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Discussion Starter · #1 ·
When we first looked at this 1972 RR Silver Shadow, we noticed that it was equipped with a battery disconnect switch in the boot that the previous owners hadn’t used, and the battery was depleted.

With the battery topped up, and a functioning charging system, the car starts/runs/operates/charges correctly.

But with the car turned off and the battery disconnect switch left in the connected mode, sitting quietly in the garage, I understand that there is a constant 100 mA load on the battery (130 Ohm system resistance) which will inevitably kill the battery unless a trickle charger is applied.

Is this quiescent load normal for all cars of this generation, or if not what do people expect to see if they put an Ohmmeter or Ammeter in the electrical system with the engine off and the car turned off?

Thx!

IR
 

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Ammeter is used to test current flow and it is connected in series with the POS cable to test.
I do not think you will find any drain on a SY.
 

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Discussion Starter · #3 ·
Wraithman - thx very much - helpful - well, ours has a 100 mA drain as indicated by Ammeter so open to anybody’s experience with likely circuits/appliances to check first?

best

IR
 

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Leave the ammeter connected, while you pull fuses and look for the load drop indicating it is on that circuit. It can be something like a courtesy lamp, glove box lamp, etc.
 

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With that low wattage (1.2) it will likely be a small bulb like in the glove box. The other low draw could be a failing diode in the alternator. Like Rob says pull fuses until you find the circuit with the draw. If no luck with fuses remove connections to alternator to see if the draw is there. A quick test for a failing diode in an alternator is to tune an AM portable radio off a station so just static, then hold the radio near the alternator and rev the engine. If you hear a whine in the radio in time with the revs that tells you that there is a leaky diode in the alternator.
 

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Our 1973 Corniche DRH14434 was prone to this battery drainage problem if not driven regularly.

The cause was the cycling of the Kienzele electric clock when the car was not being used on a regular basis. Problem was solved by fitting a battery isolating switch on the battery earth lead and manually resetting the clock each time the vehicle was taken out for a drive.
 

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I had a problem with an alternator causing a drain on a battery whilst infrequently used, (covid related), not the diodes but the ‘stator earthing’. Exchange unit was the remedy.

Very few filament lamps would be 1.2 watts. When pulling fuses, bear in mind the alternator output will be live continuously so diodes or other fault could cause this.if not found by pulling fuse, try disconnecting alternator output. If a filament bulb (or a diode) or most electronic devices - such as an alarm, are most likely non-linear so may read several k ohms yet still consume the 100m/a.

A 100A/H battery would be 50% discharged - as low as you want to go for a car battery for reasonable longevity (that is not new, and can still start a 6 litre engine) after 500 hours, say 20 days. I use a timer and charge for 1 hour a day at a modest rate. I will check what residual drain I have.

Switching the master switch off is a pia having to reset the clock, so was planning on a 20 amp resettable fuse (I.e.) in parallel with the switch that would trip under fault or thief attempting to start without the battery connected. Anyone done this?
 

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I've read in the past of owners who have wired a fuse parallel to the master switch, although usually a lower rating fuse to just protect the clock. I get where you are coming from with a 20 amp fuse - this will protect against the high amperage unfused circuits i.e. the thick power cable from the battery to the starter motor and onwards to the ammeter. It will certainly stop the annoyance of having to reset the clock each time you turn the master switch on, with minimal sacrifice to the safety of the car since the fusebox will still be protecting the lower rated circuits.

For me, I've got used to living with the clock going off each time I switch the master switch off, which is every time I put the car back in the garage. I like having the car completely isolated when it's not in use, given the wiring is now 47 years old.

On a more general note, I think a master switch is an absolute must for all cars of this age. I know it's highly unlikely, but if ever there was an electrically induced fire, it would not only take the car out, but possibly the whole house as well.
 

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What’s stealing my Amps? IR, did you find where your 100m/a is going? I said I would check my residual load. It’s 630m/a. After a two hour journey, two days later and it failed to turn the engine! I had the lights on for a few minutes and a door was ajar for a while. But not enough to flatten a good battery. That’s 3 1/2 years old - still under warranty. Tested it this morning it was OK after a night’s charging. So out came the AVO multimeter. 7 amps to unlock, 2.5 sidelights, but astonishing 0.63 for clock, temperature gauge and alarm. Seems excessive, so down to fuse pulling to isolate, not forgetting the alternator! No wonder the previous owner always turned the master switch off.

Geoff, the reason I was proposing a 20 Amp trip was that’s the lowest of the type I linked - which I have found very good. It wasn’t so much for the fire risk, although important, as an anti theft measure and keep the alarm / central locking / clock working. But no point until I locate the residual current whilst idle.
 

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Is this quiescent load normal for all cars of this generation, or if not what do people expect to see if they put an Ohmmeter or Ammeter in the electrical system with the engine off and the car turned off?


I have found the normal drain is 0.03 amps, Kienzele clock, when stopped and locked. I forgot to turn battery isolator off when the car was off the road all of December and January but the car started up with no problem, engine spun over as fast as normal after 9-10 weeks.

Jake.
 

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The answer seems the quiescent current should be less than 50m/a according to YouTube videos if you search for parasitic current drain.

https://toolboom.com/en/articles-and-video/detecting-leakage-current-in-a-vehicle/ says :-

Acceptable limits of leakage current - 20-80 mA. Typically, current consumption for OEM devices is as follows:

  • car radio memory - 5-10 mA;
  • alarm system - 20-25 mA;
  • electronic control unit - 3-5 mA
It also emphasis aftermarket accessories are more probable culprits, and alternators / diodes or starter solenoids are both without fuses. As might badly installed aftermarket audio systems etc.

Chris
 

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Chris

That's useful information to know. I checked the leakage current on my car this evening - 30mA, which is well within range. That's for the clock and remote central locking receiver (non standard).
 

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When I bought my 80 Spirit last week I noted the vendor turned the master switch off every night. I thought it an abundance of caution, fire risk etc. I wasn’t even aware the installed switch was standard I sort of assumed it was after market. When I first posted ref the 100m/a drain I wasn’t aware I had the 630m/a quiescent drain problem. He certainly wasn’t.

That’s 7 watts, so until located is a potential fire risk as something will be getting a little warm. If the alternator it’ll be dissipated - unless suddenly deteriorates - it’s unfused. So until I get back to work on it the master switch is off. That means I have to lock manually and no alarm therefore no theft insurance cover, or leave on float charge continuously. With my quotes ranging (on one comparison site for identical parameters) from £6,019 to just £83 for fully comp I don’t want to claim........

On another vehicle I fitted a combined digital ammeter and voltmeter. Found that was very useful. In fact as it had a winch I didn’t use a shunt but used the negative earth lead to the battery as the shunt. A good £20 quid’s worth of monitor. I’ll be doing same on this motor. It only reads either charge or discharge, so I have a simple changeover switch.

Chris
 
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