Charging an 8 Volt tractor battery

Most old tractors use 6 volt batteries. It is common practice to stick 8 volt batteries in them to give them a little more zap. No changes are necessary - their systems can handle it.
My problem is how to charge an 8 volt battery when it has been sitting for a while and has been worn down. I have three battery chargers - a 12V 10-30-50 amp, a 12V 2 or 10 amp, and a 6 or 12 volt 4 amp.
I went to a local battery plave and the guy told me to use 12 volt low amperage. Thoughts ?
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I would charge it on 6V. Did you adjust the tractor's voltage regulator to charge the battery at the higher voltage?
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I am told that most 6 volt regulators charge at 8 volts.
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That is true. An 8V battery needs a 9V charge, and a 12V battery needs 14V. Your 8V battery will not be fully charged on 8V and will be not much more powerfull than a 6V battery.
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Be that as it may, they turn the motor over faster and last longer.
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