My Dual Battery plan! Battery charge current draw?

[mburrows]

I am in the process of designing a dual battery system for my 98 Suburban that I will be driving to South America. I have sourced most of the hardware required and got a good deal on 2 330 Ah batteries. The only issue I am having is how to calculate the amperage the auxiliary batteries will draw when being charged by the alternator. I found the internal resistance on the battery spec sheet to be 0.003ohms, however I find this to be low for a partially charged battery as the following calculation shows I could never get this much current.


I am assuming ideal charge conditions, in reality the Rinternal will be in series with wire resistance etc.
I= (Valternator - Vbattery)/RInternal
I = (14.5V- 12.7V)/0.003ohms
I = 600 Amps

I suspect that when charged the internal resistance is higher. Is this assumption correct? I am concerned I will be overloading my alternator by drawing so much current and would like figure out what the current draw will actually before I wire it up.

Here is a schematic of what I plan to install.


-Marshall

 

[Scoutn79]

I may be off here but I think you are over thinking the issue.

How many amps the alternator will only put out, given a healthy alt and proper wire size (in reference to charging the battery(s)) is based on either:
1) Max of the alt capability or max at given alt rpm.
2) only as much as the batteries will take.

So if you have 15 330ah batteries almost fully dead a 30 amp alt will charge them it will just take a very long time, and likely overheat the alt if it isn't a heavy duty unit due to the heat output over such a long time.
Also a tiny 12 volt battery will get properly charged from a 200 amp alt.

Good luck finding a 600 amp alt and the belt system that would be required to drive it and the 60 hp or so required to drive the alt at that output, and even then the batteries will only take that for a very short period of time before the amps the batteries will take. The closer to a full charge the less amps the battery will allow.
When charging a battery with a big 150+ amp alt. 80% of the charge is put back into the battery in the first few minutes, the rest is a decreasing amp requirement.

For a real world example. I use a home brew DC generator to charge our batteries when hunting.
Between the furnace, lights and my friends heated CPAP unit we really put a dent in the batteries over night. I have a 53 map max (measured) alt driven by a 4.5HP pressure washer engine. When I first fire it up it will put out the full out put for only 10 minutes. After that the amps start being less and less. The batteries are getting only 25 amps after about 20 minutes and then it just keeps dropping. There is probably 45 minutes spent getting the last 10 amp back in the batteries before they are fully charged.

Clear as mud?
HTH

Darrell

 

[DiploStrat]

Where to Begin?

At the risk of being snarky, where are you going to put over 150kg. of batteries in a Suburban? :Wow1:

To your next question, the alternator cannot produce more than its rated output. Looking at your diagram with a guesstimated 5M of 4AWG wire, I doubt that you would see much over 50A of charge at the batteries, probably less. (That assumes that your '98 Suburban runs at over 14v. If it is a 13v system, then you will see much less.) But, assuming that you upgraded the cables so that the alternator could actually "see" your camper battery, the question would be, how much can it produce in the real world and how long before it overheated and failed?

Shoot me an e-mail and I will send you a small paper that I wrote that wrestles with some of these issues.

Basically, my truck has a 600Ah battery bank and I consider that 2x125A alternators + 500w of solar is about the minimum to keep them charged. (I also have a 125A charger for use when I have 110v shore power.) In your case, I would suspect that you are in the market for a 200A, marine grade alternator and some properly sized wires, 2/0 at least. Similarly, I would not bother with a shore power charger that puts out less than 25A.

N.B. 600Ah of battery will give you approximately 300Ah usable at 20C, less at lower temperatures, such as you will encounter on the Altiplano. While I think you have over estimated the amp draw of your fridge, you may still get only 1/2 the autonomy you are projecting.

I would not bother with a dash mounted ammeter, but would invest in a serious battery monitor like the TriMetric or, if you want to avoid a shunt, the SmartGauge. (Available in the US/Canada through Balmar.)

For news you can really use, peruse this forum a bit. Several of us have posted (at length) about what we have found to actually work.

 

[wirenut]

Your math completely leaves out the resistance of the wires, connections, windings in the alternator, etc.
I agree, you're over thinking. The alternator will put out whatever it can as long as the batteries will accept the charge. As they fill up it will taper off.
If you're going to be trying to fill up those batteries very often I would definitely get a bigger alternator. Maybe a dual alt. set up with one dedicated to those big batteries and one powering the stock vehicle wiring.

 

[mburrows]

I guess I should have explained the purpose of asking what the internal resistance of 12V batteries is. The purpose was to size my alternator and wiring. By over-thinking the matter I am just trying to come up with an estimate of load on the electrical system. This to me, is important because:

1. The load on the alternator may cause failure. If I theoretically need 600 amps to charge a mostly dead battery then my alternator will struggle to put out 100amps for a long duration and would likely fail. Example: 660Ah battery 80% discharged = 528Ah required to charge. My alternator would try to output 100 amps for 3 or 4 hours until the current is reduced as the battery charges. Sounds like failure to me.

2. Determine Wire size. I need to get as close to 14.5 volts at the batteries to fully charge them, therefore knowing how much amperage they draw is important. My current alternator will output 100 amps max and 14.5 volts. If I use 2 gauge wire than I'm looking at a 1.6 volt drop at 100 amps (= 12.9 volts at the battery). Of course if I upgrade to a 200 amp alternator than I am looking at a 3.2 volt drop (= 11.3 volts at the battery). You see the problem? I need to know what my batteries want before I upgrade my alternator and then I can determine my wire size. I realize that the 100 amp draw will occur when the batteries are almost drained and as they charge the current draw will decrease to 0 at fully charged.

For those of you that have upgraded their alternators, how did you determine the size? Was it a best guess that you needed to upgrade or a chart etc?

DiploStrat, I will be storing 150kg of batteries above and behind the wheel well where the rear HVAC unit was.



Wirenut, I realize I omitted wire resistance etc, for the purpose of determining what the maximum current draw will be.

I am assuming ideal charge conditions, in reality the Rinternal will be in series with wire resistance etc

Thanks for all the info, you guys have definitely given me some food for thought on how I'm going to build this system.

 

[Scoutn79]

As I am sure you know. Your wire size will be determined by the highest amperage that will go through it. So if you are running a dedicated charge wire from the alt. to the battery then you need to size the wire to the alt. As stated, even if the batteries will take 500 amps, if the alt. is only rated for 100 then that is all that will go through the wire so size the wire accordingly.
However if you are charging from another point (junction block to keep from running redundant wiring due to long wiring runs) but running a sense wire to the battery, but not the actual charge wire, you need to size the wire for your biggest load, say a winch. Your wire from the alt. to the charge point needs to be as big as the alt. requires.
As you know the bigger the wire the less voltage drop you will experience. You may not get full voltage to the battery at full alt. output but as the amps drop the voltage will go up so you will still get a good charge at the end of the charging cycle.
Are you running a separate alt. for these batteries, or able to switch charging duties from chassis to house batteries?
I think you already answered your own questions. If you have heavy, frequent charging to do bigger is always better..higher output alt. and bigger than required wiring with soldered on connections.

There is nothing wrong with over-thinking a modification before you do it, saves you from having to redo it later, but I think you have your answers already.

If you are really worried about over heating an alternator look for a commercial duty unit that is 100% duty cycle. They will cost more and are quite a bit bigger but will handle the work.

One other thing to consider, If you are going to be stationary for long periods of time and charge the batteries from an idling engine you won't be putting out full alt. load, so heat won't be as much as an issue.
Example: my CS144 puts out a max of 150 amps but only puts out 45-50 amps at idle.

If you do infrequent charging at full load while driving the alt will handle this type of work.
Newer cars run larger alternators than the olden days. This isn't because we need more amps for charging but need more amps for running all of the on-board electronics. So they are designed to handle more output for longer periods of time. Better heat sinks and fan designs in the newer alternators goes a long way to adding life to them.

When I upgraded my alt I used a chart...guessing doesn't work here as there are way to many variables (amps, length of run, in free air or bundled, voltage drop for electronics etc.)

The charts I have show (for automotive use with a .5 volt drop) for a 20ft run and 4AWG wire you can run 185 amps. Since you Suburban will likely be a longer run and 200 amps possibly then a 2 AWG wire will carry 303 amps 20 feet.

Hope some of this helps

Darrell

 

[DiploStrat]

It's Not That Hard

Hmm. I had a pair of gasoline tanks there. Do be sure to vent your batteries, especially if you are using open FLA.

I have almost exactly the kit you are describing. A quick "Search" of this sub forum will show that I have posted, almost ad absurdum, as to what I think works. Dig around on this sub forum a bit; there is a wealth of good information and it won't take you long to discover who has actually done what you want to do. And gotten it to work.

The first thing you need to know is the output voltage of your alternator/regulator. Grossly, if it is 14v and above, you are home free. If it is near to 14v, then you can tweak it. If it is 13v, then you are looking at various options, such as two dedicated alternators or something like the Sterling Alternator to Battery charger.

Assuming that you have a single GM alternator of 100A or so, you can:

-- Simply live with it.

-- Replace it with a 150-200A model, preferably one with a high duty cycle and thermal protection, or,

-- Install a second alternator using a junk yard or aftermarket bracket. (The HiFi nuts make a lot of multi-alternator brackets to power their silly subwoofers.) In this case you will need to slave it to the primary alternator or install it with a dedicated, multistage regulator. There are advantages to both approaches and you could even use both approaches.

Your wire run is much shorter than mine; guesstimate about 75A down a 1/0 cable. I run a pair of 1/0 cables, one from each starter battery to the camper battery bank. That passes 150A easily. (Yes, I measured.)

Finally, your engine alternators will NEVER fully charge your camper batteries for many reasons, starting with the fact that you will never drive the truck enough hours. You will need to top off with shore power and/or solar.

While my truck has not been plugged in for months, I use a large Magnum Inverter/Charger, rated at 125A. Mostly, however, I depend on a 500w solar array to complete the charge, a task which it performs admirably. Were I thee, I would try to make sure that I had at least 200w on the roof, 400w would be better.

Finally I would replace the key controlled solenoid with an intelligent relay, rated at 500A. This will help tend your starter batteries and give you the option of self jumping, should you ever need to. (As on a cold morning on the altiplano when the automatic choke has closed and your forgot to hold it open by hand. Be glad you are working with a more modern truck! )

Again, you are on the right track, merely worrying too much about the wrong details.

 

[wrcsixeight]

If I were more dependent on my alternator for battery charging, and travelling to more remote areas more often, I would make a cold air feed tube to the back of my alternator. My alternator pulls air through the back and pushes it out by the front pulley so in theory I should be able to use the Alternator fan to suck in cool air from outside the engine compartment.

I've been wanting to put a thermometer on my alternator to see how hot it does get when feeding depleted batteries and compare it to when the batteries are already full.

The back of my alternator is only an inch or 2 away from the exhaust manifold. I fabricated a heatshield between the 2 and my hot idle speed amperage rose by 5 amps when the batteries were depleted..

 

[mburrows]

Well I got to work last weekend and installed my dual batteries. I'll spare the details of the install but I'm getting 14.3 volts at the end of 20 ft of 2 gauge wire. Happy with that. I didn't install the ammeter so I guess I'll never know how much the batteries actually draw. Waiting for the fridge to test out how long the batteries last.

 

[unseenone]

Kind of neat, but it seems like overkill to me. I 50ahr battery and a solar panel would accomplish the same thing for less money and weight. What am I missing?

If you're dead set on spending money, I know a company that can build you a high output single, dual or triple alternator, in case you run into a small town you want to power.

Most 12 decent fridges are going to use 30-40 amp hr a day max.

I'm not trying to be a smart alec, just trying to understand.

 

[lysol]

Kind of neat, but it seems like overkill to me. I 50ahr battery and a solar panel would accomplish the same thing for less money and weight. What am I missing?

If you're dead set on spending money, I know a company that can build you a high output single, dual or triple alternator, in case you run into a small town you want to power.

Most 12 decent fridges are going to use 30-40 amp hr (Ah) a day max.

I'm not trying to be a smart alec, just trying to understand.

A - Amp
V - Volt
W - Watt

Ah - Amp hour
Wh - Watt hour (My favorite measurment as this encompasses all types of Volts)

You need to understand that everyone's daily power needs are different.

My case is going to be different than most because I want to be able to power about 90% of my needs without needing to run the engine/alternator... ever... this immediatelly requires a large battery bank + solar and/or wind. My fridge will pull about 1A/12V for 24 hours a day (300Wh). Both of our phones charging from 0-100% once a day will pull combined about 4A/5V for 3 hours a day (144Wh).

This is just 2 things that will pull power daily so for the sake of time, I'll keep it simple with these 2 devices (fridge and 2 phones). 300Wh + 144Wh = 444Wh. Now, adding in an additional 15% for inefficiency of wiring/voltage conversions, I'm up to 510.6Wh or let's make it an even 510Wh.

My plan is to use a 100Ah 12V battery in the back of my Jeep. Most people would think they can use the entire 100Ah range.... WRONG!!! Now... following normal practice of not allowing the battery to discharge below 50% SOC (State of Charge), I'm only able to use 50Ah at 12V with this big battery. ***This is very important if you want your batteries to last. So, to get my Wh's for this battery, 50Ah * 12V... I get 600Wh's of useable, safe power.

Could I squeeze out more than 600Wh? No problem with a deep cycle. Using the entire SOC range, I could use 1200Wh's.. Though this would likely kill the battery very quickly. Most deep cycles can be taken down to 20% SOC. So using 80% of the battery would give me about 960Wh's. But, I would rather try and stay safe and keep my minimum SOC at 50% so 600Wh's it is for me.

In order for me to refill this power with solar power, I would need to replenish around 600Wh's per day. Now I could get a 600W panel (NOT!!!!) and run it for an hour (and most likely cook the batteries by pushing upwards of 50A max power...), or a 100W panel and collect for 6 hours (more reasonable and closer to 8A max power). Most batteries don't like to recieve more than 30% of their power capacity. So for a 100Ah battery, you don't want to pump more than 30A into it (refer to each battery's manual for the exact number/%).

This is just me getting off topic... but remember that you can either slash your battery and/or solar setup to save money and try to justify it with words, but when you need to power everything at once one day, you'll quickly understand that planning for the right amount is worth it. Also, another hard part is remembering to plan for extreme temps. In the summer, my fridge my get up to 25Ah per day from cycling so much, but in the winter I wouldn't be suprised if I didn't get over 15Ah/day. Just something else to consider. Lot's of flexible pieces to this puzzle of getting the right amount of power for your needs. Another could be solar irradiance, or the difference in the amount of useable sunlight per day for your geographic location + time of year (season) + the orientation of the solar panel(s)... lol... list keeps going...

At the bare minimum if I was just wanting to run the fridge with no engine/alternator nor solar, I could run it off the 100Ah battery for about 3-4 days straight under extreme conditions and maybe even upwards of 5-7 days under normal to excellent conditions. This is somewhat a cool factor but could be usefull in running the fridge over a weekend with no input whatsoever. OR... if you are Overlanding and put your vehicle in a shipping container for a few days when in between countries.... you could have cold beer waiting for you on the other side... lol.


This is why my setup is taking so darn long....

 

[Scoutn79]

Nice looking install. They look like AGM batteries that won't require a vent, are they?
You can definitely run a fridge, TV, stereo, coffee maker for a few days with that set up.
Over kill? Yes definitely but you won't have to worry about not having enough battery power.
I can run my pop up with a 43 qt Engel, furnace (runs only a few times during the night), lights and water pump for about 4 days with one group 24 deep cycle battery.

Darrell

 

[unseenone]

Thanks for the detailed explanation. It's a great setup if you have space for it. Can't you cut the losses due to wiring by over specification of the cable sizes, to avoid losses? If you could mount a 250w 24v house solar panel, you will never have to run the vehicle.

 

[david.lemke]

what kind of batterys are those?
around here everyone is only telling me the run time in min and trying to sell me on this
no one ever tells me the aH

 

[mburrows]

Wow lysol that's a great explanation and example.

I realize it's over kill, however I got a great deal on the batteries so why not go big.

Also we plan on driving to a location and parking for 3-10 days while we explore. It's too difficult to anticipate power consumption accurately enough to size a battery setup on minimums. If I have grossly over estimated then its peace of mind that I won't have to worry about running the truck just to charge the batteries.

The reason for not going solar- Cost is high, I paid $150 for the batteries, a solar setup will be $300 or so. With solar I will still need a battery, wiring, regulator and charger.

The batteries are AGM an therefor won't vent or leak. I purchased them from a company that was replacing its battery backup bank. They are 2 years old, however they have never been used, as the backup was never required. I did test them prior to installation and am happy with their output.

David.lemke,
They are DataSafe HX330. I was confused at first too by all the different jargon used to rate and sell batteries. I am no expert but from what I understand you can convert any rating to Ah or vice versa. Ah are common for deep cycle but so is RC which is Reserve Capacity. RC is the time in minutes it takes to discharge the battery to 10.5 volts outputting 25Amps. If I was purchasing a battery with an RC of 792 minutes then....
792 minutes * 25amps = 19800 Amp minutes / 60 minute in an hour = 330Ah

I suspect there is some reason that the conversion isn't as simple as I have made it out to be. Perhaps at a different discharge rates the efficiency of the chemical reaction changes but for my purpose this works.