Solar Charging

If you simply connect your solar panels directly to the battery, you will over charge it and ruin it.  12 Volt solar panels can produce over 22 Volts in direct sun and this is too much.  Also when a battery reaches full charge the charging current needs to be removed or reduced to a trickle to protect the battery.  That is what a Charge Controller does.  It monitors the battery voltage and limits the solar voltage and current as needed to safely charge the battery.  Once the battery reaches full charge, then a trickle charge is used to maintain the state of charge.

I had thought that 40 Watts would not be enough, but have found in practice that it is enough to re-charge the battery in 1 or 2 days of full sun after 20-30 minutes of use.  So the mower is always ready to go by the time I need it.

You will need enough power from the panels to charge your mower before the grass gets too long.  There are a lot of variables to consider if you are using different parts than I did:

• Your average sun hours per day (solar insolation)

• Size of your battery

• Current draw of your motor

• Frequency and duration of use of the mower

I happen to have 40 Watts of panels on my workshop roof that I use to run my solar heating system pumps in the winter months, so I can switch them over to charging duty in the summer.  In the overlap months where I need both heating and charging I just use a battery charger (see below).  Since we have opted to get our electricity from wind power it is clean power by default.  If you have the option of selecting your electric supply then consider switching to green sourced electricity.  We also have a 4kW solar array that produces enough power to spin the meter backwards on sunny days - so even using a plug-in charger is "solar powered" in our home.

If you get an average of 4-6 clear sun hours per day and use the mower every few days, I would suggest starting with 40 Watts or so and adding from there as needed.  There are many complex and confusing formulas for calculating the required panel size for any solar need, but I favor the empirical method, blended with some rule of thumb guess work to get started!

The other way to charge the mower that I use on cloudy days is a good old fashioned automobile battery charger.  Mine is an automatic unit that is relatively inexpensive.  You can find them at Sears or auto parts stores.  My 6 Amp unit will recharge the mower in 3-6 hours depending on how depleted the battery is.