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Old 01-09-2014, 08:27 AM   #10 (permalink)
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Answers starting from the OP on:

1. Capacitors will take and give up charge with high 90's% efficiency. Chemical cell batteries are closer to 60-70% efficiency at best. i.e 0.7 X 0.7 = 0.49 or <49% for the cycle with chemical cells vs closer to 95% for the capacitors. That's ignoring any loss anywhere else.

2. Self discharge? On Maxwell or other manufacturer's site somewhere? I'm not sure that they do self discharge faster than Lead acid (Where to? Across the charged plates?) but rather don't hold as much charge in the first place, and have a steeper drop off in voltage for a given discharge.

3. A minimum of 10V seems to be fine. As you have seen, engaging the starter will cause a battery's voltage to fall to ~ that anyway and is routine.

4. Is there a current draw spec. for the capacitors available from the manufacturer?

12g wire has a resistance of ~1.8 Ohms/1000ft. Does that help?

Perhaps a bench test with a variable resistance/load while measuring the current drawn and observe the temp. of the wires?

5. Lifetime ^.

6. ?

7. Probably not.

You can work out how much cranking time you have in your capacitors by working out the difference between the charge stored at 10V and at 14V (or whatever your alternator charges at). With your measured discharge current while starting, ~efficiency (try 95%), 1 Coulomb is 1A supplied for 1 sec and you can find your cranking time.

Also consider that the charge stored across a capacitance is non-linearly proportional to the voltage; a capacitor charged to 10V is holding less than 10/14 of it's charge at 14V. The Maxwell Engine Start Module incorporates a DC-DC converter. 12 2.7V capacitors will charge to 32V. If the truck system is 12V (14.2V) there is a step up and step down occurring within the Start Module. I expect that's why the Mazda system - which will also use DC-DC conversion - uses 24V to charge the capacitors. (Comment in the linked thread in the OP.)
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