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[redpoint5]

3x of these would put you at 48 Ah at $200, and 15v.

https://hobbyking.com/en_us/multista...lipo-pack.html

[EcoCivic]

Originally Posted by redpoint5

3x of these would put you at 48 Ah at $200, and 15v.

https://hobbyking.com/en_us/multista...lipo-pack.html

Well that's pretty cool, thanks! I don't know that much about lithium batteries, so these concerns may be invalid. However, I have a couple of concerns about using those batteries:
1. I am not sure how well they would handle heat. Since those are RC batteries, they probably don't need to be able to handle that much heat. However, my battery is installed in the trunk, and I am not sure how hot the trunk gets on a hot day, but I'm guessing pretty freaking hot.

2. How would those batteries be managed? From my understanding, lithium batteries need a fancy management system to protect the battery and operate safely. From my understanding, these BMS systems limit charge and discharge rates and provide overcharge and overdischarge protection. I am not sure what kind of battery management systems are available, but a BMS that could handle the current draw of the starter would probably be very expensive!

3. How long would these batteries last? So far my Optima battery is 11 years old and still works fine, which is pretty good IMO. How long would these lithium ion batteries last when used in a car? Could they match that kind of longevity?

4. You said these batteries can't be charged below freezing. My car gets parked in a garage overnight and stays above freezing, but what if I am not home and I need to park it outside overnight for whatever reason in the middle of the winter? Would the batteries self destruct or explode or something once I start the car and the alternator attempts to recharge them?

[redpoint5]

Good questions. I intend to test myself on a cheap 5 Ah battery sometime in the future.

I've got a LiFePO4 battery in my Prius, but it lives in the cabin so it isn't subjected to very extreme temperatures. It's been running great for over 2 years with no maintenance at all.

[Ecky]

I'm using a LiFePO4 battery in my Insight, which has been converted to a conventional gasoline car. Mine lives in the cabin also, and Vermont's summer weather is far milder than Tennessee's. I have supercapacitors to handle starting current and subsequent charging during winter and though I haven't done a load test, so far so good, seems I haven't killed it.

https://www.batteryspace.com/li-ion-...lead-acid.aspx

Different chemistries have different characteristics. BatteryUniversity says that thermal runaway on LiFePO4 is ~270C (~520F), though I'm inclined to believe that you wouldn't want to be anywhere near that for longevity.

https://batteryuniversity.com/learn/...of_lithium_ion

I believe Lithium Titanate would be the safest choice if you could find some cells for a reasonable price - NASA uses them in their spacecraft, and they handle the temperature swings found in space.

[EcoCivic]

Originally Posted by redpoint5

Good questions. I intend to test myself on a cheap 5 Ah battery sometime in the future.

I've got a LiFePO4 battery in my Prius, but it lives in the cabin so it isn't subjected to very extreme temperatures. It's been running great for over 2 years with no maintenance at all.

Cool, thanks! Just PLEASE be careful while you are experimenting! These batteries can blow up very easily if they are overcharged, discharged too quickly, overcharged, charged too quickly, or shorted out, especially if they have no protective circuit. I am not sure, but for all I know maybe the draw of the starter would be too much for a little battery like that. The alternator could probably charge it way too quickly as well. So have fun, but please be safe

[EcoCivic]

Originally Posted by Ecky

I'm using a LiFePO4 battery in my Insight, which has been converted to a conventional gasoline car. Mine lives in the cabin also, and Vermont's summer weather is far milder than Tennessee's. I have supercapacitors to handle starting current and subsequent charging during winter and though I haven't done a load test, so far so good, seems I haven't killed it.

https://www.batteryspace.com/li-ion-...lead-acid.aspx

Different chemistries have different characteristics. BatteryUniversity says that thermal runaway on LiFePO4 is ~270C (~520F), though I'm inclined to believe that you wouldn't want to be anywhere near that for longevity.

https://batteryuniversity.com/learn/...of_lithium_ion

I believe Lithium Titanate would be the safest choice if you could find some cells for a reasonable price - NASA uses them in their spacecraft, and they handle the temperature swings found in space.

Awesome, thanks! Out of curiosity, does your battery pack have any type of BMS?

[Piotrsko]

Originally Posted by Ecky

I believe Lithium Titanate would be the safest choice if you could find some cells for a reasonable price - NASA uses them in their spacecraft, and they handle the temperature swings found in space.

Actually both of my sattelites had internal temps averaging 60 degrees f, ditto for all the other stuff I had the need to know on. The military prefers passive energy management, saves thruster fuel, Nasa does too.

[Ecky]

Originally Posted by EcoCivic

Awesome, thanks! Out of curiosity, does your battery pack have any type of BMS?

There are LED balancers between the cells. I doubt anything more is needed. The capacitors + battery can easily handle any current I might throw at them, either in or out, at any temperature the car might reasonably see, just need to keep the cells in balance.

[EcoCivic]

Originally Posted by Ecky

There are LED balancers between the cells. I doubt anything more is needed. The capacitors + battery can easily handle any current I might throw at them, either in or out, at any temperature the car might reasonably see, just need to keep the cells in balance.

Awesome, thanks! So just out of curiosity, what would happen if you somehow drain the battery completely (like by leaving the lights on or something) and then jump it and go for a drive? I can't imagine that the batteries would be too happy about the extremely fast charge rate, but I am not sure how they would fail, if at all. Would the batteries blow up or something?

[Ecky]

Originally Posted by EcoCivic

Awesome, thanks! So just out of curiosity, what would happen if you somehow drain the battery completely (like by leaving the lights on or something) and then jump it and go for a drive? I can't imagine that the batteries would be too happy about the extremely fast charge rate, but I am not sure how they would fail, if at all. Would the batteries blow up or something?

LiFePO4 would not explode. That chemistry doesn't melt down, it tends to just... fail. Loses capacity, loses cranking amps, etc.

I've run it down a few times in winter and jumped it, before I figured my wiring out. On the truly cold days I brought it inside to warm up before charging but it *has* been charged, probably slowly, a little below freezing. Couldn't tell you exactly what goes on having supercapacitors in parallel but my educated guess is that the battery's internal resistance must limit charge rate somewhat, and the caps handle transient loads.