# UltraCapacitor Charging Voltage

1. Mar 24, 2013

### newman180

I am working with a hybrid car which charges a bank of ultracaps from regen braking.
I read an article which I believe was written by someone who is misinformed, but it had me thinking...

Just to clarify some theory about charging voltage,

Scenario

Max ultracap voltage= 14V
Current ultraCap Voltage= 10V
Motor BEMF Voltage = 20V

It is safe to charge the ultracaps with the 20V source until the caps are at 14V, correct?
Potentially, the greater the difference of voltage (potential) is related to how much current can be transmitted. V=IR

2. Mar 26, 2013

### eq1

Sure. In theory it is safe to charge 14V capacitors with a 20V source as long as the charging is stopped before the capacitor reaches 14V.

In practice this is much harder that it might seem. The most important thing, as you mentioned, will be to limit the charge current. This protects both the capacitor and the power source. A properly sized resistor will do this.

It's not the most elegant solution and will cost efficiency for its simplicity so you might want to do a rough dimensional analysis to figure out if the regenerative breaking system you intend can reclaim enough energy to justify hauling around all the extra hardware.