12v battery charging question (lead-acid vs. lithium)

[California98Civic]

Originally Posted by oil pan 4

A normal truck sized lead acid battery soaks up about 2 amps to float it from full charge 12.8v resting voltage to 14v.

It's actually 20°F. You don't want to charge LiFePO4 batteries when their below 20°F. Freeze they should be fine.

My LiFePO4 batteries have lasted so long because they are not in the engine compartment.

I've been planning to relocate the battery to the former spare tire well in the trunk. It doesn't look like these lithium batteries can take the high heat of an engine compartment, from what I'm reading and what you all are saying.

I am in Coastal Southern Cali... It almost never ever freezes here, except high in the mountains. 20*F has quite probably not happened at my garage location in 20,000 years! Lol

[oil pan 4]

The leaf LiFePO4 batteries were heat damaged by AZ summer heat, which isn't any where near underhood level.

[redpoint5]

I'm running this in my Prius with no balancing.

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

There are coupon codes for 5% off to basically give you free shipping. Run 2 in parallel if you need more capacity. The code I found is E7SEA

Originally Posted by oil pan 4

It's actually 20°F. You don't want to charge LiFePO4 batteries when their below 20°F. Freeze they should be fine.

My LiFePO4 batteries have lasted so long because they are not in the engine compartment.

Everything I've read says not to charge below freezing. I replaced the Prius battery with a LiFePO4. The stock battery compartment is inside the cabin, so that helps it to stay warm, or cool depending on the weather. With our 26 degree days, I'm guessing the battery is getting near freezing. No issues in the past 2 years.

[Stubby79]

Originally Posted by oil pan 4

It's actually 20°F. You don't want to charge LiFePO4 batteries when their below 20°F. Freeze they should be fine.

20f isn't that far below freezing, IMO.

The data sheet for my LiFePO4 cells shows a charging range of 0°c-30°c, which prompted me do more research. After reading what I could find, I have no intentions of charging below 0°c.

It may just be that they are giving me a 6°c buffer (20°f vs 32°f), but why chance it on something this expensive?

Anyone looking for cold temperature versions, check out LiFeYPO4 cells.

[serialk11r]

Originally Posted by redpoint5

I'm running this in my Prius with no balancing.

LiFePO4 Prismatic Battery: 12.8V 20Ah (256Wh, 10C rate) (24.0) - UN38.3 Passed (DGR)

There are coupon codes for 5% off to basically give you free shipping. Run 2 in parallel if you need more capacity. The code I found is E7SEA



Everything I've read says not to charge below freezing. I replaced the Prius battery with a LiFePO4. The stock battery compartment is inside the cabin, so that helps it to stay warm, or cool depending on the weather. With our 26 degree days, I'm guessing the battery is getting near freezing. No issues in the past 2 years.

This is exactly the battery to get. For a car, you need the higher C cells because at 1 or 1.5 or even 2C, you'd need a massive battery to prevent the alternator from frying it. That pack at 20Ah can absorb perhaps a 60A charge (3C charge rate, pretty normal for cells optimized for power) without hurting it too badly. On older cars with 80-90A alternators, after subtracting the amps needed for the ECU/fuel pump/relays, you'd be right around there if the engine is revved a little and a bit under at idle. I think it is a good idea to idle for a bit to bring the voltage up if the car has been sitting for a few days to reduce the rate at which the battery charges. Preventing 1 dollar worth of battery damage is worth a lot of gasoline.

On a car that comes with a bigger battery/alternator/starter, I would probably put two of those together for 40Ah.

Supposedly some LiFePO4 cells can take charging at maybe 25F but not much below that, if you have cold winters I think it is smarter to run lead acid. I would certainly skip the lithium if it snows where I live.

[redpoint5]

Originally Posted by serialk11r

On a car that comes with a bigger battery/alternator/starter, I would probably put two of those together for 40Ah.

Supposedly some LiFePO4 cells can take charging at maybe 25F but not much below that, if you have cold winters I think it is smarter to run lead acid.

I wouldn't get larger/multiple batteries just to provide enough cranking amps to the starter, or to absorb large alternator currents. The solution to either too much cranking draw from the battery, or too much charging from the alternator would be to run supercaps in parallel to the battery, and placing a high watt, low ohm resistor between the battery and supercap. This would limit draw/charge on the battery, and offload big spikes to the supercap. It would also allow the battery to be moved inside the vehicle using relatively thin gauge wire. Moving the battery inside the cabin would help regulate temperature, and make it more convenient to disconnect and put on a charger if you wanted to run an alternator kill switch.

Here's a poorly done video I made explaining the battery/resistor/supercap concept.

https://www.youtube.com/watch?v=LX_o3hl2Dv4

[California98Civic]

That's cool, redpoint. I have been wondering about capacitors ever since someone posted their "boost pack" capacitor thread here. I have no experience with capacitors. But a question... a main purpose for my battery set up in charging the battery from the grid, using the deep cycle capacity for all electrical needs as I drive the car, and occasionally doing some regen braking by turning the alternator on while in deceleration fuel cut-off mode. The capacitors would probably be better than a battery for regen because they will absorb more energy from the 75 amp alt more quickly than the battery can. But the grid charging? The capacitors would be problematic maybe? Could I continue to run separate charging lines straight to the battery from my on board grid charger and recharge the battery without problems?

[serialk11r]

The problem is not the cranking amps, that 20Ah should have no trouble cranking most engines.

The problem is that parasitic draw while the car is sitting lowers the battery voltage, and causes increased charging current when the car is started. This is bad for the battery's life.

Ultracapacitors make the problem worse because they have leakage. I believe they are best paired with an undersized cheap lead acid since those are the batteries that tend to have trouble providing enough power to crank an engine.

[Ecky]

I wish I could install lithiums. Sometimes we have a day where it hit's -40... both C and F. This morning it's 8f/-13c, and it's going to be a lot colder in January and February.

Any ideas? Simply relocating it to the hatch isn't going to be enough if the car sits in the cold for 16 hours.

And, FWIW, I'd probably use these:

https://www.ev-power.eu/LiFePO4-smal...e-CE.html#tab2

4 cells = 12.8v nominal, 14.6v peak, same as the cheaper battery you were looking at. 8 cells would = the more expensive one. You'd need to put bars between them but it's also easy to take the cells apart and re purpose them.