Classic cars forum & vehicle restoration.

[Ray White]

When my battery is being charged using the 'low' amp function the indicator might show nearly full but if I switch it over to fast charge the needle jumps back to half charged.

Why is this please?

[ukdave2002]

Because the High setting will be a higher voltage than the lower voltage that a trickle charge, this is a bit crude in manually controlled chargers, the high charge will always be trying to charge to a higher voltage.

Smart chargers will automatically control this in several stages.

Dave

[Ray White]

Thanks Dave. I wasn't thinking about voltage. Just amps.

So perhaps the adage... double the voltage= half the amps?

Sorry, I am just no good with electrical stuff... no matter how hard I try.

I think the P38 must have a voltage leak ...and it's getting worse. The new battery now goes flat after just a few days.

[ukdave2002]

[Ray White]

[petelang]

It's a laborious task Ray, I'm afraid , but you could try connecting an ammeter in line to battery positive to measure what is being pulled out of the battery and then, one by one, pull each of the umpteen fuses and see which ones have a significant impact.
If you use a 0-10 amp meter you will have a better chance of seeing small amounts of discharge but DONT switch anything on likely to draw more than 10 amps.
Most likely it's a battery circuit, like interior lights, boot light (favourite) glove box lights, clocks, radios, etc. Alarm systems is another. Once you narrow down which fuse, with the aid of a diagram you can start splitting circuit connectons from any connected relays etc to track it down further.
This is by no means foolproof but if you pay a leccy it could take time and money and it is something you can do yourself.
If, God forbid, its wiring chaffed, pinched by a screw, or corrosion of a device or fusebox etc it gets even harder to trace. I've done this on articulated busses where bodybuilders trapped wires. Took me a week to find one and in the end had to cut both ends of a few cables and run in a new bit as finding where it was was beyond economical.
Others, more experienced with the P38 might have more clues where to look or look on model forums.
I feel your pain.
Oh, and don't get shut in the boot checking the boot light goes off! Ask me how I know. My workmates thought it was hilarious.
_________________
Daimler Fifteen 1934
Armstrong Siddeley 15 Long 1933
Daimler V8 250 1969

[Ray White]

Thanks Pete. One suggestion on a forum (P38 section) was to replace the fancy EAS relay with a basic one. Did that. No change.

I will have to check out the fuses as you say.

[ukdave2002]

I meant Parasitic draw not Sacrificial !

As Peter suggests with the ammeter in place you can firstly establish the existence and size of the parasitic draw, the size may give a clue i.e. if its 0.5A, is there a 6W bulb in a glove box, boot, bonnet that's not switching off? removing the bulbs is less risky than getting stuck in the boot!

Does the fault change dependent on whether the car is locked or not?

I had a small parasitic draw in an alternator, it would flatten a battery over a couple of weeks, yet when running the alternator was performing normally.

Has any work been performed on the car recently where wiring may have been disturbed? are there any known loom issues where chaffing may cause the insulation to fail over time?

Cars of this era had a BeCM (Body Electronic Control Module), essentially it manages the non-engine vehicle electronic modules. When the ignition is off it should go into a standby mode after a minute or so, firstly when looking for the fault give it a couple of minutes to get in to standby mode, you should see the current drain drop when this occurs, if its not going into standby, it could be because it thinks something still requires its attention; an open door or bonnet , electronic suspension not doing what it should, alarm fault or the BeCM could be faulty.

[Ray White]

It was the BeCM standby function connected with the EAS that someone said could be the problem; hence remove the fancy relay under the seat and replace it with a simple one. Apparently this is common.

[ukdave2002]

Ray

Take a look at this video, apparently there is a known problem with these cars, where all sorts of signals from things like electric gates, video doorbells etc, get picked up by the remote key fob receiver, cause the BeCM to come out of standby and drain the battery.

Simple 2 min fix is to disconnect the aerial from the Remote Fob Receiver, it does mean that the key fob will only work when you are close to the car, or there is an updated remote fob receiver.

Wouldn't have been as much of an issue a few years ago as there were far fewer wireless devices around!

https://youtu.be/KvU8wM9gQxY?si=fiZudRXqXBunczQ6

[Ray White]

Dave.

Thanks for that. I have not seen it before and now it all makes sense. I couldn't understand what the "experts" were saying.

(They assume everyone understands the jargon.)

I will disconnect the blue wire and see what happens.

The only thing is that I don't lock the car at home. Does that make any difference?

[ukdave2002]

[Ray White]

If it is that then the first suspect must be our security system. In addition to CCTV cameras we have exterior lighting. We have been having a problem with the sensors being over sensitive and lights being set off by many different things; birds, bats, even bushes moving in the breeze. My next job (if it ever stops raining) will be to remove that ariel wire and see if it makes any difference.

Someone suggested that I could have a slight leak in the AES. Apparently, the system "wakes up" if it detects the car is sitting not level and this activity over a period of time will drain the battery. Changing the fancy relay for a basic one was supposed to fix it but it didn't.

I really appreciate all the help you forum posters are prepared to give.

[ukdave2002]

I'd try removal of the aerial lead first as it a quick test, make sure the connecting wire is at least an inch away from the receiving module as you are dealing with RF signals that can cross small air gaps.

If that cures the drain great, if it doesn't you need to establish if it is the BeCM that's causing the drain, and the challenge will be that it could well be intermittently waking up, you don't want to spend hours staring at a multimeter screen!

Presumably the BeCM has some form of diagnostics? do you know if can it be read via the OBD port? OBD diagnostic readers are dirt cheap these days.

If the BeCM can't be read from the OBD you could set up an alarm that is triggered when the BeCM wakes up, a simple relay triggered when the BeCM comes to life that sounds the cars horn, may not make you popular with the neighbours at 3:00 am!
Or you could get a cheap data logger, I have one like this: https://shorturl.at/sCKY9
Connects to a phone or PC and records up to 90 hours of activity.

Dave

[Ray White]

I removed the wire but then noticed an interior light was on. I expect the light was the culprit but I may have accidentally knocked it on and not realised. I can't think how I wouldn't have noticed it before but who knows?

All I can say is the battery is staying up now!

Happy days!