Lithium-ion 12v Battery Replacement

[ME_Andy]

Bioennopower is having a 10% off sale. Code: 2018SPOOKTACULAR

[Ecky]

Question about wiring: I have 2 gauge copper running from battery to fuse box and starter. From the pictures of these caps, I can’t imagine being able to connect a wire that large to them, one that can handle a brief 2-300 amps. How does that work?

[redpoint5]

Originally Posted by Ecky

Question about wiring: I have 2 gauge copper running from battery to fuse box and starter. From the pictures of these caps, I can’t imagine being able to connect a wire that large to them, one that can handle a brief 2-300 amps. How does that work?

Short run of 10 gauge should be fine, especially for brief bursts. I'd just use as large a gauge as fits in the connector you choose for the caps. I start my motorcycle off 10 gauge, and have started my Acura on 10 gauge with no problems.

[Ecky]

Awesome.

Care to give an example of an LED balancing array? And, how would one go about calculating the correct resistor for protecting a lithium battery from too much current? Chances are good I'm going to build this in the next month or two.

[redpoint5]

Originally Posted by Ecky

Care to give an example of an LED balancing array? And, how would one go about calculating the correct resistor for protecting a lithium battery from too much current? Chances are good I'm going to build this in the next month or two.

Probably not the correct way to precisely calculate this, but I just used ohms law to determine what size resistor made sense.

Determine the maximum acceptable charge/discharge rate for your battery. I used charge rate since that was the most limiting factor. Lets say you have a 5 Ah battery with a 10C charge limit. That's 50A max.

Now estimate worst case what the voltage difference will be when your battery is at the most discharged state, and subtract that from the max charging voltage. So maybe my battery drops to 8v, and the alternator can charge at 14v, for a 6v difference.

V (voltage) / I (amps) = R (resistance)

8v / 50A max = 0.16 ohms.

I'd round that up to 0.2 ohms and find a resistor that can dissipate up to 40 watts, though it probably doesn't need to be able to handle that continuously. Mount it to something metal so it doesn't melt anything should it overheat.

[Ecky]

And the LEDs? The diagrams I've found use several transistors, looks like it's the design used by most of the "protection boards" sold on eBay.

Is it simply a small LED with an activation voltage of ~2.5v on each capacitor, designed to very gradually run them all down to an equivalent level?

EDIT: 6 caps ordered.

[Ecky]

Looks to me like this LED might be a good choice for balancing:

https://www.mouser.com/datasheet/2/4...500-368976.pdf

[redpoint5]

Originally Posted by Ecky

And the LEDs? The diagrams I've found use several transistors, looks like it's the design used by most of the "protection boards" sold on eBay.

Is it simply a small LED with an activation voltage of ~2.5v on each capacitor, designed to very gradually run them all down to an equivalent level?

EDIT: 6 caps ordered.

It's been a while since I tried balancing caps. Basically you want the LED and diode to equal about the maximum voltage of the cap. So you want a voltage drop of about 2.5v. When a cap goes above about 2.5V, you want the LED burning that voltage so it levels out.

I don't really think any of that is necessary in a super cap bank for automotive purposes, but putting in the balance diode and LED is cheap insurance. The higher charged caps will bleed off charge faster anyhow, creating a natural balance.

[Ecky]

I consider myself now far more educated on the topic, thank you.

Ordered everything but the lithium battery, going to try the super capacitor bank along with my lead-acid first.

[Ecky]

Doing some reading:

400F = 0.3 amp hours

My starter is rated 83 amps, though I know realistically it peaks much higher for a brief time.

0.3 amp hours is 1080 amp seconds, or good for supplyimg 83a for 13 seconds. However, after 6.5 seconds, it would be down to ~7 volts.

This doesn't take into account that caps in a series have less capacitance.

1 / C total = 1 / C1 + C2 + ... C6

1 / C = (1 / 400) x 6, C = ~67 Farads

So, with 6x 400F caps in series I'll have an effective capacitance of 66F. I should be able to run my starter for 1.2 seconds before the caps hit 7v, and 2.4 seconds before they're completely empty.

We'll see how it goes. I'll report back with how well it starts the car in reality.