Sizing a BMS

littleharbor2 · August 2021

I'm looking at purchasing a BMS for a lifepo4 battery bank I'm assembling and am wondering what criteria I use to size it. I see they seem to be rated in discharge current and the charging current data looks to be half of the discharge current rating. My charging source would never exceed 75 amps. I doubt my discharge current would ever exceed 75 amps as well. Which number is the one I should base the BMS rating on. BTW the battery bank is 280 amp hour 8s 24 volt nominal

RCinFLA · August 2021

There is a large number of 'cheap' bms's out there. Most cannot hold up a constant current load of half their rated amperage. Should at least double your required current drain for BMS sizing.

I usually try to find an ad picture I can read the part number of MOSFET's so I figure what the BMS series pass resistance is. This determines how much heating they will generate at a given current.

Most have resistor dump balancer with dump current of 50 mA, 100 mA or 200 mA max per cell. Get a bms that allows you to change high and low cell cut off voltages. These typically do not balance dump until a cell reaches 3.4v. This does not give a lot of balancing time until single cell high cell overvoltage shuts down the bms. You have to charge to at least 3.5v per cell (for LFP) to get any amount of balancing time.

The greater the current you load or charge batteries, the faster the balance diverges on the cells.

Third item is system monitoring. Many have Bluetooth to smartphone or UART to computer interfaces.

littleharbor2 · August 2021

Good info. This lithium battery tech is new to me. I have a lot of reading to do. Every bit of Information is most welcome.
Fortunately I figure I have 2-3 years left on my lead acid battery bank now so, plenty of time to get the lithium battery right.
Any particular brand(s) that you'd recommend?

Horsefly · August 2021

If you are going to go with a MOSFET-based BMS (which is mostly a reasonable choice unless you've got some really high current loads), you may want to consider OverkillSolar.com or CurrentConnected.com. They both sell BMSs from JBD in China. They stock them in the US though, and the test and calibrate them for you. You pay more than if you would buy directly from JBD on AliExpress, but you will get it lots quicker and you have US-support if something goes wrong. In fact, Overkill Solar gives a no questions asked refund policy if something goes wrong. Or at least they used to have that policy - I haven't dealt with them for quite a while. Get the Bluetooth interface, which will allow you to set all the parameters as @RCinFLA was mentioning.

The MOSFET BMSs with a common port (single wire for both charging and discharging) have the advantage that they can cut off charging or cut off discharging independently. So if you discharge too far and the trigger a LVD (Low Voltage Disconnect) in the BMS, the BMS will cut off discharge, but leave the charge direction on so you can recharge. Likewise, if you charge to the point that you get a HVD (High Voltage Disconnect) the charge will be turned off, but discharge will still be enabled so you can burn off some of that overcharge.

As @RCinFLA mentioned, these BMSs will attempt to balance the cells by burning off (via a resistor) a small amount of energy from the highest energy cell. This is referred to as passive balancing, and by its nature is pretty limited. On large cells (maybe more than 100Ah) this is pretty ineffective. Active balancing actually moves energy from the highest cells to the lowest cell. I'm a believer in active balancing, which can be performed with a separate device that wires to the cells like the BMS does, or some BMSs have active balancing built in.

I've purchased directly from JBD, and also from Overkill Solar. From my reading these JBD BMSs probably have the best reputation among the relatively low cost MOSFET BMS. You will see lots and lots of pretty red BMSs from Daly, but the Internet is full of stories where they fail to operate as expected.

There are a ton of BMS options out there. Two of the more expensive are Batrium (built in Australia, I believe) and Orion (US based). Both are meant for pretty large systems, and I think Orion has a leaning towards Electric Vehicles. Off the top of my head the names of some of the others you could Google are Ant BMS, Chargery, REC, Heltec, and probably lots of others that don't pop to the top of my head. I've got three of the JBD and really like them. I got one from Overkill.

littleharbor2 · August 2021

Horsefly said:

If you are going to go with a MOSFET-based BMS (which is mostly a reasonable choice unless you've got some really high current loads), you may want to consider OverkillSolar.com or CurrentConnected.com. They both sell BMSs from JBD in China. They stock them in the US though, and the test and calibrate them for you. You pay more than if you would buy directly from JBD on AliExpress, but you will get it lots quicker and you have US-support if something goes wrong. In fact, Overkill Solar gives a no questions asked refund policy if something goes wrong. Or at least they used to have that policy - I haven't dealt with them for quite a while. Get the Bluetooth interface, which will allow you to set all the parameters as @RCinFLA was mentioning.

The MOSFET BMSs with a common port (single wire for both charging and discharging) have the advantage that they can cut off charging or cut off discharging independently. So if you discharge too far and the trigger a LVD (Low Voltage Disconnect) in the BMS, the BMS will cut off discharge, but leave the charge direction on so you can recharge. Likewise, if you charge to the point that you get a HVD (High Voltage Disconnect) the charge will be turned off, but discharge will still be enabled so you can burn off some of that overcharge.

As @RCinFLA mentioned, these BMSs will attempt to balance the cells by burning off (via a resistor) a small amount of energy from the highest energy cell. This is referred to as passive balancing, and by its nature is pretty limited. On large cells (maybe more than 100Ah) this is pretty ineffective. Active balancing actually moves energy from the highest cells to the lowest cell. I'm a believer in active balancing, which can be performed with a separate device that wires to the cells like the BMS does, or some BMSs have active balancing built in.

I've purchased directly from JBD, and also from Overkill Solar. From my reading these JBD BMSs probably have the best reputation among the relatively low cost MOSFET BMS. You will see lots and lots of pretty red BMSs from Daly, but the Internet is full of stories where they fail to operate as expected.

There are a ton of BMS options out there. Two of the more expensive are Batrium (built in Australia, I believe) and Orion (US based). Both are meant for pretty large systems, and I think Orion has a leaning towards Electric Vehicles. Off the top of my head the names of some of the others you could Google are Ant BMS, Chargery, REC, Heltec, and probably lots of others that don't pop to the top of my head. I've got three of the JBD and really like them. I got one from Overkill.

Thanks Horsefly. More good info from people with practical, hands on, experience Good to hear from you. BTW I am now living in Baja California. I moved off Catalina 18 months ago. Been to the Jazz festival recently?

Horsefly · August 2021

littleharbor2 said:

Thanks Horsefly. More good info from people with practical, hands on, experience Good to hear from you. BTW I am now living in Baja California. I moved off Catalina 18 months ago. Been to the Jazz festival recently?

Yeah, I think you had told me you moved. The Jazz festival was obviously cancelled last year, but we are going this year. I don't go to Baja much, so much lower chance of buying you a beer!

RCinFLA · August 2021

Stay away from split charger port BMS's. Many cheap ones just split the back to back MOSFET's so you lose the bi-directional blockage. You cannot put parallel battery packs with dual port charger-load bms's as one pack will backflow to other through MOSFET body diode.

I am advocate for active balancers but good ones are more expensive. The cheap capacitor tranfer ones work but the electrolytic caps are subjected to high ripple current so they are unreliable long term. They are directly connected to cells so little safety protection if something goes wrong. Not worth putting expensive LFP cells at risk to save a few bucks. If you get a weak battery bus bar connection the high voltage drop can blow the balancer cap 6.3v breakdown.

Sizing a BMS

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