# Car Related, not specifically a Car Question.

#### cjdelphi

Joined Mar 26, 2009
272
Basically I just had a thought, I'm sick of Car 12v batteries, the last one Died on me after being on float charge for almost 2 years solid... now the car battery just died!

So, forgetting about parasitic drain (not that it's an issue anyway)... is there any real reason one day a super capacitor could not replace a battery?...

#### Wendy

Joined Mar 24, 2008
21,840
Yes. A battery internally regulates voltage. A supercap doesn't. An electric car might have a supercap with 8KV on it.

#### Jaguarjoe

Joined Apr 7, 2010
767
The starter motor on an IC engine consumes hundreds of amps at 12 volts. That would be a pretty big cap. If you converted to a 42 volt system you might have a better shot. One novel arrangement converted the flex plate into a dual purpose 42 volt starter/alternator.

#### tom66

Joined May 9, 2009
2,591
Umm... what?

You cannot easily overcharge a capacitor. Ever tried it?

I put 35V on a 6.3V electrolytic cap quickly. With the multimeter attached it showed 35V. But within ~100ms it had dropped to just 12V and slowly decayed.

A super cap will never get 8kV across it, it would simply arc over or damage the capacitor. Similar for a battery.

I think the main problem is that super caps are expensive. You'd spend $500 on a super cap starter battery for a small car when you could just use a lead acid for less than$100 (I think... I have never bought a battery.) A small car might require 60 to 100 amps to start for 5 seconds at the most. At 12V, that is 1.2kW, or 6kJ over 5 seconds, required to start a car on a cold day. A 2500F 2.5V supercap cell has about Q=CV joules in it, or 6.25kJ. You'd need 6 cells to get 15V. 6 cells may just about be able to manage it. Now upgrade your charging circuitry and battery monitoring circuits. Then you are ready. But as I said, at $80 a pop, that would be almost$500.

#### SgtWookie

Joined Jul 17, 2007
22,201
If a float charger that is not temperature compensated is used on an automotive battery where the storage temperature is not controlled, it can kill the battery.

Lead-acid batteries need to be float-charged at a higher voltage when the temperature is low, and a lower voltage when the temperature is high. The float voltage is nominally 13.65v @ 25°C internal temperature, but this changes at a rate of -18mV/1°C offset.

#### debe

Joined Sep 21, 2010
1,139
With car batterys Ive found when purchasing them i check the spesific gravity of all cells before using the battery. The reason is ive found a lot af batterys will have at least one cell well down on the others. I charge these batterys un till all cells are equal before fitting the battery to the vehicle. In winter time with short runs i check battery ocasionally & give a topup charge. Both my vehicles there batterys are 6Yrs old & still going, I think if a battery gets a good start in life & looked after should give good service.

#### #12

Joined Nov 30, 2010
18,076
I had to store a mini-pickup truck for a year, so I attached a wall-wart to charge the battery. After the battery was charged full, I added resistance until the bubbling stopped. Checked the water level every couple of months. Just a few ma kept it healthy through both summer and winter. Not very "millivolts per centigrade", but it worked.