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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 "/myink/ViewPage.php?file=docs/Installing%20a%20Light-Weight%20Car%20Battery">Installing a Light-Weight Car Battery.) But battery technology has drastically improved ever since.

Now, there are 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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Experience

It's fun to talk about sourcing and installing these lightweight batteries. But how do they really perform 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 be 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?

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