Will I have charging problems? (solar, 2 batteries, auto-bridging)

[Herbie]

Hiya experts. I'm at a spot where my experience and electrical theory meets my lack of experience in the realities of fancy AGM battery charging. I'm a bit worried that I'm setting myself up for less-than-optimal charging with my current setup.

For a couple of years now I've had the T-Maxx dual battery setup. Sears Platinum AGM batteries for both starting and house use. Group 34M deep cycle for the house battery, and Group 34 starting battery.

The T-Maxx has the usual feature of monitoring voltage on the battery terminals, then closing a solenoid (after a delay) to bridge them when it senses a charging voltage. This happens both on the alternator and with a charger connected. After about a year the cheap solenoid that came with the kit failed. I got a bit nervous that it was partly due to the solenoid being engaged for VERY long periods if my van was plugged into a charger for weeks at a time while parked. Also, as the charger would do its "thing", the solenoid would periodically open and close as the "maintenance" charge dropped below the threshold the T-Maxx uses.

This is question #1 - I've since replaced the solenoid with a proper Cole Hersee 200A continuous duty unit. Should I worry about the impact this VERY long activation and the periodic cycling, or just chalk up my previous failure to cheap components and rest easy now that I've got a quality solenoid?

After recently installing my solar setup, I realize now that the connection behavior also happens if the charging voltage is applied to the Aux (house) battery. I have a 1st-gen Morningstar SS-6 connected to the house battery, and it too will cause the T-Maxx system to periodically connect then disconnect the starting battery.

This is question #2 - I'm worried that the charger may not be able to do a proper full charge if it's seeing a changing "load" because the second battery is connected then disconnected frequently.

I'm less worried about "in the field" performance, since just bulk charge during the daylight hours will already be a huge boon to my system, but I'm concerned that I may be shortening battery life or at least wasting possible capacity when the van is parked for long periods of time.

This is question #3 - Should I add a manual switch to override the T-Maxx behavior and either (a) force the solenoid to stay closed when parked (thus presenting a constant battery load to the charger(s)) or (b) force the solenoid to stay open and "divorce" the charging systems when parked for long periods? This will mean going back to manually plugging in the house battery to a charger and letting the solar charger act on the house battery alone, but ok, fine.

Or am I worrying about nothing?

Thanks!

 

[Howard70]

Hello Herbie:

I leave my starter and auxiliary batteries "unparalleled" (perpendicular?) when not using my truck and I leave the solar panels charging the auxiliary battery through the Morningstar controller while the rig is parked. If it sits for several weeks I'll parallel the batteries for several days to get the starter charged up and then "unparallel" again. I don't leave an AC charger connected for long periods - I don't have one that I trust to deliver an effective "float voltage." My auxiliary and starter batteries are not the same size, chemistry, or manufacturer so I wouldn't want to leave them paralleled for long periods. Your system might do OK with the two batteries paralleled for weeks, but I'd make sure the AC charger was a high quality unit that would keep the float voltage and current appropriate for the total of the two batteries. I agree with you that when one component is connecting and disconnecting the two batteries while another component is trying to apply a three step charge sequence, neither will do their jobs optimally.

Howard

 

[DiploStrat]

Methinks you are worrying about the wrong things. Sounds to me like you have exactly what you need. You certainly have exactly the system I have, albeit smaller and with different brands of gear.

This is question #1 - I've since replaced the solenoid with a proper Cole Hersee 200A continuous duty unit. Should I worry about the impact this VERY long activation and the periodic cycling, or just chalk up my previous failure to cheap components and rest easy now that I've got a quality solenoid?

I don't know the Cole Hersee relay, but my bet would be "no." I use a 200A Blue Sea unit rated for thousands of cycles and equipped with a coil economizer.

After recently installing my solar setup, I realize now that the connection behavior also happens if the charging voltage is applied to the Aux (house) battery. I have a 1st-gen Morningstar SS-6 connected to the house battery, and it too will cause the T-Maxx system to periodically connect then disconnect the starting battery.

This is question #2 - I'm worried that the charger may not be able to do a proper full charge if it's seeing a changing "load" because the second battery is connected then disconnected frequently.

I'm less worried about "in the field" performance, since just bulk charge during the daylight hours will already be a huge boon to my system, but I'm concerned that I may be shortening battery life or at least wasting possible capacity when the van is parked for long periods of time.

Your system is exactly what you want to tend your batteries when your truck is parked for long periods - don't mess with it!

Assuming that the T-Maxx has reasonable settings, e.g., connect at 13+ volts and disconnect at 12.7 volts, you should not see that much cycling. Morning Star is a very well regarded brand. Unless your unit is somehow atypically bad, it should do a great job of floating your batteries. You WANT to keep both starter and camper batteries connected together as long as you are getting a charge from either the alternator or the solar kit.

Shore power is another issue. If you have an old "converter" then you probably need to disconnect it from time to time. If you have a modern inverter/charger, then you can leave it connected. But if you have any sunlight at all, you probably won't need to.

This is question #3 - Should I add a manual switch to override the T-Maxx behavior and either (a) force the solenoid to stay closed when parked (thus presenting a constant battery load to the charger(s)) or (b) force the solenoid to stay open and "divorce" the charging systems when parked for long periods? This will mean going back to manually plugging in the house battery to a charger and letting the solar charger act on the house battery alone, but ok, fine.

You can certainly add a manual switch, should you desire, but you probably only need it for a force combine/auto jump function. Unless your solar kit is much too small or malfunctioning, it should hold your camper battery well above 13 volts, at which point the relay will close and the solar kit will tend both of your batteries. This is certainly how my system works.

The most common problem with a key controlled relay is that the starter battery never benefits from solar or shore power charging, so, after a long, cold winter, your truck won't start.

Final notes:

-- Your camper battery is very small, only about 30Ah usable and you would get better performance with a real, deep cycle battery.
-- Beyond your relay sizing, make sure that your cabling is large enough to carry the full output of your alternator or solar charger, whichever is larger, over the distance between your two batteries.

 

[4RunAmok]

Diplo nailed it (again), Herbie.

 

[Herbie]

Yeah, relay is 200A and cables are 2 gauge jumper cables. That has worked so far.

The relay will cycle a dozen times or so during the day, then a bit more frequently in the evening hours as the solar array starts to fade.

Battery is what would fit in my old cabinet, and does pretty well for weekend trips, especially with the solar. I made sure to build my new cabinet to accommodate a larger unit, so eventually I'll upsize.

Edit: the T-max system already supports force-close of the relay for self-jump or winching, I'm just worried if I need a "defeat" switch.

Sent from my Nexus 5 using Tapatalk

 

[007]

So the Tmaxx is connecting at 13v and disconnecting at 12.7v or less?

I think the traxide sc80 soleniod disconnects at 12.0v and that seems like a better setting to me if solar charging with variable light. I believe the traxide system has an adjustable voltage setting so you can set it to what you need. I'm just learning this stuff, so don't take my word on it.

 

[DiploStrat]

State of Charge

12 volts is much too low for a disconnect voltage as that would be something on the order of 75% discharged. The goal here is to disconnect BEFORE either battery is discharged. The resting voltage of a fully charged 12 volt battery is somewhere near 12.7 volts at 70F. Thus, if the voltage of the battery is over 13 volts for any length of time, it is a strong indication that the battery is under charge and the batteries should be combined so that both batteries can benefit.

This link may help. http://www.energymatters.com.au/renewable-energy/batteries/battery-voltage-discharge.php

In my case, I often hear the batteries disconnect after I turn the ignition key as the truck demands power for glow plugs and other loads. The batteries reconnect about 30 seconds after engine start as the starter batteries recover and the voltage rises above 13 volts.

 

[007]

I know my vehicle starts with plenty of room to spare at 12 volts. I understand not wanting it to sit at that voltage for an extended time, but if its going to get recharged in the next day or two, what is the harm? The advantage to keeping your batts combined untill 12volts, is that your house battery would be reinforced, and not be drawn down as much during the evening and night.

Its my understanding that batteries prefer to not be discharged too much. Keeping them combined untill 50% of capacity seems to be a viable option since you get a larger overall battery bank to pull from. In addition, your house battery should be healthier as it has less discharge, and its faster to recharge two batteries at 50%% than it is one at 0%. Why should the house batt take all the discharge? I think if you have similar batteries, and you purchase them and replace them as sets, it makes more sense to treat them similar and share the power load to a point.

If you are running the vehicle often, or maintence charging from shore, these 12volt cutout soleniods seem like a viable choice.

If there is any truth to batteties needing to "flex" or exercise there voltage, then this system might help the starter battery out.

 

[DiploStrat]

Seeking a Level

When batteries are combined, they seek to equalize their voltage level. So, grossly, a battery at 13v combined with a battery at 12v would (all things being equal, which they are not) end up at 12.5v. That is why you are always told not to mix old and new batteries, different types, etc.

The Traxide appears to be a smart combiner or relay, programmed to allow a 50% discharge of the starting battery. Scholars might differ as to whether this is wise or not. But, by allowing this deeper discharge of the starter battery, they are increasing the logical size of your total battery array and helping to reduce the Peukert effect. The Magnum SBC that I use allows me to do this, but only to 12.5 volts. An interesting aspect of the Traxide web site is their advocation of a relay based system as opposed to a Battery to Battery system.

In the case of automatic or "smart" combiners or relays, the usual program works this way:

-- The batteries are separated until either battery rises above the combine voltage (usually 13.2v) and stays there for some period of time. This is to confirm that there really is a charge source present.

-- When the combine conditions are met, the batteries are combined. They will stay combined as long as the voltage, again at either battery, stays above the disconnect voltage (typically about 12.7v). Most systems also have a high voltage disconnect, either adjustable (mine is set at 15v) or fixed, usually 16v.

-- As long as there is a real charge source connected, both batteries will stay connected. They may disconnect early if the charge source is too weak to hold both batteries above the connect voltage. (The SmartBridge offers a "Hold" option to force a connect for a minimum of up to 90 seconds.)

-- Ideally, your charge source(s) will stay connected long enough to get both batteries to their float voltage. As a practical matter, it is unlikely that you will ever run your engine long enough to do this, so the final, acceptance/absorb charge will come from solar, shore, or genset.

-- When all charge sources are disconnected, the batteries will seek to equalize their voltages. When EITHER battery drops below "full" resting charge, again, typically around 12.7v but adjustable on some devices like the Magnum Smart Battery Combiner and the SmartBridge, the batteries will separate. This is what you want as it prevents either battery from discharging the other.

-- The batteries will stay combined until a charge source is connected and the voltage, at EITHER battery, rises to the connect voltage, etc.

This system works very well, but it sometimes confuses people as neither the connection nor the disconnection is immediate, as it is with key or ignition controlled systems. When combined with a modern solar controller or inverter/charger with a proper float function, it is safe to leave the batteries combined indefinitely.

Technically, lead acid batteries do not have a "memory" effect like nickel cadmium batteries. (Ni-Cad batteries must be fully discharged if they are to take a full charge.) But, lead acid batteries must be fully recharged as quickly as possible. If they are allowed to stand discharged, or if they are not fully recharged, then they will loose capacity. (An AGM battery will also loose capacity if it is charged at too high a voltage as this will cause it to vent moisture. With an open lead acid battery, you can add distilled water; with an AGM you cannot.)

The news you can use is this:

-- Lead acid batteries, sealed (AGM) or open, need to be fully recharged as quickly as possible after a discharge or they will lose capacity.

-- It is unlikely that you will ever drive long enough to accomplish this with your truck's alternator, so a solar kit is extremely useful to complete the absorb charge phase. While I would always recommend the largest solar array you can fit or afford, it is important to remember that the absorb/acceptance stage requires relatively little current, so even a single 100w panel will do a lot of good.

-- Absent a solar kit, you can make up the difference by plugging in shore power or running a genset. Here again, time is important, Lifeline batteries, for example, call for about two hours of absorb charge after they reach "full" charge.

-- The advantage of a bidirectional system is that it cares for your starter batteries as well as your camper batteries.

N.B. If you REALLY want a key controlled "dumb" system, Blue Sky solar kits, and probably others, offer an "auxillary" output which can be routed past the relay to the starter battery. Similarly, there are products like the Trik-L-Start http://www.lslproducts.net/TLSPage.html that address this problem. Of course, this begs the question, why not simply do it right from the start.

As always, all of this is correct to the best of my knowledge, belief, and observations on my own truck. YMMV.

 

[Herbie]

OK DiploStrat, you've convinced me. I'll leave the system to operate in "automatic" mode and learn to ignore the periodic clack of the solenoid as the T-Max couples and un-couples the starting battery in response to the output from the Sunsaver.

I may sit down with a meter at some point and try to figure out the hysteresis points for coupling and decoupling. Can't seem to find a published spec on that (not in my instruction sheet, etc.), and I have to admit I'm curious.

 

[007]

Diplostrat -

Great explanation.

My theory is that you want to avoid big discharges, and recharge as soon as possible, I think everyone agrees on this.

The next part is where it gets confusing....

Suppose you set up camp and the sun goes down. You have two group 31 batts. (starter & house) Your fridge, lights, radio, heater, t.v. etc. all start to consume power.

Expo man A. has an isolated system that disconnects the starter batt. instantly after sensing a drain, leaving the house battery to bear the full brunt of discharge (hypothetically 50% In this case.)

Expo man B. has an isolated system that disconnects only after the starter battery has been discharged to 50% (12v). On this night, he ends up with two batteries that stayed connected and now have discharged 25% each.

The next morning, Expo man A has to recharge one battery back up from 50% Discharge, and Expo man B has to recharge two batteries up from 25% discharge.

In my experience, its faster for the alternator/solar panel/ gen set, to recharge two batteries up from a small 25% discharge, than it is to recharge one battery up from a large discharge, because one battery can only absorb so much amperage.

It seems to me that Expo man B will have longer lasting batteries, because his system is better for the house battery, (less discharge, faster recharge) better for the user, (more available juice from his batt. system), and it would also increase the longevity of the battery set since both batts would live a more similar life.

If you are one that purchases and replaces "like" batteries as sets, I would think that abusing one, and not the other would cause a premature life of the set.

 

[wrcsixeight]

In my experience, its faster for the alternator/solar panel/ gen set, to recharge two batteries up from a small 25% discharge, than it is to recharge one battery up from a large discharge, because one battery can only absorb so much amperage.

Not saying you are wrong here, can't argue with personal experience, but in mere terms of replacing amp hours, a single battery should be able to do a 50 to 80% recharge faster than 2 batteries trying to do a 75% + recharge, Once that 80% threshhold is reached the batteries accept much less current, so if the recharging source is able to exceed this relatively small amount of current, then the more depleted single battery will be able to take advantage of this greater current and replenish more of the amp hours, if of course the cabling is adequate to pass the current.

If the current source is not very big, like solar, then discharging 2 batteries together will yield better life from the batteries as the overall discharge is less per battery.

 

[007]

It depends on the size of your alternator I guess. An alternator typically puts out more amps than one battery can absorb. If you have an 80 amp alternator, one battery at 50% discharge isn't going to absorb all that amperage at once, some of the amps will be bled off. Two batteries that are 25% discharged (at 75% capacity) could be absorbing all the alternators charge, (40amps ea.), so it would be quicker for the alternator to recharge these two batteries that started at 75% capacity, than it would be to recharge one that started at 50% capacity. I'm talking bulk charging up to say 95% capacity.

 

[4x4junkie]

I also agree two 25%-discharged batteries should recharge faster than a single 50%-discharged one.

Here's my reasoning:
A single 50%-discharged battery will have to be bulk-charged for a good 30% of it's capacity before absorptive charging can commence.
Two 25%-discharged batts OTOH might need to be bulked for only 5% of their total capacity before absorptive charging begins.
I also would think the absorb charge time is also slightly lengthened if the battery is coming from a much deeper discharge.

So keeping the batteries paralleled for a length of time before they auto-disconnect makes plenty of sense. Of course this does require your "starting" battery be capable of deep-cycle use also, otherwise them disconnecting right away makes more sense.

 

[rblackwell]

Comments:

Observation of my 3x255 amp hour AGM house pack is that the deeper the discharge the more current the batteries will accept (up to the chargers max).

Once into the acceptance phase charging slows down as voltage remains constant and current falls.

Thus you can replace more (though not all) AH faster is a battery is deeply discharged.

Hence I would expect that Expo man A with the 50% discharged house batteries would put more AH back into his rig in a given time frame than Expo man B.

Final thought - I would NEVER use my truck starting batteries for domestic supply (like Expo man B) when camped in a remote place. All you need is some piece of kit (like the battery isolators) to fail to work correctly and you have a truck that wont start. In some places in the world that could be fatal.