Off Grid/RV Lead Acid Maintenance, Charging, and Failure Modes

It’s 2018, and if you need power storage, there’s a great range of battery options out there.  You’ve got your bog standard flooded lead acid, AGM lead acid, lithium iron phosphate (LiFePO4), and of course the newer lithium ion chemistries (Tesla’s PowerWall being the most well known here).

This is a companion discussion topic for the original entry at

(Comments from Blogger)

2018-04-15 by Shaz-Au

Thanks for the handy post, it will be a great reference.

I’ve got a couple of questions:
- Do you ever need to add acid to a battery instead of distilled water and if so why and when?
- I was always taught when maintaining car batteries to “just” cover the top of the plates. I’ve been under-filling all these years right?


2018-04-16 by Russell Graves

You shouldn’t ever need to add acid to a battery. The only times one should do this are if you get the battery “dry” (at which point you’ll add the supplied electrolyte), or if the battery spills a significant fraction of the electrolyte out. But at that point, matching it to the rest of the pack is an exercise in driving yourself up the wall, so try not to spill them in the first place.

You lose a bit of acid through the gassing process (the gassing is just hydrogen and oxygen, but it can bring acid vapor out and you can lose some that way), but it’s not enough to worry about compared to the total volume of the battery. It just makes a bit of a mess of the enclosure. You can get caps that help drop that mist back into the battery, if you care.

As far as car batteries, I’m really not sure about the water height, but that sounds low to me. If it’s only barely covering the battery plates, it means that if you park on a slope, part of the plate will be above the electrolyte, exposed to the air. That’s a great way to corrode plates in a hurry. So I’d fill higher. For deep cycle batteries (at least the Trojans), you fill to a bit below the lower lip of the fill well (1/8" is the spec in the Trojan literature), or to whatever the battery watering can does, which is about right. It’s more important to be consistent between the cells than it is to be exactly on spec.

2018-04-29 by Shaz-Au

Thanks for the detailed reply!

2018-10-31 by Unknown

Quick question for you any answers would be appreciated. I have an RV with a PV system with 8 Crown CR 235 battery Morningstar controller and a 1400 watt solar array. Went on a long road trip very far off grid and lost a lot of water from all of the cells in the batteries from a very long bumpy road apparently it’s lost out the only water source we had was well water that has a softener hooked to it. So I refilled it batteries and now I am not getting as much charge as I was as in they are not lasting as long. Not sure what I should do. Maybe dumped out all the batteries and add fresh battery electrolyte premix to all of them. Anything I can do to save the batteries would be great thank you as they only one year old

2018-10-31 by Russell Graves

If you lost liquid in the battery bank due to physical sloshing from a bumpy road, you lost electrolyte, not just water (unless the cells were completely drained, which would be a separate issue). So, when you added water, you got back to a weaker (more dilute) acid, at best.

Except you added well water, through a softener, which has a wonderful mix of all sorts of stuff that’s somewhere between “not helpful” and “actively harmful” to lead acid batteries. So you have weak acid with ions and such that are probably toxic to the battery.

Honestly, at this point, either live with them as they are until they die, or replace them. You’ve likely ruined them completely, and spending time fiddling with pouring acid back in is unlikely to work. You might try adding acid until the fully charged specific gravity gets up to the rated level, but that’s often hard to find, and you still have all the well water ions and metals destroying the insides.

When you replace them, which is likely to be soon, get better caps so you don’t lose water like that again. And carry a few gallons of distilled water - at least you won’t ruin them with random tap water.

Sorry, there’s no good way to fix what you’ve done to them.

2018-11-01 by Unknown

Thanks for the response. This sounds like a costly mistake I’ve made. To the tune of $20 per battery for a premixed electrolyte solution that is sold for batteries that are shipped dry, I think I’m going to dump the existing acid and neutralize it out of all of the batteries give him a good rinse with distilled water and refill all of them. At least then they will have the proper acid ratio I’m going to try anything at this point hopefully get another year out of them. any advice on that strategy would also be appreciated. Thanks again

2018-11-01 by Russell Graves

I can’t say I expect that to work terribly well. But make sure they’re as fully charged as you can get them before you try that and it might work. The damage is almost certainly done already, though.

2018-11-01 by Unknown

Thank you… what brand of battery caps do you recommend.

2018-11-01 by Russell Graves

I use the ones that come with my Trojan T105REs, because they’re stationary. They seem like they’d tolerate a lot of sloshing, but… if my battery bank is sloshing badly, there’s an earthquake, and losing water from them is very, very low on my list of immediate concerns. Check the RV forums for recommendations.

2019-04-13 by shourya

What happens if I charge by lead acid batteries on-grid with higher than rated current in the bulk phase? Does it impact the battery cycle life? I read up about it here: Fast charging of sealed lead acid batteries, SLA, VRLA batteries can be charged fast. and the article cites very respected sources to say high current charging does not have any detrimental effects on FLAs.

2019-04-14 by Russell Graves

How much higher than the rated current are you talking?

For FLA, in general, they do tolerate “abuse” very, very well - it’s part of why I like them so much. The reality is that for a deep cycle battery, the internal resistance is high enough that if you start pushing much beyond rated charge current (C/8 or C/10, typically), you’ll hit the absorb voltage very quickly unless the battery is very drained - and that will then determine your charge current, as the current tapers down over time.

If you can push C/2 into one without hitting absorb, it should take it just fine, but you won’t be doing that for very long at all before you hit absorb - so it’s just not a big problem.

You are far more likely to kill a FLA pack trying to baby it than by beating the crap out of it. For off grid systems, a big, powerful solar array works wonderfully with FLA. Just make sure they stay watered, set your absorb voltage at the upper end of recommended, and let it run. I would suggest some way to drop to float based on ending amps, though. Letting them sit in absorb all day, with a big array that can get them there early, will go through a ton of water and chew up the positive plate more quickly than is needed. That said, it’s still better than sulfating them!

2020-02-11 by D Strait

Russell: I have just recently discovered your blog and this is a fine piece of writing. I do see a couple of instances where I am sure that what you intended was not quite what got typed. In a couple of places you wrote >During the absorb stage, when the current drops to 1-3% of the C/20 rate<. Your T-105-RE’s have a capacity of 225Ahr at a C/20 rate. A C/20 rate is therefore 225/20 = 11.25A. One 1% of this is 113 mA. Even with the old Trojan values for absorb voltage you will never get this low no matter how good the battery health. Clearly you intended to say a rate equal to 1-3% of the C/20 capacity. Looking forward to exploring your blog further. Great stuff here!

2020-02-12 by Russell Graves

Good catch - I’ve fixed that. Thanks!

2020-12-14 by OneMorekWh…

I have a very small total off-grid solar system (5-5 1/2 kWh/day charging and 7.5 kWh storage)at 12 volts. For charge controllers I used the cheaper 3 stage PWM things you get on Cheesebay. I used Interstate GC-2 6-volt golf cart batteries and hooked all 3 sets of 2 in parallel, making sure the wires are all the same length and are of equal resistance to avoid overcharging the set in the middle and undercharging the set on the outside. I also found a multi function voltage/ SOC/ current /resistance meter that I installed which is useful for the daily quick glance (also a $20 Internet special). It has been about a year and 5 months since I built it.

I went in this evening to check electrolyte level and happened to tap the battery with the toe of my boot and noticed bubbles rising to the top. (This is 20 minutes past sundown) Out of curiosity I kicked (gently)kicked it again and released more bubbles and a brown solid powdery substance from that cell into the acid. Yours was the most comprehensive article I read, and my guess is positive plate corrosion?


2020-12-18 by Russell Graves

Probably along those lines. Now stop kicking your batteries!

Bubbles will cling to the plates for a while after charging ends, and it’s no problem.

This is a great article, thanks.
I have a question about adjusting charge controller settings in the presence of non-trivial loads. I run an air conditioner in the summer. I believe my 3800 W of panels are sufficient to run the air conditioner and charge my eight Trojan L16E-AC 370 Ah batteries, but only if I adjust the charge controller settings beyond what Trojan recommends, to account for the power given to loads rather than to charging the batteries. Should I adjust the max amps, the absorb voltage, the absorb time, or some combination?

It was about a year of research, and probably should have been broken into a few articles. But I’m glad you found it useful!

Should be. I run my office on not that much more, and it’s very, very overpaneled for my bank and loads. At least in the summer. Nice battery bank, that’s roughly what’s in the solar trailer prototype 1 right now, just the solar version.

You should get a better charge controller, and have a shunt tracking current into the battery bank. If you don’t have that option, the best you can do is absorb time - set that, and monitor specific gravity for a while to make sure the bank is getting properly charged.

My office (and solar trailers) use a Midnite Classic charge controller with the WhizzBang Jr charge monitoring shunt. The shunt is the first thing off the negative battery terminal - my inverter connects to everything else outside that shunt, so the shunt sees the net current in/out of the battery - after everything else has gone through the charge controller and out. Right now, for instance, I have about 21A flowing into the inverter. The charge controller shows 18.5A into the battery bank, and total charge controller output is 39.5A. So my end amps are based on that 18.5A net current into the bank - not on the ~40A total output current off the charge controller.

If you don’t have that capability, you just have to guess on absorb time, and hope it’s right. But with that large and expensive a battery bank, I would really suggest putting a competent charge controller on it (which I define as one that supports a remote “net battery current sensing shunt” here).