LiFePO4, Large Wheels and H-Bridge

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Sergei Grichine

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Feb 28, 2025, 7:59:47 PM2/28/25
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Hi All,

I had interesting couple of days fixing my robot, and wanted to ask for the group's input.

My robot (Dragger) has 15" wheels and scooter 24V 350W gearbox motors. It is rather heavy. The 12V power bus with 35Ah lead-acid battery supplies plenty of power, with current varying from 3 to 7A depending on slope and surface. An old motor controller was used and worked fine for years.

All was good until the battery died (after just a year of use) and so I put a 50Ah LiFePO4 battery to work as a replacement. It worked when the battery was half-charged, but when I topped it to full - the controller gave up the ghost (no visible grey smoke, sorry). Well, I had an identical spare controller - it lasted for a couple minutes.

I had to get to work - made a much beefier controller out of two IBT-2 H-Bridge modules. While the wheels worked fine, the main V/A meter burned, and the replacement showed voltage spikes reaching 20 volts (probably, much more, as the old meter was rated to 30 volts).

The effect was expected - brushed motors generate power when left idle, and the PID controller does let it happen. The old lead-acid battery provided constant resistance, taking the charge, so the voltage spiked, maybe, within a volt or two. But the LiFe's BMI, seeing the spike above 14.6V, promptly disconnected the battery, leaving the robot's 12V bus "feathered" - at the mercy of the spikes.

I solved the problem by putting a 30A 50V diode between the main 12V bus and the H-Bridge supply line, and adding a couple of 4700 mkF capacitors to both sides. That works, but there might be a better way of dealing with it. I'd assume that the MOSFET chips used in the IBT-2 modules (BTS7960) already have all imaginable protective circuits, and can't figure out a better way in my surrounding circuitry.

Any thoughts, ideas? 

Take it as a cautionary tale. LiFePO4 batteries are fantastic, but might cause side-effects.

Here is more info:





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Best Regards,
-- Sergei
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