Main Battery Isolation Switch

[Slartybart]

Morning all....
Knowing that the car is likely to be parked up for sometimes weeks on end, I want to install a main starter battery isolation switch. Can I simply put it between the main Ground from the negative side of the second battery (circled), or are there other paths to Ground? Would it be better to put the switch instead on the Positive? I'm intending to use a big red 300amp 'marine' style rotary switch.

Thanks in advance!

For background:
The car is a 2002 100 series with a split charge system. The three batteries (2 starter and one leisure battery) are unlinked by default at start. It all works fine, but there is phantom drain somewhere on the main starter batteries, almost certainly from the alarm system. The previous owner simply pulled the fuse which solved the problem but also stopped the remote locking and interior lights from working. I've reinstated the fuse and disconnected the siren from the under front driver-side wing to achieve the same result - avoidance of nuisance noise - I tried first to adjust the sensitivity of the alarm, but to no avail.

 

[karl2000]

Presumably the starter batteries are wired in parallel (12V), so you'd need to isolate the negative from both batteries, not just one?

 

[Slartybart]

Thanks… yes they are in parallel and operating at 12v. I’ll check, because I while I saw the first battery neutral was connected to the second I didn’t see a connection from the first battery to the chassis. I’m trying to ensure that whatever I do is nincompoop proof….. one big red switch is probably ok…. but two might be too confusing for anyone else that uses the vehicle…… if I’m right, and the first isn’t connected to anywhere but the second, is it still necessary to isolate the first as well?

 

[Slartybart]

I suppose both negatives can go to the same switch, I guess. Thanks!

 

[karl2000]

The negative on the 'first' battery must end up connecting to a common ground somewhere, otherwise they're not wired in parallel?

 

[Slartybart]

Yes, absolutely. I’m not with the car now, but even from the pictures I can see there’s a very small ground to chassis from the passenger side (second) battery, and I can’t be sure but it seems that’s it’s only connection. There must be another connection from the first somewhere to ground somewhere, as you say. If necessary I’ll put the switch on the first negative and extend the second battery ground through the same switch. Thanks for your patience.

 

[karl2000]

See what you think when you look at it again, always difficult when you haven't got the car in front of you!

On my 120, one battery (passenger) has its -ve connected to the body, the other battery has its -ve connected to the engine block. Obviously it doesn't really matter where they're connected, as long as the connection is a good one - there should be very little resistance between any two points on the engine / body - as they are all supposed to be common ground.

The engine block -ve on my 120 had a dodgy wire, and I put an extra wire in between -ve and the body for a while until I could get underneath to replace the original.

 

[karl2000]

I'm intending to use a big red 300amp 'marine' style rotary switch.

By the way, not sure how many amps your engine needs to turn over, but obviously your switch needs to be rated for at least that. (Your switch might have a constant and an intermittent rating, I'd expect the intermittent rating to be OK for the starter, but others will give their view...)

 

[Tractionman]

Not sure if any relevance here, but an average diesel can take 400-500 amps to turn over for a jump start. Bigger commercial engines more.

 

[Slartybart]

That’s really kind to mention it. The one i’ve ordered has an intermittent rating of 455amps, 275 continuous. Hopefully it won’t melt!