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 Is it safe to solder directly to batteries?

Q. Is it safe to solder directly to batteries?
A. Steve Lewin

It depends who you listen to. In the safety instructions of every major manufacturer of NiCad and NiMH cells that I have seen there is a very definite instruction that you should never solder directly to the cells. The reason for this is that the cell has a number of components which are made of plastics and which can be damaged by excessive heat. In particular there is a vent designed to allow excess hydrogen to be safely removed from the cell. If you damage the vent, oxygen from the air can enter the cell and the internal chemistry will be severely degraded, dramatically reducing the life of the cell.

However these same manufacturers will tell you that you must not charge a cell in less than an hour and you must not discharge at higher than 15C (7.5A absolute maximum for 500AR cells !). By their standards we are already abusing their cells horribly.

Most of us do not have access to the spot welding machines recommended by the manufacturers. Most cells available to us with spot welded tags have only a few welds and thin tags, usually of the wrong material. They will not take anywhere near the currents we require. If you have access to battery packs made with thick pure nickel tags and at least 6, preferably 8 welds per connection these are probably the best of all possible worlds. For most of us the only method available for constructing packs is soldering.

To make soldering as safe as possible you must use a HOT iron with a large heavy tip so that it keeps its temperature when you put it in contact with the cool cell. 40Watts is the practical minimum. The objective is to have the iron contacting the cell for as short a time as possible. NOTE : soldering guns are bad for this use because they have very small tips and cool down too much meaning that you hold them in contact for far too long giving the heat time to spread to all the plastic parts. Each soldering operation should ideally take less than 2 seconds. 5 seconds is about the limit. Clean everything well and degrease carefully. First tin the contact and the wire or bar separately. Then let them cool. Then reheat the wire, then the cell and put them together. Hold them in contact until the solder has solidified, which only takes a few seconds. Practice with some scrap cells until you are comfortable that you can do each operation as quickly as possible.