First please note the cheapest way to start is to check if your mower blades are damaged. This may cause the battery to drain faster and be the real cause of excessive battery drain. I ordered a “2 Pack – USA Mower Blades® 18-1/2″ x 1-3/4″ x 0.152, 1″ Center Hole Residential fits Black & Decker® CM1936 CM1936ZF2 SPCM1936 90541433-01″ off of ebay for $17.99. My blade needed to be replace so I figured this would be better to try first before spending money on a battery. In my case the blades were not the issue.
By this point you are probably fed up with the Black & Decker battery powered 36v mower. It is no longer cutting all of your crabgrass like it used to. Plugging in between the front and back lawn over two days isn’t the most efficient way to mow. You could buy battery complete for around $130 or you can spend a little time and take apart your battery you have now and just replace the 3 batteries that are inside the case.
The batteries that were inside my battery case were 3 Long WP12-12 12v 12ah batteries. I ended up buying a set of 3 Casil CA12120 12V 12AH batteries off of ebay for $60 total that were an exact match for the old batteries. Proceed at your own risk from this point on.
To open the battery case you only need to use a standard screw driver and remove all the screws from the bottom.
Now the hard part is when you separate the two halves of the case be sure to note what cables go where or you will have a bad time getting this back together and could possibly pinch a wire in the process. Some of the wires sit in slits to keep the wires from being pinched by the heavy batteries. You may need to you a needle nose or screw driver to remove the plug from the terminals. The batteries with be taped together.
Copy the wiring and battery configuration with the new batteries. Be sure to tape the batteries together to minimized battery movement. Once everything is in place screw the case back together and give it a try. You may have to do a complete charge before you use it to see battery life similar to the original battery life.