# Is soldering wires directly on a NiMh battery safe?

I'd like to know if soldering two wires directly on a NiMh battery is considered as safe or not.

My fear is that battery would explode (right in my face) because of excessive heat caused by the soldering iron. Other possibility would be the battery slowly inflating and then spreading toxic fumes (or corrosive materials) trough a hole (like a capacitor under excessive voltage).

The battery I want to use is made of 10 units of 1.2V (thus generating 12V)

• There are NiMh batteries with soldering fans instead of regular terminals avialable. Much easier to solder, but you might not like their price tag. Jul 6 '13 at 14:14
• The below answers are good. I'll just comment that I have soldered to batteries before to create my own banks. In fact, that is how the packs are made - someone soldered the metal tabs across the ends. Just be careful no to overheat the battery and follow the other great guidelines listed below. Jul 6 '13 at 19:05
• Questions on safety of welding (err. Soldering) to battries are not electronic design, right guys, right? Apr 8 '14 at 18:57
• @Passe: No. This is a legitmate issue related to designing with batteries. Apr 9 '14 at 14:09
• @Passe: Huh? This question is about soldering and/or welding connections to batteries, and I think it is on topic. It's strictly speaking not electronic design, but it is a issue that comes up as part of electronic design. We've always treated general soldering questions and even questions about soldering irons as on topic. I don't see how this is different, and I'm not voting to close. I even upvoted the question. It's really not clear what point you are trying to make, and I'll be surprised if there will be enough votes to close this. Apr 9 '14 at 16:47

It's probably safe enough from your point of view, but not from the battery's. You really shouldn't solder to batteries unless they explicitly have solder tabs for that purpose. Most batteries, and NiMH are no exception, are damaged by soldering temperatures.

The way to make a permanent connection to a battery that doesn't have solder tabs is to use spot welding. This presses the battery terminal and the contact together, then zaps them by discharging a capacitor thru this connection. That heats the two parts enough for a little metal to melt and bond. However, the zap is very short and localized, so the total energy is low and high temperatures diffuse well before they get to sensitive parts of the battery.

Note that no solder is evident in the picture you show. That is because the tabs were spot welded, not soldered.

• I found cheap batteries on ebay that are made to be soldered. They are provided with a tab already and they boast the ability to resist high temp. Much cheaper and practical than buying a spot welder :). Those batteries are from Honk Kong and they also come cheaper than most normal AA in western countries. I can't tell about the quality, though, yet. Just ordered a couple. Very likely stock batteries provided in all home devices are still from there though. Aug 25 '13 at 7:20

An important concept, when talking about spot welding batteries, is to realize that the welding electrodes used are arranged close together and applied to the top of the tab.

This ensures that the welding current flow is very localized at the "head" or "foot" of the battery, only through the tab and the relevant battery terminal cap The welding current does NOT pass "through" the battery at all, from one battery electrode to the other, but ONLY along the very short, closely-spaced spot-weld/solder tab.

The net result on the battery electrodes is no effect, and no electrical current flows through the battery at all during welding, only through the millimetre-or-two separated spot-weld area of the two pieces-to-be-welded.

I have terrible nightmares of people reading thread like this for the first time, grabbing using 2 large crocodile clips and passing 300 Amperes though a tiny AAA cell from one end to the other. This would be extremely dangerous, so I think should be mentioned because I've seen it attempted before, with disastrous results.

• This is a GREAT Point that isn't mention more often even when it should. Apr 9 '14 at 2:50
• I thought this was obvious, but good to get out there anyway just in case. +1 Apr 9 '14 at 14:12

Yes, it's safe as long as you don't use excessive heat. NiMh batteries are relatively stable. Practice some with plain wire and make sure your soldering is up to par. To help with the soldering, you can use some flux such as Chip Quik Flux. You should not have to hold the soldering iron for more than a few seconds before the flux activates and the solder melts. Any longer than 10 seconds is likely too long, and you should reconsider how you're soldering.

The batteries that are likely to be a danger are Lithium Ion coin cell batteries:

These batteries can be soldered to using the same technique, but you should definitely be careful and wear protective eyewear. I've made dozens of blinky LED lights this way without a problem. However, when I allowed people new to soldering to build the batteries they routinely overheated them and let the explode. I don't do that any more (we tape them).

While I'd hesistate to say something was 100% safe I'd consider the risks with such large batteries as minimal to zero if you use a good technique. The way I'd go about it to minimise the risk with those batteries is:

1. Under the tab you're about to solder place something like a steel ruler or other small piece of metal so that the tab isn't in direct contact with the battery while you solder. I see you've updated the photo with a new configuration that doesn't allow this so you may skip this step.
2. Use a small file to give the tab a good clean and remove any outer protective coating that may have been applied and any oxidation.
3. Using a solder that includes a flux core with your iron set to a slightly higher temperature than you might normally use on a PCB and you should be able to form a good joint within a few seconds.

After doing that you should find the body of the battery has only been heated enough to be barely detectable with your fingers if at all. For most NiMH batteries 60C is considered OK during charging (as a maximum) so unless it feels quite warm you'll be well under that limit.

If you are agile enough to do it with some few seconds (1-3) of applying the iron, it should be ok. At least, use safety googles, just in case. For something to explode, pressure is needed and this is normally achieved by heating a gas, so think about how hot can the battery become during the process. Small ones will get hotter soon (One button battery exploded to me while playing with it trying to charge it - it was not the rechargeable type - by explosion I mean the lid just popped some 40 cm up). If possible try to cool down the battery body while soldering, for example with a metal body or even a wet cloth

It should be fine as long as it is done very quickly. If you can, I highly recommend spot welding.

You should never solder directly on a battery. It might not be dangerous, but you will hurt the cell. If you must solder, do it where I have marked in red, and squeeze a fishpaper or something that can take the heat away from the cells.

The best solution is to spot weld a new nickel-plated steel tab and than solder.

Good luck!

After the solder sets (less than a second) dab the joint with a cotton ball soaked in alcohol. The evaporation of the alcohol will cool the hot spot before the heat soaks into the battery.

• Now we need to add flaming cotton balls to the list of dangers... (For the record, this is a terrible idea. Isopropyl alcohol spontaneously combusts at 399C/750F, which is not an unreasonable temperature for a soldering iron. Water has twice the specific heat and will cool it to safe temperatures much faster. But the heat damage would likely be done long before you cool it off anyway.) Feb 1 '16 at 17:51