Lithium-ion 12v Battery Replacement

[Ecky]

I'll report back if I ever get to the bottom of it. I really shouldn't be using lithium at all with operating temps more than 30 degrees below their rated absolute minimum, but I don't have evidence yet.

[redpoint5]

The cold is why I intend to test with a cheapo Li-ion 5Ah battery first. If it dies, it was a cheap learning experience.

[ksa8907]

The resistor wont drain anything. It is merely a throttling device and will only cause an effect when energy is "flowing".

[Ecky]

Originally Posted by ksa8907

The resistor wont drain anything. It is merely a throttling device and will only cause an effect when energy is "flowing".

I understand this. When a certain amount of energy does have to flow across it wastes some of that as heat, but I agree that ultimately it shouldn't cause the battery to run dead when the car is sitting.

[ksa8907]

Originally Posted by Ecky

I understand this. When a certain amount of energy does have to flow across it wastes some of that as heat, but I agree that ultimately it shouldn't cause the battery to run dead when the car is sitting.

I didn't really think about who I was replying to, you built the circuit, clearly you know how it works. No offense intended.

[IamIan]

Originally Posted by Ecky

1) There's an intermittent parasitic draw which happened this weekend.

Originally Posted by Ecky

4) The battery is gradually losing charge over time due to the resistor.

I see 1 & 4 as two sides of the same coin.

Well .. LEDs and diodes don't truly go to zero .. they just go to high resistance .. thus to very low parasitic draw .. but it is always there if they are always connected.

If you have the part number and such you used .. you should be able to look up what amount of that effect would be reasonable to expect in your setup over __Hours.

Originally Posted by Ecky

2) The caps are draining the battery.

Unless you have something to prevent them from doing so .. it's only a question how fast is the self discharge rate for the caps .. If you have their part #s you should be able to look up and see what would be reasonable to expect over __ Hours.

Originally Posted by Ecky

3) The battery is draining the caps.

If there is a small internal short in a cell , they could be self discharging faster than the caps .. thus your #3 .. but .. generally speaking caps usually have much much higher rates of self discharge than batteries do .. especially Li Family batteries without small internal shorts.

Originally Posted by Ecky

5) Temperature change is causing voltage changes.

Yes , we would expect this to happen .. but it's kind of indirect.

a temperature change .. can effect the values of components like the resistors .. and the SoC of the sitting battery .. etc .. so , yes a change in voltage can cause effects that , will in turn cause changes in voltage / current levels.

Originally Posted by Ecky

I'm quite surprised a 20Ah battery emptied out over a 36 hour period.

20ah / 36 hours = ~555 mA rate.
But .. keep in mind a 20Ah battery isn't a 20Ah battery when it gets very cold .. if .. because of how cold it was .. it was effectively only a ~15Ah battery .. than 15/36 = ~416mA rate .. and if cold enough maybe it was only effectively a ~10Ah battery / 36 hours = ~277mA rate.

For example .. even brand new Grade A strong A123 LiFePO4 pouch cells that test as a true 20Ah at +25C temps , can drop to being effectively only an ~11Ah cell by the time they are -20C temps , and at the same time it's internal resistance can go up 6-7 times higher than it was at +25C temps.

[Ecky]

Prior measurements show parasitic draw going from capacitors to the car is 25ma. I have not yet measured between caps and battery.

These are the caps:

https://www.mouser.com/ProductDetail...hgVZbUDv3h0%3d

Max leakage is rated at 5ma. It should be lower than that when they aren't full?

The caps and batteries both came with pre-installed balancing resistors/LEDs. I have no idea how aggressive these are.

Will need to wait for the weekend to do much diagnostic, I get home after dark most days.

[redpoint5]

Yes, caps have way less bleed as they fall below max rated voltage.

[TexasCotton]

Okay
I am considering this application on a 08 prius any ideas /suggestions on diy install

[Ecky]

Originally Posted by TexasCotton

Okay
I am considering this application on a 08 prius any ideas /suggestions on diy install

08 Prius doesn't start from the small battery, so capacitors aren't necessary or even desirable. A small lithium battery would be more appropriate, preferably from a safe chemistry (LTO, LiFePO4). I would say minimum 10Ah.

EDIT: This is what I have in my car:

https://www.batteryspace.com/lifepo4...assed-dgr.aspx

There is a cheaper version which lacks a balance circuit. You may be fine without it.
https://www.batteryspace.com/lifepo4...assed-dgr.aspx