BMS inside a sealed case

rg12 · Feb 16, 2018

Hey Guys,

Why is it ok to have a BMS located inside a closed battery pack and wrapped in PVC (heatshrink) and stuff?
It has a heatsink that is clearly not meant to be covered in plastic...
What are your thoughts?

Does the BMS get hot when in use or only when balancing?

Also, what role do the fets play in a bms? I know that for balance theres the load resistors to bleed single cell rows but why are the fets needed?

Addy · Feb 16, 2018

The FETs in a BMS are used to connect/disconnect the battery pack from external loads and chargers. This is how the BMS protects the battery. If the battery is nearly dead, the BMS will turn off the fets so that the battery is disconnected and can't be drained further. The BMS can also do this to prevent the battery from being charged too much.

Because of this arrangement, when the battery pack is being used, current is flowing through the fets. The fets will produce heat when current is flowing through them, which is why heatsinking is needed and why there are often many fets in parallel, to reduce their overall resistance and decrease the amount of heat produced.

So, it depends. A BMS might be fine when wrapped up if it doesn't get very warm under use, but this is not always the case. If possible it would be better to have the BMS exposed so that it can cool down better.

rg12 · Feb 16, 2018

Addy said:
The FETs in a BMS are used to connect/disconnect the battery pack from external loads and chargers. This is how the BMS protects the battery. If the battery is nearly dead, the BMS will turn off the fets so that the battery is disconnected and can't be drained further. The BMS can also do this to prevent the battery from being charged too much.

Because of this arrangement, when the battery pack is being used, current is flowing through the fets. The fets will produce heat when current is flowing through them, which is why heatsinking is needed and why there are often many fets in parallel, to reduce their overall resistance and decrease the amount of heat produced.

So, it depends. A BMS might be fine when wrapped up if it doesn't get very warm under use, but this is not always the case. If possible it would be better to have the BMS exposed so that it can cool down better.

Thanks for the explanation

I guess in most cases it's almost impossible to avoid wrapping the BMS since it has so many wires and can't be exposed to water etc...

dustNbone · Feb 16, 2018

Most of the wrapping I've seen on packs isn't particularly good insulation. It can dissipate a certain amount of heat to the outside, as well as the cells themselves being able to absorb some amount of heat as well (and eventually dissipate it through the wrapping).

Charging/discharging cells also causes them to throw off heat, as long as it can dissipate fast enough to avoid anything getting hotter than it's rated to be, everything is fine.

e-beach · Feb 16, 2018

I really depends on how you ride and charger your pack combined with the ambient temperature you are riding in. If the heat generated by your riding style and slow charging doesn't warm up your pack much, then you will be ok. If you ride hard and fast charge and live in a hot place, then you might get things hot.

It depends on you.

I personally keep my BMS heat sync uncovered inside my battery box so it can cool if needed.

BMS inside a sealed case

rg12

100 kW

Addy

100 W

rg12

100 kW

dustNbone

10 kW

e-beach

10 MW

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