Lithium-ion 12v Battery Replacement

[ksa8907]

Do you have any relays involved?

[Ecky]

Originally Posted by redpoint5

Seems like the battery should be topped up well enough at that low resistance. Would be curious to throw some meters in there to see what's going on. I picked up a few for something like $10 each.

Similar to this:

https://www.amazon.com/HTRC-Precisio...ne+power+meter

Agreed, it should be pretty much symmetrical, but whatever. Most of my driving is less than 20 minutes, and I found it took a full day to charge the battery up to full with my 800ma trickle charger, so it looks like it still has most of its original capacity. I rewired it all today with heavier gauge and added some more 100A breakers, will see how it goes.

When I connected the caps and battery back together, the battery was at ~13.25v and the caps were at ~13.65v. It took several minutes for their charge to equalize when I reconnected them. It did not trip the 100A breaker, so presumably the battery has enough internal resistance to prevent huge rushes of current.

Normally when I start my car the DC-DC runs full tilt for around 15 seconds and then the caps are full, but it took the car a good 20 minutes to charge the battery up to the point that the DC-DC went into a low power mode. Hopefully the internal resistance of the battery will let the caps handle most of the transient loads and it won't be hammered too badly when it's cold. I believe cold increases the battery's resistance anyway?

Originally Posted by ksa8907

Do you have any relays involved?

None. The smallest 100A relay I could find had an active power draw of ~12w, and most of the nicer ones are ~20-30w. To me that's pretty unacceptable.

[IamIan]

Originally Posted by Ecky

I believe cold increases the battery's resistance anyway?

Yes.

If they are anything like the performance curves for A123 style LiFePO4 cells .. the attached are graphs showing the effect of SoC and Temperature on a cell's effective ohms.

Even if (as a different brand) the curve is a little different than A123's , I would still suspect similar overall shape / trend.

Originally Posted by Ecky

None. The smallest 100A relay I could find had an active power draw of ~12w, and most of the nicer ones are ~20-30w. To me that's pretty unacceptable.

You could move away from mechanical relays , to solid state stuff instead.

Although the premade solid state such disconnects are a bit expensive .. they do show how significant of a drop solid state options give for the switch's power consumption rates.

Random example of such pre-made solid state (Mosfet) low voltage disconnect.

Amazon
Victron Spec pdf
$40 with S&H
Load = 65A continuous , 250A for 30sec
Self Consumption = 1.5mA on , 0.6mA off

[Ecky]

Solid state sounds nice, I'll probably use them for any future projects.

After rewiring my battery and caps (deleting the resistor, lower gauge between battery and caps) I measured the capacitor voltage Friday afternoon a few hours after parking the car: 13.34v. Saturday evening I measured again: 13.34v. Looks promising. It's been cold so I'm not super inclined to mess around in the car for an extended period.

[Angel And The Wolf]

Your 12 v car battery charges whenever you are driving. Why do you need anything else?

[Ecky]

Originally Posted by Angel And The Wolf

Your 12 v car battery charges whenever you are driving. Why do you need anything else?

Extreme cold weather in Vermont kills expensive 12v batteries every few years. Super capacitors weigh a fraction as much as lead acid batteries, have far higher cranking amps, and last virtually forever.

[Angel And The Wolf]

I'm sorry, I was responding to the OP and Lithium 12V batteries. Capacitors sound promising.

[hayden55]

For the most part longevity and weight reduction. But yeah here lately I've noticed they really don't like being near 32F especially not below. lol

[Ecky]

Accidentally drove to work this morning without turning my DC-DC converter on. Weather was 15F, drive is ~22 minutes, I was blasting the stereo, headlights, rear defroster, medium blower. Voltage dropped from ~13.3 to ~12.0v over the drive.

[redpoint5]

I'd forget my head if it weren't attached. I need to automate as many things in my life as possible because my memory isn't reliable enough. It's probably my main motivating factor to be very organized with where things go. Everything has a place, and that way I don't have to remember where I put things.