I have the LM2000 20" mower, purchased in May of 2016. I have (2) 2.0 AH batteries, and the CH5500 rapid charger. The batteries don't seem to last as long as they did, so was considering getting a larger battery but am confused as to what I could upgrade to. My manual says to use only the BA1120 or BA2240 battery with this mower. I don't know what the 2240 is, or if I can even use one with more capacity, like the 4.0 or 5.0 AH. They appear to be different sizes, dimensionally. What can I use with the mower and charger I currently have?