Charging with BMS and Non BMS pack ?

[expresso]

BM i may have asked you this before i dont recall the answer if i did

i am just curious - on my bounder - it has 100ah inside with a BMS - My add on pack has no BMS - i always charged them on there own - disconnected from each other

BUT- what if i left them connected as one pack - and just connect the PL 8 to my ADD ON pack - and charge it using the PL 8 as i always did but this time have them both connected -

what would happen - thinking about this - it shouldnt be a problem since the inside pack has a BMS which does the balance for the chair pack and the ADD ON would be fine also using the PL 8 balance as always

do you think this would be OK to do - how would the PL 8 react -

[shirley_hkg]

it shouldnt be a problem since the inside pack has a BMS which does the balance for the chair pack and the ADD ON would be fine also using the PL 8 balance as always

Very likely you would get a BALANCED add-on , and a UNBALANCED bounder pack.

BMS will switch on & off dozen / dozens of time to complete balance (with charger ON).

In your case , I guess your add-on will finish balance SOONER , because of smaller capacity and bigger balance current of the PL8 than that of a BMS.

As PL8 saw the add-on balanced and termination current reached , it would stop and would NOT restart again. That renders the BMS no time to finish.

You might argue to set the termination current low enough to extend charging process, but that will also put the ADD-ON being cooked for long period of time undesirably.

[Burgerman]

Correct. And to a large degree that bms causes the bounders charger a similar problem. The charger doesent know when the balance ends or what balance even is, and may or may not stop (a minute to days) too soon... Depending on its internal logic.

[expresso]

ok i though that also - Add on would finish and PL 8 stops - chair pack wont be finished not enough anyway

i will try that once - one day curious - when it ends - i will then reconnect only the chair with PL 8 and see how far it went - and continue just the chair

to see how much longer

i been using the PL 8 to charge the bounder also - at the very start i used there supplied charger - first summer maybe - then the PL 8

another question about the stock supplied charger - when charging it reaches a stage where light goes yellow - gives off a different sound or lower one -

thats the balancing part - i recall maybe 15 min at that stage and then done -

what exactly can the charger actually be doing differently during this so called balance stage yellow light ?

[Burgerman]

nother question about the stock supplied charger - when charging it reaches a stage where light goes yellow - gives off a different sound or lower one -

thats the balancing part - i recall maybe 15 min at that stage and then done

No. It cannot know what state the balancing is at. It may be balanced already, or it might be days away from being balanced if theres a low cell. The charger has no way to tell. It cant tell if 6 cells are at 3.6V one is at 3.0V and one is at 4.5V. As long as it all adds up to 28.8 it thinks thats great! Thats all it knows.

what exactly can the charger actually be doing differently during this so called balance stage yellow light ?

It will be the CV stage, where the voltage AT THE CHARGER has reached whatever voltage that it was programmed to reach. Probably a too higg 29.2V or something. Then it holds it steady. Thats CV. It will do so for what length of time?

The correct length would be when the cells are all balanced, plus around 15 to 30 mins. But it cannot know.

[ex-Gooserider]

Actually if using a BMS, would it be better to use a voltage limited power supply (that can do lower voltage if the current draw is higher than it's max output) set to the desired termination voltage, instead of a charger? (I.e. a 'Shirley-supply, instead of a PL-8 or the stock charger)

I'm thinking a constant voltage supply would just let the BMS handle the on-off, and otherwise limit the maximum voltage the pack as a whole sees...

ex-Gooserider

[Burgerman]

What controls the length of time that the charger is sat at CV though?

Heres what we try to achieve. For a fast efficient charge while having all cells balanced. Without sitting too long at the higher than natral relaxed charged point.

EACH cell needs to be brought to the same 3.55 to 3.60 (or 3.65V according to manufacturer)*** fast so that the cell doesent sit for long at this elevated voltage. This means a high balance current capability is much better. It remains at this (ideally 3.55V) untill all cells are a) balanced voltage, and b) all drawing just a few mA. At this point the CHARGER should turn off.

Question. How can the charger know when this has happened?
In the case of a power supply it can be set at a lower voltage say 3.55 but some BMS will not balance at all or not well at this voltage as they kick in and out over a wide range of voltages per cell, and per pack.

The PL8 and other hobby chargers dont have to do this as they throttle output in stead proportionally to be sure that the balance current is adequate to prevent the cells needing to be repeatedly disconnected/reconnected. And then when the CHARGER sees all cells have very low balance current, and are at the same voltage, the charger turns off.

Beause of the way a BMS works it has to have a wide voltage range while balancing so it can turn on/off all the time. Since you cannot balance below 3.55V reliably, that means they have to allow a too high 3.65V to allow a BMS to work properly. Not good for the cells. Esp if it has low balance current and takes days to fully balance a pack.