Battery Isolators

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528i-1981
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Battery Isolators

Post by 528i-1981 »

After reading about Derek's lock controller melting down, I started investigating battery cutout switches. I had a situation recently where a wire connecting a thermal breaker for the aux fan circuit disintegrated, broke and then contacted ground. It was alarming and a reminder of two things: 1) batteries have a LOT of energy; 2) when you buy cable from Amazon that's allegedly pure copper strand, it may not be, so for hi-amperage applications use wire from a reputable electrical supply source.

There are a flood of options for these, many are panel-mount for racing or RV applications. Despite the hassle of the E12's battery location and the need to open the hood to access a battery-mount option, I wanted an easy install. Most of the time the isolator would be used when doing work on the car. The top-post battery-mounted options come in two flavors, knife-blade and knob-style. I liked that with the knife-blade style you could visually confirm the connection status. You can also remove the knobs on most of the the knob-style rather than just loosening it, but then you have something loose to lose. I also figure in an emergency opening a knife-blade switch would be faster than loosening a hot knob from a short-circuited battery.

Unfortunately, most of the knife-blade products operate vertically and are rather tall for something sitting atop a battery.

Many of the products I found are cheaply made Chinese products using, at best (and if you can believe the description) brass plating over a zinc alloy. They're cheap and easy to install but we're talking about something that needs to work reliably, endure a fair number of duty cycles, and not break or corrode in an E12 engine bay.

I ended up going with a relatively expensive unit from McMaster-Carr at about $40 that is supposed to be made of brass. The knife-blade operates horizontally and the whole unit is low profile. I bought one for my 1990 F150 as well. I'll report back with impressions on whether they're worth the extra money.

Cheers,
Eric
(oo=00=oo) Eric
1981 528i Manual
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dsw99a
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Re: Battery Isolators

Post by dsw99a »

Hey guys, here is a shot of my cut-off setup. Its pretty simple, but seems functional to date. I have a parasitic draw somewhere so i cutoff battery power whenever parked for a few hours or more. Also a inline fusible link on the battery cable.
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528i-1981
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Re: Battery Isolators

Post by 528i-1981 »

That's the rear strut brace I've seen that was sold by Top End Performance - NLA.

The battery isolator sold by McMaster-Carr is a Battery Doctor model 20108, which is available elsewhere for less than McMaster-Carr, but you can't fault McMaster on their lightning-fast shipping. My problem is that the Interstate battery currently in the E12 has recesses where the battery posts are, which I had forgotten about, so anything mounted atop the battery will ride above the post. This could be an issue for any of the knob-style isolators as well, because there is very little clearance around the battery (the hood support is on the side and the fusebox/aux fan breakers aft). I installed one in the F150 without any issue. The isolator seems well built.

It would be silly to replace a battery to accommodate a disconnect switch. Maybe there's another way .... there's always another way ...
(oo=00=oo) Eric
1981 528i Manual
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dsw99a
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Re: Battery Isolators

Post by dsw99a »

"Chuckle" - Funny, you mentioned battery size. This is not the "called for" size of the battery either. I had the correct battery, old and i didnt know its history, but what became the biggest issue was it was a tad small for the battery box. and i had a battery hold down installed to keep battery from shifting, it wasnt a whole lot - but enough to bother me. Another note the box does include threaded holes front and back to accomodate a battery hold down, which i thought considerate. When buying the new battery I looked for a bigger battery, figured as long as the cold cranking amps and everything met or exceeded the "regular" battery and it fit, id go for it. I think i had to dremel just a bit, snaps in like it was made for it, but it makes for a much cleaner look. Silly :wink:

I think i found that strut brace on a little used part site and couldnt believe it; it fits one other chassis, and I read the small print, cause it wasnt listed as an actual e12 part. And the site assured me it would fit. I dont recall what chassis it was listed for. I wanna say e21, but im not totally sure about that.
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528i-1981
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Re: Battery Isolators

Post by 528i-1981 »

I found an adaptor kit on Amazon for fitting an SAE standard battery into a car having Japanese pencil post (JIS) leads, which have a smaller diameter. That essentially served to elevate and offset the post on the Interstate battery. Then I used an adaptor sleeve to convert the pencil post back to SAE for the negative lead. The adaptors come in pairs, and the first picture shows the unused positive set. The isolator fits now, although the effect is a bit steampunk-Frankenstein.

Image
Image
(oo=00=oo) Eric
1981 528i Manual
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