Parasitic Drain on Battery

I have a 1975 MkIII Commando that has a parasitic drain on the battery, albeit very small. It takes 3 - 4 weeks to completely drain the battery. The current flow is so slight that I have been unable to trace the source. I'm now keeping a Noco charger continuously hooked up to the battery to maintain the charge.

Do you still have the original charging equipment, or an aftermarket regulator? What kind of battery (lead acid, AGM, Lithium, etc.)

The 2MC capacitor if fitted or (Lucas?) rectifier would be the likely suspects. The 2MC is probably the most likely.
Have you tried removing the fuse?

You can remove the capacitor and run a jumper wire between the two live terminals. On a earlier bike those would be the brown/blue wires. Not sure on a MKIII.

htown16 said:

On a earlier bike those would be the brown/blue wires. Not sure on a MKIII.

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Yes, brown/blue.

Just like tracking down a short circuit ..... divide and conquer!

Connect a Volt Ohm meter in series from the battery negative terminal to the frame ground terminal. Then with the meter measuring milliamps, remove the power feed to branch circuits until the meter measures zero. The parasitic drain will be in the last branch so removed..

Slick

I had the same, tracked it down using @texasSlick method and it was a Wassell 200W 3 phase rect/reg unit, draw was 40 mA but enough for battery to drain over 2/3 weeks.

The bike is 100% original including points ignition and charging system. If I pull the fuse, there is no parasitic drain on the battery. I'm going to start disconnecting things to trace the problem. I've put my meter on the circuit measuring milliamps and nothing is measured with the key off. Obviously, something is happening as my almost new Motobatt MBTX14AU AGM battery slowly dies over 3 - 4 weeks and doesn't if the fuse is disconnected. Whatever is causing the problem is a very slow minute drain.

glcaines said:

The bike is 100% original including points ignition and charging system. If I pull the fuse, there is no parasitic drain on the battery.

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In which case the two things that could potentially be causing the drain is the 2MC emergency starting capacitor (blue can on a spring in the battery compartment) or the rectifier.

kommando said:

I had the same, tracked it down using @texasSlick method and it was a Wassell 200W 3 phase rect/reg unit, draw was 40 mA but enough for battery to drain over 2/3 weeks.

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This is also the same for 3 phase AN regulator/rectifier i'm led to understand... that why mine is constantly on a battery tender (optimate battery conditioner)

glcaines said:

The bike is 100% original including points ignition and charging system. If I pull the fuse, there is no parasitic drain on the battery. I'm going to start disconnecting things to trace the problem. I've put my meter on the circuit measuring milliamps and nothing is measured with the key off. Obviously, something is happening as my almost new Motobatt MBTX14AU AGM battery slowly dies over 3 - 4 weeks and doesn't if the fuse is disconnected. Whatever is causing the problem is a very slow minute drain.

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A few things:
1) You may not have your meter on a low enough range
2) Disconnecting the red wire at the capacitor eliminates the capacitor and it is not required for normal operation
3) Lead/Acid and AGM batteries lose charge over time - AGM not as bad but they do. Are you saying that your battery goes flat or just low (like 10 volts).
4) About three years ago I bought two brand new AGM batteries (not Motoblatt). One is working fine today. The other seemed to charge fine but lost it's charge almost completely in about two weeks.
5) The rectifier setup on a MK3 does bleed a small about of power. Actually, all do but the MK3 might be a little more. On a MK3 the flow is through reverse bias on one of the diodes and its corresponding Zener. Without knowing the part numbers of those devices, I cannot lookup the reverse bias current but it is small.

See this post: https://www.accessnorton.com/NortonCommando/charging-system-leakage-current.31273/

Virtually any solid state device that is used to rectify and regulate the power produced by an alternator on an English motorcycle will have a parasitic drain. I would venture that any Commando with original power handling parts are not as efficient as when the bike(s) were delivered. What Greg said.

Pulling the fuse is the least expensive way.

Plugging the motorcycle into a Batter Tender Jr, or equivalent is my favorite.

A battery switch of sufficient current carrying capability works, but you may have trouble finding a place to mount it.

If you install a solid state reg/rec, some of which have a built in capacitor, you can install a switch or relay between power out and battery.

Best.