Charging parallel different battery with different capacity questions/help

[justlooking808]

Looked high and low and went trough 10 pages of forum topic and couldn’t find my answer so here goes.
Setup = two battery connected in parallel with different capacity one is 10.5ah and the other is 20ah
Both batteries has own BMS charge port with different max amp rating 10.5ah has 15 amp max (OEM) 20ah has 30amp max rating but both output will be connected together in parallel.
10.5ah battery BMS can handle 5a max for charging and 20ah battery BMS can handle 3a max for charging.
Charge is a 4a charger, might buy a bigger one later depending how long it takes to charge the batteries.
Questions:
1. When charging, can I use a Y adapter to charge both batteries or do I need 2 separate chargers?
2. Can I solder the two BMS charge wires in parallel to charge both at the same time always or is it better to keep each one separate and use a Y adapter?
3. While both batteries are connected in parallel, can I just charge though one BMS and let the other battery get a charge trough the output port?
4. If #3 is possible, do I just alternate charging trough one battery to keep both batteries balanced via charging trough BMS every other charge?
5. If both charge port wire was connected all the time(soldered together) to create one charge port, would there be any negative effects? ( like auto balancing/charging from charge port and output port, since both sides is connected in parallel)
6. If bike pulls 20 – 30amps, would that affect battery as one battery can give more current than the other? Would it cause the one battery to be discharge more than the other and when I let off the throttle, would that cause a surge in energy from one battery to the other to balance the voltage between the two batteries?
7. Any other areas or things I should be concerned about?
8. Should I change the second battery BMS to one that charges and output on the same port/connector, would this be a good solution to adding a second battery?

Thank you for any and all help, once again, I did try searching and didn’t find anything too specific. I did learn a good deal about BMS and it’s functions but still unclear if BMS can only balance trough charge port or can it balance through output as well? If this has been covered, please point me in the right direction as that would be much appreciated.

 

[gobi]

I am giving you MY version of how I charge 2 batteries with DIFFERENT capacity.

Voltage:
You did not provide the batt voltage, for my example we will use 40 volts. 10s
Batt A = BA

Batt B = BB

1. Charge each battery to say about 36v INDIVIDUALLY
2. Connect BA and BB in parallel
3. Then charge BA and BB at 10A, if in hurry, 15A
4. Wait till current drop to 1Amps
5. If you KEEP BA and BB connected in parallel then next time you skip step 1 and 2 and go to step 3.

If you are in hurry and need to charge at higher amp, then charge at max amps of the lower capacity battery, Parallel connection will divide the current.

Connect an inline meter to see current and other stats,

have fun!

 

[ijr9005]

yes,why,no,n/a,no,no,consider fitting leads to indivual strings with JST connecters,i have 10s pack,leads willbe wired double 5s, i will fit an iinexpensive bms as a belt and braces safegaurd,but bms only passive top ballance cells, with jst connectivity monitering/intervention is easy and cheap

 

[justlooking808]

I am giving you MY version of how I charge 2 batteries with DIFFERENT capacity.

Voltage:
You did not provide the batt voltage, for my example we will use 40 volts. 10s
Batt A = BA

Batt B = BB

1. Charge each battery to say about 36v INDIVIDUALLY
2. Connect BA and BB in parallel
3. Then charge BA and BB at 10A, if in hurry, 15A
4. Wait till current drop to 1Amps
5. If you KEEP BA and BB connected in parallel then next time you skip step 1 and 2 and go to step 3.

If you are in hurry and need to charge at higher amp, then charge at max amps of the lower capacity battery, Parallel connection will divide the current.

Connect an inline meter to see current and other stats,

have fun!

sorry about that voltage is 48v

so what your saying is once you connect the output in parallel and the BMS charge port in Parallel, when i charge i just charge at the BMS input port?

 

[justlooking808]

yes,why,no,n/a,no,no,consider fitting leads to indivual strings with JST connecters,i have 10s pack,leads willbe wired double 5s, i will fit an iinexpensive bms as a belt and braces safegaurd,but bms only passive top ballance cells, with jst connectivity monitering/intervention is easy and cheap

not sure what you mean by this? BMS as a belt and braces safeguard? with jst connectivity monitoring/intervention? how or what would i do and is this even a concern?

i know the BMS is used to balance cells and the batteries comes already with a BMS on it and i will be charging trough the BMS, im just hoping that the BMS will be able to keep the cells balanced when charged in parallel and keep both battery charged at the same time.

 

[ZeroEm]

Not telling you what to do. Most would just bulk charge the batteries in parallel up to 80%-90%. You can do this without connecting to the batteries charging ports. Just put a charge connector at your Y splitter/junction (main power). You can use a bigger faster charger than for a single battery. This would be for most of your riding/charging.
To balance you would disconnect the pair and charge up thru the charge port of each battery. then bring both batteries back into balance before connecting them together again. You should not need to do this but several times a year.

Hope that helps.

by justlooking808 » Oct 02 2021 2:22pm

ijr9005 wrote: ↑Oct 01 2021 12:52am
yes,why,no,n/a,no,no,consider fitting leads to indivual strings with JST connecters,i have 10s pack,leads willbe wired double 5s, i will fit an iinexpensive bms as a belt and braces safegaurd,but bms only passive top ballance cells, with jst connectivity monitering/intervention is easy and cheap
not sure what you mean by this? BMS as a belt and braces safeguard? with jst connectivity monitoring/intervention? how or what would i do and is this even a concern?

i know the BMS is used to balance cells and the batteries comes already with a BMS on it and i will be charging trough the BMS, im just hoping that the BMS will be able to keep the cells balanced when charged in parallel and keep both battery charged at the same time.

 

[justlooking808]

Not telling you what to do. Most would just bulk charge the batteries in parallel up to 80%-90%. You can do this without connecting to the batteries charging ports. Just put a charge connector at your Y splitter/junction (main power). You can use a bigger faster charger than for a single battery. This would be for most of your riding/charging.
To balance you would disconnect the pair and charge up thru the charge port of each battery. then bring both batteries back into balance before connecting them together again. You should not need to do this but several times a year.

Hope that helps.

by justlooking808 » Oct 02 2021 2:22pm

ijr9005 wrote: ↑Oct 01 2021 12:52am
yes,why,no,n/a,no,no,consider fitting leads to indivual strings with JST connecters,i have 10s pack,leads willbe wired double 5s, i will fit an iinexpensive bms as a belt and braces safegaurd,but bms only passive top ballance cells, with jst connectivity monitering/intervention is easy and cheap
not sure what you mean by this? BMS as a belt and braces safeguard? with jst connectivity monitoring/intervention? how or what would i do and is this even a concern?

i know the BMS is used to balance cells and the batteries comes already with a BMS on it and i will be charging trough the BMS, im just hoping that the BMS will be able to keep the cells balanced when charged in parallel and keep both battery charged at the same time.

thank you, i'll try to experiment with this and see what happens. much appreciated.

 

[gobi]

I am giving you MY version of how I charge 2 batteries with DIFFERENT capacity.

Voltage:
You did not provide the batt voltage, for my example we will use 40 volts. 10s
Batt A = BA


have fun!

sorry about that voltage is 48v

so what your saying is once you connect the output in parallel and the BMS charge port in Parallel, when i charge i just charge at the BMS input port?

Yes,

The main thing to watch for is BEFORE connect the batteries in PARALLEL, pls ensure they are nearly at the same voltage.

Once you connect them in parallel, you are good to go.
[sorry I was out up north salmon fishing in the streams]

 

[john61ct]

within 0.1V per S-count is a decent rule of thumb

So for 13S, 1.3V max difference when paralleling

 

[justlooking808]

I am giving you MY version of how I charge 2 batteries with DIFFERENT capacity.

Voltage:
You did not provide the batt voltage, for my example we will use 40 volts. 10s
Batt A = BA


have fun!

sorry about that voltage is 48v

so what your saying is once you connect the output in parallel and the BMS charge port in Parallel, when i charge i just charge at the BMS input port?

Yes,

The main thing to watch for is BEFORE connect the batteries in PARALLEL, pls ensure they are nearly at the same voltage.

Once you connect them in parallel, you are good to go.
[sorry I was out up north salmon fishing in the streams]

will do, thank you! have fun fishing!

 

[justlooking808]

within 0.1V per S-count is a decent rule of thumb

So for 13S, 1.3V max difference when paralleling

thank you, this makes it easy to remember. :thumb:

 

[gobi]

within 0.1V per S-count is a decent rule of thumb

So for 13S, 1.3V max difference when paralleling

thank you, this makes it easy to remember. :thumb:

Sparks make me nervous, 1.3v is a lot in my books,
I would try to make it as close as possible pls, I try for 0.1v difference,
I charge BA to say 35v
Then charge BB to say 35v
then I charge BA again to 35v (as it will sag), wait 3 mins or so, for quick juice up to 35v.
then say a prayer and connect them in parallel,
If I did not swear in swahili = success

Fishing > thanks, I switching to deep jigging for lake trout instead, to hot here to wade and fish for salmon as the fishing is slow.

 

[john61ct]

Staying within 0.1V per S-count is simply to prevent inrush current transfer high enough to heat things up, hurt the sub-pack cells etc.

Obviously a 24V pair does not need the same caution as an 80+V pair.

Preference wrt sparking is a separate issue, so of course a closer delta is fine.

Anti-spark connectors would also help.

 

[ZeroEm]

Sparks bother the normally intelligent. Hooking up my lights last night. Remembered which plug went where but could not see the DC-DC voltage labels and pugged it in. The arching was disturbing and my mind said "you fried that one". All was fine as 72v just sounds like that. Next time will drop the battery down to 36v first.

Sparks make me nervous