Frequently Asked Questions

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Below are some questions that come up regularly. If you have more to add, feel free to contact me.

Charging

With lithium batteries, should you leave the charger plugged in after the battery is charged?

If your charger does a float charge, like the SC-48 charger, it will charge your lithium battery, but must be unplugged when the battery is finished charging because it will continue to top off the battery. This is good for lead acid, but not good for LiFePO4. You probably want to consider a different charger.

If you're using a charger that does not do a float charge, such as the OP48RM or P3648, then the charger will shut down when it's finished. You don't have to unplug the charger, but there is a benefit. The main benefit of unplugging is it protects your mower from lightning strikes. There have been several Ryobi mower owners who had some parts get fried when lightning struck nearby.

What I do is plug the mower in after I finish mowing. Then, the next time I'm in the garage and notice it's finished, I unplug the charger from the mower. I usually leave the charger plugged into the wall all the time.

Take a look at charge profiles and find out about different chargers on the Charger Options page.

Can I use my stock charger after converting to lithium?

There are two different stock chargers. If you have the Ryobi OP48RM charger, it will work fine with your new battery. No need to get a new charger.

If you have the Ryobi SC-48 charger, it will charge the battery, but will continue to top off the battery indefinitely. This is not good for LiFePO4 batteries, so it should be unplugged when charging is finished. For this reason, it's recommended to replace it with an aftermarket charger.

For more information on stock and aftermarket chargers, see the Charger Options page.

What amperage should I use when charging my LiFePO4 battery?

In general, a lower amperage is better for batteries because it reduces dendrite formation. Dentrites are needle-like growths of lithium that can eventually lead to battery failure. I recommend charging at 0.1C, or 10% of the battery's capacity. This means charging at 5 Amps for a 50Ah battery or 10A for a 100Ah battery. If it's a little higher or lower, that's fine. You can always charge at a higher rate if you're in a hurry. Occasionally charging at a higher rate is not going to have a huge impact on battery life.

For more information on stock and aftermarket chargers, see the Charger Options page.

Part Failures

Why do controllers fail after a battery upgrade?

A common scenario is someone does an upgrade to LiFePO4, then immediately, or shortly after, finds a controller has failed. There are three to five controllers, one per motor, depending on the model. Sometimes the failure is a drive controller, sometimes a blade motor controller. This has nothing to do with the new battery. There are three main reasons this happens.

The first is the failure happened before the upgrade, but was masked by other symptoms. It's difficult to diagnose a failure without proper power.

The second is the upgrade was delayed, causing the mower to operate at a low voltage for an extended period of time. The blades will cut off if the voltage drops below 48V, but the drive motor will continue to operate so you can limp back to charge. The lower the voltage, the higher the current. The higher the current, the more heat generated. The higher current and undervoltage can cause stress on electrical components leading them to fail prematurely.

But I think the third reason is the most common. Unless you use a stock charger or modify an aftermarket charger, you loose charge lockout when upgrading. Charge lockout prevents the main contactor from operating when a charge plug is in the charge port. This keeps you from driving away while the mower is charging and protects the mower's electronics from charge voltage. While I generally recommend charging at 0.1C or 10% of the battery's capacity, many chargers put out more current than this. At the peak of the charge cycle, an 18A charger will be pushing a full 18 Amps at 58.4 or more Volts into the battery. If the mower is turned on at this point, that current is pushed to the capacitors in the controllers because the resistance of the capacitors is lower than the resistance of the batteries. This can overload the capacitors and cause them to fail.

So what can you do? I generally recommend using the stock OP48RM charger if you have one. This lets you retain charge lockout and saves you from having to buy a new charger. If you have the old stock SC-48 charger, it continues top off the batteries indefinitely and should be replaced. Some aftermarket chargers can be modified to enable charge lockout. You can find an example here. If you can't do either of these, be very careful about turning on your mower when the charger is attached.

For more information on stock and aftermarket chargers, see the Charger Options page.

Storage

What should I do with my battery durring the winter?

Most LiFePo4 batteries are rated for use down to -4 °F (-20 °C) and can only be charged above 32 °F (0 °C). A good BMS has multiple temperature sensors and will prevent you from operating or charging the battery below these thresholds. An unused LiFePO4 battery can discharge up to 2% per month. If the battery discharges enough, it will may go into protection mode.

Keep in mind , even an unheated shed or garage  will be warmer that the outside temperature.  In my case, my garage is often 10 - 20 degrees warmer than the outside temperature, and, due to a stream near my house, the temperature at my house is often different than what is reported for my area. I use Switchbot sensors to monitor my garage and outside temperature. I got the SwitchBot WiFi Thermometer Hygrometer 3 Pack with Hub Mini and put a sensor in garage, outside, and in my basement. This lets me set alerts in case the temperature gets too low.

If you're not planning to use your mower during the winter, it's best to leave it charged to 50% - 70% and disconnect the Anderson connector that connects it to the mower. If the battery has a power switch, you should make sure it's off. If the temperature where you're storing your mower will often be below 32 °F, it's best to remove the battery and store it inside. Reconnect and charge the battery to 100% before using again in the spring.

If you are planning to use your mower during the winter for plowing snow or hauling things, then be sure to charge when the battery is above 32 °F and do not operate it if the battery is below -4 °F. If you need to store your mower in an unconditioned space and live somewhere with very cold winters, you may want to look at battery heaters or a self-heated battery.