BMS shutoff or battery top-up/partial drain for storage?

[wayover13]

I'm grappling with the issue of battery storage--think over the cold, upper-midwest winter, where biking outdoors is at best unpleasant, at worst impossible: during these intervals, the battery must sit largely unused and thus in storage. My LiFePO4 battery might have to sit idle for 3 months per year or more due to these weather conditions. So I need to develop a storage strategy. The basic premise I've learned, by the way, is that partial discharge prior to storage is a must, so that will be a given in what's discussed below.

First, a discussion of the problems I've been informed about that arise in storage. Some batteries (apparently my Ping is among them) have a poorly-designed BMS, one that draws the current to keep itself active from just a few cells. Long term storage under this scenario--as I've discovered this year--will result in an unbalanced battery; the longer the storage interval, the more unbalanced the battery will become. The bottom line regarding these sorts of batteries is, as it's been expressed to me, that they've been designed for use, not for storage. So what's at stake here for us cold-climate dwellers who must take our batteries out of service for some part of the year is, in a sense, finding a way to thwart the design philosophy.

I've been told about, and have actually tried, disconnecting the BMS as a storage strategy. An appealing solution since, on the surface it seems so simple: just get the battery down to an appropriate charge level, pop those connectors apart, and store. In practice, though, this sort of solution was obviously not a consideration in the design philosophy and, for that reason, could be a recipe for disaster; you see, on my Ping battery that connector is pretty fragile and fidgety--almost like it wasn't made for regular disconnecting/connecting. If it ever broke on me I'd be in a real pickle. So, though I like simple, crude-but-effective solutions like this, this particular one doesn't look advisable as a storage strategy.

The other strategy I've been informed about involves topping up the battery periodically over the storage term--apparently anywhere from weekly to once per month could suffice. This, I'm told, keeps the unit balanced while in disuse. Also simple and thus appealing for that reason, but perhaps also not too advisable owing to the fact that, as I understand it, these batteries can suffer degradation if stored at full charge.

Which gets me to thinking that maybe a better storage regime for me might be to top up the battery on some regular basis as described, but to find some means of discharging the battery by a certain amount immediately after the charging session. That strategy, though involving an extra step, also seems an acceptably simple and practicable one: it mimics, to some extent, the constant-use scenario for which these batteries seem to have been designed. And since I know of nothing along the lines of a BMS shutoff switch that might make the first storage option the more attractive one, this is the strategy I am currently inclining toward implementing.

The main question I have in regard to it is what type of load might I use to partially drain my battery after topping up? My sketchy understanding of electronics tells me anything that draws 36 volts or more DC, and whose amperage rating is not going to be overwhelmed by the battery, could be a viable candidate. The bike motor is the obvious choice but is not a terribly practical one owing to the fact that riding outdoors usually won't be feasible during the period in question (prop up the wheel inside, open the throttle and let it whiz away for part of the day? ride the bike in circles around the inside of the garage for an hour or more?). So I'm looking for suggestions as to how, once I've topped off the stored battery, to drain 20% or a bit more from the stored pack--the quicker the better.

Input, suggestions, and corrections of any misconceptions, will be appreciated.

 

[Ykick]

This is what I use. Warms lunch too -



Any sort of heater element from toaster, range top, space heater, etc. will work on DC. I usually bypass control circuitry and only expect 1/4 or less wattage to your lower voltage DC pack based on the AC 110V ratings. But, you can also parallel several more heater elements together to increase DC load.

 

[999zip999]

A hair dyer, try it on low first. Some say 1,500 watts. Read the watt rating on ping website for your battery. Run your bike at ping rating. For long life. I would charge and unplug bms. Worry about cycle life. Take it down 2 volts. 75- 80% charge on lifepo4 for 4 months. I rather have some charge in it than zero.

 

[dogman dan]

Well, you could look at things this way,,,

9 months of the year its warm,, and you want to ride, so you may be storing the battery full for a lot of that time.

Warm and full,, what ages a battery fastest for 9 months,

On the other hand,, the winter it will be stored cool. Full maybe, but cold,, and only 25% of it's lifespan.

Honestly,, you do want your battery to last, but how you use it in the warm months is going to be the big deal, not storing it full and cold. I just think you are fretting more than the real damage justifies.

Here's a compromise,, before it gets that nasty,, charge full and balanced, then ride a block or two. Store it like that for 30 days. Then charge full. Charge full again in 30 more days.

Now you have and easy way to do it, and it's only full 2 months instead of 3.

 

[teklektik]

My LiFePO4 battery....
The basic premise I've learned... is that partial discharge prior to storage is a must...

Some batteries (apparently my Ping is among them) have a poorly-designed BMS, one that draws the current to keep itself active from just a few cells. Long term storage under this scenario--as I've discovered this year--will result in an unbalanced battery

LiFePo4 has unique storage and charging attributes differentiating it from all Li-ion chemistries resulting in a lot of speculation and mis-applied advice. Authoritative studies on storage of LiFePo4 are hard to come by, but there appears to be evidence that three month storage at 20%-80% charge will result in about ~50% of the voltage-induced cell deterioration as storage at 100%. However, we are talking about a net degradation of 1% vs 2% overall capacity so we are looking at chasing down a very small benefit. This amount of deterioration is arguably less or at best, similar to, cycle-induced deterioration from normal use for the same period. This means the net improvement for an ideal storage strategy would be better than no loss at all, but not materially different than if the battery was in constant use.

If you want to pursue this: owing to the difficulty in using open cell voltage as a SOC indicator with LiFePo4, you really only have the option of doing coulomb counting by timed discharge after a full charge to achieve an 80% SOC (or use a CA V3 SOC indicator which does the calculations for you).

Since you have one of those wonky BMSes that causes cell imbalance, then periodic charging is a good plan. Realize though, that to achieve balance, the charge must be left running for some time - perhaps a day or more. However, achieving perfect balance isn't that critical for the periodic storage re-charges where you are primarily trying to prevent over-discharge of the BMS-powering cells. That said, a balance charge is probably a good precaution immediately before launching your new riding season. There is no downside to battery health in doing so.

Short form: don't worry about it too much. You particular BMS makes periodic recharge a good idea, but a partial discharge regimen appears unlikely to give you a huge payback. The important thing will be to make sure your battery sits on the charger a day or two before you launch your Springtime riding season to fix up any BMS-induced charge issues that may persist from abbreviated non-balancing storage charging.