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#115077 - 12/07/07 09:24 PM Backwards Batteries: Fact or Myth?

More than once I've heard of putting one or all of the batteries in a flashlight backwards from their proper position to prevent voltage leaks or induction from draining the batteries while the light is in storage. I didn't put much faith in this until I read it in the SAS Survival Guide. I'm still skeptical however.

Do any of you practice this or have you heard of it? with the price of lithiums I'd like to keep them as fresh as possible in the emergency lights.

#115079 - 12/07/07 09:49 PM Re: Backwards Batteries: Fact or Myth? [Re: ]
Arney Offline

Registered: 09/15/05
Posts: 2485
Loc: California
You can fry an LED from reverse polarity if you put the batteries in backwards unless the light specifically has circuitry to protect against it. However, I don't think most incandescent lights are harmed by reverse polarity, which is what the SAS manual is likely referring to.

Very few lights will draw any current when off, and even then, it's usually miniscule (usually these lights have electronics for multiple levels or multiple modes). The SAS manual is probably referring more to accidental activation of the switch, like in your pocket or in a pack. Does the manual mention voltage leaks or induction? Because I don't really see that being a problem with modern lights. If the light doesn't come with a lock out tailcap, like a Surefire, you can also loosen the head or tailcap so that activating the switch cannot physically close the circuit.

Personally, I would avoid putting batteries in backwards in anything electronic.

#115080 - 12/07/07 09:54 PM Re: Backwards Batteries: Fact or Myth? [Re: Arney]

The SAS manual specifically prescribes turning one of the cells backwards to prevent accidental switch ons. Like I said though I've heard many variations over the years...to me there would be no benefit to doing this.

If I purchased an LED light that didn't have reverse polarity protection I'd toss it and consider it junk...or return it and get my money back.

#115083 - 12/07/07 10:39 PM Re: Backwards Batteries: Fact or Myth? [Re: ]
raydarkhorse Offline

Registered: 01/27/07
Posts: 510
Loc: on the road 10-11 months out o...
If you turn only one of the cells backward it won't cause any harm to your light LED or otherwise, it wil only blow your LED out if you turn both of them backwards there wil be no effect on your incandesent bulbs at all. It's a PITA though if you need your light in a hurry.
Depend on yourself, help those who are not able, and teach those that are.

#115094 - 12/08/07 01:12 AM Re: Backwards Batteries: Fact or Myth? [Re: raydarkhorse]
unimogbert Offline
Old Hand

Registered: 08/10/06
Posts: 836
Loc: Colorado
One backwards battery out of a pair will not discharge if the light is accidentally turned on. The batteries will oppose each other and it acts like the switch isn't on.

One backwards battery out of 3,4,5,6 or more won't protect teh batteries against a switch on but will just produce less light and will be a waste of the reversed battery.

Turning both batteries backward is an incandescent bulb light will make little difference except that connection might not be very good as the contacts are designed differently for each end.

I've done this for flashlights that have poorly protected switches.

(I'm an electrical engineer.)


#115102 - 12/08/07 02:06 AM Re: Backwards Batteries: Fact or Myth? [Re: unimogbert]
dougwalkabout Offline
Crazy Canuck
Carpal Tunnel

Registered: 02/03/07
Posts: 2537
Loc: Alberta, Canada
Yes, the word is that for some LED lights reversing both batteries will fry the LED.

One time, when travelling, I cut a circle of white plastic from a yogurt container lid and put it between the endcap spring and battery (in one of those AA Minimag-style lights; in this case, a Fenix L2T). Perfect electrical isolation, and I could put the light back into action in less than ten seconds, even in the dark.

#115112 - 12/08/07 03:31 AM Re: Backwards Batteries: Fact or Myth? [Re: dougwalkabout]
Art_in_FL Offline

Registered: 09/01/07
Posts: 2432
It works fairly well. Key here is that it only works with lights that use an even number of batteries because you have to invert exactly half of them for this trick to work.

It also is important that the batteries all be of the same type and relative strength, age, amount of energy remaining.

This was far more of an important insiders trick when flashlights generally had switches that were easy to bump into the on position. Most modern flashlights have rotary or recessed push-button switches that are pretty darn hard to activate accidentally.

Packing for critical situations where the packed flashlight is going to be banged up in transit I leave the batteries out. I also tape over the ends of the batteries with tape and keep them in a bag to eliminate any risk of them shorting out.

This is a trade-off. In some ways with a high-quality flashlight there is less risk of the flashlight accidentally turning on than of the batteries being lost, damaged or shorted out by moisture, sweat and salt water are the worse, or conductive gear.

Inverting half of the cells would give you an additional level of protection. But so would taping the switch off or inserting a non-conducting disk that could be removed when you need the light to work. Off and on I have inverted half the batteries but one day when reinstalling them in a four cell light it dawned on me that in a desperate situation and less than perfect conditions it would be easy to drop the batteries and have to hunt them down at the worse possible time.

I can see this quite clearly. I'm cold, wet and standing in a puddle up to my ankles during a freezing rain at night when I drop one or more of those batteries in the murky water. Screwed in a situation of my own making. Unhappy camper.

Be careful about that insulating disk also. I have never used this method but I could picture that bit of cardboard falling down into the flashlight where you can't get to it easily. Given time, some light and a handy stick you could remove the offending bit but this would be a hell of a job to have to do under pressure.

Years ago I got carried away taping the spare batteries and it took an act of congress, and a sharp knife, to extricate the batteries. There was no rush at the time but since then I try to make sure the tape can be removed quickly without need of a tool.

I don't know about all LED flashlights but all of the ones I have used show no ill effects from reversed polarity. The diode blocks the voltage. No harm done. But be sure to check your light. Murphy's law says that those batteries will be inserted the wrong way in the heat of the moment. You want to know how your light or device will react before your in the middle of an emergency so you can leave vulnerable devices at home where such weakness is more acceptable.

I wouldn't keep a device around that was so sensitive. I would give it away or otherwise pension it off. Even at home if I can't count on it I don't want it around.

#115113 - 12/08/07 03:32 AM Re: Backwards Batteries: Fact or Myth? [Re: dougwalkabout]
Alex Offline
Old Hand

Registered: 03/01/07
Posts: 1034
Loc: -
Originally Posted By: dougwalkabout
Yes, the word is that for some LED lights put it between the endcap spring and battery

That's the best way. Reversing one of the batteries even in a 2 battery light still leave room for electrical potential, which will drain the batteries if the switch fail or leak (that happens too due to a hidden defect). All cells are slightly different in voltage and resistance.

I'm second to your recommendation.
Though, for a small kit (like EDC) it is impractical to keep batteries and empty flashlight separately. Except if you consider to stuff the latter with something precious for protection...


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