LiFePO4 replacement for Ride On Mower Lead Acid Battery - Progress

I tried using my programmable low voltage disconnect "backwards" as a proof of concept.

Expectation versus Outcome:

The big P channel mosfets will help manage battery voltage - Yes, but more work needed.

When the battery voltage is low, the mosfet is on allowing full current in either direction. - Yes, but it took a few goes to be happy with the on and off settings.

Once the charging voltage hits say 13.8V turn the mosfet off and allow the intrinsic diode to drop the voltage to the battery. - No. While I settled for a lower off voltage of 13.3V, or 80% charge, the intrinsic diode had a much higher voltage drop than 0.7V. It was several volts and I was way off on this point.

 

Next steps:

I'm going to fit a silicon diode in parallel with the intrinsic diode. Since the mosfet turns off at 13.3V (80% charge) I am less concerned with reaching 100% charge and more troubled by keeping the alternator voltage low enough to stop the mower trying to shut down due to a high voltage. With a silicon diode the alternator voltage can rise as high as 15V before I have the 100% state of charge voltage of 14.4V. This will give me some peace of mind that I'm not damaging anything.

From measuring voltages and currents while the mover was running at working rpm  the charging current once the mosfet turns off will only be a few amps so no need for a massive power diode. A 10 amp diode would be overkill.

Now that rain has set in for a few days I will get myself ready for another mowing onslaught in a few weeks.

In the meantime I will continue waiting for some larger cells to arrive to help overcome the peak compression challenge. And a charge shuffling balancer to ensure that when I go to mow the lawn all the cells are at the same voltage or state of charge.