Mike,I need a new battery for my 71 Triumph. The bike only runs a head and tail light.
I want to use a sealed maintenance free AGM unit but not sure what size. I found two that should work YTZ-7S and YTX7L-BS. Both will fit but are only 6 amps. Is that enough or too small, will the bike over charge them?
As long as they fit without shorting out the posts on the seat pan and they are 12V, they should work fine. The size of the battery and the amperage rating has nothing to do with your bike 'overcharging' it. You don't need much current from the battery on your bike if it only runs the ignition, headlight and tail light. If your charging system is in order, it should supply current to the battery as long as the voltage is under a given threshold. Once the batter reaches it's design voltage, the charging system should stop providing current to it. Your bike should have an Ammeter in the headlight shell. Upon startup it should show a positive rate of current then taper off after running for a few minutes. If it constantly charges, you either have something drawing too much power or your regulator is faulty. If you haven't changed out the stock regulator, the modern solid-state replacements are a very good improvement.