Leave charger plugged and connected or not.

[deronmoped]

Whats the best way to charge a Lithium battery pack with a BMS.

1) Charge it till the charger says it's done and disconnect it.
2) Charge it till the charger says it's done and leave it connected.
3) Charge it till the charger says it's done and only leave it connected indefinitely, only occasionally.
4) Charge it till the charger says it's done and leave it connected, only when you suspect you have a low cell.


The reason I'm asking is, it's possible I have a cell or two that are low and the pack is cutting out on me because of a under voltage. One of the members on this forum told me to leave the pack connected for several days to recover any low cells. Would it be smart to charge the pack for days on end occasionally.

 

[knighty]

hi

if it's convenient, just leave the charger connected and plugged into the wall whenever possible

with the bms there's no way to over charge the battery

 

[dogman dan]

Sure it can, but not if the bms is working properly.

I advise people to charge the pack, and leave it plugged in at least overnight frequently when they have a bms. Particularly if checking for low cells is hard to nearly impossible.
If the bms is good, it won't overcharge the pack, but it will have the time it needs to fully balance the pack, giving you the best possible capacity in the morning.

If you are having problems, then put that thing on the charger every hour you are not actually riding it. I did this with my pingbatteries for years, commuting 2 hours a day, and leaving it on the charger 22 hours a day. I'd unplug on the weekend if not using the bike, but always overnight charge it if I would ride the next day. I just wanted to always have the maximum balanced charge, since I had a long and tough commute. If I ran low, it was always going to be an uphill ride home. So I took no chances on having a low charge.

The above assumes that your charger will restart, once the bms lowers the voltage of the high cells. Some chargers do not do this. Those designed for lifepo4 usually do.

Also worth repeating, there is no memory effect. DO NOT run your battery as low as possible before recharging. Instead, recharge at every convenient opportunity. Batteries love shallow cycles.

So why do you do the opposite with the cell phone? Depends on the charger. Above 4.2v is overcharged for a cell phone lico battery. But I have owned phones who's chargers always charged the battery to 5v. Needless to say, this severe overcharging damages the battery, and there was a definite limit to how many of these overcharges a battery could stand. With that phone, I quickly learned to try to limit recharging to once a week.

Again, if your bms is working, then you need not worry about this happening with your bike battery.

Should you leave the charger on the bike, and leave it plugged in to the battery when not riding? Emphatically NO!

There is a very good chance you will vibrate the charger to death if you carry it too much. If you must carry it, in a bag on your body like a pack or messenger bag will vibrate it least. Leaving the battery plugged in when it's not powered up means the battery is powering up the charger. This could leak away power, and left long enough destroy the battery.

 

[deronmoped]

What about if the bike is not going to be rode for several weeks, or a month or two. I heard that storing a lithium battery at half charge is good for longevity.

Leaving my battery pack charging for several days did a good job of recovering the low cell(s). The charging light did not cycle between red (needs charging) and green (charged), but the charger held the voltage at almost 42 volts, for a 36 volt pack. The individual cell voltage did not show any change with the additional charging. They were anywhere from 3.4 to 3.7.

 

[fechter]

If the bike is in 'storage' it might not be bad to leave the charger on.
Most chargers will detect end of charge and essentially turn off until the pack voltage drops a certain amount. In storage mode, this might take weeks or months. It would then come on again and top off the pack and turn back off.

It would probably not be a good idea to leave the charger connected to the pack but not turned on (no AC), since it would probably drain the pack slowly.

 

[dogman dan]

What I have done for storage, was to put the battery on the charger about once every two weeks, overnight. Or, if convenient you can unplug the bms. Then you can store for long term at less than fully charged. Do check on it periodically though, it could have some tiny trickle short discharging it somehow. So watch it for self discharge at first, till it proves that it won't self discharge.

It can be very hard to catch the charger with the red light on, when balancing. It often goes like this, green for an hour, red for about 30 seconds, then green again since the low capacity cell overcharges really fast. That 30 seconds of green does not do much to bring up that low cell, so it can take days, even weeks to get it fully caught up.

It can look a lot like nothing at all is happening. And if you charger is not the right type, nothing is happening.

 

[999zip999]

If in storage mode I unplug the battery from the controller.

 

[dnmun]

your battery is in bad shape if it is that far out of balance. too bad your voltmeter doesn't work well enuff to measure the voltages more accurately. i already tried to get you to measure the voltages while it was charging since you said the battery was cutting out on you from being so outa balance but you just ignored it.

 

[deronmoped]

I did measure the voltages while the pack was on charge. I explained that they did not change much. The low ones stayed at 3.4 volts and the higher ones stayed at 3.6 and one would go back and forth between 3.6 and 3.7.

It's a Micronta handheld digital voltmeter, not sure what the tolerances are, but it wasn't calibrated or does it measure into the hundredths of a volt.

If the pack is in bad shape, it does not show any problems when I using it, well, at least since I changed my charging style.