Karl Zimmermann

You can build your own battery pack for a fraction of the cost if you can solder.  Match the battery type in your current pack(s) and you should be fine.  I went to a larger capacity battery and have had no problems.  Amazon sells in bulk. 

Here is another little tip:  Once you build or buy your new battery pack(s), let the mower charge it/them for at least a couple hours before you set the mower free on the grass.  Set a program to run for three to four hours when you can be there to monitor it.  Ours ran for about 90 minutes on one charge, then took about 35 minutes to recharge.  We also determined that one cycle was not enough to mow the lawn well, so I had to program it to run for two cycles on the days we chose to run it (twice or three times a week based on growth rate).

This is where the math comes in, but first some background.  I am using NiMH batteries which, I believe you will discover with research, do not develop run time memory like the older NiCd  batteries, BUT they do have a finite number of charge cycles.  Since I like to optimize, I have set the program time based on the combination of run and charge times.  For me that was 90 minute run time x 2 + 35 minute charge time or a total of 3 hours and 35 minutes.  I set the program time to 3 hours 45 minutes to make sure I got two full cycles.  i.e. If I had randomly programmed my run duration to be 4 hours and 30 minutes, the third run cycle would have only been 20 minutes and my battery life would have dropped from 7 to 4.5 years. (Some assumptions in there, but I hope you get the idea.)

Also note that batteries are temperature sensitive.  Charging under freezing is not usually recommended and NiMHs don't really like to be charged under 5 C according to the web.  High temps should be avoided as well.  I also unplug the batteries after fully charged, including the round motherboard battery, before I put it in winter storage.  If you store where temps go under freezing, then bring your battery packs inside for the winter.

HTH!