How hot do BMS get?

[Indolent58]

I'm planning to build a 4s2p 12v 560ah out of EVE 280ah cells and have two Overkill BMSes. Because of space constraints the series packs will be stacked inline in a long narrow steel case. I'd like to have the BMS's mounted to a rigid plastic shelf over the cells but I don't know how hot the heatsink plates might get in normal (or abnormal) operation. I would imagine that having the plastic melt and the BMS heatsinks shorting out the cells would be bad.

 

[BiduleOhm]

It depends on a lot of factors: BMS design, if it has resistive balancing or not, if there's forced airflow or not, how much current you use, ...

 

[Zil]

and more factors: are the cells top balanced? are the cells matched?

 

[HRTKD]

While I have yet to stress test my 4s2p system, so far the Overkill BMS is the least of my concerns. They have stayed near ambient temperature. The inverter on the other hand really likes to heat things up.

 

[Indolent58]

I plan to top balance the cells before installation. The cells are coming from Basen - I wont know how well matched the cells are until I get them. I understand that the BMS will generate heat in the course of resistive balancing but my concern is the heat not melt plastic (for ex HDPE ~ 110C). In my application the two 120amp BMSes in parallel will have a max combined draw of 180amps for short periods. I may be overthinking this as several tear down/assembly pics I have seen of plug and play LIFEPO4 batteries show the BMS living on top a plastic sheet over the cells In a sealed case.

 

[GXMnow]

My 200 amp rated JK BMS gets a little warm while running 80 amps through it. The rise over air temp was only 10 degrees C which is not bad, but since it is in my garage, and the air temp can be over 100 F ( 38 C ) I did add an additional heat sink to the aluminum case that cools the FET's. A good high current BMS should be under 0.001 ohm and should not dissipate more than 10 watts as heat from the FET's at 100 amps.

Passive resistor balancing can make a little more heat, but it should not be a problem. 60 milliamp balance current on 6 cells could add up to just over a watt, and 200 ma on 6 cells is still only about 5 watts of heat. The worst case would be having just one higher capacity cell that takes a while to catch up to all of the other cells. 15 balance resistors trying to hold the voltage down as just one cell in a 16S bank is still topping up. At 250 ma of balance current that works out to about 13 watts on LFP cells at 3.5 volts each. You will want some air flow, but it is not a huge amount of heat.

 

[Indolent58]

Thanks for the replies. I just found a remnant sheet of 1/16" FR4 fiberglass for cheap at my local plastics store that will go over the bank. I'm not going to worry about that melting.

 

[Luthj]

If the bms is hot enough to melt plastic, it won't survive very long...

 

[jvbutter01]

So does this beg the question, Should one add a PC cpu heat sink to the surface of the BMS to keep it little cooler. Heat sink only, not fan powered.

 

[GXMnow]

Mine got a bit warm while running 80 amps and it was over 100F in my garage. So I did stick a CPU cooler on mine. I used thermal conductive double sided tape, but it only held for about a week so I have to devise a mechanical bracket to help hold it on. Mine is a vertical surface. Even at it's hottest, the BMS app was only reporting the FET's were at 50C and the shut down temp is 75C, so it really was not a problem.

 

[jvbutter01]

yes most of the older heat sinks were glued on with that white paste, now they have a spring holding it down.

 

[GXMnow]

When I first stuck it on with the thermal tape, it held solid. It was not until we had another 105 day for several hours, it got soft enough it started hanging away from the BMS case. It made for such a clean setup with just the adhesive holding it. Oh well, I have to fab up some form of bracket.

 

[BiduleOhm]

There's thermal glue and thermal epoxy if you want, far more effective than thermal tape

 

[Jesshaines]

I'm considering trying this but with the motivation of keeping my battery compartment overall more cool. The BMS consistently runs 5 to 10° hotter than the battery temps are reading.

I'm not sure if it will have an impact though. It just seems like something putting out heat in a small area is going to bring up the overall temperature by the necessity.

Has anyone tried this? Any positive results?