Battery with huige capacitor bank in parallel with it? (electric vehicle)

[zenerbjt]

Supposing you have a 5kW-15kW battery operated electric vehicle, or drone, or ship. Battery voltage is 14S or 24S lithium. (48V or 82V). Why would you ever put a very large electrolytic capacitor bank right by the output of the battery? (ie, in parallel with the battery)

I have seen this on a product, and wonder why they do it?

 

[rjenkinsgb]

It would allow the load (motor or whatever) to draw short-term current peaks rather higher than the batteries can supply.

If the caps are near the load rather than near the batteries, it's also a way of reducing the effects of cable resistance, so somewhat smaller cables can be used for parts of the system.

 

[zenerbjt]

If the caps are near the load rather than near the batteries,

Thanks, these caps are some 1-2 metres from the motors/inverters.

In this cases the capacitors were all large electrolytic capacitors....each cap was some DxH of 1.5cm x 7cm and there were about 200 of them right by the battery.

Would you agree, that by using such electrolytics, they are inviting the electrolytics to fail since the trace impedance may mean that each motor/inverter pulls most of its ripple current from only those caps that are nearest to it....so those caps will overheat.

I mean, surely one should just use thick bus bars from the battery...and no caps?

I must admit iver never heard of a battery having a high Z at high frequency....at least not any more so than caps...but of course, i suppose there is the battery connection wiring inductance, yes......but i've never heard of anyone using a big cap bank for this......there's the inrush problem of course...and the large inrush limit circuit then needed.

It would be nice to see if Formula E electric cars have such a big cap bank just by the battery....i bet they dont.....but i never sneaked into the pits to have a sneak peak i must admit.

 

[Nigel Goodwin]

Such a bank of capacitors will cost a LOT of money - if they weren't needed they wouldn't be fitted - as already suggested they will be capable of providing a VERY high short term current to the motor.

 

[picbits]

We design and build EVs and one setup we've recently used has required some large capacitors near the controller (specified by manufacturer) - I say large, they are a few thousand uF at 500V.

Large for me was the 1000F we used as a storage medium for stop starting engines on a test bed ...

 

[rjenkinsgb]

I must admit iver never heard of a battery having a high Z at high frequency

It's not necessarily a matter of frequency, just internal resistance.

Power loss is proportional to the square of the current.

PWM systems inherently draw significantly higher peak currents than the average load current, so having a reservoir separate from the battery could drastically improve overall efficiency and reduce heat dissipation.

eg. At 25% duty cycle, the battery current could be reduced to near a quarter the peak drive input current, with a large enough capacitor bank.
That's 16x lower peak dissipation in the batteries themselves so an overall 4x reduction in battery heating.

It is all somewhat guesswork though, without knowing the exact details and layout of all the parts involved.