Battery voltage and charging modes.

[E-HP]

This question might be better placed in the battery subforum, but it’s not too complex, I think.

I rarely charge any of my batteries to 100%, and have never discharged any of them to LVC. I bought a new battery and have been using it for a week. I’ve been charging it to around 90%, discharging to around 40%. I decided to fully charge it this weekend, and it stopped at 54.6v as expected. I unplugged it and let it rest. 3 hours later it was still at 54.6v.

My observations from charging to 90%: After taking the battery off the charger, the pack voltage drops about a volt within an hour.
My observations from fully charging, as stated above, 54.6v after an hour or more.

I think this is due to the battery at 90% is still under constant current charging mode when removed from the charger. The 100% battery is being charged in constant voltage mode for a good while near the end of charge. In constant current mode, the voltage is artificially higher while charging as the constant current is pushed into the battery, raising the voltage, but settles back down when the current stops.

Can someone confirm this is the case? I confess the terminology may not be correct, but asking whether that’s what’s happening conceptually.
Also, I’m concluding that the battery seems healthy if it fully charges with the charger cutting off the charge cycle vs the BMS, and is able to maintain the fully charged voltage. Yes, no?

 

[amberwolf]

My observations from charging to 90%: After taking the battery off the charger, the pack voltage drops about a volt within an hour.
My observations from fully charging, as stated above, 54.6v after an hour or more.

The most likely thing is that you have cells that have already reached full (because they are lower capacity than the others) and then have to be drained down by the BMS (which drops their voltage).

If it is a charge-transfer (active) BMS isntead of passive balancing (resistors) then it would transfer some of the voltage to other cells that were lower, but depending on the SoC of the lower cells, they might not show much voltage rise out of that vs the drop from the higher voltage cells, giving a net voltage drop.

I think this is due to the battery at 90% is still under constant current charging mode when removed from the charger. The 100% battery is being charged in constant voltage mode for a good while near the end of charge. In constant current mode, the voltage is artificially higher while charging as the constant current is pushed into the battery, raising the voltage, but settles back down when the current stops.

If the voltage rise was from charging current, it would drop instantly as soon as the current stopped, because it would be form internal resstance of the cells,e tc.

If it was from an artifact of the charging process, it would drop after a full charge just as much as a partial one, or even more.



To find out what is actually happening, you'd have to measure cell votlages before, during, and after the event.

Also, I’m concluding that the battery seems healthy if it fully charges with the charger cutting off the charge cycle vs the BMS, and is able to maintain the fully charged voltage. Yes, no?

To say if it is healthy depends on your definition of healthy, and what tests are done to verify that.

If healthy means it is still original capacity, resistance, etc., and that all cells are identical in properties, you'd have to verify all those things to know if it is healthy.

If healthy just means that it fully charges and stays at that voltage after charger is disconnected, then if it does that, it's healthy.

If healthy means something else, you'd have to define that, then test those things, to know if it is.


My personal definition would be:
-- all cells are identical properties so that they always stay balanced from full down to empty without requiring any device to do this,
and
-- it doesn't sag in voltage enough to affect acceleration especially under heavy load,
and
-- it has enough of the original capacity to still do the worst-case range I require
and
-- it doesnt' have internal leakage causing self-discharge
and (something else I lost track of after dozing off).

 

[E-HP]

The most likely thing is that you have cells that have already reached full (because they are lower capacity than the others) and then have to be drained down by the BMS (which drops their voltage).

So is this a case where the BMS is performing balancing before the pack reaches full charge? Would this disconnect the charging circuit, causing the charger to stop charging? I thought most cheap BMS's that actually balance, usually balance at the top, not at 52v, for a 48v battery. When I manually unplug it, when it's reached 52V, the voltage will drop. Maybe it happens more quickly; I just unplugged it, went to do something and came back to see it at around 51V.

On the full charge, I took the battery off the charger as soon as the light went green and voltage hit 54.6V. Didn't budge from there.

I'm not sure that this BMS does any kind of balancing. Actually that's my assumption. I'm pretty sure it will cut off charging if a cell group goes over 4.2V. Hence my logic is, the highest voltage the battery could charge to would be 54.6V, and if the BMS cut off any group(s) early, it would never achieve 54.6V. 54.6V would also imply the groups are in balance, since no groups could exceed 4.2V (or fall below).

 

[docw009]

Google AI has a believeable explanation for the drop in voltage when the charger is removed. To paraphrase, it's mostly about the cell dropping back to open circuit equilibrium when the charging potential is removed. Maybe less drop near max charge, at least initially,

I don't fully see how non balance BMS can allow 54.6V to happen, given that cells can't be that uniform. Must be something in the cell chemistry and how cells charge.

 

[Diggs]

The 100% battery is being charged in constant voltage mode for a good while near the end of charge.

This is most probable. The current at the end of the tapered charge cycle is almost nothing so that when it does finally end there is no drop in voltage as there was really no current being supplied at the end anyway. Most chargers start tapering off current quickly after 90% charge is reached.

 

[amberwolf]

I don't know why this didn't post yesterday...ws still in the editor.

So is this a case where the BMS is performing balancing before the pack reaches full charge? Would this disconnect the charging circuit, causing the charger to stop charging?

Depneds on the bms design. They can be made so that the balancing starts at a cell voltage below HVC. Then HVC where the BMS turns off the charge port. Then another voltage below which the cell must drop before it will turnt he port back on.

Or they can be made so the BMS tursn off charge at any point where balancers are on, and tursn back on as soon as balancers are off as long as hvc isn't reached.

And that's assuming resistive balancers, which are only on, or off.

If it's a charge-shuffling type, it could operate that anytim, all the time,


Some BMS HVC is higher than you'd expect--it might be as much as 4.3v from one case reported around here a whil eback.

There might be different levels of HVC, too, just like some might have different LVC levels, so that if it goes above some point it turns off the ports and won't turn them back on until ti's been reset, to protect against use after overcharge or overdischarge that could've damaged the cells.

I thought most cheap BMS's that actually balance, usually balance at the top, not at 52v, for a 48v battery.

Depends on teh design.Personally I'd expect the simplest possible behavior set, but more ocmplicated stuff gets cheaper all the time....

if it doesn't have any manual or data about it's behavior, you'd have to test to see what happens for each type of event, to know if it is still operating the way it should later (and making the assumption that when tested it is operating correctly).

When I manually unplug it, when it's reached 52V, the voltage will drop. Maybe it happens more quickly; I just unplugged it, went to do something and came back to see it at around 51V.

If the drop is pretty immediate, it's just from the ceasing of current thru the resistance of cells. Current falls off at end of charge so there wouldn't be a voltage drop when charger is removed when full. (EDIT: someone else already covered this)

If it takes significant time to drop, it's almost certainly either from cells with self discharge or the BMS draining them for balnacing reasons.

On the full charge, I took the battery off the charger as soon as the light went green and voltage hit 54.6V. Didn't budge from there.

I'm not sure that this BMS does any kind of balancing. Actually that's my assumption. I'm pretty sure it will cut off charging if a cell group goes over 4.2V. Hence my logic is, the highest voltage the battery could charge to would be 54.6V, and if the BMS cut off any group(s) early, it would never achieve 54.6V. 54.6V would also imply the groups are in balance, since no groups could exceed 4.2V (or fall below).

The ways to verify these assumptions are
-- measure the cell voltages at various SoC to verify balance among htem all
-- charge it with a charger of too high a voltage to see if it overcharges to veriify BMS HVC operation (I don't recommend doing this, if it doesn't work you can damage the cells or start a fire)
-- examine the BMS for either a type of control chip that does charge shuffling, or for balance resistors / shunts per channel (usually "obvious" by the repetitive nature of the circuitry equalling the number of channels)