[SOLVED] Repairing battery pack

[atha]

Hi!
My power drill battery pack suddenly woulden hold charge. I opened it and one of two of the cells are dead. One of them gets quite hot. I want to switch them with new ones. They are 2600mah. Not so common. 2500mah and 2700mah are easier to find. What will happen if I use them instead?

 

[ads-ee]

mAh is the capacity of the battery, so using the 2500mah will reduce run time, using the 2700mah will increase run time. In either case run time differences will likely be negligible. Just buy the best quality batteries as many of the manufactures are producing substandard stuff that won't even be close to the rated capacity.

 

[atha]

So it wont hurt the batteries if they are not all the exact same capacity?

 

[BradtheRad]

Your old 2600mah cells have declined, so a 2500mah should be okay. That is approximately the capacity of AA rechargables available today.

Do you intend to clip away the bad cell and solder in the new one? If so then the new one really ought to have tabs for soldering. It is hazardous to heat a battery for a long enough time that high temperature can reach its internals. The battery might break a seal, or worse.

Nevertheless I have connected batteries by soldering wires to the terminals. I tin all surfaces beforehand. I use a high wattage soldering gun. I do not hold the hot tip against the surface. I let the tip heat for a few seconds, then touch it to the parts, for the shortest time that melts the solder.

 

[Audioguru]

You forgot to tell us what chemistry is the battery. Ni-Cad, Ni-MH, Lithium or lead-acid?
If the cells are in series and are charged like that then they should have the same age, the same manufacturer and the same capacity.
Some cheap batteries will not produce the power that is needed. The capacity (if it is the truth) is how long a charge will power the product, a higher capacity will run the drill for a longer time.

I have some very cheap Chinese AAA Ni-MH cells from solar garden lights. Most of them produce low maximum current. They are marked 600mAh capacity but I measured less than 100mAh for most of them.

 

[atha]

Yes, it is nimh. and I intend to replace the two bad ones. But I hadnt thought of the capacity being not what it says on the label.
I gather it would be better to replace all cells with identical cells both in brand and capacity althrough more expensive.
I'm thinking of buying the ones from panasonic with tabs.

 

[ads-ee]

You should replace all of them not just two of them. Mixing batteries, old/new or differing mah, is never a good thing to do as the lower capacity batteries will discharge before the higher capacity ones.