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My question is why the original battery, which was rated at 17 Amp-hours, lasted twice as long as this second battery that is rated at 22 Amp-hours. The two batteries are the same physical size, externally -- so it's not the case that the 22 A-hr battery is simply a "scaled up" larger version of the 17 A-hr design.

Some other details...

17 A-hr battery: lasted 3 years, at which time I

*thought*it had died, which turned out to be wrong. One of those years, I was mowing a friend's lawn as well as my own, so it's pretty reasonable to say this battery lasted the equivalent of 4 years and was still in working order.

22 A-hr battery: has lasted 2 years, and is noticeably degraded (as I mentioned earlier)

Also, the two batteries are different brands, which might be an explanation for the different lifetimes. However I am wondering if there is some fundamental design difference that would allow a 22 A-hr battery to be the same size as a 17 A-hr one, but also would result in shortened lifetime. Mainly I am thinking about the size (area or thickness) of the electrodes, or the total number of electrodes per cell being different -- or something along those lines.

Thanks in advance for any insights.