Lithium-ion 12v Battery Replacement

[redpoint5]

I'd go with 6x Maxwell BCAP0350 (350 farad). It's just big enough to start a car on its own. Any bigger probably isn't necessary.

This site supposedly has them for $14 each plus shipping, but you have to ask for a quote:

BCAP0350E270T11 Maxwell Technologies Ultracapacitor- Richardson RFPD

This site also has 'em for $14:
https://www.verical.com/quote-cart

Mouser has them for $17 each:
https://www.mouser.com/ProductDetail...20Technologies

They come in 2 flavors: solder post, or weld, meaning you have to spot weld the connections.

Ebay might be the best place to shop:

Balance board (although balancing is not necessary)
https://www.ebay.com/itm/Six-In-Series-MaxWell-350F-2-7V-T11-T10-Supercapacitor-Balance-Protection-Board/263928360582?epid=1950825530&hash=item3d735b2e86:g :g-sAAOSwqrZa1pLZ:rk:25f:0

EDIT: Just found these 400 farad Eaton for $12:

https://www.mouser.com/ProductDetail/Bussmann-Eaton/XV3560-2R7407-R?qs=sGAEpiMZZMuDCPMZUZ%252bYl72C5N2%252bSzT2g2g0v E9ei8Y%3d

Also found this 400 farad for $12 and it has radial leads to solder to. New item it seems:

https://www.mouser.com/ProductDetail...yX1%2fDaf4o%3d

I use octopart.com to keep an eye on electronic prices at various retailers.

[Ecky]

Can you explain to me why balancing is not necessarily when running 6 caps in series? I'm getting conflicting information online about this.

[teoman]

I have a cheapo chinese pack 350F each. 6 series. And they have vastly different capacities.

Charging from fully discahrged caps one can reach 2.7 while another is at 2.0V

[redpoint5]

Originally Posted by Ecky

Can you explain to me why balancing is not necessarily when running 6 caps in series? I'm getting conflicting information online about this.

Quality caps are closely matched so they don't drift very much. The fact that higher voltages exponentially increase leakage means they self-balance.

I'd do the initial charge with them in parallel to 2.25v, but after that they should be fine.

Originally Posted by teoman

I have a cheapo chinese pack 350F each. 6 series. And they have vastly different capacities.

Charging from fully discahrged caps one can reach 2.7 while another is at 2.0V

Keyword- cheapo Chinese. I've heard many people report receiving defective caps, but quality ones should contain no defective units, and be closely matched. I've not had a single bad one out of 17 so far.

That said, I did wire up LED balancing because it's so cheap, and I was paranoid, and it's an easy way to visually see state of charge for each cap.

[Ecky]

The downside being that it increases parasitic draw a lot? I imagine there isn't a good way to actively or passively balance the caps without increasing drain.

[redpoint5]

Originally Posted by Ecky

The downside being that it increases parasitic draw a lot? I imagine there isn't a good way to actively or passively balance the caps without increasing drain.

Define a lot. My linked spreadsheet had a cap leakage chart showing an average drop of 0.09v over 9 days from an initial 2.22v charge. To be honest, I can't remember if I had the LED balancers on or not, so much of that could be attributable to that, but that's only 4%.

I'd be happy to run the test again with a single cap and chart the drop.

As I said, caps near the rated voltage will leak much more rapidly than those a few tenths of a volt below the peak rated voltage. This tends to self balance them.

By the way, LED balancing barely increases parasitic drain. They too rapidly reduce power draw as the voltage drops below their rated maximum. They will be dimly glowing months later, with cap voltages around 2v each (12v total).

[Ecky]

Looking at the datasheet for the AVX capacitors, it appears max current on them is 180A:

http://datasheets.avx.com/AVX-SCC.pdf

The Eaton capacitors are rated for 26 amps continuous and 220 amps peak:

http://www.cooperindustries.com/cont...-xv-series.pdf

The Maxwells have an "absolute max" current of 170a:

http://www.maxwell.com/images/docume..._1017105-4.pdf

Looks like a 6s2p configuration would be safest, so as not to pull double the current rating on the caps?

[redpoint5]

I don't think those current limits matter much unless you're welding with them, which I accidentally did with no noticeable harm besides slightly melted aluminum.

Starter motors might draw 220A for a fraction of a second. No big deal.

I've been running Eaton 400 farad caps in my motorcycle for a number of years now with no problems. Perhaps I should have measured the capacity when new vs now, but at least I haven't noticed any degradation. My guess is caps tend to fail more catastrophically than gradually, unlike batteries.

[Ecky]

How many cranking amps is your motorcycle?

[redpoint5]

About 80

https://www.youtube.com/watch?v=qweEl8Wypzg