# Charging and discharging Ultracaps with a 12v DC Brushed Motor

I am trying to store small amounts of energy from a 12v motor with an ultracap. I'm thinking of a 3000F 2.7v cell from Maxwell. I am trying to find out info on charging ultracaps, specifically the current required/advisable, as in my situation it may fluctuate. I also need to boost the voltage to 12v on discharge, but with a motor stall current of 60A (although I do wish to put a PWM in to control it to about 30A), I fear that combined with the near 5x boost, I'm drawing some serious current (270A worse case without PWM) over the 30 seconds it's active. Will the ultracap take it? I read some can output up to 1000A safely, provided I make the wiring thick enough it doesn't melt. Or do I take the capacitance hit and put 2 in series?(For cost I don't really wish to buy 4 and put another 2 in parallel) Thanks,

• The datasheet of the capacitor will explicitly list charge and discharge current. – scld Feb 26 '16 at 14:08
• How is this not answered directly and clearly in the datasheet? – Olin Lathrop Feb 26 '16 at 15:05
• As a mechanical engineer not an electrical one, the terminology is not obvious and best to ask a dumb question than assume and get it all wrong. What about the rest? It does not state what the effect of a flucatuating charge current has, not great on a battery, what about a cap? – andyEngineer Feb 26 '16 at 15:48
• @andyEngineer your question is not dumb. Unfortunately newcomers can expect a bit of bitterness on this SE, probably because of the large amounts of poorly written questions. – Dejvid_no1 Feb 26 '16 at 16:12
• At an energy storage capacity of about 3 wHours, how long are you planning on running your motors? – R Drast Feb 26 '16 at 18:46

It's easier to answer these kind of questions if you supply the components manufacturer part no. I assume you have Maxwell BCAP3000 P270 K04.

The absolute maximum current is 1900 A.

The maximum continuous current is 210 A with a ∆T=40 °C

The data sheet also tells us that the temperature rise at a given current is:

∆T=IRMS^2 x ESR x Rca

∆T=270^2 x 0.00029 x 3.2

∆T=67.7 °C

The maximum operating temperature is 65 °C so you will be a bit over the specification if you start at room temperature. But doing this in short bursts and with sufficient cooling time, will probably not kill your capacitor but may affect its total lifespan.

• Good answer although I'd question if the operating current was telling you the maximum component temperature or the maximum ambient temperature. Each part may have a specific note on its datasheet but I usually assume that "max operating temperature" is ambient....the idea being that the user is the one controlling the environment and the part, given all other max ratings i.e. current, should be able to meet all specifications in that environment. The temperature rise is usually factored in and the appropriate derating of operating temperature already takes that into account. – scld Feb 26 '16 at 20:02
• Yeah, I'm with you but the data sheet says "Cell case temperature" at the maximum operating temp section. – Dejvid_no1 Feb 26 '16 at 20:52
• Interesting, perhaps that's common among supercaps. I've used them but never near enough their maximum ratings to raise the temp, so can't say I've experienced any thermal issues with them. But that datasheet would probably make me call the vendor for clarification. – scld Feb 26 '16 at 21:58