Drain on the batteries

[Rob]

I have been trying to locate a drain on my batteries for some time without success. Could someone please confirm what is the best way to find the drain and what equipment to use. I have an 80 with 24v starting system and a leisure battery connected in parallel to the main battery. The leisure battery does not go flat only the main ones so the split charger is doing its job. So the test the current drain on the batteries I disconnect the positive terminal on all 3 batteries and I use my Maplin Unit-T multimeter to measure the current draw on the main battery on the positive terminal. Now this is were it gets tricky as either my multimeter is crap or the drain constantly varies. On the 0-20A setting I get below 0.5 apms after it has settled but on the 0-200mA scale I usually get around 50mA but I have blow the fuse on that setting twice as the current draw is high initially and then it reduces. I removed all the fuses and added them back one by one but I could not find the current drain due to varying results from the multimeter. Do I need a better multimeter, or current clamp or is my method flawed?

 

[goingdownsouth]

The long winded way is by process of elimination.

Split the fuses into halves or quarters. Pull them a quarter at the time and check where the biggest variance is.........just an not checked or tried

 

[Rob]

Doing it by halves is a better idea , statistically its a much faster way.

 

[Jon Wildsmith]

binary search, I use that quite a bit To measure current draw on your dual battery set up you need to disconnect one of the batteries or measure current at both batteries at the same time, otherwise you're only measuring what one of the batteries is supplying and will get variable results. If your meter can only manage .5 amp resolution on the 20amp setting you might need to borrow a better meter. Check the voltage of each of the main batteries while they are disconnected and not recently charged, they should be near enough identical. If they're not the same then the lower voltage battery will drain the higher one and often it's because the lower voltage battery is faulty and so they never equalise.

 

[Rob]

Both my main batteries always show an identical voltage, leisure battery is usually different but that is expected due to it being isolated from the main ones and having no drain on it. I have figured out that you need to disconnect both main batteries and measure current from one but thanks for confirming that Jon. One thing that is odd but not unexpected in the electrical world is that when I wire up my multimeter in series there is a surge of current initially which then usually levels off at a lower value but its difficult to tell with my multimeter. Might have to look at what I can use from work, its either another cheapish multimeter or a Fluke current clamp with no display and a National Instruments NI-DAQ card plugged into my laptop as I don't have access to anything in between...

 

[Jon Wildsmith]

How about connecting a link wire, then your multimeter, then remove link wire to get round the initial surge problem? The higher inrush current is normal, mostly charging capacitors in the ECU's, radio etc.

 

[Nuclear Chicken]

Fluke clamp meter might not measure DC current as it's not signwave based, unless I'm mistaken. Clamp meter might not measure current that small anyway, or at least with any accuracy. Most clamp meters ahve the option of using leads and bits with them too so have a look at it. Worth buying a decent quality meter if you can stretch to it. Will last for years and are extremely useful.

 

[Rob]

Found a decent multimeter that I can borrow for the weekend so I should be able to do it now. No chance of borrowing £2k worth of equipment I mentioned above so will just use a decent Fluke multimeter.

Link wire is a good idea thanks Jon

 

[Rob]

Fluke clamp meter might not measure DC current as it's not signwave based, unless I'm mistaken. Clamp meter might not measure current that small anyway, or at least with any accuracy. Most clamp meters ahve the option of using leads and bits with them too so have a look at it.

I occasionally use this current clamp at work mainly for DC and it gives good results and has a decent sampling frequency so perfect for data acquisition. No leads needed for this one just the clamp but you do need some data acquisition equipment as it has no screen which can be ££££ so that's why I found a decent multimeter to use instead.

 

[Rob]

I checked the system today and got 1.25A and I traced it to the dome fuse. The current draw on the batteries is 0.1A without the dome fuse but I later realised that I was testing with the drivers door open and the dome fuse powers the interior lights so I will do it again tomorrow. I still think a current draw of 0.1A is a bit on the high side, what should it be? Also how long should I wait before the current settles? I waited about 30 seconds and it seems to have levelled off.

BTW using a decent multimeter helps a lot!

 

[Jon Wildsmith]

0.1A does sound a bit high but it depends what extras you have connected to the main batteries? Do you have an alarm? Sometimes when the backup batteries in those are failing they draw extra power.

 

[Crispin]

120 draws 20mA when asleep.

Worth getting one of these?
http://mobile.maplin.co.uk/module.aspx?moduleno=217881
could make finding simpler

Sent from TapaTalk

 

[Rob]

I do have an alarm which was factory fitted, otherwise on those main batteries its all standard now. It used to have other electrics before I got the truck but I removed it all. There is an aftermarket radio but I think that runs off the dome circuit so that cant contribute to the 0.1A.

 

[Rob]

I have located a 80mA drain on the fusible link labelled Glow Main on the positive terminal of the main battery. I assume this is the air preheater system... will have a look later today. The air preheater relay is under the main battery right?

 

[Rob]

Had a look and it was the air preheater relay, was slightly melted on top and once removed the current draw was down to 24mA. took it out and taped up the wires and will order an new one. Seems that this problem is quite common on the 24v, would strongly recommend checking this relay if you have one or else your pride and joy will end up like Chris's old 80.

 

[Gav Peter]

I wonder if this is actually what 'did' GW Rob? You said its a common problem on the 24v models; any intel to suggest its a problem on the 12v trucks? Or does the 12v actually have one even???

Cheers

 

[Jon Wildsmith]

Think this is only fitted on the 24v Gav but we've seen a few pictures of melted units over the years

 

[Gav Peter]

Cheers Jon

 

[Rob]

Here is a pic of mine, not as bad as others but still worrying, the cover had mostly away this is the relay:

 

[Rob]

I meant to say the cove mostly melted away...