soldering AAA batteries

[ericmark]

This is what most replaceable batteries look like. The problem with old equipment is charging them. Old phones would have NiCad not Ni-MH and the charger is not the same. What seems just a battery often has some electronics built in

the early Ni-cad would have a peak voltage when charged and this was used to tell the charger when the battery was full, as soon as the battery voltage dipped the charger would turn off. But as we moved to Ni-MH the single cells had some electronics in them, but where they are charged as a group this control is for the group as a whole. So with my cordless phone I can swap Ni-cad for Ni-MH cells without too much of a problem, but the AA Ni-Cad was 600 mAh where the Ni-MH is 1900 mAh so the charger which was timed to switch off after it had delivered 600 mAh turns off too soon with the new cells. So every so often I have to remove the batteries and put them in a stand alone charger, and my phone does tend to go discharged to early, as a house phone this is not too much of a problem, I can grab another phone, and manually recharge the batteries. With a mobile phone that is not so easy.

So it depends what type of cells the phone had to start with.

 

[JohnW2]

..... Old phones would have NiCad not Ni-MH and the charger is not the same....

The OP was talking about "mobile phones", not cordless landline ones. Heven't all mobile phones (in recent decades) had lithium-ion batteries, rather than either NiCad or Ni-MH ?