Matt Gross
New member
- Joined
- Oct 23, 2017
- Messages
- 13
Warning: Im not that knowledgeable about batteries, BMS, ect but Ive done a lot of reading, but have no practicable usable of this stuff. However Im going to buy a van that already has a system so thats what Im working with. If I say something that doesnt make sense, please correct me. I also understand this isn't powerwall related, but I didn't really know where else to post this as I assume a lot of you guys are knowledgeable about BMS.
A little background before I ask my questions.Im going to be purchasing a campervan that has a 4s4p setup with Sinopoly 100ah 3.2 LiFePO4 cells for 400ah at 12v. The BMS and cells came from AmSolar and it looks a heck of a lot like a minibms v3 from CleanPowerAuto:
Im not sure if its exactly the same or not, but either way the seller of the van is running into some problems with a specific cell group where the balancing board is sensing a high voltage and is shutting down the system. It has a single relay for shutting down the charge and discharge as they are hooked up to the same relay. This is obviously a problem as the battery essentially shuts off. They have replaced a balance board and swapped the cells order in the groups but there is still a problem where the bms shuts down due to high voltage that happens at the end of the charge cycle. The system is still under warranty and the seller wants to fix it before handing it over to me but Ive got non-refundable tickets to fly down and pick up the van this friday, but the seller is still running into problems. I had suggested to take it as is and continue to work with AM Solar, or lower the price of the van by the amount of purchasing a new BMS. This is where my questions comes in.
The current BMS has individual boards on each sell for balancing. Ive been looking at other BMS systems for 12v setups and came across a few. Some of balance boards, but the one that interested me the most was REC-BMS (looks nice as it has two relays for discharge cut off and charge cut off) that does not have balance boards. What is the difference of a BMS that has a balance board vs one that has wires going to the cells? What do the boards do that just a central BMS cant?
Second question is charging. By the looks of the system thats currently installed (the photo above), the balance boards are limited to a charge rate. The manual assumes youre using a Magnum inverter which has a 100amp charger. This was taken from their manual:
*We set the current limit to 30% when connected to shore power because this allows for a more gentle charge, which reduces the amount of heat that has to be dissipated from the cell balancing resistors.
So thats 30amps going into the battery. 90% of the time, Id charge with solar, but if there ever was a rare case where Id like to plug into the grid for shore power, 30amps would take a very long time to charge. If I could use the full 100amp charger, that would be preferred. The batteries look like they could handle 100 amps no problem (132amps should be safe with 4p), but as the manual stated, they want to you charge at 30amp.
Like I said up top, Im not really knowledgeable, but if it is a minibms v3, it looks like they have a shunting current of .75amp with a shutdown at 1amp:
Shunt balancing has a maximum shunting current of 0.75 Amp. Shunting slows down charging process on cells reaching HVC sooner than other cells, allowing other cells to catch up. This is referred to as top balancing. PTC resettable fuse protects from overheating during shunting as well as short circuit
conditions. Fuse opens if temp reaches 80C or if shunting current reaches 1 Amp. Open fuse results in same alert as full charge, shutting down the charger. Fuse returns to normal conditions automatically.
Would this be the limiting factor for faster charging? The Rec-BMS states the following about Shunt Balancing:
high efficiency 2amp bi-directional active balancing (pack to cell and cell to pack). BMS unit cell balance fuse rating (SMD) 3amp
Thanks for any help you could provide. Im working on a tight time crunch as Ive got non-refundable tickets.
A little background before I ask my questions.Im going to be purchasing a campervan that has a 4s4p setup with Sinopoly 100ah 3.2 LiFePO4 cells for 400ah at 12v. The BMS and cells came from AmSolar and it looks a heck of a lot like a minibms v3 from CleanPowerAuto:
Im not sure if its exactly the same or not, but either way the seller of the van is running into some problems with a specific cell group where the balancing board is sensing a high voltage and is shutting down the system. It has a single relay for shutting down the charge and discharge as they are hooked up to the same relay. This is obviously a problem as the battery essentially shuts off. They have replaced a balance board and swapped the cells order in the groups but there is still a problem where the bms shuts down due to high voltage that happens at the end of the charge cycle. The system is still under warranty and the seller wants to fix it before handing it over to me but Ive got non-refundable tickets to fly down and pick up the van this friday, but the seller is still running into problems. I had suggested to take it as is and continue to work with AM Solar, or lower the price of the van by the amount of purchasing a new BMS. This is where my questions comes in.
The current BMS has individual boards on each sell for balancing. Ive been looking at other BMS systems for 12v setups and came across a few. Some of balance boards, but the one that interested me the most was REC-BMS (looks nice as it has two relays for discharge cut off and charge cut off) that does not have balance boards. What is the difference of a BMS that has a balance board vs one that has wires going to the cells? What do the boards do that just a central BMS cant?
Second question is charging. By the looks of the system thats currently installed (the photo above), the balance boards are limited to a charge rate. The manual assumes youre using a Magnum inverter which has a 100amp charger. This was taken from their manual:
*We set the current limit to 30% when connected to shore power because this allows for a more gentle charge, which reduces the amount of heat that has to be dissipated from the cell balancing resistors.
So thats 30amps going into the battery. 90% of the time, Id charge with solar, but if there ever was a rare case where Id like to plug into the grid for shore power, 30amps would take a very long time to charge. If I could use the full 100amp charger, that would be preferred. The batteries look like they could handle 100 amps no problem (132amps should be safe with 4p), but as the manual stated, they want to you charge at 30amp.
Like I said up top, Im not really knowledgeable, but if it is a minibms v3, it looks like they have a shunting current of .75amp with a shutdown at 1amp:
Shunt balancing has a maximum shunting current of 0.75 Amp. Shunting slows down charging process on cells reaching HVC sooner than other cells, allowing other cells to catch up. This is referred to as top balancing. PTC resettable fuse protects from overheating during shunting as well as short circuit
conditions. Fuse opens if temp reaches 80C or if shunting current reaches 1 Amp. Open fuse results in same alert as full charge, shutting down the charger. Fuse returns to normal conditions automatically.
Would this be the limiting factor for faster charging? The Rec-BMS states the following about Shunt Balancing:
high efficiency 2amp bi-directional active balancing (pack to cell and cell to pack). BMS unit cell balance fuse rating (SMD) 3amp
Thanks for any help you could provide. Im working on a tight time crunch as Ive got non-refundable tickets.
