The 'E' Type Forum

My S3 battery goes flat if I leave it for a few days. It came with a 44Ah own brand battery, so I have ordered a new Varta E23.

However there must be a reason and I have noticed my volt meter is usually just into the red when running. So I have purchased a USB cigarette charger which shows the voltage. I checked it on my wife's car and it reads exact as expected. (BMW) It should show if it is charging or not.

So my next job is to it the fit the new battery. At the same time I will brisdge my 10A multimeter from the + post to the lead to see if there is a significant current drain when everything is off.

Assuming the charging circuit is OK (This was a nut and bolt rebuild) what are the potential sources of current drain?

With the old battery disconnected does it still go flat?

I swapped out the old battery with the new Varta. Any current measured between the new battery and the positive terminal with the ignition off was negligible. I put the battery on charge for a few hours and the following day we headed out.

We did choose a very warm day, high 20s in our part of Denmark. The little digital voltage indicator showed a constant 12.6 to 12.7 volts, even with the lights on. We drove for about 130 miles, stopping 5 times for sightseeing and ice creams. During this. Period the car was flawless, returned 16.14 imperial mpg and just did everything I wanted of it.

This morning, the digital display showed 12.7 volts.

So it looks like this was just a battery issue. The factory battery indicator still sits slightly left of centre, but it must be charging.

Thoughts anyone?

I've just gone through issues with battery charging etc, from what I know, at idle you should be reading 14volts across the battery terminals, if not, try adjusting the regulator, that would be a starting point, you will get some expert advice from the wonderful people on this forum.

Before any useful advice can be offered we need to know what Alternator and regulator system your car is fitted with. It is at least a 50 year old car so could have had a totally different alternator fitted. Your car should have a Butec unit with a matching Butec reg. fitted. cheers John.

It is a BUTEC. I found an article which suggests running the car st 1000 rpm and then turning the adjustment screw clockwise to increase the voltage. I tried this with a digital AVO, but a 1/4 turn in small increments made no difference and I am not sure if it is a multi turn or dingle cturn pot, do I stopped.

Can anyone clarify this please?

EDIT image added

Just a thought.... should that white plastic screw be removed first, if so, how does it come out?

I feel very dim here!

You probably need to know whether you have an original regulator or a repro. The originals are very simple and easy to repair and the web has plenty of info already published for you to achieve this. With a repro, you need to go back to he original vendor and get a circuit diagram for it. For all we know, what you have there may not be openable, settable or even alterable.

Here is what you should be able to see inside if it is an original:- kind regards
Marek

Hi David,
the plastic plug needs to be removed to expose the adjuster screw, it is approx, 20 mm inside the case and you need a small electricians screwdriver and torch. If you have no luck give the terminals a good clean up and try again. The earth wire on your photo looks a bit sus, check it is contacting the regs. case and it not insulated by the paint. if you cannot sort it you can send it to me for checking and refurb, I carry all the parts and a test rig, Also worth checking that the field connector is dead with your ignition key in the off position. More info. if needed just ask. cheers John.

Thanks John, how is the white plug removed?

EDIT. After more research it looks to be a replica ftom Martin Robey. I will remove it and check the earth, etc.

Morning David, the plug looks to be a plastic or nylon screw, if it will not screw out then a hot screwdriver pushed into it should provide enough for its removal. I don't know about the origin of the reg. but it will be an easy and cheap repair I'm sure, nothing to get too excited about. John.

As an update, I spoke to the sales team at Martin Robey. They tell me the unit is not adjustable and my 12,8 volts "Seems about right"

I will still remove it, and check the connections as I am not convinced.

If you still are running the V12 Butec alternator this should be regulated at 14.00 volts anything less will not fully charge your battery. Have you got a matching pair of drive belts fitted? If not you could be getting slip as one of them will be doing all the work. Did they offer info. on the battery drain? My view is that these parts suppliers are just number checkers and unable to offer any valuable advice on technical matters John.

Yes, the belts are matching. The discharge issue seems to have gone with the correctly sized battery. I am stopping 4 or 5 times on a 50 mile run with the battery running at an indicated 12.7 to 12.8 volts, that includes with main and side lights.

So it looks like it is charging, just not at the 13+ volts I would expect.

Have you managed to remove the blanking screw on the front and if so is there an adjustable pot? Did you manage to measure the current draw from the battery when everything is off (after a 10 mins. wait)? Measure the charging voltage with just the engine running and them turn on everything electrical including heated rear window and other heavy draw items such as main beam headlights, wipers and heater motor. What voltage do you now have at the battery? John.

Jack the lad wrote: ↑

Tue Jul 22, 2025 8:35 pm

Have you managed to remove the blanking screw on the front and if so is there an adjustable pot? Did you manage to measure the current draw from the battery when everything is off (after a 10 mins. wait)? Measure the charging voltage with just the engine running and them turn on everything electrical including heated rear window and other heavy draw items such as main beam headlights, wipers and heater motor. What voltage do you now have at the battery? John.

I have not had a chance to look properly yet. Hopefully we will get some spare time over the weekend.

If you're getting 12.8 volts with all lights on, that sounds ok to me - although I may be wrong! The key thing is as mentioned above; with lights and other heavy discharge units off, and with engine running at about 1500 revs or more, you should see 14 volts or a little more at the battery. Does this happen?