Manually Balancing Ultracapacitors

Manually Balancing Ultracapacitors

In my post “Resisters and Diode To Balance” is the hardwire attempt to “auto” balance a set of ultracapacitors (CAPs). In reality, I don’t even need an electronic balancer, just common sense. Each CAP has its own internal capacitance. Due to age, loading, heat stress, etc. each cell varies. When I rack a set of say, seven together, the CAP with the lowest capacitance would be subjected to higher voltages by the mating CAPs in the set. The rated over voltage for a CAP can also change with age and stress. I always over size my CAPs to the total applied voltage. If each CAP is rated at say 2.7 volts, and I will need a starting storage voltage at say 18 volts, then I will use (8) CAPs that has the total capacitance of about 22 volts. If my CAPs are aged, I may use (9), totaling 24 volts. This “2.7” volts is theoretical when the CAP were newly manufactured, hence I mix and match actual capacitance.

I have a box of lose CAPs of the same type. Maxwell re-designs their CAPs every few years. I cannot mate CAPs of different designs. The newer CAPs hold even more capacitance. To set up a bank of like capacitance is simple. Group a set of say (9) CAPs at (0) volts, charge to 18 volts. Check each CAP individually for it’s holding voltage. Rate and tag them. Those with the lowest voltage, group into one set. Most multimeters can check capacitance, but these instruments are designed to test small circuit board CAPs and if I read the documentation on my testing equipment, it is stated clearly that testing ultracapacitors is not reliable due to the heavy amp draw that they are designed to hold. The only sure way to know the health of a CAP is by subjecting them to a load. See Figure (1).

Figure 1

After I combine a set of like capacitance, I will drain the set of all voltage, re-charge to 18 volts and test each cell again making sure the voltage is far under the rated 2.7 volts. I am talking survival here, and this is a manual operation. Say I use this set for a LED lamp load and the CAPs drain down to 8 volts then I top the set back up to 18 volts. The capacitance will change. After so many cycles I will check the voltage of each cell to watch for overvoltage. If so, I will drain the set  back down to (0) volts and recharge. Depending on how I use this storage power, this would be a full time job. The use of integrated charge controllers are ideal. BUT, as I have to repeat in every post, I do not rely on microprocessors in a true survivalist situation. Microcontrollers are not repairable and subjected to fail by heat, static electricity or EMP natural or man made. Once these fail, any device you relay on is now totally useless!

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