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Use three Gates Cyclon 2.5 A-H ("D" size) cells.
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Powersonic and several other people make direct replacements for the old Panasonic battery. It is a fairly common battery used in a lot of medical and other equipment. Replacements should cost less than $15.
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I have replaced these. You can get almost direct replacements online. Do a search on "lead acid" batteries and look for a cross reference.
If you can't find the replacement, find someone selling a good supply of lead acid and ask them.
I got mine that way.
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Send them an email with the battery number you have. Get the dimensions of the battery and try to find a match.
http://www.batterycountry.com/ShopSite/sealed-lead-acid.html