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.

LiFePO4 replacement for Lead Acid Battery

My ride on mower needed a new battery. For about half the cost of the cheapest lead acid battery I could find I purchased 4 14Ah LiFePO4 cells. Mucked around with them a bit then when I thought they were top balanced and reasonably charged tried them in the ride on mower.

Initially the mower engine must have been at the peak of compession cycle and the cells, less than fully charged, had a tough time getting the engine to crank. But once the engine was past the peak compression point the cells had no problem starting the mower.

As I mowed I kept an eye on the battery voltage. When it rose above 14v I got nervous I could be overcharging a cell since there was no balancing between cells. I shut down the mower, waited a minute then easily started it again. After stop starting a few times when the voltage rose over 14V I put the mower away.

Conclusion:

Even small LiFePO4 cells are suitable in the mower.

Some sort of voltage regulation is needed to adjust the lead acid charging regime to be suitable for LiFePO4 cells.

Next steps:

I have a programmable low voltage disconnect I can use "backwards" as a proof of concept. The big P channel mosfets will help manage battery voltage. When the battery voltage is low, the mosfet is on allowing full current in either direction. Once the charging voltage hits say 13.8V turn the mosfet off and allow the intrinsic diode to drop the voltage to the battery. 

In the meantime I am 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.

Joule Smasher Led Flasher - Second Trial Results

Results

A quick update because the previous blog regarding the interim results covered many of the findings from the second trial.

 

Despite some mishaps with battery holders and boost control failures it was the a surprise when the battery that did a lot of extra work with flashing and verification lasted the longest. Note the led was hanging off the MISO line so data transfers were always flashing it. I'll be changing this in due course.

Taking the life of the best AG3 cell to be 18.6 days I divided the time axis for the first fresh AA cell I started testing by 193 (365*10.18.6),  Compare the AG3 battery with the results to date for the AA cell I get the following:

It's early days but the AA cell is putting in a good start. Nothing so far to suggest it will not achieve 10 years of life.

Boost Control

Having dispensed with boost control I went back and modified the boost control boards to be always on. I then re-ran a trial for that board. In every instance the "Always On" version lasted longer.

Watchdog Oscillator

I learned that the watchdog oscillator was running slower than expected because of the lower voltage. In fact, it never reaches 128kHz at any temperature or voltage so I'm amazed it's called a 128kHz oscillator. All my calculations and code have been adjusted for this. 

Near Final configuration:

A 3v boost convertor with better efficiency than that used in the trials.

3ms (measured) at 9.6mA (measured) of led current.

Flash every 2.3 seconds

Overall power consumption of 52uA.

Expected Life exceeds 10 years from a AA battery.

There is still a little more hardware optimisation to be done. Something I'll be pursuing as time permits.

Housing the Joule Smasher Led Flasher

It amazes me that often the enclosure for a project costs more than the rest of the project. Here is one way of keeping the costs down: plastic centrifuge tubes. Perfect for indoor use.