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What voltage reading should I be getting at the battery on a 12 volt DC system. Without engine running the voltage reads 12.6 with engine running it varies from 12.8 to 13 volts.

12.6v is the output from the battery (but you had guessed that).

Ideally, 14.4v when the engine is running, but anything over 13.6v is good.

Yours sounds a bit low.

Knowledge wrote:12.6v is the output from the battery (but you had guessed that).

Ideally, 14.4v when the engine is running, but anything over 13.6v is good.

Yours sounds a bit low.

+1, is that with or without the lights on?

Technically...... A lead acid accumulator should have 2.10V per cell when fully charged open load. So times that by 6 and 12.6V is the answer. Its a bit arbitrary though because there are a host of scenarios that can affect this slightly.
Charging voltage for wet cell lead acid batteries should typically be between 2.24 - 2.40 V/cell (13.44 - 14.40V) overall.
Too much charge voltage destroys the battery or at very least shortens its potential life.
The simple shunt design regulator for a Lambretta doesn't regulate particularly well. If it did, then the batteries on our bikes would last as long as the ones on our cars!

coaster wrote:

Knowledge wrote:12.6v is the output from the battery (but you had guessed that).

Ideally, 14.4v when the engine is running, but anything over 13.6v is good.

Yours sounds a bit low.

+1, is that with or without the lights on?

Did not try it with the lights on. It's a shorai gel battery that performed well up to the Larling weekend when I had to cut the journey short because of exhaust failure. The heat from the hole in the exhaust burnt the paint off the footboard and head cowling so I am suspecting it also affected the battery which is in the toolbox. The battery is coming up to 3 years old but it seems a coincidence so I connected to over=heating.

If it is a full DC Wassel type conversion I would expect to see slightly above 14 volts without the lights on and a volt or so lower with them on. You could be right about the battery failing, what voltage do you get if you disconnect it? You will probably need the lights on to provide a load in order to get a meaningfull reading.