Lithium-ion 12v Battery Replacement

[redpoint5]

I'm not sure exactly what it would take to start a diesel, especially factoring in glow plugs or grid heating.

I've got a project to start my Cummins motor with supercaps and bought the biggest, most energy efficient caps I could find. BCAP3400 which have a 3400 farad rating, and more interestingly, a 2.85 maximum voltage compared to the other caps that are rated to 2.7v.

I bought 5 and put them in series, compared to 6x of the lower voltage caps. The reason I went with 5 is they are super expensive ($50 each) and the farad rating decreases for every additional capacitor in series. 2.85v times 5 series caps is only 14.25, which is extremely close to the output of many alternators, so you'd want to be very careful to measure what an alternator would charge them to, and maintain a robust balance circuit.

My diesel truck has an extremely low parasitic draw of only 5mA, and that is entirely due to maintaining the memory in the aftermarket stereo. My thinking was I could put an extremely small lead acid battery in the truck, use a 5 watt solar panel to keep it topped off, and use the supercaps to start.

Unfortunately my old ways thinking dad didn't listen to my instruction to not buy new batteries for the truck, and installed them anyhow, at a cost of probably $300. I've got no need for the supercap now, and haven't spent the time to test the setup.

[Ecky]

Originally Posted by teoman

Do you have a link to a bigger one? To start a diese for example.

Hop on Mouser Electronics, and sort super capacitors (they have their own category) by how many Farads. I believe Mouser carries them up to 3000F.

[redpoint5]

Originally Posted by Ecky

Hop on Mouser Electronics, and sort super capacitors (they have their own category) by how many Farads. I believe Mouser carries them up to 3000F.

Once you find the one you want, search octopart for the best deal.

It seems the Bcap3400 is difficult to find these days.

Be mindful of voltage differences. Most supercaps are rated to 2.5v each, but the BCAP line is 2.7, with BCAP3400 only rated at 2.85v.

[Ecky]

My 12v battery very rapidly declined the last few days. On Monday it started the car fine. Yesterday the engine cranked once and then nothing. My temporary solution was to grab a pair of discarded 7Ah alarm panel batteries from work and wire them in parallel. Car started up fine this morning and this evening, cranked faster than it has in a while.




Caps were at my door when I got home. Funny thing, I couldn't tell which box had the capacitors and which one had the resistor and LEDs in it; they weigh practically nothing!




redpoint5, what did you use to hold your caps?

[redpoint5]

Time for some photos:

350F caps in series with LED balance circuit built into the board. 4.2Ah LiFePO4 battery connected to supercap board with a series resisistor to limit current. Power leads cut at different lengths to minimize chance of accidentally touching them together.


400F supercaps in series, with ghetto tape holding it together and ghetto LED balance leds in parallel across the caps. This is what has been my motorcycle "battery" for the past 4 years.

[Ecky]

I think I may have chosen the wrong LEDs - they went from bright to dark in a matter of minutes, with the cap bank stabilizing around 12.5v. I'm thinking rather than 30ma continuous forward current (100ma peak), I might want something more like 2-5ma, and/or a slightly higher voltage.

On the other hand, they'll probably very aggressively keep the caps in balance.

https://www.mouser.com/ProductDetail...rrencycode=USD

Maybe it was because it was the first time I'd charged them? I'll keep an eye on it and see.

[redpoint5]

Originally Posted by Ecky

I think I may have chosen the wrong LEDs - they went from bright to dark in a matter of minutes, with the cap bank stabilizing around 12.5v. I'm thinking rather than 30ma continuous forward current (100ma peak), I might want something more like 2-5ma, and/or a slightly higher voltage.

On the other hand, they'll probably very aggressively keep the caps in balance.

Maybe it was because it was the first time I'd charged them? I'll keep an eye on it and see.

Yeah, keep the caps on the charger for half a day and then see what they settle out to. Measure each cap to see how closely the voltage matches the others.

If you had an adjustable power supply (I highly recommend one), you can take a spare LED and measure current draw at various voltages. The ideal activation voltage would be somewhere around 2.5v, with lower voltages barely flowing any current at all, and higher voltages flowing higher mA. A settled voltage of 12.5v suggests your LEDs flow pretty high until about 2.1v each.

I had tested various LED and diode combos in my parts bin before settling on the ones you see in the photo with the yellow caps. BTW, there is an additional diode in series with the LEDs because the LED alone was not a sufficient voltage drop. It would have been too much voltage for those red and green LEDs without the diode in series.

Other color LEDs might be closer to the voltage you want.

I'd also charge all caps up to the same voltage and let them sit for several days to see if they bleed at the same rate. You want to identify a dud before putting these to work. I know Maxwells to be very high quality, but many of the other brands have been known to ship duds.

[Ecky]

Seems that even after just an hour, they're holding a lot more energy. They all had some small charge in them when they arrived. I charged them in series using a trickle charger I have, then used a resistor to manually discharge them all to within a 0.01v of each other. I think I'll probably start using them right away, but keep a very close eye on the LED brightness. These LEDs might actually work out, I just won't know right away.

[Ecky]

Moment of truth!




Caps were around 14v. Turned the key, car started right up. Caps dropped to around 8.5v under load then bounced up to around 9.5v with a single start though, so they're literally good for just one start. Good enough if I pair them with some other battery that will top them back off, but I think if I were to recommend this project to anyone else, it would be with caps of at least 800F.

[Ecky]

Correction: Might not be workable, at least with my DC-DC converter supplying 12v. After starting the car, the DC-DC kicks on, then its overcurrent protection trips and it shuts back off, car runs for about 15 seconds then dies when the caps get too low. I'll need to think about this.