I've got a 1300 Locost. Brand new battery. Car has a battery isolator switch. If I don't disconnect the battery it drains to nothing over a few days. No ignition on. All lights etc turned off. Any ideas?

Disconnect one of the battery terminals and then connect an ammeter in series between the lead and battery. This will then register the drain. Pull out your fuses one at a time until the drain stops. You now know the relevant circuit to investigate further.

Alternator diodes are the prime suspect , try disconnecting the plug at the rear of alternator bu don't run the engine with the alternator not connected.
If not you need to check the current through each fuse invidually, you can get little ammeters off eBay the plug into blade fuse holders off eBay for a good price these measure up to 30 amps.

If using a digital multi Meyer be aware the amps measuring socket on these has 10amp internal fuse, if the fuse blows the volts range will work but the amps ranges will read zero.

If it is the alternator diode what do I do to correct the fault?

Thanks for the replies so far

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Originally posted by Monty2556
If it is the alternator diode what do I do to correct the fault?

Thanks for the replies so far

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recon your alternator or fit a second hand one, on my last Xflow engined kit i fitted alternators from the Rover 214. it cost me all of £8 and that was only 2 years ago. the alternator was an easy fit, and lasted until the car was sold on.