Can’t trust battery charger

I have a small Diehard battery charger I use from time to time. Has 3 settings. 2 amp, 10 amp, and 50 amp start.
Also has switch for standard and agm batteries. It has always worked great till yesterday and today. I put a 12v
650cca battery on charge yesterday for a few hours. The charge meter said it was almost done charging and I called
it a night. Just put the charger back on it and it was charging the battery like crazy. Without the charger on the
battery is showing almost 15v. The charger is still trying to charge it. The charger is supposed to be automatic and
shut off when the battery is charged. I also tried the charger on another battery and it did the same thing. I believe
my meter is accurate. I checked the batteries in our vehicles and they were around 13v to 14v. Guess the auto shut
off on the charger isn’t working. Glad I didn’t leave it charging overnight !

Charging voltage is probably controlled by an SCR or triac, which has likely shorted and gone to semiconductor heaven.

There is smoke in that heaven.

Probable cause is shorted diode and/or wiring internally. Your trusty local starter/alternator shop guy can troubleshoot and most likely fix if he knows what he is doing.

Tim Daley(MI)

Treasure that charger. If you put a dead battery on a new charger the stupid smart device will say it's sulfated and stop charging. You have to charge it first and then put it on the new chargers. I've had to start using a battery maintainer for a charger even if it takes days to charge.

Most cheap battery chargers don't actually regulate their output. Changing the charge rate just changes which transformer taps are used, with highest voltage on the "start" range and lowest voltage on the 2 amp range. As long as you don't try to use your charger as a battery maintainer, it should be fine.

I have a practically new stupid smart charger that has 4 amp and 15 amp settings. I have set it on 4 and come back to find it jumped to 15 amps and the battery gassing. Not acceptable.

Year before last I bought 4 or 5 different brand smart chargers and ended up taking all of them back because they wouldn't work. Have an old tractor which has a short somewhere I can't find. If I don't disconnect the battery when I'm done for the day it will completely drain the battery. Even though it's a good new battery the smart charger will say it's sulfated and turn off. If I put the battery on a battery maintainer for several hours where it's up to at least 10v the smart charger will then work. I don't need double the work to charge a battery.

Tips on battery care and performance:
Here is my advice and info for my FORD TRACTORS but most applies to any battery. As the battery gets weaker, the first thing to fail is your spark. The more current you use to spin the starter, the less you have for the ignition. It doesn't really matter much if the battery is "fully charged" until you test it correctly. 4 Volts (on a 6V unit) is an almost dead battery. Bench test under load; specific gravity must be tested. Your trusty local starter/alternator shop can bench test it on their special machine, usually at no charge –no pun intended. No matter what else you do, the battery must be fully charged and maintained under load. Just because you apply a ‘battery charger’ does not mean your battery is good and will be charged. Cheap brands are prone to poor lifespans. Some units can be junk out the door. The TSC and Wally World ones are the worst. 7 years is a good run for a 6V battery. Briefly in a nutshell, you want a GP-1 6V AG battery -no Deep Cycle or RV/Golf Cart types. I suggest you invest in one of the better brands. Those would be DEKA, INTERSTATE, EAST PENN/DURACELL, or EXIDE. I'm not sure who makes the NAPA one but I've heard good things about it. Get at least a 550-750 CCA with the average cost at about $125.
A fully charged 6v battery should read about 6.3 - 6.6 volts. In order to get that charge level, your v/r must allow 7.2 volts to the battery. Specific gravity should be 1.24 - 1.28 on each cell. A hydrometer is used but your shop has the machine to test under load. Battery cables are important too. 6V cables are thick as your thumb due to current used and the ground is a braided flat strap. 12V cables like the smaller diameter gauge ones used on your car & truck can’t conduct the amps for a 6V system.
A float charger is VERY helpful; and NOT a trickle charger, but a float charger, like the DELTRAN Battery Tender Jr.® ™. It is a popular brand, available at Walmart, $30. A battery charger, even a "trickle" charger, left unattended will eventually boil out a battery. Connect the battery Tender to your battery when the tractor is idle for periods over a week. This ensures a full charge so it’s there when you need it. The thing that depletes battery life is constant charging and discharging. The loss of electrolyte weakens the plates.
FORD TRACTOR BATTERY INFO:

BATTERY TENDER JR (6V) BY DELTRAN:

Tim Daley(MI)

Regarding your meter, I have come across digital meters that will read high on the DC volt setting when the meter battery gets low.

You said you checked your other vehicles with your meter and got a reading of 13-14 volts, that sounds high to me unless that reading was taken with the engine running.

It’s analog meter and were not running. Charger was still put
more into a battery that was already overcharged !

Simple, reliable, understandable, set charge rate with screwdriver.