Replace your car battery with capacitors!

[serialk11r]

I was looking into this some more and had a realization: Deep cycle lead acid batteries have better energy density and cycle life, and pairing them with ultracapacitors (to provide starting current) is more cost effective than lithium titanate if you have a small engine.

For example:
YTX14L battery weighs ~10lbs, 14Ah capacity
This 14Ah battery probably has higher internal resistance, but it weighs 7.7lbs:
https://www.amazon.com/Chrome-Batter...3125925&sr=8-6

350F Maxwell ultracapacitors are 60 grams each and have leakage current on the order of 0.3mA, and a set of 6 is probably around 1lb all in with the balancing board.

100F ultracaps are more like 20 grams each, so this package is probably only 200 grams all in: https://www.ebay.com/itm/US-CDA-16V-...5/313179060140

I believe you can get away with the 100F capacitors because they'll be fully charged by the battery, which can also provide a few dozen amps of starting current.

The whole package performs almost like lithium titanate that weighs 3lbs more, but it has a cheap lead acid battery that you can replace for 30 dollars.

[redpoint5]

Originally Posted by serialk11r

I believe you can get away with the 100F capacitors because they'll be fully charged by the battery, which can also provide a few dozen amps of starting current.

The whole package performs almost like lithium titanate that weighs 3lbs more, but it has a cheap lead acid battery that you can replace for 30 dollars.

I don't know what the Pb-acid battery can contribute, but 100F rated caps probably aren't close to being able to start most engines. Worth experimenting with, but I wouldn't think anything less than 300F rated caps would work reliably.

400F barely seems adequate for my 0.6L motorcycle, but then again I have no battery assist.

[serialk11r]

Originally Posted by redpoint5

I don't know what the Pb-acid battery can contribute, but 100F rated caps probably aren't close to being able to start most engines. Worth experimenting with, but I wouldn't think anything less than 300F rated caps would work reliably.

The original Youtube "Laserhacker" ultracap guy got his Honda V6 to start on 400F caps when it was full, and started his 1NZ-FE on 350F caps at 10.4V.

The battery alone should be able to start most 4 cylinder engines if it's fully charged. I've seen some guy run a Harbor Freight AGM (rated at 10Ah) in his car. You easily have something like an intermittent 50 amps available even if the battery is only half charged.

The 11mohm ESR of the 100F cap means you'll see approximately 1V drop at most, so your usable energy is at least the difference from 12.4 to 8.2 volts or 86.52*16.66/2=720 joules. That's 100 amps available for almost a whole second, which is enough.

Certainly the 58F pack is better, but I think you could get away with the 16F pack if you want to save money. The point is to not add extra power you don't need to the battery to avoid leakage current, cost, and bulk. The more money you spend, the closer you get to the intro price of ~200 dollars for a lithium titanate pack with its greater ease of use.

The 16F bank can also be added to a small lithium ion battery for cars that don't sit very long of course.

[redpoint5]

It's been a while since I did the math with these, so I was avoiding it. 100 amps for a second isn't much though. My 2.5L 4 cylinder Acura pulls 250+ amps peak, but cranks for about 2 seconds. Someday maybe I'll measure the voltage drop and calculate the energy consumption, but I still don't think going under 300F is reliable enough unless the battery can contribute something substantial.

As an aside, with my ~6 ft long run of 10 gauge wire to the glovebox, I'm unable to start my Acura with the LiFePO4 battery alone. It certainly needs the ultracap bank, though I haven't found out what the lower limit is for sizing purposes.

Looks like there are 120F caps for $6.90
https://www.mouser.com/ProductDetail...G33T0UjA%3D%3D

Why mess around with those when you can get 400F with low ESR for $11?
https://www.mouser.com/ProductDetail...yegxchbA%3D%3D

These are $11.69 for 150F, but you only need 5 since they are rated for 3v.
https://www.mouser.com/ProductDetail...g0qA9fIQ%3D%3D

Too bad nobody makes a 15v ultracap so you only need one.

[serialk11r]

Buying the individual caps like that on Mouser is a sure way to lose money lol. You can get an assembled 500F/6 => 83F pack for only 40 bucks.

Yea I'm not surprised you can't start the car with all that extra wire. 250 amps is only momentary; the sustained starter current is more like 80, and even a wimpy battery is capable of supplying most of that. You only need the extra amps for that momentary surge at the beginning where the motor needs to start turning, then the current demand rapidly drops as the motor's back-emf rises.

Of course, with cold weather I think it's probably a good idea to get a bigger bank since you'll probably need several thousand joules to crank the engine, and the lead acid battery is going to crap out at low temps. The warmer it is, the less capacitor you can get away with.

[redpoint5]

The balance boards I've seen all show them paired between 2 caps on a single "end". I've got 5 caps, so I wonder if I can put balance boards on both ends as if I'm creating a series circuit? In other words, 4 balance boards for 5 capacitors.

I might buy another cap so I'm not pushing voltage so close to limits.

[serialk11r]

All the capacitor balancing circuits I've seen just use a voltage regulator or LEDs across individual cells, that one appears to do the same. In that picture, you can see those red wires going down to the bottom, so I think there are 6 boards total.

The most ghetto thing you can do is use diodes and resistors across the terminals, but I feel like these days when you can get inductive active balance circuits for 10 dollars, you might as well do it properly.

And yea I think not pushing close to the max voltage is a good idea. 6 in series leaves some comfortable margin which should help the electrolyte last longer.

[redpoint5]

Originally Posted by serialk11r

All the capacitor balancing circuits I've seen just use a voltage regulator or LEDs across individual cells, that one appears to do the same. In that picture, you can see those red wires going down to the bottom, so I think there are 6 boards total.

The most ghetto thing you can do is use diodes and resistors across the terminals, but I feel like these days when you can get inductive active balance circuits for 10 dollars, you might as well do it properly.

And yea I think not pushing close to the max voltage is a good idea. 6 in series leaves some comfortable margin which should help the electrolyte last longer.

I bought a 4s balance board that moves high charge to the lower cell for my LiFePO4 battery. It kicks on whenever the voltage differential exceeds 0.1v.

I inquired about this balance board to see if it would work for ultracaps, but apparently not. I'm not finding anything else except the boards I posted above.

https://www.amazon.com/6Series-Lithi...6R4PA75ME4K544

[serialk11r]

Originally Posted by redpoint5

I inquired about this balance board to see if it would work for ultracaps, but apparently not. I'm not finding anything else except the boards I posted above.

I don't see why it can't be used? It only works off the difference right? I found a bunch on aliexpress and ebay that say they're okay for LTO (2.7v full charge) and one I saw yesterday said "LTO/ultracapacitor" as a mode (the other mode was "LFP/LCO"). A small balance current should be all you need.

https://www.ebay.com/itm/Li-ion-Life...oAAOSwKWhdvSB-

[redpoint5]

Sounds like it has a 2.2v threshold, so there's a chance a 6s config wouldn't raise cell voltage enough to trigger balancing, but as long as it's below 2.2v it probably doesn't matter. Cheap enough that I'll give it a try.