Old automotive battery charger question

[23hotrodr]

I have an approx. 40 year old 12/6 volt automotive battery charger. When set for 12 volts and not connected to a battery, it reads 12.81 volts. Set to 6 volts and not connected to a battery, it reads 8.1 volts. When I connect to a 12 volt battery, and set for 12 volts and battery fairly well charged I read about 16 volts and the battery starts boiling right away.

Does this seem normal? Should the difference of charger output boltage be that much different when connect to a battery?

Thanks for any info you can provide. -- Mick

[bgum]

I'll venture to say that I have the same charger with possibly a different name. Bought and used to charge boat batteries.

[fourthclassC]

Agree 100% I gotta keep an old "dumb" charger around for batteries that are to depleted for my modern smart charger to work.

[Gdetrailer]

YES, that is all normal for the old school battery chargers.

Those old chargers are nothing more than a transformer and a rectifier (diode) with no filtering or voltage regulation. Because they are not filtered or regulated the voltage without a battery will read low (8V DC) and when a load like a battery is connected it will read much higher than normal battery voltage depending on the discharge state of the battery.

The battery acts like a filter capacitor smoothing out the choppy DC voltage from the rectifier an also regulates the voltage to a certain extent.

When you connect a very discharged battery the charger voltage will drop considerably since it can only supply a limited amount of current and when the battery is fully charged the charger voltage will be much higher than normal battery voltage since the battery will be drawing very little amount of current from the charger.

These old chargers are GOOD to have in an emergency backup situation but not good for say 24/7/365 always on situation. The old chargers will boil a battery if you were to leave it connected and turned on for too long.

I keep one of the old ones laying around just in case I run into a very flat battery, newer standalone chargers are "smart" chargers and are designed to not output unless a battery is attached which has enough voltage to trigger the charger on.. Typically 9V or so is required to turn on a smart charger.

If you accidentally ran down your car battery low enough that the lights don't light, a smart charger will not turn on to a charge the battery! That is where the old school chargers shine!