relays versus mosfets for BMS

[tarunsmehta]

Hi,

I have been trying to understand analog BMS's design for some time. One thing which has been bugging me is that why do BMS designers use a bunch of power mosfets for switching instead of something like a mechanical relay.

The switching operation here would be fairly rare (only in case of a fault). In any case it is not something so fast that a mechanical relay cannot handle. Plus, a mechanical relay will eliminate most of the heating issues with mosfets. Isn't that worth shooting for?

 

[liveforphysics]

Use a quality relay/contactor if you have budget and space.

 

[tarunsmehta]

so why do existing BMS's don't use it already?

i was discussing the same with some of my friends here and they were wondering whether the relay 'bounce' might be a problem for upstream or downstream circuits..

 

[amberwolf]

Two physical/functional reasons for it might be:
1) Relays are much larger than FETs for the same job.
2) Relays can bounce under vibration, disconnecting and reconnecting their contacts--and if this happens under load, it can then weld the contacts together from arcing. Then the relay can't operate at all. It can also just deposit enough soot on the contacts to prevent them from making any contact, or good cotnact, anywya, and that's almost as bad (worse in some situations).

A third reason, money, is that relays that are less likely to have the problems above are going to cost more (probably significantly more than the FETs would have, I'd guess).

 

[dnmun]

relays use a lot of current to drive the coil and historically they were never needed. now you can buy a 24S lipo BMS capable of 150A for less than the cost of a single relay.

 

[tarunsmehta]

thanks amberwolf and dnmun! i had no clue relays could have that been expensive in mass also. maybe then it's back to fets then..

 

[liveforphysics]

The above posters brought up some good points. For low currents (from an EV perspective), like say sub ~100A, fets are pretty attractive. Get into the many hundreds of amps game, and you find just about everyone goes with a contactor.

 

[powersupply]

You could still use latching relays to avoid power to be drawn constantly.
As you mentioned, the switching operation would be fairly rare.

But I agree that the large current, especially during turn on, plus the high voltage needs (sparks/plasma) would need a very expensive (sealed/gas filled) relay as used in aviation f.i.

 

[Punx0r]

150a bms for the cost of a relay? Where are they available from?

 

[dnmun]

i have been selling the D131 from Bestechpower.com for $75 in the US.

http://www.bestekpower.com/888v24spcmbmspcbforli-ionli-polymerbatterypack/PCM-D131.html