Storing Lithium over winter

[jakew009]

But as you are using them for standby applications you don't really have the option to maintain them at 60%, or you'd need twice as many cells, if keeping them at 100% shortens their life a little, so what, it isn't going to halve their lives. But keeping boat batteries at 100% SOC for Winter storage would make no sense, it shortens cell life for no benefit whatsoever.

Agree. The cost / benefit analysis is massively in favour of keeping them at 100%. We also routinely discharge to essentially 0% (cut off at 10.8V).

My point was, it's not worth worrying about too much. LFP is pretty damn robust.

 

[jakew009]

What temperature are you storing the standby batteries at? Have you done full certified capacity tests on the batteries since they were installed?
The design of the standby pack should really be based on 13.4v not 13.65v in my opinion. This would give way better life. Also storing at circa 15degC would.d help battery life but would reduce available Ah when you need it

It varies a lot, we do have some insulation in the enclosures but they get hot in the summer and cold in the winter. The batteries are heated if they are charging below 5, we discharge down to -20 celcius then cut off.

We track all the power into and out of the batteries. There is not really any noticeable degradation that you couldn't just account for as normal calendar aging.

We settled on 13.65v because it takes account of various voltage drop etc in real life. The batteries are "online" (ie . in parallel with the loads and the charger). 13.65V on the chargers will eventually take the battery to 100% and keeps it there with our typical loads in parallel.

The actual logic is after a mains power cut, we bulk them to 14.4V to quickly recharge and then switch to 13.65V. We never go back to bulk, so they just stay at 13.65v forever.

Obviously it's all a commercial trade off, and extra complexity vs simplicity.

 

[Dockhead]

There is a recent article on Attainable Adventure Cruising on this subject, somewhat based on this other article: -

Don't murder your batteries, tips for winter storage of LFP batteries

Conclusion from the above Panbo article: -

You need to subscribe to Attainable Adventure Cruising to read their conclusions.

Commenting on the Panbo article, John of Morgan's Cloud goes on to write:

"Rather, it seems to me the best solution would be to select batteries with an external BMS in the first place and simply charge them to say 70% and then disconnect them from everything: BMS, monitoring, and other batteries in parallel (serial are fine since no current can flow)."

I totally agree with him. Ridiculous, and wrong, to leave a charger on them when the boat is laid up over the winter, categorically wrong if your batteries don't have automatic heaters. Because cold doesn't harm them; charging them when they are cold, on the contrary, kills them. And, unlike lead batteries, leaving them partially charged not only doesn't harm them, they prefer that. You do not want to leave lithium batteries for long periods at 100%; this greatly accelerates capacity loss. The author of the Panbo article assumes that BMS's are left on during storage, and that this can lead to totally discharging the batteries during storage, and killing them.

I have 2x 280AH x 24v lithium batteries with JK MOSFET-type BMS's. I just switch the BMS's off, and then physically disconnect both batteries using contactors, when I'm not using the boat. But even if you leave the BMS's connected, so long as they are switched off, they can't discharge your batteries measurably during a few months of storage. They only draw 0.2 to 0.5mA in standby, which is only 5 to 12 mW, which is nothing. Self-discharge nominally is 3% per month. If you leave your batteries at 60% and leave the boat for 6 months, they will still be at at least 42% when you come back.

If you leave the BMS's switched on, they draw up to about half a watt each. For my setup, that's about 5% a month for each 280AH battery. That plus 3% is now already more significant, but even that can still be planned for. Three months of that is 24%. I'm never away from my boat for more than two months at a time; YMMV. But I would never leave the boat alone without the batteries completely isolated in any case.

I have a separate lead battery bank which maintains bilge pumps etc., and this is kept on a charger at all times when I'm off the boat.

 

[Dockhead]

Agree. The cost / benefit analysis is massively in favour of keeping them at 100%. We also routinely discharge to essentially 0% (cut off at 10.8V).

My point was, it's not worth worrying about too much. LFP is pretty damn robust.

It's true that the difference between 8000 cycles and 4000 cycles, even 2000 cycles, is pretty academic to those of us who cycle our batteries less than several hundred times a year. Unlike solar farm or data center users.

For us, calendar aging is the main thing, and the rate of calendar aging might not be academic. Storage at 100% will double or triple the rate of calendar aging compared to storage at 50%, reaching 6% to 8% capacity loss per year at higher temperatures.

So I agree with Paul Rainbow here; storage at 100% when off the boat brings zero benefit and has a definite cost, even if it's relatively small, especially at lower temperatures. I wouldn't do it.

CYCLING to 100% and STORAGE at 100% are two completely different things. Nothing really wrong with cycling to 100% in order to use the full capacity of the battery when it's in use. I don't -- as 90% to 95% allows me more conservative charge profile settings which make the batteries easier to manage with my system, but that's a different consideration to calendar aging.

 

[Dockhead]

Keeping batteries half charged on a boat used half the year. Might be better value overall to use everything to its potential

Storing the batteries at 50%, and CYCLING them at 50%, are two completely different concepts. You can cycle them to 100% when they're in use, and "use everything to its potential", but that doesn't mean you have to store them at 100% when they're not in use.

As to using the boat only half the year -- I agree. It's a waste, and winter cruising on the UK South coast is so lovely. Especially when you have a great power system!

 

[geem]

It varies a lot, we do have some insulation in the enclosures but they get hot in the summer and cold in the winter. The batteries are heated if they are charging below 5, we discharge down to -20 celcius then cut off.

We track all the power into and out of the batteries. There is not really any noticeable degradation that you couldn't just account for as normal calendar aging.

We settled on 13.65v because it takes account of various voltage drop etc in real life. The batteries are "online" (ie . in parallel with the loads and the charger). 13.65V on the chargers will eventually take the battery to 100% and keeps it there with our typical loads in parallel.

The actual logic is after a mains power cut, we bulk them to 14.4V to quickly recharge and then switch to 13.65V. We never go back to bulk, so they just stay at 13.65v forever.

Obviously it's all a commercial trade off, and extra complexity vs simplicity.

That makes sense. I used to get involved with data centre battery design but in those days we had AGM. We used to heat the battery rooms but no mechanical cooling. We did have mechanical louvre to keep them cooler in the summer.

 

[jakew009]

Commenting on the Panbo article, John of Morgan's Cloud goes on to write:

"Rather, it seems to me the best solution would be to select batteries with an external BMS in the first place and simply charge them to say 70% and then disconnect them from everything: BMS, monitoring, and other batteries in parallel (serial are fine since no current can flow)."

I totally agree with him. Ridiculous, and wrong, to leave a charger on them when the boat is laid up over the winter, categorically wrong if your batteries don't have automatic heaters. Because cold doesn't harm them; charging them when they are cold, on the contrary, kills them. And, unlike lead batteries, leaving them partially charged not only doesn't harm them, they prefer that. You do not want to leave lithium batteries for long periods at 100%; this greatly accelerates capacity loss. The author of the Panbo article assumes that BMS's are left on during storage, and that this can lead to totally discharging the batteries during storage, and killing them.

I have 2x 280AH x 24v lithium batteries with JK MOSFET-type BMS's. I just switch the BMS's off, and then physically disconnect both batteries using contactors, when I'm not using the boat. But even if you leave the BMS's connected, so long as they are switched off, they can't discharge your batteries measurably during a few months of storage. They only draw 0.2 to 0.5mA in standby, which is only 5 to 12 mW, which is nothing. Self-discharge nominally is 3% per month. If you leave your batteries at 60% and leave the boat for 6 months, they will still be at at least 42% when you come back.

If you leave the BMS's switched on, they draw up to about half a watt each. For my setup, that's about 5% a month for each 280AH battery. That plus 3% is now already more significant, but even that can still be planned for. Three months of that is 24%. I'm never away from my boat for more than two months at a time; YMMV. But I would never leave the boat alone without the batteries completely isolated in any case.

I have a separate lead battery bank which maintains bilge pumps etc., and this is kept on a charger at all times when I'm off the boat.

I thought the same when I read that Panbo article. However, when I thought about it more I actually agree with him.

He is talking generally, for the 99%. Those who just expect their batteries to work rather than discussing LFP batteries on an internet forum...

There are a lot of really crappy BMSs out there that allow the battery to discharge so far that the BMS can't even allow it to charge up again. You hear stories of people 'jump starting' batteries bypass the BMS etc. I've seen BMS that basically became a brick as they lost their settings. Combine that with sealed batteries with glued on lids and you have a mess.

So faced with that, if you have a choice of potentially letting a battery discharge so far that you can't recharge it, or holding it on charge at 100%, holding it at 100% is the lesser of two evils.

 

[jakew009]

That makes sense. I used to get involved with data centre battery design but in those days we had AGM. We used to heat the battery rooms but no mechanical cooling. We did have mechanical louvre to keep them cooler in the summer.

Ours are all in things like road signs, CCTV towers, traffic lights etc. The batteries get abused 100% -> 0% and fast charged back to 100% over and over again and they keep coming back for more.

 

[lustyd]

We are
Back to full-time liveaboard for thr next couple of years or more.
The never ending season

Very envious. Stuck in Falmouth getting smashed against the pontoon today in what feels like sub zero temps! Scotland was nice but I feel I made a mistake going north!

 

[geem]

...temperatures here are pleasant here in the mornings and sticky by afternoon. No complaints though

 

[Dockhead]

I thought the same when I read that Panbo article. However, when I thought about it more I actually agree with him.

He is talking generally, for the 99%. Those who just expect their batteries to work rather than discussing LFP batteries on an internet forum...

There are a lot of really crappy BMSs out there that allow the battery to discharge so far that the BMS can't even allow it to charge up again. You hear stories of people 'jump starting' batteries bypass the BMS etc. I've seen BMS that basically became a brick as they lost their settings. Combine that with sealed batteries with glued on lids and you have a mess.

So faced with that, if you have a choice of potentially letting a battery discharge so far that you can't recharge it, or holding it on charge at 100%, holding it at 100% is the lesser of two evils.

If there are really BMS's that bad, then you could be right. But I would argue that such a BMS is not fit for purpose!!

JK BMS's are cheap as chips and certainly don't have this problem.

 

[PaulRainbow]

If there are really BMS's that bad, then you could be right. But I would argue that such a BMS is not fit for purpose!!

JK BMS's are cheap as chips and certainly don't have this problem.

Yes, there are some bad BMS's and they are not restricted to cheap batteries Lilfo 105 LiFePo4 batteries flat.

 

[Dockhead]

Yes, there are some bad BMS's and they are not restricted to cheap batteries Lilfo 105 LiFePo4 batteries flat.

When the BMS is packaged with the cells as a drop-in battery, then that makes them disposable, and naturally the makers will do everything possible to keep the cost down.

Why it's still better, in my opinion, to build your own, at the current state of the art.

 

[Dockhead]

Another consideration, relevant to this discussion: As it turns out, self-discharge rate is not a constant. That's new informatio for me. It's faster at higher temps and SOC, otherwise slower. EVE wrote somewhere that at below 50% SOC and <25C, self-discharge is less than 1% a month.

So I think the only thing which could kill stored batteries is some kind of parasitic load left on them.