Automatic Battery Cut-Out Switch Recommendation

I’d looking for recommendations from direct users for automatic battery switches that disconnect the battery from a non-running car when the voltages goes below some threshold (~ 12.0 vt.), protecting the battery while leaving sufficient reserve to start the engine. A friend has a Chevy truck with a parasitic drain problem the best minds in Rochester NY have been unable to fix, he’s lost 3 batteries to this, and to hold him over I’d like to recommend something that others have had good experiences with. I’ve been using such a switch (a “Battery Buddy”) successfully for ~15-20yrs on an older car that doesn’t see much use, but it seems these no longer are made.

One concern is that the switch should be designed in such a way that should its electronics fail it won’t disconnect the battery while the engine is running, which could cause much havoc with modern electrical systems. It’s okay if it requires a manual reset.

A web search turns up several such items, including on Amazon, but too many of the the “1 Star” reviews of at least one model (Priority Start) give me pause. So I’d like to hear from those who have recent, direct experience.

Just have your friend install a manual disconnect.

Of get better mechanics who can actually fix the truck.


Your friend already has an electrical problem and you want to add another layer of potential problems .

I did a simple web search ( battery cut off switch ) and found that Walmart and Harbor Freight have them . I would imagine any parts house would also .

I’ve never heard of a battery disconnect switch that does the job automatically, based on battery voltage. The ones you switch on and off manually – under the hood – those are pretty common. I’ve seen them at Harbor Freight I think, doesn’t cost much. It would be a clever work-a-round to prevent no-cranks due to a dead battery though, when you have a perplexing battery drain problem and nobody can figure out what’s causing it. The battery current to start the engine is over 100 amps, so the automatic switch (presumably a relay) would have to be pretty hefty device. It seems like it would have to have some kind of mechanical latching mechanism to hold its position without using much current, as if it were a simple relay configured normally on, then it would take a lot of current to turn it off; and if it were normally off, it would take a lot of current to turn it on. Neither of which seems viable.

By using a battery disconnect your friend will also lose radio/clock presets and memory power to the car or trucks’s ECM.

I can’t answer the why as to those “best mnds” can’t figure this out. It’s a simple matter of narrowing it down by installing a VOM and pulling fuses one at a time until assumed too high current draw goes away.

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Responding to ok4450: At 2800 mi removed I’m not in a position to monitor how my friend’s truck has been evaluated, but I’m told those helping him have measured current at all of the fuses, which is the logical place to start. Until he can figure it out, loss of radio codes and engine mapping are acceptable trade-offs for reliable starting and not ruining batteries.

Not all potential leakage paths run through the fuse box, and worse, they can be intermittent as is the case with the car on which I have a disconnect switch. (Some day I may put a recording meter on mine to see whether I can determine a pattern.)

My original question remains: does anyone have direct experience with automatic switches one currently can buy?

Rather than going through the trouble why not pull a battery cable? Maybe this will help

Reply to George_SJ1. The no longer available “Battery Buddy” automatic disconnect switch I’ve been using for over a decade appears to be what you describe. It’ mounts at the battery and when in its open circuit state following triggering on low voltage, a mechanical button pops up. Pushing the button down resets the system with a palpable click until the next low voltage event (one does want to either drive the car or connect a charger after resetting or it can discharge all the way). My guess is when triggered it might apply a brief pulse to release a “one-shot” mechanical latch which might allow it to consume less power than continuously holding in a relay, though a solenoid with many turns of very fine wire could generate a sizable magnetic force to hold a relay without consuming much power (it doesn’t need to pull in the relay). Also, the contacts aren’t switching cranking loads, they just require sufficiently low “on” resistance to pass them. Bottom line: one can find several versions of these on the market (Amazon has several), the question is whether any of the current units are both reliable and fail in a safe mode.

All of this can be solved by a competent mechanic… That is truly the long and short of this issue. If there is a draw…a real mechanic will find it and fix it.

hmmm … I must not understand how that gadget works. I don’t see how a switch that connects at the battery post could be configured so it could disconnect the battery from the rest of the car in the “off” state but wouldn’t be required to pass cranking current in the “on” state.

I don’t consider those people the “best minds” if they cannot determine in which general area an excessive current draw exists. Almost everything on a car runs power through the underhood and dash fuse boxes. The only things not run through the fuses is the starter motor and part of the alternator. Even the alt. has a fusible link on the power lead from alt. to battery…

I’m a proponent of fixing the problem instead of relying on a crutch but then again that’s just me.

George SJ1: Not switching while carrying cranking loads (maybe 100A, sometimes more) means that the switch isn’t being used to change from the closed circuit to open circuit state while high currents are flowing. It does pass cranking currents while it is in the already closed state. It is easier to design a switch to carry a high current that is introduced after the switch is closed (it already has a low resistance, contacts aren’t making or breaking and there is no arcing) - for this reason some switches carry two current specifications, one for a continuous load and a lower limit for switched loads. In the case of auto-disconnect devices, the idea is for the switch to go from from closed to open while only relatively low currents are flowing, maybe a few hundred ma. in the case of parasitic discharge ranging to a dozen or so amps if the headlights were left on.

Thanks to all for your interest and I’m happy to try to clarify questions, but fear this post has been unproductive - I was hoping to hear from those who have had experience with these devices.

As for my friend with the truck, he’s elderly, has had multiple handicaps since birth, works extremely hard for subsistence wages and is dependent upon the charity of members of his church (including professional auto mechanics) for things like this - they have gone way beyond the call of duty in fixing a number of problems, including another source of electrical drain located with the one circuit at a time method, but something still eludes them. They are donating otherwise billable time and, from my remote viewpoint, may be well past the point of diminished returns. This is a potential stop-gap that could allow continued use of the vehicle until either the problem can be solved or it dies for another reason. I fully understand that it is not ideal, in many ways.

Used to have a 95 Ford Ranger that had an intermittant battery drain that mechanics and the Ford dealer could not pin down. After one episode, the engine would not turn over, even though the battery had been recharged and it had to be transported to the dealer. The dealer had to replace the ECU, expensive, that had somehow become damaged, but I still had the intermittant battery drain problem. I replaced all the relays, JIC the contacts on one of them was hanging up, and the problem seemed to be gone, but due to the very intermittant nature of the problem I was never sure that it was fixed.

Jerome_S. Thanks, it may be something subtle like that. The next step for this job may be to hook up a data logger and try to catch it in the act, or at least to determine whether there is a pattern to it.