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redpoint5 · 02-01-2015, 03:08 AM

My trucks 2 group 27 batteries are on their absolute last leg. With a solar panel keeping them topped up, the truck barely starts on a cold day.

I wanted to see if it would be safe to run 5x of the BCAP3400 supercaps in series considering they have an individual voltage rating of 2.85v, and a series rating of 14.25v. My concern is that the alternator might over-volt the caps and cause premature deterioration or even perhaps catastrophic failure.

I started the truck, connected a small bank of 6x 350 farad capacitors, and disconnected the lead acid batteries. This tested the trucks output when only supercaps are connected. One thing I noticed is that almost no current flows into, or out of the caps. The voltage seemed to be tightly regulated and the alternator supplied just enough to meet the electrical demands. At idle with all electrical loads turned off, the alternator maintained an average of 14.17 V. It very briefly peaked at 14.37 v when I revved the engine with no load, and had a minimum of 13.88 when all electrical loads were turned on.

I measured individual cell voltages of all 6 caps and found a 2% difference between the highest and lowest value cells. Since my truck has a peak of 14.37v, that would make the average cell voltage of 5 cells 2.874v. 1% more than that would be 2.903 v.

Since the caps have an absolute peak of 3v each (15v total), it seems I could just barely make them work in this application.

I could buy 2 group 27 batteries for $250, or I could experiment with 5x BCAP3400 for $340. My trucks extremely low 5.7 mA parasitic draw would not be a problem, and the caps would be kept topped up with a small solar panel. They would be balanced and trimmed by either LEDs or a Zener diode circuit.

What do you think? I'm leaning towards the caps because I think there is a good chance they will work, and will at least satisfy my curiosity if they don't work so well. I could re-purpose them if I find them regularly operating outside of their design envelope.

Oh, and I already purchased the aluminum z-strap bus bars. You can tell me I'm crazy and I won't be offended.

02-01-2015, 03:08 AM	#52 (permalink)
redpoint5 Human Environmentalist Join Date: Aug 2010 Location: Oregon Posts: 12,814 Acura TSX - '06 Acura TSX 90 day: 24.19 mpg (US) Lafawnda - CBR600 - '01 Honda CBR600 F4i 90 day: 47.32 mpg (US) Big Yeller - Dodge/Cummins - '98 Dodge Ram 2500 base 90 day: 21.82 mpg (US) Chevy ZR-2 - '03 Chevrolet S10 ZR2 90 day: 17.14 mpg (US) Model Y - '24 Tesla Y LR AWD Pacifica Hybrid - '21 Chrysler Pacifica Hybrid 90 day: 43.3 mpg (US) Thanks: 4,327 Thanked 4,480 Times in 3,445 Posts	My trucks 2 group 27 batteries are on their absolute last leg. With a solar panel keeping them topped up, the truck barely starts on a cold day. I wanted to see if it would be safe to run 5x of the BCAP3400 supercaps in series considering they have an individual voltage rating of 2.85v, and a series rating of 14.25v. My concern is that the alternator might over-volt the caps and cause premature deterioration or even perhaps catastrophic failure. I started the truck, connected a small bank of 6x 350 farad capacitors, and disconnected the lead acid batteries. This tested the trucks output when only supercaps are connected. One thing I noticed is that almost no current flows into, or out of the caps. The voltage seemed to be tightly regulated and the alternator supplied just enough to meet the electrical demands. At idle with all electrical loads turned off, the alternator maintained an average of 14.17 V. It very briefly peaked at 14.37 v when I revved the engine with no load, and had a minimum of 13.88 when all electrical loads were turned on. I measured individual cell voltages of all 6 caps and found a 2% difference between the highest and lowest value cells. Since my truck has a peak of 14.37v, that would make the average cell voltage of 5 cells 2.874v. 1% more than that would be 2.903 v. Since the caps have an absolute peak of 3v each (15v total), it seems I could just barely make them work in this application. I could buy 2 group 27 batteries for $250, or I could experiment with 5x BCAP3400 for $340. My trucks extremely low 5.7 mA parasitic draw would not be a problem, and the caps would be kept topped up with a small solar panel. They would be balanced and trimmed by either LEDs or a Zener diode circuit. What do you think? I'm leaning towards the caps because I think there is a good chance they will work, and will at least satisfy my curiosity if they don't work so well. I could re-purpose them if I find them regularly operating outside of their design envelope. Oh, and I already purchased the aluminum z-strap bus bars. You can tell me I'm crazy and I won't be offended. __________________ Gas and Electric Vehicle Cost of Ownership Calculator Give me absolute safety, or give me death!