Are momentarily shorted batteries still safe to use?

Using Arduino General Electronics
1

Hello everyone,

I've got what might be a stupid question, but I thought I'd check for some advice nonetheless, as I'm far from an expert. I've been trying to debug a soft latching switch for a small light chasing robot I've been tinkering with. I used some empty space on my breadboard to test one of the small 6mm push button momentary switches that came with my kit to see if it was working correctly by hooking up a completely separate and simple circuit to switch an LED on by pressing the button.

As basic as it gets. Except that I managed to get the push button switch turned 90 degrees the wrong way, so that the battery + and - terminals were connected on the same side of the switch, immediately shorting out my battery pack (eight 1.2V 2500mAh NiMH AAs in a plastic holder with 9V-style snap on leads). By the time I pressed the button once, saw no effect on the LED, and looked up, wisps of white smoke were making their way out of the holder.

After quickly disconnecting the pack and cursing a bit, I inspected the holder and found that some of the internal contacts appeared scorched, and a bit of the surrounding plastic was slightly melted (I assume at least one source of the smoke). The batteries themselves appear normal - no scorching, bulging, or any other obvious defect. I dropped them two at a time into a penlight, and they seem to work OK. The batteries didn't seem warm/hot a few moments after the incident either.

Still the "Do Not Short Circuit" warning on the side of each is leaving me with a Here-Be-Dragons sort of feeling. But I'd hate to throw out perfectly good batteries, because they're kind of expensive (for me anyway).

I did some searching and found this previous thread, where a participant states that one should "be aware of the danger of hot lithium ion batteries (place them on a ceramic or metal surface outside just in case..) Other batteries (NiMH / alkaline) aren't a fire hazard" (emphasis added by me).

So to sum up with two questions:

  1. Is there a way to tell if these batteries are still good/safe to use?
  2. Why aren't NiMH batteries a particular fire hazard; or, are they reasonably tolerant of these sorts of screw ups?

Thanks for your advice!

2

Your batteries are probably just fine.
Lithium burns.

3

Those 4-pin square switches are a curse.....

4

jgladieu:

  1. Is there a way to tell if these batteries are still good/safe to use?

Likely, as mentioned previously - they are fine. If you really wanted to know, you would want to discharge them to about 0.9 volts per cell (or 7.2 volts for the pack) - don't go any lower than this! - then recharge them at their proper rate (I encourage you to purchase a "smart" charger if you haven't already - they are more expensive than a dumb wall-wart charger, but worth it; some even come with discharging capabilities).

Once they are recharged, you can then put them on a load (sand resistors or similar - sized for the current draw needed) and measure the pack voltage; if it dips fairly quickly down or below 7.2 volts - then one or more cells in the pack is no good (you could do the discharge/recharge/load test again to each cell to see if any are working properly).

If, however, the voltage lowers at a steady pace consistent with the load sizing (it would need to be timed), then it is ok to use.

jgladieu:
2. Why aren't NiMH batteries a particular fire hazard; or, are they reasonably tolerant of these sorts of screw ups?

Every battery is a potential fire hazard when shorted (max current draw!) or when a heavy load is placed on them; some batteries can withstand fully max current draw and max recharge current - the pack will get warm, and the life may be shorter, but it will continue to work.

Other batteries (well, the individual cells, actually) have a max current draw you shouldn't exceed (and a max current for charging that shouldn't be exceeded either) - if you do, the battery could be damaged or catch fire; the best way to figure this information out is to contact the manufacturer of the battery and/or cells. If you purchased cheap chinese no-name cells, though, you are likely out of luck in finding this information (and you might want to assume that the lettered mAh on the side of the cells is not correct either, but inflated by some percentage).

5

If you shorted Ni-Mh batteries for a minute or two and they did not get hot, all you have done is given them a little bit of exercise.

They are used in things such as rechargeable soldering irons drawing thirty amps or so, and are quite rugged. Your wiring resistance was probably quite significant so that it heated up rather then the batteries, and it will probably burn before any harm at all happens to the batteries. I would not even concern yourself with testing their capacity - if they were not warm, they have hardly noticed the discharge.

Now one has to be careful with Lithium batteries because if they heat beyond a certain point, the internal separators can break down causing a chain reaction despite being disconnected at that point and they have of course, a much higher energy density than Ni-Mh so that chain reaction is guaranteed to be catastrophic. They would appear to be at even more risk when being charged as the power absorbed during charging is added to the potential of a fully charged cell.

6

LarryD:
Your batteries are probably just fine.

cr0sh:
Likely, as mentioned previously - they are fine.

Paul__B:
If you shorted Ni-Mh batteries for a minute or two and they did not get hot, all you have done is given them a little bit of exercise.

Yeah, the batteries themselves look and seem to work OK. Had to toss the holder though because one of the terminals became warped, causing unreliable contact. I think the sight of smoke just got me a bit spooked.

cr0sh:
If you really wanted to know, you would want to discharge them to about 0.9 volts per cell (or 7.2 volts for the pack) - don't go any lower than this! - then recharge them at their proper rate (I encourage you to purchase a "smart" charger if you haven't already - they are more expensive than a dumb wall-wart charger, but worth it; some even come with discharging capabilities).

Once they are recharged, you can then put them on a load (sand resistors or similar - sized for the current draw needed) and measure the pack voltage; if it dips fairly quickly down or below 7.2 volts - then one or more cells in the pack is no good (you could do the discharge/recharge/load test again to each cell to see if any are working properly).

I'll give this a try and see what I get. The smart charger sounds like a good idea too.

cr0sh:
Other batteries (well, the individual cells, actually) have a max current draw you shouldn't exceed (and a max current for charging that shouldn't be exceeded either) - if you do, the battery could be damaged or catch fire; the best way to figure this information out is to contact the manufacturer of the battery and/or cells. If you purchased cheap chinese no-name cells, though, you are likely out of luck in finding this information (and you might want to assume that the lettered mAh on the side of the cells is not correct either, but inflated by some percentage).

Paul__B:
Now one has to be careful with Lithium batteries because if they heat beyond a certain point, the internal separators can break down causing a chain reaction despite being disconnected at that point and they have of course, a much higher energy density than Ni-Mh so that chain reaction is guaranteed to be catastrophic. They would appear to be at even more risk when being charged as the power absorbed during charging is added to the potential of a fully charged cell.

Thanks for the info/tips. I'd considered lithium batteries, but decided they were more expert-mode. Probably a good choice, considering my propensity for trying to set things on fire.

JimboZA:
Those 4-pin square switches are a curse.....

They are. In fact, I switched to a 9V temporarily, so that I could continue working on my latching switch, and nearly got the push button the wrong way around again. Luckly, I used my DMM's continuity test to double-check which pin was which first. I think I'm going to make that a general rule...

Thanks everyone!

7

I don't understand - if you have +ve and -ve battery terminals connected to the
switch in any manner you're going to short circuit the battery, whether its
turned 90 deg or not....

8

MarkT:
I don't understand - if you have +ve and -ve battery terminals connected to the
switch in any manner you're going to short circuit the battery, whether its
turned 90 deg or not....

Pairs of pins are always connected. Only way to guarantee a switch, is across a diagonal.

edit: annotated version of OP's pic attached

short.PNG

9

JimboZA:
Pairs of pins are always connected. Only way to guarantee a switch, is across a diagonal.

All very well, but Mark's query was far more fundamental.

Why would you ever have - in any form whatsoever - a switch connected directly across the power supply?

10

Paul__B:
Why would you ever have - in any form whatsoever - a switch connected directly across the power supply?

Beats me, Squire.