Advice on BMS & Battery Questions

[Goboatingnow]

High Voltage Disconnect (Any Cell)
Low Voltage Disconnect (Any Cell)
High Temperature Disconnect
Low Temperature Disconnect
High Current Disconnect

I would add that high temp should be cell based using multiple sensors in the battery

 

[curiouscarbon]

I would add that high temp should be cell based using multiple sensors in the battery

indeed. unfortunately few affordable options seem to include cell level temperature monitoring, which can catch a number of failure modes that are possible.

one thermometer much better than zero still ??

 

[Goboatingnow]

indeed. unfortunately few affordable options seem to include cell level temperature monitoring, which can catch a number of failure modes that are possible.

one thermometer much better than zero still ??

Sure but temp rise is a classic Li indicator of trouble and a good bms should monitor every cell.

 

[rbanic]

@curiouscarbon I noticed the same, the JBD BMSes also only include one probe and have one connector. Unfortunately I cannot find any affordable ones that monitor multiple probes and make them fit even with stretching the budget by 50 euros @Goboatingnow. Unfortunately it seems we'll just have to omit that part.

 

[Goboatingnow]

@curiouscarbon I noticed the same, the JBD BMSes also only include one probe and have one connector. Unfortunately I cannot find any affordable ones that monitor multiple probes and make them fit even with stretching the budget by 50 euros @Goboatingnow. Unfortunately it seems we'll just have to omit that part.

Hey you pays your money and you take your chances.

 

[rbanic]

I'd gladly donate more from my own paycheck but it seems like you have to go into the hundreds of euros to get that feature. We simply don't have that kind of money available to allocate. The shelter gets around 70 EUR per dog per month and basically nothing is left after the end of the month (90 mid-large sized dogs).

 

[rbanic]

Just for information though, if I were to build a pack for myself, which in the future I probably will, what kind of "affordable" BMSes even offer every cell temp monitoring?

 

[curiouscarbon]

When it comes to per cell temperature safety, I have always just assumed to DIY a per cell temperature cutoff to save money.

The JBD BMS has a "switch" input of two terminals, which seems to allow manual disconnect function.

There are various relatively low cost temperature sensors, for example the MCP9808 https://www.adafruit.com/product/5027 which can have up to 8 on the same bus. I would use some sort of adhesive to affix one on the top of each cell, and prepare a simple firmware to read them all and activate the JBD switch based on if any cell reading goes out of range. There are definitely less expensive sensors, but that's the general idea.



I too found that the BMS that offer per cell temperature monitoring are above the price range that I am looking for.

edit: clarify about per cell function

 

[curiouscarbon]

from overkill solar's JBD BMS manual:

2.4.6 External Switch
An optional external switch can be wired to the BMS via the included 2-pin pigtail (JST-XH, red/black wires). If the configuration option is enabled within the BMS, discharging will be disabled when the switch contact is open. When the switch is closed (or a jumper is in the place of the 2-pin connector), the BMS will operate normally.

4s BMS 120a LifePo4 with 12″ 8ga Wires

Comes with 12 inch long 8ga wires soldered to the board as shown. BMS For 4 cell, 12v volt Lithium-iron-phosphate DIY batteries 120 amp continuous charge or discharge current Low Temperature Chargi…

overkillsolar.com

https://github.com/FurTrader/OverkillSolarBMS/raw/master/Overkill_Solar_BMS_Instruction_Manual.pdf

 

[curiouscarbon]

Just for information though, if I were to build a pack for myself, which in the future I probably will, what kind of "affordable" BMSes even offer every cell temp monitoring?

none that i am aware of ? all the ones with that feature were like >> 200 usd when i looked. would be happy to learn of one though!

 

[rbanic]

@curiouscarbon thanks for all the very useful information, I am really grateful. If it has an external switch I'll get my EE colleague to whip up something and we'll make some arduino solution. We might just take these: https://www.amazon.de/-/en/AZDelivery-stainless-temperature-waterproof-compatible/dp/B07KNQJ3D7 and since they're 1-Wire sensors we can set them up on a bus and just iterate over all of them to check that all have the correct temperature.

 

[curiouscarbon]

great idea of sourcing 1Wire sensors that are easier to scale on the bus ?️? I2C is inconvenient in that way

 

[curiouscarbon]

Sure but temp rise is a classic Li indicator of trouble and a good bms should monitor every cell.

regarding the algorithm of temperature safety disconnect, is it helpful to disconnect on a max allowed temperature difference between all cells, in addition to simple min/max per cell limits?

the concept being that it might catch failure earlier? depending on thermal conductivity around the battery, a larger or smaller delta range would be seen, i assume.

either way it would be neat to thermally detect a failure early?

 

[rbanic]

Good idea!