winter storage

[Peter Kuczynski]

I wanted to share my recent experience with storing our RV solar battery cells over the winter.

Last year I upgraded my 8 cells from 270 to 310AH, I used them about 6 months. I then stored our RV over the winter, all power was off, but SBMS0 was connected.

In the spring, all cells were dead, measuring 1 volt at most some had 0 voltage.

I wound up using a portable solar batter system the remainder of the season.

I am now reconditioning the cells using a cc/cv charger.  Its taking a week of step charging to bring them back to full 3.65 voltage, so far I lost one cell completely.  Im charging each cell on a CC/CV charger starting at 3.4, them 3.6 and 3.6, finally 3.65 volts. It takes a week per cell to reach 3.4, and about several hours from 3.4 to 3.65.

I'm challenging the recommendation that cells can be disconnected for storage and left to drain completely, I feel a large system cant be simply brought back up to usable power without first recharging each cell, which would be time prohibitive in order to use quickly.

Instead, all cells need to be maintained, at relative room temp and at at least 50% state of change, else you risk damaging the cells and taking a very long time to bring a system from near 0 to full charge and eventual use.

So once my system is back on line at full change, I'll place it into our RV,  and start using it in May-June, at the end of the season, Ill remove it, and maintain it at home at 70 degrees and relatively full charge while its being stored..

Hope this helps someone.

[Dacian Todea (electrodacus)]

Peter,

The 8 cells have a capacity of about 8kWh if fully charged.

The SBMS0 if the WiFi is ON will use around 0.8W so  0.8W * 24h * 30 days * 6months = 3.45kWh

So if the SBMS0 was the only load and battery will have been more than 50% charged you should not have had any problem.

That said for long therm storage (months) with no charge source connected the SBMS0 should be disconnected.

With WiFi OFF the SBMS0 uses less around 0.35W in that case 0.35W * 24 * 30 * 6 = 1.5kWh so should be able to handle even a full year if battery was left above 50% SOC

If is just a few weeks it is fine but if is many months the SBMS0 should be disconnected as while consumption is not high it adds up for long periods.

Unfortunately your cells as completely damaged ans should not be used. Anything below 2.5V per cell will result in permanent damage and they can not be reused.

[Peter Kuczynski]

Thanks for the quick reply Dacian. I agree with your here"   if is many months the SBMS0 should be disconnected as while consumption is not high it adds up for long periods.".

Ive ordered 4 new cells but wanted to pick your brain on reconditioning 4 of the cells from the batch of 8.

So far Ive been able to bring 4 cells up to 4.65 so far, again, they were new prior to this event last summer. There is no bulging, They seem fine. 

What would make them "completely damaged" as you stated above? Is it the dandrite issue, would those form on depleted cells  or is it another reason. Ive not been able to locate any article that links dandritic formation to depleted cells yet: https://www.sciencedirect.com/science/article/abs/pii/S0378775398002420

[Peter Kuczynski]

Answered my own question w AI. I'm interested in the "damage mechanism" as often this isnt mentioned, its only mentioned that you shouldn't try, to recondition cells. 

Take away for me is, as its stated "some cells CAN be recovered w low current, which I have done. What we dont know or cant tell is to what extent are the battery's damaged internally if at all. I suppose the only way is to monitor these 4 cells in comparison to new cells. Dendrite formation isnt an issue here then,

LiFePO4 (lithium iron phosphate) battery cells dropped below 2.5 volts per cell enter a deep discharge state that risks permanent damage.​

Damage Mechanism

Dropping below 2.5V causes copper dissolution from the anode current collector, a process where the protective SEI (solid electrolyte interphase) layer breaks down, leading to irreversible structural changes in the cell. This dissolution and electrolyte decomposition reduce capacity and can make the cell unsafe for recharging due to internal short risks.​

Recovery Challenges

While some cells can be revived with very low-current "jump-start" charging (e.g., 1/2000C rate until above 2.5V), success varies and often results in severe capacity loss or instability. Most experts advise against reuse in critical applications like trucking or solar, as lifespan plummets and safety margins erode.

​

[Dacian Todea (electrodacus)]

I'm guessing you charged them to 3.65V not 4.65V.  You should do a cycle test discharging them to 2.8V and then charge them again then see if they are bulging.

I recently (just a few days ago) tested two batteries an 8S 100Ah GBS battery and a 8s 20Ah A123Systems both of them about 14 years old.

The 100Ah GBS was my main house battery for about 4 years and since then it was mostly stored the smaller 20Ah was used just a few times when I needed portable power mostly during the house construction and since then it was also stored for more than a decade.

The GBS battery was tested new 2012 had 91Ah then 4 years later in 2016 (was used daily) had around 74Ah and now after a very long period of storage they where around 42 to 50Ah (I tested all cells individually) just charged the cells to full (they where fairly close to full anyway).

Then I took one specific cell the one that measured 50Ah and made 6 cycles discharge to 2.8V and charge back to 3.55V and with each cycle it was lower and lower capacity 

1) 50.09Ah

2) 48.98Ah

3) 43.879h

4) 39.77Ah

5) 36.30Ah

6) 35.72Ah

I was ready to do another cycle when I decided to unscrew the compression straps (they had factory compression straps and where never removed) and I noticed that they where under significant tension and the cell I decided to test multiple times was swollen so I discharged it (was about 10Ah in to last charge) but it remained swollen and it is a bit scary since the plastic case of this batteries are very crumbly (not sure how to exactly describe). The plastic quality may even have to do with how badly they degraded over time. Likely the worse LiFePO4 battery.

The other battery of same age was 18.3Ah in 2012 and now after about 14 years is 17.4Ah (did 3 full cycle tests with exactly the same result) so just about 5% degradation in 14 years witch is likely the best possible LiFePO4.

So the point is that cells where not swollen after first capacity test after a very long storage but just the cell that had multiple capacity test with decreasing capacity measured has swollen. So you should do at least 3 or 4 cycle tests on those 4 cells you want to save and see if they maintain shape and there is no internal gas pressure. Also capacity should be close to spec and remain consistent over multiple cycles. Maybe if they only got to above 1V and for just a short period of time few days they may be fine.

[Peter Kuczynski]

Thanks Dacian, correct, I step charged  from 3.4 to 3.65, observed no swelling on those 4 cells, Ive had a couple that showed spelling, those I wont use.

Next step is to perform:

 " 3 or 4 cycle tests on those 4 cells you want to save and see if they maintain shape and there is no internal gas pressure." 

"Also capacity should be close to spec and remain consistent over multiple cycles"

I realize Im in a bit of gray area here with relatively new cells, that were completely discharged, lesson learned.

Thanks for your insight again.

[Peter Kuczynski]

Im looking for a programable load tester w a 2.5 volt cut off next. If anyone has a line on a inexpensive unit.

[Dave McCampbell]

Peter, there are many 10-20a small load testers and cell chargers on the internet.  Just google what you want.  After buying cheaper separate load testers and bench top chargers we finally bought a combination ZKETECH battery cell charger and  load tester.  It can do multiple steps of both charge and discharge LFP cells according to what you tell it to do.  It also produces a nice graph of what it has done so you can record results for multiple cells.  It is rated for 5 v and 40 a.   It is excellent quality especially for doing capacity testing and top balances.

[Peter Kuczynski]

Here's what I ordered to load test,  EbC‐A20 Battery capacity tester : https://www.ebay.com/itm/365773229506?ul_noapp=true

The bigger version is here:https://tinyurl.com/mvahmpvs