Quote:
Originally Posted by Nautilus
This is very much possible. Balance circuits are factory made, they employ a small IC with a power transistor and resistors. ...So the circuits may be dropped altogether to save energy.
Lithium batteries are barely usable at freezing temps. Even high tech Varley Lithium are rated by factory for no less than -10°C (14°F).
The cheap 10Ah AGM I've used had no issues in -5°C to -6°C. But gave up at -11°C.
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My guess is that either your balance circuit or a defective cap is placing too much parasitic draw on your battery, which then damaged it (your AGM might have a short). You shouldn't be losing so much voltage in such a quick amount of time on a supercap pack when it isn't connected to the vehicle. Like I said, my supercap can go months and only loose 1 volt, and that's with an LED balance circuit. Correct this problem first, or damage might result on the replacement battery.
I have a LiFePO4 pack in my Prius as an experiment. The allowable charge rate is very low when it's below freezing. Fortunately the battery is located inside of the passenger cabin, and I park inside a garage that never goes below freezing. Relocating a small lithium battery inside the passenger compartment might be a viable solution. Another solution is to place a current limiting resistor in series with the lithium battery to limit the charge rate to something safe when below freezing. The super caps would do almost 100% of the work of starting the engine, and the lithium battery would maintain voltage when the vehicle is parked.
Quote:
Originally Posted by ECONORAM
I was about to ask if a solar charging panel would work instead of another battery. Would you need a voltage regulator in the circuit for sunny days?
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Yes, and that was my plan for the truck, which has an extremely low parasitic draw (7mA). It's absolutely critical to have a working balance circuit on the supercaps because the solar panel doesn't limit output voltage when the caps reach their rated capacity. Capacitors by themselves have no way to bleed off excess charge, and instead just self-destruct. LiFePO4 is similar, and must have some means of bleeding excess charge if connected to a solar panel.
NiCad, NiMh, and lead-acid all have the ability to bleed off excessive trickle charge, and should be fine with an appropriately sized solar panel.
Combining a small lead acid battery with supercaps is a good way to limit voltage to the supercaps since the battery can waste excess charge if you want to also use a solar panel.