BMS's in parallel

[ridgerunner]

Good afternoon...
I have some basic questions that I am hoping this forum can answer. I would like to get a 5S3P Li-Ion battery with a BMS. I have multiple applications: A) 5S3P (x3) in series for a 15S3P configuration B) 5S3P (x3) in parallel for a 5S9P configuration.....I am trying to decide whether to do a 15S BMS that is over-sized both on the voltage and current to support the requirements of the parallel configuration OR whether to embed each 5S3P with a 5S BMS. So now to my questions:

1. If I have three 5S BMS in parallel will they conflict with each other? My understanding is parallel cells balance themselves out and the BMS is there to support the series cells, so my inclination is to think that the three in parallel will work??

2. If I go with a 15S can I hook 3x 5S3P at the bottom end of the 15S BMS and will it work (assuming of course it has been chosen to support the 9 parallel cells current charging / discharging requirements?

3. Final question...Would I be able to build the battery packs up WITHOUT BMS and then use 1x 5S BMS external to the packs to balance 5S3P (x3) in parallel if I sized it appropriately for the current for the 9 parallel cells?

Sorry if these are simple questions as I'm just getting my feet wet here in this.

Sincerely,
Steve

 

[goatman]

im far from an expert but in your situation id just get one of these hobby chargers to balance your 5s3p packs when you decide to balance them.

https://www.banggood.com/Wholesale-IMAX-B6-Digital-RC-Lipo-NiMH-battery-Balance-Charger-p-46220.html?cur_warehouse=CN

 

[goatman]

Ive never played with those chargers but this might work with above charger

https://www.banggood.com/Power-Genius-PG-Parallel-Charging-Board-Supports-5-Packs-of-2-6S-Lipo-Battery-XT60-T-Plug-p-1141232.html?rmmds=detail-bottom-viewalsoview__3&ID=510961&cur_warehouse=USA

and 5s 6pin balancing wires that go onto your battery for balancing, the link below is for 6s7pin, not exactly what you need but it shows you what to look for

https://www.banggood.com/5PCS-6S-7Pin-2_54XH-30cm-Lipo-Battery-Charger-Silicone-Wire-Balance-Extension-Cable-p-1190580.html?rmmds=search&cur_warehouse=USA

 

[amberwolf]

1. If I have three 5S BMS in parallel will they conflict with each other? My understanding is parallel cells balance themselves out and the BMS is there to support the series cells, so my inclination is to think that the three in parallel will work??

it depends.

if the bms units have the same port for charge and discharge, you may simply hook them all up in parallel and they'll operate as needed, and whichever pack reaches full during charge will disconnect itself as needed, and whichever pack reaches empty during discharge will disconnect itself as needed.

however--if they have separate charge and discharge ports, then having them paralleled means that when you charge and one tries to disconnect itself, it's charge port does disconnect, but the discharge port is still connected to the other packs, and so current still flows into it, overcharging whatever cells in it are already full.

similarly, during discharge, if one of the packs reaches empty first (low cell), it will disconnect it's discharge port, but the charge ports will still be connected, and currentw ill still flow out of that empty pack, overdischarging it's low cell(s).

you can make a wiring harness / plug that only connects the discahrge ports when you're using it to operate something, and disconnects the charge ports, and another that disconnects the discahrge ports when charging, and connects only the charge ports, and that will fix that problem.

2. If I go with a 15S can I hook 3x 5S3P at the bottom end of the 15S BMS and will it work (assuming of course it has been chosen to support the 9 parallel cells current charging / discharging requirements?

i don't know what you mean by "hook 3x 5S3P at the bottom end of the 15S BMS".

but...assuming you simply mean to connect three 5s3p packs in series, each of which has it's own bms, so that you have effectively a 15s3p pack, that has it's own problem potential:

if the fets on the 5s bms units are not rated for teh full voltage of the entire 15s pack (whcih they generally aren't likely to be, since they're meant for 5s), then when one of the bms units shuts off it's fets to protect a pack against overdischarge or overcharge, the entire pack voltage will be across those fets, and they may fail (blow up, short, etc). if they fail shorted, then the pack cannot protect itself, and the cells themselves will then be damaged by overcharge or overdischarge, depending on what's happening at the time.


if the fets are rated for the full pack voltage, then they'll work fine.


a large diode (meant to handle the full pack voltage *and* the full pack current) can be placed reverse-biased across each pack, and it will then allow current to flwo around the shutoff pack, until the system can be shut off.


if you mean something else, you'll need to clearly explain exactly what that is.

3. Final question...Would I be able to build the battery packs up WITHOUT BMS and then use 1x 5S BMS external to the packs to balance 5S3P (x3) in parallel if I sized it appropriately for the current for the 9 parallel cells?

sure. but...if you're just using it to charge and balance, a bms isn't what you're after--you would just want to get a balancing charger (like rc chargers, like the imax, etc) that can do at least 5s.

a bms is meant to leave on a pack all the time, built into it, to manage and protect the cells against both overcharge and overdischarge. no point in having one if you're not going to do that.

 

[Dui ni shuo de dui]

1. If I have three 5S BMS in parallel will they conflict with each other? My understanding is parallel cells balance themselves out and the BMS is there to support the series cells, so my inclination is to think that the three in parallel will work??

Yes, it will work.
As amberwolf explained, just get a BMS who doesn't have a separate port for charging.
My scooter's battery was made like that, two paralleled block, each one having its own BMS. Worked just fine.

2. If I go with a 15S can I hook 3x 5S3P at the bottom end of the 15S BMS and will it work (assuming of course it has been chosen to support the 9 parallel cells current charging / discharging requirements?

I suppose you are asking "If I go with a 15S BMS and plug a 5S battery in it",right?
In which case, yeah, it should work, but in this case I suggest you to get a programmable BMS, because it is possible that some non programmable ones won't work because their firmwares could be set up for working only with 15S.
If you go for non programmable units, then ask the seller.

3. Final question...Would I be able to build the battery packs up WITHOUT BMS and then use 1x 5S BMS external to the packs to balance 5S3P (x3) in parallel if I sized it appropriately for the current for the 9 parallel cells?

Theoretiaclly yes but there's very little point in doing that. In my opinion, the main use of a BMS isn't to balance the cells, it is to prevent damage during discharge. Balancing the cells is a bonus but don't be focused on that aspect since on a good pack the cells should stay in balance even without it. Also, in practice it seems like BMSs don't really like to be connected and disconnected too often, many people destroyed theirs during installation so I guess doing that everyday isn't a great idea.