Quote:
Originally Posted by freddyzdead
There will be a 25% reduction in capacitance, which is already suffering from series string disease. I don't know that it really hurts them if they occasionally nudge a little over 2.5 volts. The weak link is the dielectric layer, which in this case is very thin. Excessive voltage will cause a breakdown, which may or may not be self-healing. One day if/when I have a spare one that I don't need, I will run some tests. It certainly would be useful to know what is the failure mode of these things.
Connecting a series string of zeners directly across the capacitor/battery is a bad idea. A transient spike, perhaps from the starter motor, could zap one or more of them, resulting in disaster. The proper way to do it is to have a transistor across each capacitor, turned on when the zener starts conducting. Probably cheaper and safer to just buy a balancing board. The LED idea is appealing because of its simplicity, but it doesn't really do the job, and can't dissipate enough power to drain away the excess charge quickly enough.
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It is assumed the pass element (zenner/balancing mosfet, etc) already has a series limiting resistor, same as a LED (which too, is a form of zener) or any mosfet in a battery pack balancing system. Balancing is limited to a few mA and not meant to limit fast rising transients. The latter are dealt by the very low ESR of the cap.
I don't know of any LED that conducts at 2.5V. IRdA is about 900mV, standard types at 1.75V and white/blue/HB GREEN at 2.8V. I don't see this as an optimum solution.
Of course this is a free forum, this is just my opinion - Why not use the ubiquitous TL431 with a 100Ohm series resistor for the same effort/cost, and much better regulation?
Transient spikes are handled by the cap low ESR. This supersedes the much higher impedance of the battery in lieu of which these systems are designed, and expected, to operate.
Quote:
Originally Posted by redpoint5
LEDs are fine for balancing because caps that are already closely matched will not go that out of balance to begin with. The point isn't to bleed off massive overvoltage from an alternator, but to bleed off slight imbalances between the caps.
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I agree 100%. And a LED for such terminal voltages makes sense too, perhaps that's how this method first came to be?