 10, that'll be "bulk charge to 14.4v, absorb at 14.2v and float at 13.6v". There is a reason why those are the set points used in almost every non-programmable multi-stage charger on the market - because it'll work just fine on every battery except a GEL type (which takes the same settings, except that you usually don't want to go over 14.2v ever).

When you say "load off / load on" I suppose you are referring to the LVD (low-voltage disconnect) circuit. Those are generally rated for VERY small loads like a radio or lights and most people probably don't even use it since most of their loads are too big for the LVD circuit and stuff like most decent DC fridges and all inverters have their own LVD circuits anyway.

Ignore the alternator - it's just a power supply that will eventually get your battery fully charged by holding it above 13v. That works fine on an engine start battery, which gets drained maybe 0.2 amp*hours every time you start your truck and is fully recharged 10 minutes later. For a house battery, it'll take a LONG TIME (24-36 hours of driving) to get it to 100%. It really has nothing to do with the solar, other than adding some extra amps here and there.

To really get a house battery charged up quickly, totally and properly, you need to pump it up way above 12.8v. 14.4v normally, though a lot of people (including battery manufactureres) are finally coming around to the idea that you can go even higher as long as you keep an eye on the battery temp and don't overheat the chemistry.

 

[Stitebunny]

Thanks for the replies about the amp ratings. Will go with 200 and see if it works!

 

[dwh]

Hrmmm...

A) I probably should have been more specific, but I got in a hurry. What I was referring to was: He was using the alternator's voltage as a basis for programming his solar charge controller. Thus, "ignore the alternator". What I should have said was, "When choosing voltage set points for your solar charge controller, ignore whatever the alternator's doing because the alternator is a crappy battery charger because the voltage regulator is too simple and stupid to do a good job and a multi-stage solar charge controller is a different sort of beast."

B) 150ah over a 3 hour period is an average of 50a per hour. No doubt a big flow of amps at the beginning (probably the 150ah mentioned?) followed by a tapering off as the voltage of the battery bank rises. That's great bulk stage performance. But once the battery (or in your case, bank) reaches a surface charge equal to the bus voltage, it's just a trickle charge after that no matter how big the alternator is, and no matter how big the wires are.

C) As I recall, *you* have a special voltage regulator. Multi-stage? 14.4v? PWM? I don't recall the details. but something special yes?

This is the only place where I would, respectfully, disagree; properly wired, you can get a sustained charge rate of over 150A with 200A worth of alternator running at over 14v. I can typically recoup 150Ah into my camper batteries, as measured by a Trimetric and a Blue Sky IPN Pro, in two to three hours of driving. And I sustained this performance on a 60 day trip cooking exclusively on electrical appliances.

The big reason most people ignore alternators as a charge source for their camper batteries is that their wiring is too small. With proper wiring the alternator regulator system will, in fact, "see" the camper batteries beyond the starter battery and continue to charge them.

The big problem is that you may not drive long enough to complete the job and that is where a health solar kit is useful.

 

[dwh]

Bone stock or not, that's a helluva special system. It's a computer controlled voltage regulator with extra sensing inputs. A bit like a factory installed Sterling external multi-stage regulator.

I'd be interested to find out just how many modern vehicles have such a system. Would the same truck as yours with a gas engine have the same system? I seem to recall someone around here with a modern truck and the only thing mentioned that was special was that the regulator was a PWM.

In this case, the truck alternator will, in fact, hold that 150A charge rate until both batteries reach about 90% charge.

Two PPOTs (Pedantic Points Of Technicality): 1) The alternator isn't holding a 150a charge rate. Far as I can tell from what you posted, it's holding a voltage based on duty cycle. The amperage is determined by how many amps can overcome the resistance of your batteries at that voltage. 2) If the starter battery was fully charged when you started your truck, then it's already gonna be at or above 99%.

 

[dwh]

There will be a current flow, to be sure, but it will be like trying to fill one swimming pool from another, but with a garden hose. In theory the two pools will reach the same level, but it will take a long, long time.

Right, but the bottleneck is usually not the wire, it's the battery resistance.

Replace the garden hose with a culvert and the two pools reach the same level very quickly. How big a "culvert" do you need? Start reading here: http://www.smartgauge.co.uk/cable_type.html

To quote from the bottom of that page:

"Finally choose whichever wire is the largest from rule 3 and rule 4. On low voltage systems rule 4 will almost always dictate the wire size. On high voltage systems rule 3 will usually dictate the wire size."

Well rule 3 says exactly what I keep telling people; "size the wire to safely carry the expected amp load". I.e. if the expected amp load is 30a, then all you need is 10 gauge because that's the load #10 is rated to carry.

Rule 4 says to run this formula to figure out wire size based on voltage drop. Which, of course, is an option and won't hurt anything, and will shave some time off a bulk stage, but once the battery surface charge reaches the voltage regulator set point, you're in the trickle and absorb zone (the real time eater). The bigger wire won't help because the current flow (amp load) has dropped and the oversize wire based on voltage drop isn't needed since the voltage drop has... well... dropped. Over the course of hours of absorb, the current just keeps dropping and thus voltage drop just keeps getting less and less - making the oversize wire even more and more... well... oversized.

So yea, great big ol' wire will shave some time off the charge cycle. Off the short part of the charge cycle. It helps nothing during the long part of the charge cycle.


[Also note that his example is a bit whacked. He's using 40 METERS (!!) in the example. Maybe he was thinking of a really big boat.

"However, irrespective of the voltage the system is running at, a 20 amp load will drop 5.8 volts down a total of 40 metres of this cable. So if our load at the end is a 12 volt load drawing 20 amps then by the time the power gets there, it will now be at 12 - 5.8 = 6.2 volts. Obviously this is no use to us whatsoever!"

Um...yea...obviously. But I'll concede his point - if you are gonna supply a 12v load over a distance of 20 meters (x2 for the loop distance) - you're gonna need bigger wire.



And some may have noticed that I've repeatedly said that voltage drop is a VERY important consideration when supplying loads FROM a battery.

And one last quibble from that page. At the very bottom:

"NOTE - Always build a safety margin in to the cable sizes. i.e. increase the cable size from the calculated size by around 30%. Never try to run cables at their maximum specified current limit."

That's a bit silly, since published wire ratings already have that built in. For instance a 10 gauge is rated for 30a, but it can actually carry a lot more before it will melt. It's derated as a safety factor. Of course, if you are engineering the setup yourself and not using published ratings you'll have to build in your own safety margins.]

At this point, I am usually 125Ah from full charge. It has been as high as 200Ah. The starter batteries are at close to full charge as to automatic relay opened as soon as the camper batteries dropped below about 12.5v, which they did as soon as the microwave powered up.

Ah. You're letting the IBS regularly draw down the starter battery a bit. So in the morning when you start the truck, it's not fully charged. I see now.

-- For a moment nothing happens. Then the starter batteries attempt to recharge the partially discharged camper batteries.

I would say that the alternator is trying to recharge the camper batteries.

-- Current flows from the charged starter batteries to the discharged camper batteries at the rate of about 15a down each 1/0 cable. (2x AWG 1/0 = approx 100mm2 of copper)

You running doubled cables or do you mean one pos and one neg? Cause if it's one pos and one neg, then it'd be 15a total for the loop.

-- Being 150Ah down, the camper batteries demand a lot of current from the starter batteries and as the starter battery voltage drops, the alternate/regulator produces more current.

Well...again, I'd say the alternator is doing the work, not the starter battery. Reference http://www.smartgauge.co.uk/nosurge2.html where it says:

"The worst scenario actually occurs with a fully charged engine start battery and the auxiliary battery bank at around 40% charge state. The reason is that, although the terminal voltage falls as the battery approaches totally flat, the internal resistance rises. At 40% state of charge the internal resistance is considerably lower. The result in the above example would be that the initial current surge would be around 40 amps, and even that would only last a minute or so (thus discharging the engine battery by 40A * 1/60th hour = 0.7 amp hours) at the most until the surface charge on the auxiliary bank raised it's terminal voltage to that of the engine start battery. Once this had happened, the current would be negligible." [emphasis added - dwh]

-- Truck system takes care of starter batteries. The camper batteries simply present as a large load.
-- Solar kit is connected to camper batteries only. Solar kit is programmed to match the size and type of camper batteries. The starter batteries simply present as a small load.

Yea, with that killer voltage regulator you've got that's a sweet setup.

real world caveats:

-- The cable between the two batteries must be large enough to flow a lot of current, ideally >the rated output of the alternator with <0.5v drop.

Agree with the first part, but the second part is where we differ. I say the drop is irrelevant because by the time the battery surface charge gets to the voltage regulator set point - there is no voltage drop. "Voltage drop has left building."

 

[wrcsixeight]

Okay I think we can agree once the batteries are ~80% or more charged, fatter alternator circuit cabling will not reduce charge times in any significantly measurable way.

But below 80% charged is where almost all battery damage, capacity loss occurs.

I want my alternator, to deliver as much of its available current into my depleted batteries as possible, to get them to 80% as quickly as possible, and upgrading the cabling between my alternator and house battery has doubled the maximum current I have seen, and tripled the average current my depleted batteries see.

There is theory and then there is quantifiable repeatable results.

When I press the circuit breaker button on my 2awg parallel alternator circuit, breaking the circuit, the maximum amps I will see is 62 and after 5 minutes this is down to 25 and will keep tapering down to single digits. The engine battery will read 14.9v on the battery terminals, the voltage on the depleted house battery might only be 13.6v.

When I close this circuit the maximum amps I will see are 110 before my belt starts squealing, and 5 minutes later the batteries will still be taking 60+ amps and an hour later they will still be in the mid 30's as long as there is enough rpm to maintain these higher currents.
My "130 amp" alternator is not very impressive at engine rpms under 1400 when hot.

Long story short, the time it takes my alternator to take my batteries from 50 to ~80 % is about 1/3 the time it takes when I break this additional ~2awg parallel charging circuit. I find when the batteries are newer and healthier that they actually accept higher currents before tapering significantly well up to 90% range, when verified with my hydrometer.

If the the cabling is thick, then the voltage regulator will see overall lower average voltages from depleted house batteries and allow the alternator to make more juice to bring up the voltage the voltage regulator sees upto the maximum setpoint. If the wiring is thin, well the VR might see 14.5v, but a voltage reading taken at the house battery terminals might only be 13.2 and accepting 8 amps. If the Cabling is fat the VR might be seeing 14.5v and the and the house battery terminals might read 14.3v and be accepting 45 amps.

After I upgraded my alternator circuit, my batteries started holding higher voltage each discharge cycle each day that I drove and let the solar take over the rest of the day.
Days when I did not drive, but the solar still held 14.5v for several hours each day, the batteries read lower voltage during discharge that night. Time after time, day after day, driving with an upgraded alternator circuit allowed the batteries to perform better. Before the upgraded alternator circuit, no difference.

In my system, 14.9v is the max the VR allows, but all too early, for depleted battery charging, it will decide 13.7v is enough. While the alternator might have needed to make 65 amps to hold the depleted batteries at 14.9, only about 13 amps are needed to hold them at 13.7v.

I have the option of removing the engine battery entirely from the alternator circuit at the turn of a switch. When I do so, the VR allows 14.9v to be held for longer, higher amps make it into the thirsty batteries for longer, and they get back upto ~80% that much faster. I remove the engine starting battery often from the circuit, especially in winter or when cloudy or when I really hammered the house battery the night before, and it make a big difference, holding that 14.9v longer vs that all too soon 13.7v.

The Low and slow trickle charge crowd can thump their chests when they have all the time in the world to recharge their batteries on mains/grid power, but my next discharge cycle begins before sundown, and my upgraded oversized alternator circuit gets my batteries to 80% SOC faster, significantly faster, and allows more time for the Solar to raise the Specific gravity upto near the maximum baseline.

DWH, I believe you are drastically underestimating how long a depleted battery can accept large currents, and certainly are not taking into account large battery banks like Diplostrats. The fatter wire allows the voltage regulator to see one fully charged starting battery and x amount of depleted battery together as one partially charged battery and allows the alternator to make more amps to bring the voltage up and the result is more amps making it into the depleted battery. Flooded batteries need at least 110% and upto 150% the amount of energy taken from them to fully recharge. Its is hardly a use 30 amp hours return 30 amps hours and all is well.

With thinner wire the VR just sees the mostly fully charged starting battery with a small load on it, and this battery does not need much current to reach the maximum voltage setpoint, and as a result much much less current makes it into the battery which most needs it, the house battery tacked onto the circuit which was undersized from the factory.

It is not a matter of how much the wiring/cabling is rated to pass at such and such a length, it is about getting various batteries located a distance apart to read as One large less than fully charged battery, and this is accomplished with fatter cables than are required to pass the current which the depleted batteries are asking for at that voltage. The fatter the cabling, the more they look like one big battery which is at a lower state of charge and needs more current to raise the voltage.

In my experience, fatter cables between alternator and battery have greatly increased charging current, and increased battery performance each night when the alternator quenched them, even briefly. The mile and a half drive to the ocean in the morning can equal .10 more volts under discharge than night, all other factors being equal.

On my Vehicle/ System, Removing the fully charged engine battery from the alternator circuit entirely greatly increases the charging amps into the house battery over the course of a drive, as all the voltage regulator sees is a depleted battery 22 total feet away over 2 awg cabling and the alternator feeds it all it can at any given rpm until 14.9v is reached. This starts ( on my single group 31 when heavily depleted) at around 85 amps, and a half hour later this is still in the 40 amp range +/-8.

My AGM seems to taper less and it certainly takes even more alternator current than the flooded 31. I've seen the single AGM group 27 take 110 amps when I took 75 amp hours from it( out of 91). I jumped on the highway. 65mph, 2000 rpm 86 amps, and 11 miles later it was still taking 74.

One other point I bring up often on this forum concerns the well respected and very popular Sears Die Hard Platinum/ Odyssey battery. The group 31, rated at 100 amp hours, when deeply discharged, requires a 40 amp initial charging current, Bulk current. It requires this much current until battery voltage, measured at battery terminals, not somewhere else along the circuit, reaches 14.7v. 14.7 volts is then to be held for 4 hours. The more the battery is discharged the more important it is that this battery sees a 40% minimum bulk rate, and Odyssey will tell you that one can feed it a 300% rate safely too, just keep it below 14.7v. Lifeline, perhaps the top dog AGM manufacturer, says feed their batteries as much amperage as you possibly can, just keep it below 14.4v.

Try feeding this expensive battery 40 amps with 20 feet of 10 AWG wire from your solenoid, and that person will choke it to death and be one of those ignorant indignant consumers who ruin a good product and demand the manufacturer replace it for free.

 

[Kevin.Hutch]

Prior to around 2000 nearly all bog standard alternators had an archaic, cheap voltage controller called a regulator that simply maintained a preset voltage of around 13.8v or some specials at 14.2v.

In the 90's we started swaping the cheap ($17) alternator regulators for after market multistage ($299+) ones on boats to improve house battery life.

All of us that worked on automotive/marine electrics from the 60's and before, new crude regulators were the norm and I did not realise that the game plan had changed until I got a "Euro 6" Bosch EGM regulated alternator in my 2012 sprinter, that is now seen on many vehicles.

The higher (15v) cooked my standard MPPT solar regulator and forced me to do a lot of research, with help from this and other forum contributors I was able to diagnose the problem and fortunately the solar regulator manufacturer agreed replacing the regulator complete with upgraded firmware.

With the advent of electronic engine management systems the smarts became present to feed the exciter field of the alternator with a real multistage regulation voltage, time, temperature, current and voltage compensation, so now my years of learning/understanding batteries and charging from the 50's had to be revised for new vehicles.

Even smart devices that sense 12 or 24v functionality often use the 15v as the mode switch point, yet another layer of confusion.

The battery/alternator world has changed, the voltage settings for Wet cell, AGM and Gel cells differ and should never be charged in parallel unless they have dedicated charge controllers, hence the programmable voltage settings on modern regulators.

The cut-in/out voltages are a subject of there own and established by the need to separate load from charge circuits when low voltages are present.

Kevin H

 

[Eurovan2003]

DWH and others
Thanks for the info, I really appreciate the sharing of so much on a complicated topic. Its been a great help in understanding more on this.
Phil

 

[superbuickguy]

ummmm, Dave.... a slight issue.

what happens when you drain a full glass into an empty glass? both glasses will actually have less-than 1/2 the fluid (because you can't fill to the rim, and you'll spill a bit when you pour).

Same issue with a dead battery - if 1/2 the charge isn't enough to start you car (such as an electric fan comes on as well); you'll have two dead batteries and no way to get a jump start. The reason the battery controllers, like the T-Maxx I have on my H3 is so cool is it doesn't hook the batteries together when the ignition is in the on position. It waits until it detects a charge from the alternator, then turns the relay on so recharging can occur. Similarly, if your main battery is dead (like happened to me at the Rally), when you link the two batteries together, you're using a fully charged battery (backup) plus the residual in the main battery to allow a jump start.... with yours, it will immediately drain your main battery and potentially leave you dead in the water. The entire T-max kit costs ~130 and includes the controller, solenoid, and connections. Honestly, that's a pretty amazing deal because the material (beside the controller) alone cost about 1/2....

Sorry man.

 

[dlichterman]

ummmm, Dave.... a slight issue.

what happens when you drain a full glass into an empty glass? both glasses will actually have less-than 1/2 the fluid (because you can't fill to the rim, and you'll spill a bit when you pour).

Same issue with a dead battery - if 1/2 the charge isn't enough to start you car (such as an electric fan comes on as well); you'll have two dead batteries and no way to get a jump start. The reason the battery controllers, like the T-Maxx I have on my H3 is so cool is it doesn't hook the batteries together when the ignition is in the on position. It waits until it detects a charge from the alternator, then turns the relay on so recharging can occur. Similarly, if your main battery is dead (like happened to me at the Rally), when you link the two batteries together, you're using a fully charged battery (backup) plus the residual in the main battery to allow a jump start.... with yours, it will immediately drain your main battery and potentially leave you dead in the water. The entire T-max kit costs ~130 and includes the controller, solenoid, and connections. Honestly, that's a pretty amazing deal because the material (beside the controller) alone cost about 1/2....

Sorry man.

That's why I added a timed relay to my system. I have a 3 position switch that lets me pick auto, disconnected, or manual join.

Auto uses the relay, and waits 10 minutes after I've turned the key to "ON" before joining the batteries. The relay is adjustable to any time period you want.
Disconnected is just that - the batteries are not joined.
Manual Join - sends power direct to the solenoid to join the batteries in case you need to self jump.

I can leave the switch in the auto position and not worry about it joining when starting, and it gives the truck time to charge the starting battery before the aux battery is joined in.

Relay can be found here: http://www.zoro.com/g/00062834/k-821TD10H-UNI/ the part number is 821TD10H-UNI

 

[dwh]

There is theory and then there is quantifiable repeatable results.

Great Googly Moogly. I get tied up for a few days, and what do I find when I get back? Mega-posts.

Well...okay. Here we go.

wrc, buddy...I'm a gonna make you eat those words.

First of all, if we were talking about Darwin's theory, then yea okay, theory and real world do have some differences. Einstein's theory? Well...okay, maybe. But basic electrical theory? I'd have to say, "Nah, they've pretty much hammered all the kinks out of that one."

BUT...tell you what I'm gonna do. I'm gonna show that A) you are correct, there is a difference between the theory and the real world, and B) YOU are the one who's theory doesn't match the real world. Tall order? You betcha. But I work well under pressure.

Afterward, you are gonna owe me a beer.


-------------------------------------------------------------------

I've decided that what is needed here is cave paintings. So here's a pic to set the stage:




So what've we got here? Well, we've got a pretty much dead battery, sitting at a resting voltage of 11 volts. We'll say it's a 100 amp*hour battery (at the 20-hour rate). We've got some generic 3-stage charger (let's say a Samlex) that can do 25a on the bulk stage. We've got a 40' loop of #10 wire (because we need to carry 25a, and #12 is too small, so we move up to the next size), which a handy voltage drop calculator tells me will result in a 1 volt drop at 12 volts with a load of 25 amps over a loop distance of 40'. 1v drop. Perfect - makes the math easy.

Now when we turn on the shore power, the Samlex will do a constant CURRENT bulk stage and REGULATE (and also LIMIT) the CURRENT at 25a until the battery reaches 14.4v, then will switch to constant VOLTAGE mode and REGULATE the VOLTAGE at 14.2v until the current flow rate (amps) drops below 2a and then it will stay in constant VOLTAGE mode and REGULATE the VOLTAGE at 13.6v until the energy death of the universe.

Now, I've said this before, in an argument with some guy (names withheld and all that) about (IIRC) solar and I kept feeling that I just wasn't getting my point across. I take full blame for that. So let me say this again, and maybe (hopefully, possibly) it will be clear to all:

IT'S A LOOP!

Yes, that's right - a LOOP. As in a circle. Or a circuit. If we were feeling truly pedantic, we might even say circuitous. Repeat after me boys and girls: Loop. Loopy-loop. Loop-da-loop. Round and round and round we go. IT'S A FRIGGING LOOOOOOOOOOOOP!

People seem to have this mistaken impression, this false idea, this incorrect assumption, this wrongheaded boned up picture in their noggins that voltage drop occurs between Point A and Point B. NO! WRONG! Voltage drop occurs between Point A and Point A. Actually, from Point W, to Point X, to Point Y, to Point Z - AND THEN BACK TO POINT W.

BECAUSE IT'S A LOOP!

Ahem.

Okay, so what's gonna happen when we hit the switch and supply some 2000 megawatt Hoover Dam shore power to the input that charger? Here is what is going to happen:





Now, would someone please whip out their handy-dandy ThinkGeek GoalZero titanium whiffle bat with integral laser pointer and lay that nifty little red dot on the voltage drop? C'mon.

<Jerry Macguire> SHOW ME THE VOLTAGE DROP!</Jerry Macquire>

You can't. I knew that. The reason you can't is because there IS NO VOLTAGE DROP! Even though THE THEORY says there will be voltage drop - in THE REAL WORLD there IS NO VOLTAGE DROP. In the words of the great Master Electrician, Jungian: "Voltage drop is a figment in the imagination of the collective unconscious."





I'll have that beer now. What? Oh sure, Miller is fine. Oh! A fourty! A 40oz. all for me? Why thank you! Too kind, too kind.




Stay tuned for our next episode of "Life, the Universe and Everything", where we'll answer that age old question that has plagued and baffled mankind since the dawn of time: "Dude...did Doogie Howser just steal my voltage drop?"

 

[dwh]

Okay, so what happened to the "theoretical" voltage drop?

Well, the thing is - voltage drop is an IDEA. A CONCEPT. There is no voltage drop, but what there is, is resistance. Now, you can play with the numbers. You can INTERPRET the data. You can say, "Well, with this resistance and this voltage potential and blah blah blah, we'd have X voltage drop." You could also spin the numbers around the big Wheel of Math Fortune and convert the resistance into mega-joules. Or British Thermal Units. Hell, you could even describe it as tension. (Ever hear of "high-tension" power lines? They aren't talking about the stretching of the wire.)

The common wisdom today (common largely as a result of too many people spending too many hours trying to figure out what the hell solar electric engineers are talking about) is that you need to OVERsize wire to reduce or compensate for voltage drop. Well, it's true. In CERTAIN SITUATIONS you will want to do that. In other situations, it just don't matter.


So, what happened to the voltage drop in the second cave painting? Oh, it's there alright, it's just not there in the FORM of voltage drop. It's there in the FORM of resistance. But it doesn't matter because the charger is operating in constant CURRENT mode. In constant current mode, the charger is NOT REGULATING THE VOLTAGE. It's REGULATING THE CURRENT.

[Analogy Part 1: Think of it this way: The throttle on your truck is stuck wide open. There's nothing to can do to fix it, so you just man up and head out. But you don't want to go too fast, so you ride the brakes to REGULATE the SPEED of the truck.]

That's constant current mode. The current is regulated, the voltage is UNregulated. The voltage of the CHARGING LOOP *is* being regulated, but not by the charger. It's being regulated by the battery's voltage. The charger is going to run with the voltage throttle wide open and REGULATE the CURRENT FLOW until the voltage at points W and Z reaches 14.4v. The voltage at those points won't reach 14.4v until the voltage of the LOOP reaches 14.4v. The voltage of the LOOP won't reach 14.4v until the BATTERY reaches 14.4v.

(Diplo and wrc, both of you guys have described what the voltage regulator "sees" - and you've both made some false assumptions. The voltage regulator "sees" the voltage of the loop. It "sees" nothing else. It knows nothing of chassis battery or house battery or combined banks or voltage drop or anything else. (In the case of Diplo's computerized voltage regulator, it also knows the duty cycle of the alternator and I believe he also mentioned temperature, but that doesn't change what it "sees" in terms of the voltage of the loop.))

So, could you decrease the overall resistance of the loop (wire+battery+wire) by using bigger wire? Sure. Would it matter? No, because the voltage throttle on the charger is running wide open and thus supplies enough POTENTIAL to OVERCOME the resistance and FORCE 25a through the charging loop. It doesn't matter if the resistance is a little higher or a little lower - the wide open voltage POTENTIAL is more than enough to overcome the resistance. In other words: The voltage drop (extra resistance) IS being compensated for. It's not being compensated for by having larger wire, it's being compensated for by having a higher voltage potential at the power supply.

[Analogy Part 2: Your truck with the stuck throttle has a trailer. Is it going to matter if you unhook the trailer and leave it or if you drag it along? No, it won't matter - you'll still have to ride the brakes to regulate the speed, because the throttle is still stuck wide open.]

Now, the charger is going to keep running like that, until it sees 14.4v at terminals w and z. At that point, it'll change its operating behavior. It will stop regulating the CURRENT (though it will still LIMIT the current to 25a), and will begin to REGULATE THE VOLTAGE. It will regulate the voltage at 14.2v for the absorb stage, and keep doing that until the current flow drops below 2a.

At the point where the charger switched from CC bulk to CV absorb, the voltage of the loop (regulated by the battery) was at 14.4v. So now no power is going to flow, since the POTENTIAL of the battery is HIGHER than the POTENTIAL of the charger. As the surface charge of the battery dissipates (electrons at the interface between lead and acid dissipate into the electrolyte) the voltage of the battery (and thus, the loop) starts to fall. As soon as that voltage falls below 14.2v, then the POTENTIAL of the loop is lower than the POTENTIAL of the power supply, and current begins to flow.

How much current? Depends on the RESISTANCE of the LOOP. If the voltage potential of the power supply is 14.2v, and the voltage of the battery (and the loop) is 14.1v, and the resistance of the wire+battery+wire is X - then Y amps will flow. It's like a gradient. You could picture it as electrons flow down hill from higher voltage (potential) to lower voltage (potential).

BUT! The RESISTANCE of the loop is NOT FIXED. IT IS A VARIABLE. The higher the CURRENT FLOW (amps) the HIGHER THE RESISTANCE. But right now, the gradient isn't much, so there's not a lot of resistance. The only SIGNIFICANT resistance is in the BATTERY, NOT the WIRE - because the battery is nearly fully charged and the wire isn't resisting much because there aren't a lot of amps flowing.

So, is there any voltage drop? Nope. There is a bit of resistance, which you could play around with mathematically and CALL voltage drop. But it's not like the voltage of the loop is any lower because of the wire size.

As the battery charge approaches 100%, the resistance of the BATTERY is so high that a VOLTAGE POTENTIAL of 14.2v is almost not enough to push any current through the loop. So the current flow (and thus the resistance of the WIRE, and thus the "theoretical" voltage drop) is next to nothing. When the current flow falls below 2a, the charger is going to remain in constant VOLTAGE mode, but begin to REGULATE the VOLTAGE at 13.6v instead of 14.2v.





Okay, so that explains how Doogie Howser stole the non-existent imaginary voltage drop.






Stay tuned for our next episode, where we'll restate (perhaps even with lucidity) "dwh's Rules for Sizing Wire" and see if we can take a look at some real world case histories.