GearHack: Revision: Installing a Lightweight LiFePO4 Battery in a Car

Older Revision of Installing a Lightweight LiFePO4 Battery in a Car

Revision: 20241203_173534 | All Revisions

Installing a Lightweight LiFePO4 Battery in a Car

Quite a while back, 16 years ago, we discovered that we can install a lightweight sealed lead acid (SLA) battery in our car for daily driving and racing purposes. (See our "Installing a Light-Weight Car Battery" article.) But battery technology has drastically improved ever since.

Now, there are rechargeable LiFePO4 batteries in the same formfactor as the SLA batteries. Naturally, we wondered whether we can replace the light SLA battery with an even lighter LiFePo4 battery. Compared to an SLA battery, a LiFePO4 battery has the following advantages:

While the 12v 16A SLA battery weighs around 11 to 12 lbs, a 12V 20A LiFePO4 battery weighs around 6 lbs. That's half the weight of a SLA battery, providing further weight reduction in your race car.
LiFePO4 allows 100% deep discharge, while a SLA battery only allows 60% deep discharge.
The temperature range of a LiFePO4 is -20 to 60 degrees C, compared to -5 to 50 degrees C of a SLA battery.
LiFePO4 battery allows 4000 recharge cycles, while SLA only allows 500 recharge cycles.
SLA batteries are rated for 3-5 years of service life, while LiFePO4 is rated for 10 years of service life.

A 12V 20Ah LiFePO4 battery is currently around $50. Given the advantage above, the cost is relatively half of an equivalent SLA battery.

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Will It Work?
It's fun to talk about sourcing and installing these lightweight batteries. But how do they really perform on a car in the real world? The LiFePO4 battery is new to the scene, so we have not tried it yet. But it's a new battery technology we are ready to test in our car next. Since the 2021 Subaru WRX STi drains the battery more than the 2001 Subaru Impreza 2.5 RS, we are thinking of going with either the 12V 20Ah LiFePO4 battery and/or the 12V 30Ah LiFePO4 battery to determine their usability.

One main question with regard to whether the LiFePO4 battery will work is how does the 20A battery management system (BMS) work with the car alternator? The car alternator current is known to be unregulated and can charge the battery at over 100. Will the 20A BMS shut down and automatically turn back on when the alternator current decreases to tolerable range? Or will the 20A BMS be able to regulate a high current charge to 20A? Plus, starting a vehicle generally requires between 250 and 600 cold cranking amps. Will the LiFePO4 battery be able to provide that much power at once?

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Wattcycle 12V 20Ah LiFePO4 Battery

In order to test our thought, we purchased a Wattcycle 12V 20Ah LiFePO4 battery during Amazon Black Friday sale. This battery is roughly the same size as the 12V 18Ah SLA battery we used in our 2001 Subaru Impreza 2.5 RS for the past 16 years. And it only weighs 5 lbs 11 oz. (see photo below), which is roughly half the weight of an equivalent SLA battery!

Older Revision of Installing a Lightweight LiFePO4 Battery in a Car

Installing a Lightweight LiFePO4 Battery in a Car

Wattcycle 12V 20Ah LiFePO4 Battery

Converting to Car Battery Post

Mounting in Your Vehicle

Experience

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