Alternator spikes

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Veteran Member
Jan 15, 2020
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1989 GB 42 Classic
I noticed spikes on my stbd alternator while underway. Any ideas what could cause that?
- New AGM batteries
- New alternators - Lucas A127 (55amp, internally regulated)
- monitoring voltage using Siren Marine hooked directly to battery(s)
- In this case - stbd battery wasn't even running the loads

The other alternator/battery voltage looks 'normal'.


I am most likely no big help. If it was me, I would use a meter and see if you get the same readings. The cheaper the meter the less it will react to the changes. If the problem is still there, I would be checking connections and cables and replace if needed.

I just installed new starting batteries and replace some of the 20 yr old cables. A 55 amp ALT seams rather small. I would think bigger would have been better with a external regulator. You could swap the ALTs and see if the problem move to the other engine or just pull the ALT and have it tested.
Check that all connections are clean and tight with no corrosion and ground leads are the same. If it persists, then I'd consider the voltage regulator.
It’s not surprising to see the voltage do that, especially with internal regulators. They are pretty crude as you can see by the graphical voltage readout. I’m wondering why one has such higher volt output though. Could be many reasons, but crappy regulator comes to mind first.
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