charge at 12v low current.

Use the 12v charger output.
automotive and small gas engine 12v systems charge with approx 14V. 12 or less won't do it.
With an 8 v battery, the 12v charging at low setting shouldn't hurt the battery. Just watch the water level in the battery closely.
Never heard of an 8 volt rechargeable tractor battery. Interesting.
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I've used them in 6v Porsches and VWs. In this case it does provide more voltage if the voltage regulator is adjusted to charge at 9V. It doesn't make much sense to use an 8V battery if it will be charged to only 6V.
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Most automotive supply stores have them. And I just got one at Sam's Club - got my last one at Auto Zone - and one before that at NAPA. Sears had them but stopped carrying them a few years ago.
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