What Happens to Battery Packs that Sit Unused?

In 2020 I built a off-grid system with EVE 280AH cells, Electrodacus, and Growatt 3000.

You can see my build details on this thread.

The entire purpose of this built was Emergency Backup power.

Since I have regular grid power in my house I do not use the system very often.

It has been sitting unused - no solar connected, mostly no loads connected for ~18 months. Every once in a while I'll use it for some small load for an hour or two.

The self discharge rate, according the Electrodacus, is about 1% every two weeks or so. Very slow.

Lately I noticed the cells don't stay as closely balanced as they used to.

-Used to consistently be 10-20mv delta Max
-Now consistently 75-150mv delta

I charged it to "100%" today just to see, and this is about 30 minutes after charge disconnect.

Will the cell balance start to diverge like this if the system is not used?
Do I need to disassemble and do a fresh top balance?

Many variables.

Two cases
1) I built a 14s88p over a year ago (which includes tossing self-discharging cells) and discharged it to 3.6v/pack as part of my procedure to hook it into the battery bank. No BMS, no load, in-house temps, and it sat for over a year.... no detectable change.

2) I have 7s7p packs in my APC UPSs and they get sub-sec 'spikes' every day as the ATSs switch between grid and solar... so it's significant load but sub-sec and they seem to require ongoing balancing.

My sense is that healthy batteries with 0 load / 0 BMS load - no load - will not degrade/change very fast. However, occasional loads can cause disruption in the balance a little bit.

All anecdotal comments - batteries are 'messy' (electrically) but do fantastic jobs

OffGridInTheCity said:

My sense is that healthy batteries with 0 load / 0 BMS load - no load - will not degrade very fast. However, occasional loads can cause disruption in the balance a little bit.

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Cool thanks for the info. That is what I would have thought too.

I do have the Electrodacus always connected, with draws ~0.05A 24/7 so I guess I should say that it is "BMS Only Load"

I started out doing the same thing then decided it did not make any sense to have "free" power just sitting there not doing anything. I installed a manual transfer switch where I could use the power, my setup was making, on a regular basis on different circuits. I have certain circuits wired through the transfer switch to different breakers in the house. I can use solar or grid power at the flip of a switch.

My battery just sits at maybe 70% charged. I built it for emergency power during the rainy season to power a pump.
Will charge it back up late November to be ready for use. April I will let the inverter idle the power back down to 70%.
I consider this lower maintenance and higher reliability compared to the gasoline generator sitting around.
Will check the balance when charged but not really concerned.

After doing some reading over on the Electrodacus forum I believe I have figured out my problem.

By default, the electrodacus does not Balance on Discharge.

Since I was using my system without any type of recharge (no solar or no grid, just using the battery) the Electrodacus was never balancing, even though I was using it occasionally.

I guess eventually the caused the cells to get out of balance.

I've enabled Balance on Discharge on the Electrodacus and turned the Growatt on. on idle without powersave mode is draws about 1.7-2A and the dv between the cells is steadily falling now, so I guess it is balancing.

Now my problem is: I have no way of knowing if the cells are at wildly different SOC or not?

Do I need to take it all apart and charge each cell individually, and then top balance, to get them all back to the same SOC?

If it were me, I don't think I'd leave the battery stored long-term at full capacity as I don't think it's healthy for the battery. I'd also consider using an active balancing BMS (like the JK) so the cells are auto-balanced when it is brought up to full charge. Either way, I'd probably occasionally cycle the battery and leave it at around 80-85% SoC for storage in case of emergency use. Then again, I'd be utilizing the battery all along to capitalize on it's utility and savings potential after putting that much expense into its purchase!

Thersom1948 said:

After doing some reading over on the Electrodacus forum I believe I have figured out my problem.

By default, the electrodacus does not Balance on Discharge.

Since I was using my system without any type of recharge (no solar or no grid, just using the battery) the Electrodacus was never balancing, even though I was using it occasionally.

I guess eventually the caused the cells to get out of balance.

I've enabled Balance on Discharge on the Electrodacus and turned the Growatt on. on idle without powersave mode is draws about 1.7-2A and the dv between the cells is steadily falling now, so I guess it is balancing.

Now my problem is: I have no way of knowing if the cells are at wildly different SOC or not?

Do I need to take it all apart and charge each cell individually, and then top balance, to get them all back to the same SOC?

Click to expand...

Ideally you want to balance at the top of the curve (say above 3.4V/cell). If you're balancing in the flat part of the curve, it will be very inaccurate and likely result in cells deviating much more when at the top (or even at the bottom).

Wet1 said:

Ideally you want to balance at the top of the curve (say above 3.4V/cell). If you're balancing in the flat part of the curve, it will be very inaccurate and likely result in cells deviating much more when at the top (or even at the bottom).

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I'm don't understand - balancing the cells while they are discharging will cause them to actually deviate further?

Do you think it's a bad idea to enable "Balance on Discharge"

My primary use of this system is:
-Charge it up
-Use it purely as a battery (no charge input)
-Charge it up again when it needs it

Lithium likes to be cycled once in a while.
I think my battery spec sheet (EVE) says to cycle them every six months.
Something goes wrong in the chemistry of they just sit.

Maitake said:

Lithium likes to be cycled once in a while.
I think my battery spec sheet (EVE) says to cycle them every six months.
Something goes wrong in the chemistry of they just sit.

Click to expand...

By "cycling" meaning full or partial discharge (then recharge to 100% afterwards) ?

I use mine as UPS (no solar, only AC mains charging) and once in a while I use the batteries partially then recharge to 100%.

Thersom1948 said:

In 2020 I built a off-grid system with EVE 280AH cells, Electrodacus, and Growatt 3000.

You can see my build details on this thread.

The entire purpose of this built was Emergency Backup power.

Since I have regular grid power in my house I do not use the system very often.

It has been sitting unused - no solar connected, mostly no loads connected for ~18 months. Every once in a while I'll use it for some small load for an hour or two.

The self discharge rate, according the Electrodacus, is about 1% every two weeks or so. Very slow.

Lately I noticed the cells don't stay as closely balanced as they used to.

-Used to consistently be 10-20mv delta Max
-Now consistently 75-150mv delta

I charged it to "100%" today just to see, and this is about 30 minutes after charge disconnect.

Will the cell balance start to diverge like this if the system is not used?
Do I need to disassemble and do a fresh top balance?

View attachment 102410

Click to expand...

What is this battery monitor that you are using here?

LFP batteries last longer if they are cycled from there 100-20% at least 2-4 timers per year then charge them back up 50% if your storing them. Kind of a waste though since you could be cycling them daily and cutting your utility bill.

Thersom1948 said:

I'm don't understand - balancing the cells while they are discharging will cause them to actually deviate further?

Do you think it's a bad idea to enable "Balance on Discharge"

My primary use of this system is:
-Charge it up
-Use it purely as a battery (no charge input)
-Charge it up again when it needs it

Click to expand...

If you look at the chart below for a LiFePO4 battery cell, you'll notice the shape of the curve is not liner from 2.5V to 3.65V. In fact, it's almost flat for the majority of the "curve". Basically for most of the usable port of the curve, the voltage is 3.2V to 3.35V, and from 20% to 95%(+), that voltage range is even narrower. So if you try to balance cells in that flat part of the cure, small cell voltage deviations can represent large percentages of SoC. If this is done, it creates a lot of error, or false balancing once the cells start getting to the top or bottom parts of the curve... where large voltage changes represent very small % of SoC. For this reason you want to only balance in the steep parts of the curve, and ideally on the top part only. This is why I suggest using an active balancer and only balancing above 3.4V.


Likewise, it's unhealth for the cells to sit for extended periods of time in the steep parts of the curve. So storing them at near 100% for months on end will degrade them prematurely. I understand you're using the battery for emergency situations only and need them to be full so they are useful when the time comes, but at a minimum, I would suggest storing them at no more than ~90% SoC (so you're safely in that "flat" part of the curve.

Sorry, I don't know anything about your BMS or its settings, so I don't want to recommend settings for it. With that said, I would think you'd only want to balance during charging, and only above 3.4V (for the previously mentioned reasons).

As mentioned previously, these cells like to be exercised every so often to extend their life. They will calendar age-out over time anyway, and much quicker if stored in the steep parts of the curve. Given the expense of the battery, and the cost of energy now, it would be to your benefit (and the cells) to utilize the investment you've already made in them by putting them in use, rather than just letting them age-out sitting there waiting for that rainy day.

Hopefully this helps and all makes sense...

Wet1 said:

...

Click to expand...

Thank you for the detailed reply.

The cells were all carefully top balanced when I built the system and up until the last few weeks they have been consistently within +/-0.005-0.01V on each cell. I also capacity tested when I built it and came out within 3-5AH of advertised, which is all I would ever expect from these cells.

So unless something is wrong with a cell or a connection I do not think the SOC per cell is very far off.

But in case it is, I charged to what BMS says is "100%" and I am doing a full discharge and capacity test to see where it is.

If capacity comes out low, I'll take it apart and do a fresh top balance and clean, sand, and oxide protect the terminals and bus bars and terminals.

My main takeaway here is that I need to make a point to exercise the system more often. I can store at 90% SOC, that's no problem.


I do find it funny that everyone is worried about me "maximizing my investment" in the system. It's doing exactly what I need it to do. I can't install permanent solar panels where I live - I'm not wasting any "free power".

If it makes you all feel better about me "not utilizing" it, pretend it's mounted in a vehicle and I'm recharging with the alternator.

chess-equality said:

What is this battery monitor that you are using here?

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Electrodacus

It's the BMS, not just a monitor.

Understood on not having PV installed. Although, you've already purchased the expensive part (storage), so you still might want to consider doing the rest in the future.

The cells will drift in time, so again, I would recommend an active balancing BMS solution so no future top balancing is really ever required. OTOH, if the cells are still relatively close on the top end, I'm not sure I'd bother with rebalancing as the resulting battery capacity gained will likely be pretty minimal... unless you want to let the OCD kick in.

Wet1 said:

Understood on not having PV installed. Although, you've already purchased the expensive part (storage), so you still might want to consider doing the rest in the future.

The cells will drift in time, so again, I would recommend an active balancing BMS solution so no future top balancing is really ever required. OTOH, if the cells are still relatively close on the top end, I'm not sure I'd bother with rebalancing as the resulting battery capacity gained will likely be pretty minimal... unless you want to let the OCD kick in.

Click to expand...

Oh I would install panels if I could! It's not a question of funds, but living situation.

Unless I am misunderstanding what you mean by "Active" - The Electrodacus is an Active BMS.

It's default settings are:
"Balance on Charge" = Yes
"Balance on Discharge" = No
Only balances above 3.2V by default.

Thersom1948 said:

Oh I would install panels if I could! It's not a question of funds, but living situation.

Unless I am misunderstanding what you mean by "Active" - The Electrodacus is an Active BMS.

It's default settings are:
"Balance on Charge" = Yes
"Balance on Discharge" = No
Only balances above 3.2V by default.

Click to expand...

I would change the "only balances above" to 3.4V, but that's just me. I'm guessing balancing below that level is what got them out of whack to begin with. 3.2V is way too low to be balancing at IMO. Leave the "balance on discharge" off.

Just as an update here for anyone in the future: I did a full discharge cycle to see if there was a balance issue.

The BMS is set it cutoff when any cell hit 2.8V

Before Low Voltage Cutoff I was able to draw almost exactly 270Ah / 7000 Wh and the BMS was reading "3%" remaining.

270ah out of 280ah pack with LVC at 2.8V seems like full capacity to me, given the cells could technically be pushed a bit further to 100% drain them, but I have no need to do that.

The total pack voltage was right around 22.4-22.8V, all cells were right around ~2.85-2.9V when the first hit LVC.

Going forward I plan to leave the pack at ~70% charge when not in use and do a full drain cycle every 6 months or so.