leaving Lifepo4 sit fully charged for 2 days

[Learning Curve]

I have a simple question about maintaining the life of my 36v 15 amp hour Lifepo4 battery.

I know it is best to store them at a 50 percent charge, but will frequently leaving the battery sit with a 100 percent charge for 2 to 4 days make a substantial difference to the life of the battery?

I know it would be better to take 10 percent off the top, but I live up a long path and just getting the bike down to the road and back to do a short ride is no always convenient, and the weather or other circumstances can cause my plans to change.

If leaving the battery sit fully charged for a few days is going to create a noticeable difference in the life of the battery I will figure out how to run something off the battery that will discharge it a bit.

Otherwise, I will try to leave it at less than 100 percent charge if it will be sitting a few days, but not worry about this when it does not work out for me to do that.

 

[wb9k]

I wouldn't worry about it much unless you're putting the cells into storage. If you NEVER fully deplete the pack, you could charge to less than 100%, but if this causes you to go below 20% it's probably a wash or worse. Batteries in plug-in vehicles are going to spend much of their lives at 100% SOC, ready and waiting to be used. It's just the way it is.

 

[MeanGene36542]

+1 on that, I do it all the time. I also have a 36v LiFEP04 16Ah battery. I ride at least 3, often 6 days a week. I recharge right away. Alternative would be to wait 3+ hours whenever you decide you want to ride. Who wants to do that. No apparent ill effects on battery after 9+ months and 3500+ miles.

 

[wb9k]

The ill effect is slightly accelerated degradation of cell health. You wouldn't notice it unless you had several other packs to compare it to that had treated differently.

If you want A123 LFP cells to last forever, take them down to 20% SOC or so and store them at 10*C or lower. You won't be able to use them for much of anything in that state, but they'll be use-able forever. Having a battery ready to use is in direct conflict with its desire for immortality. You can't have both, that's just life. The only place it really makes much sense to me to consider not going to a full charge regularly is in a (non-plug-in) hybrid, where getting maximum range (or maximum EV range) does not depend on bringing the battery to a full SOC.

 

[Learning Curve]

Thanks for answering my question! And for the additional philosophical consideration of my dilemma

You won't be able to use them for much of anything in that state, but they'll be use-able forever. Having a battery ready to use is in direct conflict with its desire for immortality. You can't have both, that's just life.

I think you are right about that!

 

[amberwolf]

If you always know in advance when you need to use the pack, and for how far, you could leave it at it's "storage" SoC whenever you aren't using it, and then charge it up just before leaving, and only just to the capacity you know you need.

BUT: you won't be able to depend on the pack to help you with any unpredicted stuff enroute: headwinds, detours, extra unplanned stops outside the route, etc.

Also, you have to know how long it will take to charge to the point you need already, meaning keeping a chart of tested times to specific SoC's. And you have to know far enough in advance that you are leaving to be able to charge the pack to the amount you'll need.


So, while it would make your pack last some amount longer, time-wise, it will also be much less convenient, and in adverse conditions could actually cause you problems (if you really needed the pack to get you home/etc but had headwinds or a long detour).

 

[dogman dan]

At some point, you have to choose between having a bike that is ready to ride and use as practical transportation, and having a bike you have to constantly fuss with in some way.

You could look into a new charger, one that can partially charge the pack. Then when you do decide to ride, you could top off the last 10% with your current charger.

You will still need to occasionally charge fully overnight, so the pack fully balances.

But a couple days, here and there, it's not going to hugely affect the battery. After all, you already charge it everyday the night before when you do ride.

 

[Ykick]

One caveat here that I didn't see mentioned is your pack likely has a BMS installed. This circuit draws power from the pack 24/7 so if you get in the habit of leaving your pack close to empty and go away for a significant period of time you run of risk of over-discharging cells. Many of us would agree that BMS equipped packs either need to be charged at least every month. Or, if possible, disconnect the BMS completely for worry free long-term storage/transport.

With a power meter installed for charging you can get an idea when a pack nears full as the amps begin to taper off. The beginning of that taper is where I usually terminate any bulk charge and then wait until the next day, an hour or so before my commute to "top-off" a complete charge.

Is it gonna make much difference? Probably not but it's always better to be "interested" in what your battery pack is doing so for that reason alone, it's not a "bad" way to go about charging routine.

 

[dnmun]

i think the problem of draining the pack on the BMS circuit current is limited to the ping signalab BMS. he uses a series of opto transistors over on the side and each of those optos uses some current all the time to keep the led inside the optotransistor turned on all the time. there is also the BMS circuit current that comes off of the top of cell #4.

i do not see the same level of draining of the battery on the other BMS designs and the circuit currents for most of the BMSs i see with the other type of circuit are only in the 20uA range which should not be a big risk.

but the ping BMS should be disconnected from the pack at the sense wire plug if it is gonna sit around for months or years or even over the winter. then there is no circuit current or individual channel current for the optos.

 

[wb9k]

I highly recommend disconnecting all control electronics when putting a pack into storage. Not only are there leakage currents through components, even when they are completely "shut off", oddball failures in electronics in storage can lead to exciting events if they are left connected to their batteries. I've seen it happen. There is no benefit to leaving electronics connected in storage, only risk. Don't do it.

 

[Learning Curve]

Thanks for all the input! It is appreciated!

In my case, I won't be leaving the battery sit unused for more than a week, but as I work at home and have a large garden, it is easy to not make out of my yard for a few days, and sometimes, especially in winter, I can plan on a bike ride and get all charged up, but plans change, or the window of opportunity passes. I am glad to know that when a few days go by and it is really inconvenient to do even a short ride to discharge it a bit, it is probably not frying itself, or at least not in a big way...

The suggestion about unplugging the connections to be on the safe side, is also appreciated.