Building a LifePO4 pack for automotive applications

Hi All,

I'm trying to wrap my head around building my own battery pack for my exocet, weight is pretty important as I'm trying to offset the race chassis + steel floors a bit.

Here's my research so far:

This post details the technical aspects.

This guy built one for his rx8.

Some more details.

So I'm looking at stringing a few of these together, but the confusion lies in the fact that they "Maximum Charge Current(continuable):50A"

From what I've read online the miata alternator peaks at much higher than that. I'm not very good with the electrical side of things, so can anyone clarify? The RX-8 thread uses batteries with the same specifications and the updates indicate that he has been using the pack for a while now.

Thanks!

In reply to accordionfolder:

I'm almost done with the schematic for a circuit that'll let a LiPo battery survive in a car!

Key thing: the car's charging system doesn't charge the battery. The battery is disconnected a few seconds after start using a relay and an $30 balance charger is turned on to charge the battery from the car's circuitry. It seems hokey to have to turn this on but until an OEM builds a million of my circuits it's much less expensive than the individual cell charger chips and wiring.

If you never run it much below freezing, and you have no key-off parasitic drain, and your alternator's voltage regulator puts out at least 14V, a LiFePO4 (Zippy 8400 4s2p) will survive in the car with an unmodified charging system. Mine did in my CRX till I left my lights on!

chaparral wrote: In reply to accordionfolder: I'm almost done with the schematic for a circuit that'll let a LiPo battery survive in a car! Key thing: the car's charging system doesn't charge the battery. The battery is disconnected a few seconds after start using a relay and an $30 balance charger is turned on to charge the battery from the car's circuitry. It seems hokey to have to turn this on but until an OEM builds a million of my circuits it's much less expensive than the individual cell charger chips and wiring. If you never run it much below freezing, and you have no key-off parasitic drain, and your alternator's voltage regulator puts out at least 14V, a LiFePO4 (Zippy 8400 4s2p) will survive in the car with an unmodified charging system. Mine did in my CRX till I left my lights on!

Awesome, thanks for the info. While researching the LiFePO4 batteries, that was the conclusion I came to, I was just confused by the specs vs the alternator output. Everyone/Everything else points to it being A-OK.

Won't disconnecting the battery while the alternator is running cause widespread electrical damage?

http://www.troubleshooters.com/dont_disconnect_battery.htm

GameboyRMH wrote: Won't disconnecting the battery while the alternator is running cause widespread electrical damage? http://www.troubleshooters.com/dont_disconnect_battery.htm

Yep! That's why the relay that disconnects the battery in my circuit hooks the alternator up to a cap/resistor bank.

After some brief reading I'm wondering what the advantage is of the LiFePO4 battery over another compact lithium ion battery?

For comparison, I'm thinking of those made by Braille for example here.

Very interesting topic BTW!

In reply to paranoid_android74:

The main advantage of LiFePO4 over most other lithium battery chemistries is that it takes more energy to ignite than is stored in the cell, so they don't burn as readily.

In reply to paranoid_android74:

As I recall, the FePO4 chemistry doesn't have a habit of exploding or catching on fire. Some have had success taking some big cheap RC pack, dropping it in the car, and balancing the cells on a proper charger every few months.

You have to watch the voltages on LiFePO4 like a hawk, a single instance of voltage dropping below 13.1v and you can totaly make it useless. I would definetly keep it on a battery tender system all the time when parked...I had a functional $250 ~4LB Shorai battery, it became an expensive paperweight after 6 months use... I don't trust them, will be going back to what I know - Hawker Odyssey PC680. Sure they are 'Heavy' in comparison (still 1/2 weight of factory battery), but they can take the heat and use in a daily driver, and they are much less expensive.

Accordionfolder I just bought an Anti Gravity 12 cell Lithium Iron battery from Racers Parts Wholesale. I looked into using it and read several threads here, on another tech oriented site, Corner carvers and others. There are a few name brand and a couple types of lightweight batteries that have worked in multiple different set ups. Some have used similar batteries in daily drivers with no issues for a few years. It seems like high compression and/or V8s are the biggest challenge with cold weather or winter use being similar of a challenge. I am in the same boat with a Race chassis Exocet with steel floors while adding a not so small driver. The 12 cell Anti gravity is around 2.4lbs and fits in the palm of the hand. I got a discount at RPW and got it with brass car terminals for under $200 shipped.

Why build a pack? The $85 hobby king 8.4ah is more than enough to crank over most engines. Just dont leave your lights on and get the voltage too low and its fine. All you do is adapter 5.5mm bullet connectors to your stock battery wiring and find a way to bolt it down, and take it out and balance charge it once a year.

In reply to BrokenYugo:

Thanks for the info guys, this was a new one on me. Ill have to do some more reading.