Quote:
Originally Posted by Enki
Keep in mind that as you get closer to peak capacity, the charging rate (amps/sec) on the caps tapers off. They're usually only a dead short when bone dry of capacity.
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I was just looking at one in farnell:
XB3550-2R5307-R - BUSSMANN BY EATON - CAP, SUPER, 300F, 2.5V, SNAP IN | Farnell element14
...with 0.006Ohm, if the capacitor is at 11V after a start I = U/R
I = (14.4-11)/0.006
I = 566A
I don't suppose many car alternators can provide that nominally. Of course there are many other losses on the system, but those can't be quantified, they vary widely (alternator impedance, engine speed, cabling...)
Quote:
Originally Posted by redpoint5
Alternator output is regulated, so the strain should not exceed rated output. Perhaps the rectifier blowing was coincidence.
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Standard alternators (excluding special vehicles and marine applications) are voltage regulated, not current. They rely on the high impedance of lead acid batteries for protection. Current is reduced if the regulator temperature is too high, but provide no protection against short term overload conditions.
The rectifier could very well be coincidence, but an empty lithium battery and a constant voltage charge source are asking for trouble, regardless.
Quote:
Originally Posted by redpoint5
I like the 10 Ah paired with the capacitor idea the best. I'd go with 6x 350 farad caps since they are most common and cheap kits are available on Ebay
Solar charging would require a balance circuit since neither caps or LiFePO4 is tolerant of even slight overcharges. A small solar panel can easily exceed the parasitic drain of vehicle electronics plus self-discharge and over-volt both the caps and the battery.
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Curious if anyone tried audio caps for that purpose (the ones uses with subwoofer's)?
Solar panels often have a regulator circuit except the very smallest ones, for which the BMS can actively shunt the excess energy and balance the cells at the same time, so this isn't particularly troublesome.
Quote:
Originally Posted by redpoint5
A supercap/LiFePO4 combo makes sense if the goal is to reduce weight, but LiFePO4 is not tolerant of charging in sub-freezing conditions.
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That depends how much the subfreezing is. My battery gets warm after 5seconds at 3C to pre heat the glow plugs and 2-3 seconds at 8C to crank the engine. Further charge will warm it up a bit more. This may cover the typical usage. If not ,there are ways around it. For minus 20C or below it might be troublesome, even for lead acids. Here the capacitor would be a welcome extra.