How to Check Battery Discharge

[HarryM1BYT]

This is one that puts many people off - how to check the discharge from the battery, when it is parked and everything is supposed to be off. On the 75, it is a really easy process....




Pictured above it the under bonnet fuse panel. The red circle is where the battery feeds onto the fuse panel main bus bar, which subsequently feeds the main fuses. All of this area is permanently live, unless the battery is disconnected. The only things not fed via this panel are the starter and the alternator.

If an ammeter is connected between the battery's positive terminal and the screw marked in a blue box, and the main feed (red) disconnected and lifted clear, all current then has to pass through the ammeter. Remember these points are live and touching the body, engine or etc. with the meter leads could damage the meter or cause a fire - work slowly and carefully.

Start by closing all of the doors, switch parking lights off, closing the boot, then set the meter on its 10amp range. If the reading is less than 1 amp, you can then move to the lower current ranges, but as doing that usually involves disconnecting the meter, it is best to reconnect the main feed first, then disconnect once your meter is back in circuit.

Each car derivative will vary, but I have given some base figures for my diesel which has all of the options, except Hi-Line TV/Satnav replaced by DD...

The cars electronics go through several stages before finally going into full sleep mode. The slightest glitch in the voltage can cause things to wake back up, as I recently found, so your meter connections need to be good.

1. Once it has started its initial systems shut-down process - 500mA

2. A few seconds later - 150mA, then below 100mA.

3. Several minutes later 18mA with a brief occasional flick up to around 25mA. These final figures are the important ones, I would be concerned by any figure showing a discharge of more than say around 35mA at stage 3.

Obviously, the lower the discharge current whilst the car is parked, the longer your battery will last whilst still being able to start the car.

Keep in mind, that the above test excludes any discharge via the starter and the alternator, which have their own separate +ve connection to the battery terminal.

[FrenchMike]

Originally Posted by HarryM1BYT

This is one that puts many people off - how to check the discharge from the battery, when it is parked and everything is supposed to be off. On the 75, it is a really easy process....




Pictured above it the under bonnet fuse panel. The red circle is where the battery feeds onto the fuse panel main bus bar, which subsequently feeds the main fuses. All of this area is permanently live, unless the battery is disconnected. The only things not fed via this panel are the starter and the alternator.

If an ammeter is connected between the battery's positive terminal and the screw marked in a blue box, and the main feed (red) disconnected and lifted clear, all current then has to pass through the ammeter. Remember these points are live and touching the body, engine or etc. with the meter leads could damage the meter or cause a fire - work slowly and carefully.

Start by closing all of the doors, switch parking lights off, closing the boot, then set the meter on its 10amp range. If the reading is less than 1 amp, you can then move to the lower current ranges, but as doing that usually involves disconnecting the meter, it is best to reconnect the main feed first, then disconnect once your meter is back in circuit.

Each car derivative will vary, but I have given some base figures for my diesel which has all of the options, except Hi-Line TV/Satnav replaced by DD...

The cars electronics go through several stages before finally going into full sleep mode. The slightest glitch in the voltage can cause things to wake back up, as I recently found, so your meter connections need to be good.

1. Once it has started its initial systems shut-down process - 500mA

2. A few seconds later - 150mA, then below 100mA.

3. Several minutes later 18mA with a brief occasional flick up to around 25mA. These final figures are the important ones, I would be concerned by any figure showing a discharge of more than say around 35mA at stage 3.

Keep in mind, that the above test excludes any discharge via the starter and the alternator, which have their own separate +ve connection to the battery terminal.

Hi Harry,

Just replaced the battery yesterday and while refitting i have measured the
drain current like you and found 80 mA ..?

So after 21 days,a lost of:

0.080x24x21 = 40 Ah

Mike

[T-Cut]

Sorry. Ignore this post.

[HarryM1BYT]

Checking the discharge is the easy bit, finding what causes a higher than normal discharge is the very hard time consuming bit.

If you have given it time to settle, you would seem to have a problem. Try pulling those fuse and links out one by one in the under bonnet panel, When you find it, start tracing by pulling more fuses out from the glove box panel.

As in my previous thread, I eventually traced my discharge to my voltage sensing tow relay, constantly waking up the cars electronics.

[Woodsie]

Originally Posted by T-Cut

Sorry. Ignore this post.

What post ?

[COLVERT]

A solar charger is cheap.

The one I use puts much more back in than the minor discharges you fellas are quoting.





Ho--Ho--Ho.


Colvert.

Originally Posted by T-Cut

Sorry. Ignore this post.

I really did try----honest; but God made me do it. ( The reply I mean. Lol )

[HarryM1BYT]

Originally Posted by COLVERT

A solar charger is cheap.

The one I use puts much more back in than the minor discharges you fellas are quoting.

Will not work for me - The car is parked in my dark garage. Mind you, its on a charger and the house just over the road has several banks of solar panels, so I suppose they could be feeding into the mains and I'm using some of what they are feeding into the system

[Evil McNasty]

Strange that while reading this the alarm goes off on my ZT-T........
Flat battery
Interesting about the tow relay.... I have just had a tow bar and electrics fitted and my battery problem has only started since then, If I don't use the car for 24+ hours it won't start 'flat battery'.

[HarryM1BYT]

Originally Posted by Evil McNasty

Strange that while reading this the alarm goes off on my ZT-T........
Flat battery
Interesting about the tow relay.... I have just had a tow bar and electrics fitted and my battery problem has only started since then, If I don't use the car for 24+ hours it won't start 'flat battery'.

If you have it wired for both fridge and trailer battery charging, check the fridge output wire on the 12S is being turned off reliably, with the engine not running - if not.....

The voltage sensing relays can be adjusted via a small adjuster screw. It should make, after a few seconds with the engine running at 2000rpm and open within seconds of the engine being switched off.

[Tatra Man]

Originally Posted by HarryM1BYT

3. Several minutes later 18mA with a brief occasional flick up to around 25mA. These final figures are the important ones, I would be concerned by any figure showing a discharge of more than say around 35mA at stage 3.

Hi, Harry,

Rather than start a new thread I hope it's okay to resurrect this one.

First a question on your excellent advice quoted above:- When you say "several minutes", may I ask roughly how many? I'm still getting a quiescent drain of roughly 170mA after five minutes yet I remember it dropping to under 20mA in less time than that when everything was working okay. (I checked it because I couldn't start the car when it had been standing for 2 weeks.)

Now to my current problem which appears to be as confusing as everyone else's:-

Mine's a Mk.I ZT-T 260 V8 SE so it has the dreaded Hi-Line set-up and everything except the Sat-Nav, a relay next to it (what does that do?) and what I assume to be the Harman Kardon Audio amp. is where the spare wheel would be in a non-V8.

The intermittent faults I'm getting are occasional fast discharge of the battery when standing and misbehaviour of the Hi-Line system even when running. That misbehaviour takes the form of the entire system shutting down or just displaying a white line across the bottom with TMC on it. I've once had it say "No disc" and once "No caddy" (or similar - I forget the precise wording) yet it wouldn't release the caddy until I unplugged the CD changer and reconnected it. When it's working, everything works fine including the TV (noise on screen) and sat-nav.

I realise the CD changer keeps checking discs when the battery is low so I've tried unplugging that and I've tried unplugging the video unit which kills the screen on the head unit but doesn't appear to cut the quiescent current by much, if anything. The led on the sat-nav goes out so I haven't yet tried pulling the unit and disconnecting it.

I have changed the battery but the old one appears fine and starts my Sunbeam Alpine even after it's been standing for a month. I changed it because last May it went almost totally dead on an overnight ferry from le Havre even after a journey from the Pyrénées! - It's rather embarrassing getting a bump start on the car deck in a 260 V8 with X-Power exhausts!!!

Everything appears dry in the "spare wheel well" which, on a V8, is only half a spare wheel well and has the battery, the video unit, the CD changer and another unit I assume to be the radio tuner (plus a module which I'm told is so the Ford Mustang ECU and the BMW/MG/Rover BCU can talk to each other and a battery isolator module) yet my problems appear to be at their worst when the rear screen mists up and I suspect it's not coincidence that I arrived in le Havre in very foggy weather.

Is it possible I'm getting condensation on one or more of the circuit boards? If so, where should I look first. If not, how would you suggest I proceed?