Electrical question referance Liesure battery

[windywatson]

I'm bemused by the following situation. I'm electro/mechanical technician so have a good understanding of how the eletrics work on my van but today a came across something that I'm struggling to work out. I was fitting an isolation switch to my liesure battery and to check it actually worked I put a meter across the +ve battery terminal and earth and then operated the isolation switch. I expected the meter to drop from 12 v to 0v, however I have a reading of 2.8v. I checked across the switch on ohms range to check that the switch is a clean open circuit which it is.
I should say that the switch has been put in the negative lead from the battery to chassis, which should still work to isolate the battery just as if it were in the positive (shouldn't it?).
With the LB isolated, out of intrest I put my meter across the +ve of both SB & LB not expecting to get a reading but I did of 9v. How can this be?
I do have a wilton kit fitted to the LB that powers the radio, blinds, fag lighter etc & the usual split charge system.
Any ideas would be welcome.

Cheers

Richard

[g8dhe]

Usually leakage, DVM are very high impedance it takes very little current just a few picoamps to make them read so any leakage across your isolation terminal is visible, and that means ANY leakage just picoamps 1000,millionths of an amp and you will be reading something!

[cmm303]

Hunch rather than science ... With the LB isolated, the voltage between +ve and chassis is floating rather than zero. So attempting to measure with a digital meter will give unpredictable results due to the meters high internal impedance. Do I smell of bull sh1t? !!!

[Simon Jones]

You may have devices powered by both batteries or sharing a common earth i.e. the bodywork. For example, my stereo has the permanent feed from the main battery but I've fitted a switch to allow the feed normally switched by the ignition to got to either battery. If taking power from both batteries & then the negative is disconnected from LB you will still get a drain as it will earth by the chassis / internals of the stereo. You see similar thing with tail lights where they all come on at once due to a dodgy earth.

Putting an isolation switch in the positive lead would be the correct thing to do otherwise any device connected to it will still drain the LB via the chassis. Any reason why the negative lead was used?

[cmm303]

You may have devices powered by both batteries or sharing a common earth i.e. the bodywork. For example, my stereo has the permanent feed from the main battery but I've fitted a switch to allow the feed normally switched by the ignition to got to either battery. If taking power from both batteries & then the negative is disconnected from LB you will still get a drain as it will earth by the chassis / internals of the stereo. You see similar thing with tail lights where they all come on at once due to a dodgy earth.

Putting an isolation switch in the positive lead would be the correct thing to do otherwise any device connected to it will still drain the LB via the chassis. Any reason why the negative lead was used?

Surely if either of the LB terminals is isolated, there can be no drain on it as there is not a complete circuit through the battery? Connectivity to other batteries cannot provide a path for current through a half disconnected battery.

[g8dhe]

Not as simple as that I'm afraid! You have two batteries, and some items may have supplies from both sources. If that is the case then there will be leakage via the device between both supply lines, hence again very odd results can be obtained, if you have Solar panels as well that's a third source just to confuse the issue as well
Unless you isolate each circuit completely then be prepared for strange readings!

[Simon Jones]

Other devices are providing the earth path via the chassis. It may be that things like the blinds will still function with the negative LB lead disconnected so for an isolation switch to do what it is intended to, you must break the positive connection rather than negative.

[windywatson]

Thanks for all of the responses, some interesting points to consider. The negative terminal was used as this was the most convenient for my particular wiring set up and the terminal end of the isolator was marked up negative any way. One point I should make with respect to some of the responses is that the isolation switch will work as well in the negative as it would in the positive. As far as the LB is concerned once the switch is open circuit the battery will not supply anything as it is open circuit. The fact that it is coneccted to the same earth as the SB is of no consequence. It a case of good old sience where that's concerned. I do agree with the point about the possibility that there may be an item of equipment that has inadvertently ended up with dual supply so to speak. I shall have to start to do some isolation of equipment to track down the culprit. Watch this space as work on this progresses.
Cheers m

[Simon Jones]

Just re-read the posts, and yes you are totally correct that you won't flatten the LB with the negative lead disconnected. Devices on the LB could however drain the main battery via earth leakage which wouldn't be ideal. It depends what the purpose of the switch was in the first place. If it was to isolate the battery, then job done, but if it was to isolate the circuits connected then it may need need further tweaking.

[BigPanzer]

Hi,

Just a thought re: the odd voltmeter reading, but is your stereo connected with its main supply to the LB, but its control supply is from the SB/ignition aux. supply?

Peter

[mikeonb4c]

Just to add a general question on this, it seems to me that many spannerman mechanics prefer to steer away from electrics where possible, especially with Bongos where aftermarket wiring may have been installed. I'd like to get mine looked over at some point, but is there such a thing as an autoelectrician who will, for a reasonable charge, be competent and willing to look over 'legacy' wiring. If anyone can recommend one in the Manchester area I'd be grateful.

Good luck sorting the LB issue