Connecting 2 3S BMS units in parallel.

[Ampster]

I understand the physics part, but if you have a total of 3s 2p , then you will have 3 pairs of battaries in parallel . As the balancing system works on one cell, it will only detect the highest voltage of the 2 parallel cells.

Is it 3S2P which is two 3S batteries or 2P3S which is one battery with pared cells in series? The first has two BMSs and the second only one BMS and the balancing system sees the paired cells as one.

 

[mikefitz]

Hi Scott.

It does not matter that the two BMS 'sees' the same volts at the positive end of the connected batteries. Each BMS works independently and looks after the 3 cells in its 'battery'.
The amount of current flow during charging is determined by the 'internal voltage generator' in each set of cells. This generator increases with the state of charge. Should the SOC differ then the battery with the lower internal volts will get a bigger share. The net result is that the batteries self balance.


Mike

 

[scsn]

Hi Scott.

It does not matter that the two BMS 'sees' the same volts at the positive end of the connected batteries. Each BMS works independently and looks after the 3 cells in its 'battery'.
The amount of current flow during charging is determined by the 'internal voltage generator' in each set of cells. This generator increases with the state of charge. Should the SOC differ then the battery with the lower internal volts will get a bigger share. The net result is that the batteries self balance.
View attachment 146802

Mike

Hi Mike,
Thanks for your reply.

I see you have 2 BMS for the parallel pack. This is what I thought I had to do, but I have seen 1 BMS connected to 2 3S batteries in you tube, which I thought wouldnt work correctly with balancing. As the current into the BMS has to be controlled, can having 1 supply into 2 BMS might be an issue if both need to be charged at one time? If 3A is required to go into 1 BMS, would I have to set the supply for 6A to power both? But this would cause an issue if only one pack needed charging as 6A can go into the pack.

Cheers
Scott

 

[Ampster]

...but I have seen 1 BMS connected to 2 3S batteries in youtube.

The only way one BMS will work is to make one battery out of six cells by pairing two cells each into buddy pairs and connecting the buddy pairs in series. The two cells in buddy pairs will be the same voltage as each other because they are in paralell. I have not seen that YouTube video but it may appear to be two batteries but there may be interconnections that make it electrically one battery. That configuration is often described as 2P3S and is not shown in the above diagrams.

 

[mikefitz]

With a battery charger rated at 6 amps that's the maximum it can deliver. The charger will also have a maximum output voltage. The current through a 3 cell battery and its BMS will be limited by two facts.
Firstly the BMS will stop charging worst case if the current exceeds its design limit, in your case 3 amps.
Secondly the current accepted by the battery depends on its Internal 'voltage generator'. Since this increases with SOC the current falls to to a low value as the battery nears full because there is no voltage difference.
Where you have a 2P3S battery there's normally only one BMS. Since you have two 3 amp limited BMS, idealy the diagram above showing the two 3S batteries in parallel, Is the correct approach .
There have been situations where 2 BMS have been connected to cells arranged 2P3S . This approach is not recomended

 

[heavy-impact]

I wanted to do about the same with two cheap 40A BMSs in parallel on a 4s pack to deliver more current. I'm waiting on a answer from an EE.
I have one built and housed in a 50cal ammo can made up of 100 medical 4400ma li-po packs each being 2 cells in parallel with their own 4A BMS. so there are 25 packs in parallel x4 in series for 110ah 16.8v with a 4S 60A BMS. People tried to tell me this wouldn't work but it works perfectly and the cells deliver much more than their rating. I put the 12v trolling motor in a barrel of water and let it run for exactly 5 hours drawing a measured 11 amps. When I stopped the test the pack was still at 15.3v. It's range is 16.8v full to 11.4v empty. I balance the pack with a 4A active balancer.
I know things look a little sloppy in the pics but rest assured everything is well insulated with 1/8" silicone sheets and Kapton tape before going into a padded housing.

 

[heavy-impact]

Just got an email from batteryhookup.com stating parallel BMSs on the same battery for higher current draw is fine.