Automatic cutoff for cheap 12v battery charger

I've been given a 12v battery charger that doesn't have a cutoff on the boost mode, so there is a risk of boiling the battery if it gets left on. I was wondering if this would be very difficult or expensive to set up? I'd assume that it would be current dependent as a voltmeter would be testing the charge voltage as opposed to the battery voltage.

I'm an industrially trained sparkie with little to no experience with electronics (apart from replacing blown components on PCBs)

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No, a lead-acid battery charger is a voltage mode device. It will current limit at some level of discharge but then it will switch to voltage mode BUT the terminal voltage has to be temperature compensated.

It's probably not worth modifying, unless you want to rip its guts out and use some of the components building a better charger. If your problem is leaving it on, perhaps a timer is the best/simplest solution.

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I have seen after market regulators on some of the DIY sources. Basically it was a zener based voltage standard set somewhere around

14+v (adjustable) and a comparator that shuts down the charger when you exceed that.. An LM417 would work too. Basically you manually start the charger, reset the circuit and this circuit shuts it down.
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