up vote 4 down vote favorite
share [g+] share [fb]

If I put in batteries the wrong way (+ and - sides reversed), is there a risk that I will break them? Or the device? Or start a fire?

In particular, I am talking about rechargeable Ni-MH eneloops and cheap electronic toys for kids, and most of the time not all of the batteries are put in the wrong way, just some of them are reversed (which is probably even worse, right).

link|improve this question
2  
Check out how Microsoft has solved the backwards battery dilemma, here. – tyblu Dec 12 '10 at 11:56
1  
But it requires licensing, so no one else will ever use it. – endolith Dec 13 '10 at 5:36
As a comment, Ni* batteries can put out hefty currents. I've had a small two AA battery holder melt when the wires got shorted for a few seconds. – XTL Dec 13 '10 at 12:00
Microsoft's may be the only battery holder which compensates for reverse polarity, but many devices use the shape of the battery ends to ensure that the positive terminal will only make contact with the positive end of a stack, and vice versa. Devices with five or more batteries in a stack could be reverse-biased if one connected the end batteries in one polarity and the inner batteries the other way, but that's probably not a terribly common a scenario, and the properly-polarized batteries would help to impede the errant current. – supercat Feb 23 '11 at 16:07
feedback

3 Answers

up vote 6 down vote

Consumer Devices

Almost all consumer electronics have protection in place to prevent any damage when connecting the battery backwards. Companies know they can't trust consumer to put the batteries in the correct orientation and it is usually cheaper for them to add the protection then it is to deal with support calls and returns.

Batteries in your own device

If you are wanting to make your own device, you will need to add protection or it will be almost guaranteed that you will have ICs that smoke. The battery itself probably wont be hurt. They can output ALOT of current. All of your ICs and polarized caps will usually break in such away that they will act like an open circuit. Because of this the batter will only have a short time where it is having to put out a lot of current.

How to protect your device

The easiest way to protect your electronics is to put a fuse in-line with your battery. This is a good all-purpose fail safe. Fuses are too slow to blow to protect all ICs, but at least it prevents your project from catching on fire.

If you place a single diode in series with your battery you can prevent any current flowing in the wrong direction. Do be prepared to see a 0.7v drop across the diode.

You can also use a 4 diode configuration to allow your circuit to continue to work regardless of how to battery gets connected. This disadvantage of this method is that you have 2 diode drops (about 1.4v).

alt text

link|improve this answer
I wouldn't say that Almost all consumer electronics have protection in place. Consumer electronics is often about producing at extremely low cost. A common remote control for your stereo (produced by tens of millions) has 3 or 4 electronic components and can be produced for less than 1 dollar. I don't think there's room there for protection. – stevenvh Dec 12 '10 at 17:35
1  
I can say that I have put batteries in my remotes backwards and haven't had a single issue (other then it not working until I swap them). In a remote, the current draw is very little, because of this the diode used can easily be less then half a cent at those large quantities. – Kellenjb Dec 12 '10 at 18:14
@stevenvh here's another way to think of it. Let's say the extra diode costs $0.10. Now let’s say that 10% of people will accidentally put their batteries in the wrong direction. These people will then call saying their remote doesn't work. The company will then have to pay for all of the overhead and employee time dealing with the call. I could easily see this adding up to $1. The company would then probably have to assume that it wasn't the consumers fault and will send you out a new remote. – Kellenjb Dec 12 '10 at 18:39
This will then cost them an extra $1 for the remote plus some more overhead associated with packing and sending out the new remote. So if you look at the net numbers now for 10 people, $0.10*10=$1 extra in parts for 10 remotes, out of these 10 remotes you can guess that 1 will be hooked up wrong, earlier I showed how this could cost the company $2 (and that’s assuming the company is making the customer pay for the shipping). – Kellenjb Dec 12 '10 at 18:40
So the company put $1 up to save $2. Sounds like a good deal to me. Now, I mentioned earlier that diodes can actually be purchased for half a cent. If I use that number it’s more like it cost the company 5 cents to save $2. It’s a no brainer to me. Even if only 1% of people put their batteries in wrong it still makes sense. – Kellenjb Dec 12 '10 at 18:40
show 1 more comment
feedback
up vote 3 down vote

If you put one of two batteries the wrong way, then there's no problem, neither for the batteries nor for the device; total voltage is +Vbat - Vbat = 0V. No voltage is no current, so your batteries won't discharge. It's just that the device won't do a thing.
If you reverse both batteries the result may be worse: most likely the device will be damaged, esp. if it's electronic. The batteries may survive without too much damage unless the reversal causes a short circuit.

link|improve this answer
"esp. if it's electronic" what else can you can you power batteries with that aren't electronic? – Dean Dec 12 '10 at 13:59
2  
@Dean: small motors? – stevenvh Dec 12 '10 at 15:58
1  
@Kellenjb: lighting an incandescent lamp also involves the movement of more than a few electrons, but I don't think anyone will call the electrical circuit in your house electronics. – stevenvh Dec 12 '10 at 17:30
1  
@Dean -- You can put the batteries in a flashlight with an incandescent bulb the wrong way without any damage. I wouldn't call that electronic. – tcrosley Dec 12 '10 at 19:01
2  
@dean, @kellenjb, you all understood what Stevenvh meant. Just be argumentative for fun. @Stevenvh, I think they tricked you into a pyrrhic victory. You may be right, but the argument was not worth it. – Kortuk♦ Dec 12 '10 at 19:56
show 5 more comments
feedback
up vote 2 down vote

If you put in both batteries the wrong way and apply a negative voltage, your device may be damaged. The designers may not added a protection circuit (i.e. a diode) because it's battery powered and they need all the voltage they can get.

If a device causes a short, that can damage the batteries. Otherwise it's more likely that the batteries will damage(destroy) any IC that gets a voltage outside their maximum ratings.

link|improve this answer
feedback

Your Answer

 
or
required, but never shown
discard

Not the answer you're looking for? Browse other questions tagged or ask your own question.