LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[CatNewBee]

Originally Posted by tanglewood

This is probably part of why drop in batteries typically have pretty restrictive current limits.

Nope, it is because of the price for heavy duty relays or FET transistors.

[Checkswrecks]

Pat -

It's not uncommon for the middle prismatic lithium cells to age more quickly if stacked and under repeated loads. We saw starting motor current increase cell temps almost 10 degrees C (30F) and a long time to normalize and cool back down. So what loads are being put on the cells which may create heat? Starter motor? Windlass? And then for how long?
fwiw - Seems to me that I heard at a conference that the older Prius car cells have one end or the other age first.

Yes, some of the big ESS systems do use mineral oil to distribute heat and most EVs have liquid systems too. Again, it is for distribution for longer life, not for cooling per se, but liquid adds other issues. I've not yet built the battery for my boat but for these small packs I do plan to get to almost the same result by sandwiching a piece of silicone rubber and two sheets of aluminum between cells. And of course to not bolt the cells tightly together.

evm -

When I mentioned 5C, I picked the number entirely out of the air but it was from an event I'd seen in which the charger [on an external BMS system] had an internal MOS (iirc) failure which led to overcurrent. An EV had an inadequate BMS in which inadequate low voltage protection damaged one cell and then continued charging of the pack to 100% with overcharging of the initial cell. The over voltage protection may not work or just be out of calibration. In the 787 a 4th layer sub-tier vendor to GS-Yuasa left something out of the cell assemblies which resulted in a thermal breakdown of the terminal assembly due to localized resistance and heat. Glad that you feel you feel comfortable that you will never have a problem but my whole career has been spent meeting engineers after an event and having them tell me "That [pick a failure mode] can't happen." It's your boat so I'm not criticizing, just trying to relay experience and caution in providing for an initial cell to fail in the worst way. Peace.

For all and as mentioned earlier, heat begins to shorten life when a cell temp is less than 40C (104F). The rate of degradation is low in the 30s (>86F) but it begins at less than 40C.

[evm1024]

Originally Posted by Checkswrecks

SNIP %<

evm -

When I mentioned 5C, I picked the number entirely out of the air but it was from an event I'd seen in which the charger [on an external BMS system] had an internal MOS (iirc) failure which led to overcurrent. An EV had an inadequate BMS in which inadequate low voltage protection damaged one cell and then continued charging of the pack to 100% with overcharging of the initial cell. The over voltage protection may not work or just be out of calibration. In the 787 a 4th layer sub-tier vendor to GS-Yuasa left something out of the cell assemblies which resulted in a thermal breakdown of the terminal assembly due to localized resistance and heat. Glad that you feel you feel comfortable that you will never have a problem but my whole career has been spent meeting engineers after an event and having them tell me "That [pick a failure mode] can't happen." It's your boat so I'm not criticizing, just trying to relay experience and caution in providing for an initial cell to fail in the worst way. Peace.

For all and as mentioned earlier, heat begins to shorten life when a cell temp is less than 40C (104F). The rate of degradation is low in the 30s (>86F) but it begins at less than 40C.

No offense taken.

Extrapolating Lithium Cobalt battery failures to risks involved with LiFePO4 batteries on boats appears to be apples to oranges at best.

THis does not say that there is not a risk of LiFePO4 failing but rather that LiFePO4 has other failure modes than LiCo and that thermal runaway fires are not expected as part of LiFePO4 failure modes.

Let's stay apples to apples. Pulling any LiCoO2 chemistry into LiFePO4 safety discussion is not productive in my mind.

As for rate of degradation - of course the rate increases with increasing temp. It does not start degrading at 40 C but the rate becomes more significant above 40 C or so. And as noted by Grolleau et al the rate is less than 20% per year at 45 C and around 5 % per year at 30 C. The key finding for us is that storage at 60 C (well beyond the safe temp for humans) up around 50% per year at 100%SOC but is closer to 28% at 30% SOC.

SOC is very important when stored above 45 C.

[Dockhead]

Originally Posted by Cpt Pat

I am seeing what may be a very small capacity loss in half my cells (2) due to what may be accelerated heat-induced aging - indicated by higher aging of cells in the center of my pack versus at the outer ends of the pack, caused by what I suppose is differential heat dissipation at the pack ends versus the pack center. I have experimented to test that hypothesis by swapping the cells positions to place the center-mounted cells at the ends, and observed the same effect following the cell pack positions after repositioning the cells. This observation may prove my hypothesis. But I stress, in my case, the effect is very small. One could argue it is statistical noise. What I do know for sure is that heat accelerates cell aging. That is well proven. And that cells having limited heat dissipation capacity will experience more heat.

That is very different than memory effects, which I am not seeing and which would presumably affect all the cells equally. I had hoped I had communicated clearly in my post above that I was referring to thermal effects, not memory effects. I don't know how to be more clear. Any conflict with memory-effects is imagined, because I have never experienced nor described memory-effect capacity degradation with my pack.

The effect I am seeing is very small, but the heat generated is also very small at a maximum charge rate of equal to or less than 0.2C. If I were charging those cells at 0.5C while they were sealed in a thermally-insulating plastic case -- I strongly suspect the entire pack would undergo significant thermal aging. My point is: placing cells inside what essentially comprises an oven, as is done with drop-in batteries inside plastic cases is not good!

Heat is bad. LFP batteries generate heat. Dissipating that heat is good. Drop-in battery replacement outer cases are sealed plastic. Therefore those cases are bad. Is that simple enough? Memory-effects are a separate topic.

(Deep sigh.) Therefore, my post does not belong in the memory effects thread. There is no "thermal effects thread," though I would argue there should be. So here is my post, in the more general "...house banks" thread. And the discussion was about drop-in batteries (Battle Born), to which my posts directly pertain.

----

How much more would it cost to construct the drop-in battery container case (to make them physically the same dimensions as the AGM batteries they are intended to replace) of a material, like aluminum, that has much better thermal conductivity?

This is really interesting and maybe worth a separate thread. Great information.

Is there a consensus, by the way, on whether it is still advisable to pack the cells tightly together as, for example, MaineSail shows on his site?

[Dockhead]

Originally Posted by Checkswrecks

Pat -

It's not uncommon for the middle prismatic lithium cells to age more quickly if stacked and under repeated loads. We saw starting motor current increase cell temps almost 10 degrees C (30F) and a long time to normalize and cool back down. So what loads are being put on the cells which may create heat? Starter motor? Windlass? And then for how long?
fwiw - Seems to me that I heard at a conference that the older Prius car cells have one end or the other age first.

Yes, some of the big ESS systems do use mineral oil to distribute heat and most EVs have liquid systems too. Again, it is for distribution for longer life, not for cooling per se, but liquid adds other issues. I've not yet built the battery for my boat but for these small packs I do plan to get to almost the same result by sandwiching a piece of silicone rubber and two sheets of aluminum between cells. And of course to not bolt the cells tightly together.

evm -

When I mentioned 5C, I picked the number entirely out of the air but it was from an event I'd seen in which the charger [on an external BMS system] had an internal MOS (iirc) failure which led to overcurrent. An EV had an inadequate BMS in which inadequate low voltage protection damaged one cell and then continued charging of the pack to 100% with overcharging of the initial cell. The over voltage protection may not work or just be out of calibration. In the 787 a 4th layer sub-tier vendor to GS-Yuasa left something out of the cell assemblies which resulted in a thermal breakdown of the terminal assembly due to localized resistance and heat. Glad that you feel you feel comfortable that you will never have a problem but my whole career has been spent meeting engineers after an event and having them tell me "That [pick a failure mode] can't happen." It's your boat so I'm not criticizing, just trying to relay experience and caution in providing for an initial cell to fail in the worst way. Peace.

For all and as mentioned earlier, heat begins to shorten life when a cell temp is less than 40C (104F). The rate of degradation is low in the 30s (>86F) but it begins at less than 40C.

I have 10 horsepower bow thruster, so I'm following this with interest.

[newhaul]

Originally Posted by Dockhead

This is really interesting and maybe worth a separate thread. Great information.

Is there a consensus, by the way, on whether it is still advisable to pack the cells tightly together as, for example, MaineSail shows on his site?

Or would it be better to get aluminum cased cells.

[nebster]

Originally Posted by Cpt Pat

I am seeing what may be a very small capacity loss in half my cells (2) due to what may be accelerated heat-induced aging - indicated by higher aging of cells in the center of my pack versus at the outer ends of the pack, caused by what I suppose is differential heat dissipation at the pack ends versus the pack center.

I can't say I'm surprised (on the FLIR I can see tiny temperature differences on interior cells versus edge ones in my pack when in use), but: how do you know you're seeing aging? Did you disassemble and capacity-test each cell?

It may be that some or all of what you observed was just voltage varying due to temperature. Especially if you are checking at the top of SOC, a higher temp might "fool" you into thinking a cell has a little less capacity.

[nebster]

Originally Posted by newhaul

Or would it be better to get aluminum cased cells.

They're so far superior now in density (essentially, 2x), I think it's a no-brainer. But, DIYers do have to get over the fact that there aren't really any "brand names" except the old CALB CAMs.

[newhaul]

Originally Posted by nebster

They're so far superior now in density (essentially, 2x), I think it's a no-brainer. But, DIYers do have to get over the fact that there aren't really any "brand names" except the old CALB CAMs.

currently I am running camel brand cells in my 100ah bank. And the xld cells that I am waiting for are also aluminum cases.
( both are big names in the Chinese ev market.)

[CatNewBee]

AFAIK, the Winston cells, that are broadly uused in Europe require compression cases to prevent bulging on deep discharge. Instead of breaking the enclosure, the pressure properly releases over the sacrificial vents on top and no additional spill happens in a catastrophic failure.

On a accidential deep discharge the compression prevents degrading the capacity to some extent without blowing off the vent, if not compressed, the expansion leads to irreversible loss of capacity, so makes sense to have straight plates on the long sides. Tensioning straps are not sufficient, while they keep the cells together they do not help to prevent bulging of the outer cells at all.

[rgleason]

Very interesting theory. Maybe the compression case needs to be a little bigger with compression spacers some thickness (1/4"-1/2") and some distance apart (1 1/2"?) at the sides and bottom to allow air flow?

[john61ct]

The need for compression plates / strapping is critical

and not just Winston, nor just for high C-rate use cases.

You want to suppress any bulging at all, and it can occur in normal use even coddling care optimizing longevity.

The temperature "imbalance" issue IMO is a red herring.

Sure if you have cells where you **know** there is no chance of case bulging, ventilation within the pack might be worth putting a bit of resources in

but at this point, trying to get effective compression **and** intra-pack cooling would be overkill, completely unnecessary.

[rgleason]

John, blanket statements aren't necessarily right.
What about hoping to use higher charging rates like from a big alternator?
Some of these issues might be legitimate concerns. Of course I have NO experience about this.

[john61ct]

Brand names are absolutely necessary, critical if getting good longevity is an important goal.

But not sufficient, the supply chain back to the factory output / QA sorting process is just as important to ensure Grade A, recent mfg date, not counterfeit not secondhand.

Of course 5-10 years from now, the "top-notch shortlist" of known-good quality LFP cell makers may well have a couple more names on it

(and likely a few dropped out or changed like A123 to Lithiumwerks)

But that's how long it takes to really earn the savvy sector of the market's trust.

There are literally hundreds of shoddy makers and scamming sellers out there in every niche of this business.

If you only want 5-10years, then fine roll the dice, you might save a little money, maybe.

Note that **consistency** of cell quality is the real holy grail!

I'd be happy to save money buying a brand that I knew would only last 50% of Winston/CALB/GBS/Sinopoly lifecycles

if I knew that a given batch of cells were well-matched wrt not just in Ah capacity but "voltage vs SoC mapping" and ESR/IR.

Even more idealistically, compared to additional cells I will buy in the future.

We've seen a huge decline in that aspect, even within the top-notch shortlist, likely more a supply chain issue than their QA processes.

[john61ct]

Originally Posted by rgleason

What about hoping to use higher charging rates like from a big alternator?

Define higher.

The temperature imbalance issue being discussed here is from the impact of ambient heat affecting calendar lifespan.

Usage C-rates is completely irrelevant to that topic.

In a propulsion context where 2C even 5C are considered normal for fast-charging,

and fast-charging is critical to many use cases,

yes, there will be internally generated heating.

The reduction in lifespan from doing that regularly would overwhelm this minor imbalance issue.