1967 BSA Spitfire Flat Battery

A reader needs advice in regards to upgrades to his charging system, which keeps flattening the battery when the headlights are on.

Reader Contribution by James Wootton
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by Keith Fellenstein

Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Q: I have a 1967 BSA Spitfire. I’m continually having problems with the battery, as using the headlight seems to be flattening the battery. What upgrades do you advise?
James Wootton/via email

A: Since daylight headlights were not required when your bike was built, the charging system was set up to keep the battery well charged with the headlight off, and slightly discharging with it on, on the assumption that most of the running would be in daylight hours and so the battery would be maintained. First of course is to make sure your battery is in good condition, and if it isn’t, replace it with a good battery. I prefer AGM batteries for my classics for their superior energy density and long life. An added bonus is you never have to worry about acid leaks damaging paint or chrome. There were two different alternators used, a two-wire and a three-wire. If you have the three-wire alternator, make sure the green/black and green/yellow wires are joined together to the green/yellow wire in the wire loom. That provides the maximum power output from the alternator. As usual, there are a number of ways to avoid flattening the battery and/or getting more power out of the alternator. You could switch the headlight and taillight bulbs from incandescent bulbs to LEDs, being careful to source LEDs that are made for positive ground. That’s probably the fastest and least expensive method, but may not completely fix the problem. Slightly more expensive would be replacing the two-part rectifier and zener diode regulator with a one-piece combination rectifier/regulator, which may provide a more efficient conversion of AC to DC, but again may not completely fix the problem. The most expensive method would definitely fix the problem, and would consist of replacing the rotor with a new and presumably more magnetic rotor, the single phase stator with a three-phase stator, and the regulator/rectifier with a three-phase version. That would certainly provide enough output to keep the battery charged and the lights on.

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