[jray3]

Originally Posted by redpoint5

My truck batteries (2x group 27) have finally degraded so much that they cannot start the truck after sitting for a day. I can replace them for about $200, or I can eliminate the batteries entirely and run a supercapacitor.

Someone talk me out of spending $319 on this! The auction is for 6 used 3000 F capacitors with balance circuits.
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I've used six 3000F capacitors to boost the 9 year-old AGM batteries on my 6.9l diesel truck for over four years now. They keep the voltage up nicely during glow plug operation and starting is typically much faster due to the stiff voltage. With low batteries, if the truck does not start, then voltage recovery is longer before the next cranking attempt, but the capacitors help get the engine turning better even on low voltage. I call 'em a win-win, but spent about half that price on a fully-assembled set with balancers from seller cusdn on EBay.. (I'm due to replace those batteries now, but got 4 more years out of them than I otherwise would have!)

IMHO- you could run the six caps without balancing circuitry just fine if you periodically switch them out of parallel with the house battery and do a bottom balance (bleed 'em down to zero). However, I do not see ultracaps as a practical stand-alone battery replacement.