Craftsman Rechargeable Lawnmower battery problem

Saw a post from awhile back about a problem with a rechargeable lawnmower. No one in that thread posted a resolution to the issue, so thought I'd ask again.
I bought a Sears rechargeable lawnmower 2.5 years ago. Last summer, the lawnmower no longer would hold a charge. So I bought a new battery. Worked great for awhile, or about four months, until that one wouldn't hold a full charge. I just bought yet another battery, the second in less than 6 months. Something's not right.
One other thing: Sears no longer sells the original battery model but, instead, a replacement. Problem is, it doesn't fit. It's too tall. So I have it installed lying on its side. Is it okay to mount a lawnmower battery in a position other than upright?
Please advise if you can. This is getting expensive and ridiculous.
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