Advice on LiFePo4 bank storage

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Advice on LiFePo4 bank storage

Postby TimC » Mon Nov 22, 2021 12:20 pm

I have a 100 aHr bank in my teardrop and its been two full months since the PV panels have seen sunshine. I've noticed the voltage has dropped to 12.2v from around 13.2v when it was put away in the garage. While I realize that is still quite well charged I'm wondering if I should pull the bank and bring it indoors for a little charging boost back up to about 90% full. At this point there is virtually no chance of the bank achieving anything above 32 degrees until maybe April.

Yes, there is a slight draw on the battery which does not need to be maintained; just ammeters and the solar charge controller readouts. And I don't want to install bee hive heaters, at least this year.

I've read several topics of advice on these LFP banks that they can be stored below freezing with no harm. Am I wasting my time pulling the bank? If I reinstall after recharging should I just leave the battery leads off for the duration of the winter?

Thanks, Tim
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Re: Advice on LiFePo4 bank storage

Postby MtnDon » Mon Nov 22, 2021 4:39 pm

12.2 volts just sitting there with no big loads on it? That seems a big drop from the 13.2 you parked it at. Isn't 12.2 volts indicating the battery is at maybe 10% or less state of charge?

I believe you should have a battery disconnect and pull it when parking the unit for an extended period of time. I pull the disconnect on the cabin LFP batteries when we leave and it hardly drops at all even if it is months before we return. It is a 24 volt nominal system and if I leave it at 26.6 volts, a couple of months later it has only dropped a tenth of a volt or so.; sometimes hardly enough to read a difference. I do not worry about storing them up there in the mountains over winter freezing as they do not get charged until they warm until the batteries themselves are above freezing.
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Re: Advice on LiFePo4 bank storage

Postby TimC » Mon Nov 22, 2021 5:28 pm

MtnDon wrote:12.2 volts just sitting there with no big loads on it? That seems a big drop from the 13.2 you parked it at...


Yeah, that's why I just pulled it and brought it inside. Maybe the ammeters are effected by the cold (18 degrees last night). The ammeters are just the Bayite type and I haven't had much experience with them. They haven't been that far off before though.

I have had it indoors maybe two hours and I wonder if the ammeters were telling me the wrong voltage. So I just put a multimeter on it and it read 13.3v. I am now a little less concerned. I'll wait a few days before I decide whether put a charge on it. It might not be any safer indoors than outdoors but I'll worry about it less. :thinking:
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Re: Advice on LiFePo4 bank storage

Postby MtnDon » Mon Nov 22, 2021 5:38 pm

At 13.3 it is fine. The meters would seem to be off by a lot.

I have had a couple of LFP cells (big prismatic cells bought in bulk) sitting for a year with barely any self discharge and some Li-ion cells (smaller) sitting for two years with less than a tenth-volt drop.
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Re: Advice on LiFePo4 bank storage

Postby John61CT » Mon Nov 22, 2021 5:42 pm

Storage only is fine down to very very cold - datasheet should give a spec.

But that means isolated, including ideally from BMS.

Always get back up to 70's F before charging, warmer if fast charging.
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Re: Advice on LiFePo4 bank storage

Postby TimC » Mon Nov 22, 2021 7:11 pm

John61CT wrote:...

Always get back up to 70's F before charging, warmer if fast charging.


Holy crap! 70s? I'm going to have to wait until June before my house gets anywhere near that... :lol:
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Re: Advice on LiFePo4 bank storage

Postby saltydawg » Mon Nov 22, 2021 8:13 pm

yes john is wrong....

Anything over 32 is fine, for safety over 36-38. If they are reading 13.3 inside and warm, let them sit in the house for the winter. Maybe check monthly just incase, but they should be fine for a year if thats the correct voltage.
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Re: Advice on LiFePo4 bank storage

Postby John61CT » Tue Nov 23, 2021 12:52 am

Sure if the C-rate is extremely limited, charging slowly in those cooler temps is safe enough, say 40-50's.

But going to say a 0.4C rate or higher, not saying you need to wait for ambients to warm,

better to just pre-warm the cells like you would a diesel engine.

It is all too easy to render a bank worth thousands into instant scrap

and poof there goes your projected lower cost per year.
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Re: Advice on LiFePo4 bank storage

Postby featherliteCT1 » Tue Nov 23, 2021 8:07 pm

Battery University says:

“Li ion can be fast charged from 5°C to 45°C (41 to 113°F). Below 5°C [41°F], the charge current should be reduced,”

https://batteryuniversity.com/article/b ... mperatures

I wonder what C rate they mean by "fast charge".
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Re: Advice on LiFePo4 bank storage

Postby John61CT » Tue Nov 23, 2021 8:37 pm

Far from authoritative source.

Actual numbers vary by the specific chemistry.

For LFP at the standard 25°C

0.4C or lower is best for longevity.

Many care more for saving time, say an EV scenario, happy to sacrifice life cycles, go up to 1C or even faster.

In racing, preheating cells to 90°C is common in order to make 3+ C-rates somewhat safe, but they are buying new packs ever season.

In sweater weather, no pre-heating I'd stick to 0.2C or slower.

Anywhere near freezing I would bring inside or do pre-heating.
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Re: Advice on LiFePo4 bank storage

Postby featherliteCT1 » Tue Nov 23, 2021 10:18 pm

John61CT,

Thanks!
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Re: Advice on LiFePo4 bank storage

Postby lfhoward » Wed Nov 24, 2021 12:46 am

Tim,

I’ve noticed that with the BMS and solar charge controllers still hooked up but with everything turned off, my LFP battery’s voltage does creep down over time if it’s not being charged. At one point I estimated it was like 1% drain per day on my 156 Ah battery. The shunt/battery monitor still read 0 watts draw, so the rate of discharge was undetectable by that device (also the BMS itself doesn’t go through the shunt). I think I would charge the battery back up every couple of months. After 3 months it could get to a point where it is in the danger zone. It’s not the battery’s fault per se. It’s these fancy electronics we connected to it! Our batteries should store fine long term with the BMS and chargers etc. physically disconnected.

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Re: Advice on LiFePo4 bank storage

Postby lfhoward » Wed Nov 24, 2021 12:59 am

If it is too much of a pain to disconnect everything and bring the battery inside (it would be on mine, for sure), you could try a different strategy. You mentioned your trailer is in an unheated garage. Could you put an electric space heater inside your trailer on an extension cord, and warm the battery and trailer up as a unit? The space heater would need to have a thermostat on it so it is not constantly on, but keep the trailer warm for a day or so. The battery would eventually equalize with the trailer temperature. You would just need to get the battery itself up to 40+ degrees to charge it, and your BMS should be able to tell you the battery’s temperature.
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Re: Advice on LiFePo4 bank storage

Postby TimC » Wed Nov 24, 2021 10:12 am

Thanks Lauren.

I did pull the battery. Its not too difficult to do with my set up. And it now reads 13.28 consistently so I'm not so worried.

In my haste to finish the LFP build I hacked together a battery box. I couldn't find terminals to mount on the box so I rigged up Rube Goldberg posts to attach the wires to from some 1/2" copper pipe. I want to redesign the box. Another reason to pull it.

My wife bought a monster RV and I've spent the last month or so tricking it out with 540 watts of PV panels, a nice charge controller and all the other stuff to make it more comfortable off grid. I'm probably going to duplicate the batteyhookup cylindrical LFP build and install a new battery there as well. I really enjoyed slapping that together the first time and the cost was just a bit higher than going with an equivalent AGM bank. Thanks again for the encouragement and happy camping.
Tim
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My First Benroy Teardrop Build Thread - A 5x8 Woodie - http://www.tnttt.com/viewtopic.php?f=50&t=63575
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#3 My son's Benroy Foamie team build - Started July '20 - http://www.tnttt.com/viewtopic.php?f=50&t=72877

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Re: Advice on LiFePo4 bank storage

Postby John61CT » Wed Nov 24, 2021 11:05 pm

Any long term storage should completely isolate the cells from any parasitic draw loads

especially BMS, Battery Monitor type gadgets.

Many such devices draw their power from only a subset of the bank's groups in series

so not only quickly drain power, do so in a way that results in the bank getting way out of balance as well.

Hence the alt sobriquet Battery Murdering System, BMS can cause lots more pack's demise than what they protect.

As for preheating a bank, there are silicon-embedded DC elements that include thermostatic controls, not expensive on eBay.

The sellers will make custom sizes / power / thermostat settings for just a small quantity run.

Slabs of foam insulation lets them operate very efficiently, starting say the day before you need to start charging.

Or design your packs to be easily removable, say 30-lb or 40-lb sub pack units, and just bring them in with you inside your heated living space.

Many ways to skin a cat.

PS not sure if I mentioned, LTO chemistry can function in much colder temps, and also withstand getting down below 5% SoC without losing as much lifespan, compared to LFP.

Similarly fire safety, vanishingly small risk of thermal runaway.
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