I have a Schumacher WM-2500A automatic battery charger that is reading incorrect battery voltage. When connected to the battery and plugged in the charger runs a test mode to check battery quality,and charge status. I am unsure of the specific voltages the charger starts and stops charging, however I know that a standard car battery is considered fully charged at around 13.5 v. my problem is the charger is reading the battery voltage at around 1.5 - 2 volts higher than the actual battery voltage. Assuming the charger is calibrated to stop charging at 13.5 v, if I were to connect a battery with standing volts at 11.5 - 12v the charger sees the battery @ fully charged and shuts off. I have opened the case and looked for any kind of adjustment to calibrate the input voltage with no success. The charger worked fine when new,and is out of warranty so thats out. I paid almost $100.00 for the charger, and only used it occasionally, When connected to a very low(say 10 v) the charger will come on and charge but,will shut off at what it thinks is 13.5v, leaving me a partial charge. Any advice?

 

maybe a bad or open resitor near the voltage comparator(maybe LM339 or LM311)
hard to tell but might be easier if you had the schematics

 

I appreciate your response, however, obtaining the schematics from Schumacher will probably not happen. They don't even have a Q&A section at their web site. I was hoping that they had added a calibration mode that was accessible by pushing buttons in a special sequence or something similar. I am in the early stages of teaching myself electronics, having basic knowledge at the moment, so I wouldn't know where to start looking for the items you mentioned, and what to do when I found them! Thanks again!

 

if you could post a picture of the circuit board maybe that might help

 

With this same charger and battery, did it show a higher voltage in the past?

Have you checked the actual battery voltage with a DMM to rule out the Charger's display being defective, although the actual charging/cutoff is still functional?

The reason I ask is I have two Schumacher SE 40225 chargers here that still function, but the display isn't correct. I haven't yet gotten around to tearing them apart to fix them yet, since they still work. It may be the same problem.