Installing a Lightweight LiFePO4 Battery in a Car

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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 "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 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 100A. Will the BMS shut down and automatically turn back on when the alternator current decreases to tolerable range? Or will the BMS be able to regulate a high current charge to the charging current it needs? Plus, starting a vehicle generally requires between 200 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!

Placing the Wattcycle LiFePO4 battery behind the Uplus SLA battery in the photo below, we see that the LiFePO4 is roughly the same size as the SLA battery. It's taller by approximately an inch. But the terminal posts are actually about the same height, because the SLA is using angle brackets.

Both batteries are shorter than full size car batteries, so there is no problem fitting either batteries in the engine compartment.

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Converting to Car Battery Post

The Wattcycle LiFePO4 batter comes with M6 screw posts on top of the battery. So you simply need to get a pair of battery terminals that can screw onto the M6 posts. The part you will need is the following:

Ampper M6 Thread Male Brass Battery Terminal Charging Posts

You can source these parts in the "Related Links" below. You only need one set of these terminals for each car, because they can be re-used on each replacement battery. With these parts, all you have to do is screw them into the battery terminals of the battery (see photo below) and you are good to go.

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Mounting in Your Vehicle

Cross-Reference
Before you remove your old battery, if it hasn't died yet, you might want to save your car's computer data and settings. See our "Preserving Your Vehicle's Computer Memory" article.

Cross-Reference
We've provided a thorough explanation of how to mount a small SLA battery into the battery compartment of a vehicle in our "Installing a Light-Weight Car Battery" article. The LiFePO4 battery we covered in this article is similar in size as the SLA battery, therefore, the instruction in that article should be followed for this purpose as well.

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Experience

We were quite excited to put in the Wattcycle LiFePO4 battery into the 2021 Subaru WRX STi. We opened the door and all the expected lights came on. We sat in the car, stepped on the clutch, and pressed the "Start" button. And guess what? Everything turned off. The dash went completely dark.

Looks like the 8A BMS cut the power out when the starter attempted to draw enough power to start the car. That's unfortunate. We were hoping the BMS would allow a large current to be drawn from the battery. LiFePO4 batteries can definitely handle the current draw. It means a LiFePO4 battery with a simple BMS cannot be used as a vehicle starter battery.

The lesson learned from this experience is that you can't just use any LiFePO4 battery to start your car, unlike the SLA batteries. There are LiFePO4 batteries that are designed for starting cars. But they are about the same size as a regular car battery and cost similarly or even more so than a regular car battery. You can see some of these options in the "Related Links" section below. So when will there be a lightweight LiFePO4 battery that can start a car for $50?

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Attachments

Bittwee OBD-II Memory Saver (Small).jpg (66 KB)
Uplus LP12-18T3 vs. Wattcycle LiFePO4 Size (Small).jpg (56 KB)
Wattcycle 12V 20Ah LiFePO4 Battery (Small).jpg (21 KB)
Wattcycle LiFePO4 Weight (Small).jpg (19 KB)

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