Battery kill switch to avoid gremlin power drain
 

One of my clients has a Lexus sedan, probably around vintage 2000 that he’s really fond of. It is a good looking car, and has been pretty reliable for him. Lately he’s going through phases of needing to jump it every time he goes anywhere.

He’s taken it to a mechanic he likes a couple times to try to find the energy drain. I told him it could be tough to find those things. He had heard about some sort of kill switch on the battery so that he could isolate it from the short during his regular stretches of two or three days of not driving it. He’s an editor and does most of his work these days on Skype with clients around the world.

His man installed a device at the ground connection that does the trick, but he has to open the hood at the beginning and end of each trip. That was part of what he didn’t like about needing to jump it every time. I told him there might be some sort of relay that he could have placed under the dash so that he could do it remotely. His mechanic said he couldn’t find the hardware to do that.

I found this device on Amazon, not exactly a relay, it would require running the cables through the firewall to attach under the dash. I’m not sure how long those things can be, I know my BMW had the battery in the trunk, and it worked well enough with the long cables.

Most relays are temporary in nature it seems, they close the circuit for a second or two and then go back to default position of being off. Like a starter relay. On the other hand my BMW’s fuel pump operated with a relay that stayed on while the car was operating So what do you think? Could this device be placed under the dash and do the trick?

https://tinyurl.com/kkxcbwwc

Sure, you could come up with a heavy current relay and arrange a switch to energize the relay. That’s the purpose of a relay, to use a low current circuit to enable a high current circuit.

The first thing that came to mind was an old Ford starter relay. The problem is that the Chinese ones are all that are available today and the quality of them is horrid. It could be that you could find one from the general aviation world. If it is one that is TSO’d, meaning that it is certified for use in certified aircraft it would be stupid expensive. Many items though are available for experimental aircraft that are the same but no paperwork, thus much cheaper. Determine how much current it needs to handle and browse the Chief Aircraft website. They are a good supplier for aircraft electrical items. If that one doesn’t work try Aircraft Spruce.

Yes, the starter relays are for intermittent use, but the aircraft relay is for continuous use supplying main power and supplies current all the time when in use.

Hope this helps.

Great replies, thanks! I’ll look for an aviation device, it only makes sense that much testing and innovation is done in that area.

The second device sounds interesting also. The part I couldn’t understand was when they said that after it cut out when it reached 11.7 volts battery voltage immediately recovers. I hate to admit that some aspects of batteries are still a mystery to me. I gather that sometimes the inner chemistry will generate some electricity.

Another thing that concerned me is I think that on some cars 11.7 V is not enough to start it. The reviews are good, so perhaps these things are not the issue I think they might be.

They used to make "latching" relays .

I'd think all you need is a heavy duty continuous duty relay for the 1.5MM wire, not the positive cable to the starter .

Maybe an inverter would work .

Supposedly automotive electronics work until below 10VDC but that's from my factory training in the 1980's.....

Most breakerless ignitions work down to just above 9VDC .

? Have you done the multimeter & fuse pulling test yet ? .

There is a somewhat frequent mod in the Delica community to bypass the automatic glow plug cycles (which are based off of a coolant temp sensor). These relays handle huge amounts of amperage so I would guess it is a pretty good analogue. My glows are fine and the system works pretty well in my van (It also has dual batteries and a master cutoff switch) so no need to bypass...but if you are curious, here is a good writeup:

https://starwagon.info/delica-star-wagon/upgrades-how-to/glow-plug-bypass/

Fwiw I keep my Saturn on a battery tender since it does not get driven much. Works great and just a quick plug/unplug. I think it is a 1.25 amp unit which isnt much but takes care of the parasitic draw. It would drive me nuts to have to reset clocks, stereo, seat settings etc every time I got in the car, so a batt switch may not be the best solution.

I think the Ford solenoid would be fine, but there are purpose built battery isolators that you might find more appropriate. Same idea as the Ford solenoid:


https://www.sherco-auto.com/handles-200-amps-continuous-and-300-amp-surges-durable.html


The problem with the battery isolator on a modern car is that you wipe out memory whenever power is cut. So no codes, no radio presets, various advanced functions would require new programming, clock, etc.


Elusive battery drains can be caused by weak diodes in the alternator. Disconnect the B+ lead on the alternator and see if that makes it more better. If so, you need a new alternator. It's always the last thing you think of...

I have the same issue with our Honda Odyssey. I traced the drain to a single fuse but that circuit splits to many modules such as keyless entry, clock, power doors-seats, etc, etc and I would need to remove the interior to troubleshoot-not worth it. It can go for about 2 weeks before it won't start. I never understood why it needed a new battery every few years but the drain was ruining the battery by it being partially discharged all the time. My solution is a battery tender plugged into the ceiling with the plug hidden in the grill. Problem with battery disconnect is a lot of things need to be relearned everytime.

That could be a winner for my client. He often uses one of those jump packs, not sure the proper name, about as big as a small briefcase. And he says he often has to jump it when he wants to come home from some short trip. I explained that the trip wasn’t long enough to charge the battery. With that and the killswitch his main complaint was opening the hood over and over. Like I said, he’s 81 and one arm is weak lately.

I have a battery charger that I can fit under the hood after some hoop jumping. But I recall seeing some in the past that were pretty small. If it’s just a no bells and whistles two amp charger, to guess at a number, I imagine it could be pretty small.

How do you hook it to the battery terminals?

When was it supposedly difficult to drain the power out of a gremlin? shifiting into drive usualy took care of it :)

https://www.oldcarsweekly.com/.image...mc-gremlin.png

Mine just had generic loop terminals in a lead, which has a waterproof plug on the other end. I was able to just stack the loop terminals on top of my batt terminal bolts, and the negavtive goes to a nearby body ground (or negative terminal if you can't find one). Then it has the small waterproof plug you put the tender on. I have the leads on my car permanantly and just plu/unplug the tender from it when parked, which is always plugged in the wall. The tender came with traditional squeeze clamps too, but that would not be a good solution here. I think this would be a very good solution for the situation stated in the OP. Relays and cutoffs seem like way too much of an error-prone and tedious workaround.

Hah!

No personal experience on that matter. It’s amazing how many cars that have come and gone that I would like to have driven just for the heck of it, but never have. There’s a place in B-town called the Buggy Bank. They sell cars on consignment. I bought my first BMW 325i there. I also test-drove a Volvo P1800 and Mazda Miata during the process. Underwhelming, both.

I figured it must be something like that. The squeeze clamps really would not work of course. The charger I use most of the time is a little too busy. It has a gauge and an idiot light to tell you when you’re connected. One time I had had it on all night and just unplugged it the next morning at the outlet as I was in a hurry. Mistake. The idiot light and other features drained the battery absent 110V input.

I used the clamps and just clamped it to the battery and zip tied the cord so the clamps could not fall off or get pulled off if someone drove away without unplugging it. The cord has a plug about 2' from the clamps so the plug hangs out the bottom of the grill. The actual charger is plugged into the garage door opener receptacle.