All-Sun BT21 Battery Tester - Voltage Reading Problem

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Joined Jan 20, 2010

I have this BT21 battery tester (I think it's brand is All-Sun.) I got for my mom as something easy to use so she can find out if her rechargeable batteries are fully charged before use. I got it at a flea market and it was brand new in package. I tested it out and it gives about 3.??v more reading than what it actually should read and the numbers after the decimal change quickly and a lot so it's difficult to find out exactly just how much.

I opened it up and found that it looks like it had water damage on the PCB back. I cleaned it with isopropel alcohol with no luck. It has a variable resistor on the back that you use a screwdriver to adjust. It doesn't adjust it much, probably about .80 volts either way. That's not enough. I could probably replace it with a small resistor or something but the solder points are just too small for my iron, I would probably bridge the points. I ran it outside of the case with the battery in parallel with a DMM I have at the same time and with the jumpiness of the number after the decimal I was unable to determine if that was even accurate but I adjusted the variable resistor on back to make it match as good as I could. At least she would be able to subtract 3 from the reading...

Has anyone else had this problem with this product and have you found a fix for it? This is not a big deal, it's more of a project for me now and if I happen to fix it, great.

Here are some photos of it.

(Pictures thanks to Cannon, Picnik, and Tinypic.)