Making Replacement Cordless Phone Batteries

My 3 year old cordless phones  started to die.  They would seem fine, show full bars of charge, but then fade out, usually in the middle of a conversation,  or while the phone was ringing.   #1 went first a few months ago.  Then a couple more just seemed to be dead all the time.  One actually seemed to have some life left, but as the handsets were quadruplets, who knew which was the good one?

Replacements packs average $20 and more ready made.   Although I knew it was a rip-off  I bought one from Radio Shack a few months ago.  It is now dead, in fact the worse of four.  Fed up with the high price of incovenience and bad quality I decided to build my own.

As usual, we start in the middle.

This is what my setup looked like after  I pulled out the old packs, taken off the wrappers, pried off the battery leads or “pigtails” that plug into the phone with needle nosed pliers, and stripped about 1/8″ off the ends to bare copper wires.    These we have to solder to the new battery pack we are about to make.

Here you see the new bare cells, an old pack, a soldering iron, a roll of solder, a wire stripper and wire cutter, and a tube of flux.

I should say that this job is much easier if you already know  how to solder.  Soldering is not for everybody, but if you find you want to try, sure,  do it.   Do consider that tools make this job possible, needle nose pliers, wire cutter, wire stripper,  a soldering iron, solder,  some wire (four 2.5″ lengths of  of #22 stranded copper),  flux.   If  you buy tools you  may not save money this time around.   ( The cost of ready made batteris  =4 x $20 or $80.   The cost of 12 AAA eneloop  batteries + shipping is around $28,  a 40 watt Weller soldering  iron $20, solder ?$5,  flux ? … well, maybe you can save money).  I recommend practicing on the old battery pack first to get the  soldering technique down.   Cut off the shrink wrap because you will need the leads.

Soldering is fun and to make things even more fun, there is a little bit of danger to go with it.  You work with your hands,  high temperatures,  melting dripping metal, lead/tin and rosin fumes.   For this job, you will  need at lease a 40 watt iron.   Less than that … may not be enough heat.  It’s important to not put a lot of heat into the battery while soldering  so a hot iron is required.  The high heat make the joint heat fast before too much heat flows into the cell.    Some major soldering advice – use a little bit of  rosin flux, and apply it to all wires ends to be soldered and the battery terminals (aka “the work”) – just a thin smear is enough – this will make a huge difference in how well the solder flows.  Flux is usually a translucent amber colored fluid something like tree sap in consistency and stickiness.   It helps the solder flow out onto the work – if it doesn’t the joint won’t be any good.  OK – so if you know the soldering basics, including some way of holding the  two wires, the tree batteries, the solder, and soldering iron with your two hands  at the same time, you are good to go.

These packs are built from 3 AAA rechargeable NiMh cells.  If you buy a pack of 4 AAA rechargeables from Radio Shack or hardware store, they still cost around $20.   I went online to Battery Junction where you can buy 850 mah Sanyo Eneloops, $8 for 4 cells. Shipping raises the price somewhat.  Eneloops are among the best consumer NiMh AAA cells you can get.  Sanyo produces a good product; consistency and quality control.  I needed 12 cells for four batteries.  3 packs of four then.

Here is the finished battery pack in the phone.   It looks passably like what I took out.

Above, the bottom of the 3 cell pack soldered together.  The cells are taped together to keep things together. Note the bridge joining two cells in series with a short piece of copper braided wire.    This example is of a barely passable solder job.  The solder did melt and flow although there is a blob like appearance, the joint is not the dreaded “cold solder joint”.  You get the idea.  Make your own.

A note on battery life.  Cordless phones usually sit in a charging cradle which trickle charges the cells with a low current.  After a few hours the cradle proceeds to slowly overcharge the batteries.  The batteries will get mildly warm with the trickle charge.  Because the current is so low, there is no fire hazard and the cells will  bleed heat, but not vent or explode from gas pressure or hot electrolytes.    The overcharge is probably the reason the batteries last two years instead of five.  If the phones are used a lot, it’s possible that they won’t necessarily get overcharged – that is probably ideal.  In my household, convenience over-rides economy so the phones do tend to sit in the cradle unto death.


One Response to “Making Replacement Cordless Phone Batteries”

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