Battery charge and balance questions

[ccihon]

I have been riding my trike more lately, so also charging more often. I have a big Li-Ion 52v 22ah battery, and a 3A charger, marked "58.8v" which seems correct. I am monitoring the output voltage with an accurate meter, but the numbers there match my voltmeter as well as the KT LCD voltage display. All works well - but my question is whether it makes sense or is beneficial to "balance" the battery from time to time, or how to know if it is "out of balance". The battery has a BMS, but I'm not sure what it is capable of.

Generally, I charge it up after it reaches a low of around 25% on the meter, which equates to around 46V. I generally manually stop charging after a number of hours, and am happy if the battery nets out above 80% - maybe 56v.

My real question comes after observing the behavior of the charger. It seldom if ever shows a "green" led and a fully charged state until I disconnect the battery for measurement and re-connect. It seems that if the battery is at above 90% of the total spec voltage of 58.8, returning it to the charger will yield a green led and the charger is idle (has internal fan that is off). Is "out of balance" indicated by a condition when the full charge is less than the spec pack full charge voltage of 58.8?

I know it may be a bad practice to leave it charge to 100% - I'm just wondering if it will ever get there, or in fact the charger will continue to deliver a charge and possibly the BMS will begin balancing? Next charge I plan to leave it on well past when I think it will yield a "full" charge indicator on the charger. Does the charger enter a green "charged" state only after the BMS stops "asking" for charge voltage? Thanks

 

[john61ct]

Cannot be answered without knowing the exact specifics of the pack, charger and BMS internals.

Step one is a display that shows the exact voltages of each cell group

Step zero is the balance lead wiring brought to the outside to allow various devices that access.

Which could also allow for easy BMS removal and replacement.

Hobby style balance chargers, dedicated balancers, manual balancing with power supplies

many many ways to skin that cat.

Maybe even your current BMS can do it well enough

but without those two prerequisites you will never know.

 

[john61ct]

Seems your pack is 14S.

Do you know that for sure?

If so, holding 58.8V at the pack level overnight at a low amps rate **might** allow the BMS to balance.

However without seeing the actual group level voltages you'll never actually know.

In theory it is possible a programmable BMS and charger are fully integrated so the former controls the latter, say with CAN protocol communications

but usually that is only with much more expensive gear, used on EVs.

 

[E-HP]

I'm just wondering if it will ever get there, or in fact the charger will continue to deliver a charge and possibly the BMS will begin balancing? Next charge I plan to leave it on well past when I think it will yield a "full" charge indicator on the charger. Does the charger enter a green "charged" state only after the BMS stops "asking" for charge voltage? Thanks

Are you saying you've never fully charged your battery?

If the battery is out of balance, then once one/some of the cell groups reach their maximum voltage (4.2V), a cheap BMS will stop charging the pack, and then attempt to siphon off the charge, at a very slow rate, of the higher voltage groups to match the lower (which can seemingly take forever). If the pack is way out of balance, it may take more than forever (e.g. a long long time).

A smart BMS, or good balance charger, may siphon off the higher groups and charge the lower groups to balance, which is faster. Some may even have active balancing, to keep the pack balanced regardless of voltage level, and/or have higher balancing current to speed up the process.

A lot of this goes out the window if you have bad cells within the groups. In that case you may be able to charge the pack to 58.8V, but the voltage may drop once you take it off the charger, or experience a large amount of voltage sag when using the pack; or you may not be able to get the pack to fully charge at all, etc.

Charging to 100% isn't ideal, but there's a trade off to charging to 80%-90% for longevity, and having the pack go too far out of balance that it's a PITA to get back in balance. So the better practice is to fully charge occasionally. You don't want to create a situation where it takes days to balance your pack.

For the charger, the light will turn green and stop charging when the voltage of the pack reaches the output voltage of the charger, or if the BMS stops the charging. So as long as that's 58.8V, it will fully charge a pack that's in good condition. You can also adjust the charger voltage to something lower (e.g. 58V), and it will turn green and stop charging at that voltage. I have one higher current charger set to 58V for my normal charging, and another, lower current charger set to 58.8V which I used to balance the pack, once in a while. I charge the pack with the faster charger to 58V first, then switch to the slower charger to finish charging and balancing, ever couple of weeks.

I bought some active balancing modules a while back, so I'm considering taking out my BMS and just moving to active balancing. Mainly, I never trip the pack on overcurrent or low voltage, only run the pack in the 15%-85% range, if that, and monitor it closely when riding. I figure that way I can take balancing out of the equation and just charge the pack to 85%-90% and never worry about it. Still weighing the pros and cons.