Originally Posted by Chas Tennis
Charging: For these D cell rechargeable batteries they were removed from the ball machine and individually charged in their normal chargers designed for maybe 2 batteries. This was inconvenient especially with just one charger.
I asked the guy who installed some lithium batteries in a Tennis Tudor and here is what he had to say
"Originally it came with a small 12V lead acid. Then after that died, I replaced with 2 pairs of 4 Trustfire Lithium size 18650 from dealextreme.com that I had on hand for high performance LED flashlights that I was using for my mountain bike night lights. Lithium rechargables have a voltage through most of their discharge cycle of 3.7V with a max of 4.2 and min of about 3.2. So with my 4 in series, I had 14.8V. I also recovered 6 from my daughter’s dead laptop computer. Laptops usually use 2 pairs of 3 in series for 11.1V nominal.
Later after you had it, I bought some plastic holders to replace the copper tubes for about $2.50 each. I also bought an 18650 charger from DX. The only pain was the charger is only 2 batteries at a time. You can buy multipack chargers, but they are expensive."
Anyone pursuing should look into the safety of charging these batteries in any way not using the manufacturer's properly designed chargers.