Constructing Replacement Battery for Cordless Phone

Discussion in 'The Projects Forum' started by erotavlas, Aug 13, 2009.

  1. erotavlas

    Thread Starter New Member

    Aug 13, 2009
    Hello, My cordless phone battery died a while back. I was going to go buy a new one but found it to be too expensive (!) So I thought why not make my own battery. Basically the specs of the original battery are

    800mAh 3.6V NiMH

    There are 3 cells in the battery each the same size as a AAA battery.

    My idea was to use 3 Duracell NiMH AAA batteries and connect them in series (which gives the same voltage of 3.6 since each AAA battery is 1.2 volts) But the AAA batteries are 1000mAh not 800....does this make a difference. I was thinking it would not harm the phone, but rather give more talk time.

    Does this make any sense, and will what I have suggested work? I just want to make sure I do not damage my cordless phone.

    My other question is:

    1. will there be any problems charging the batteries when using the original charger that came with the phone? (like overcharging)

    thanks for reading and for any help
  2. someonesdad

    Senior Member

    Jul 7, 2009
    I can't give a definitive answer, but if it was me, I'd give it a try. Assuming the battery chemistry is actually the same, then you should have a safe solution. The worst that could probably happen is if the charger somehow integrates the current to limit the total charge (to avoid overcharging); then you'd wind up with undercharged batteries. My guess is they wouldn't bother; charge rate is probably controlled by cell voltage.
  3. jpanhalt


    Jan 18, 2008
    Another issue to consider is size. The positive pole on individual batteries that you get at Walmart, etc. is raised relative to the positive on welded packs. I have come across one situation in which the fit was so tight, I had to use welding rather than soldering with a braid for the over-the-counter cells (OTC) to fit. That situation is probably rare, but is worth checking on.

    Another advantage of the newer OTC cells, particularly those made by Sanyo in Japan (sold by Energizer and others under their own name...the clue seems to be Made in Japan) is that they have a very low self-discharge rate. They are often advertized as "pre-charged." The Sanyo brand is "eneloop." A supposedly similar product is made in China. I have had good luck with eneloop and Energizer, but no experience with the version made in China.

    As you will probably be soldering your pack, be sure to use a large iron to keep time and heat exposure to a minimum. Tin the battery contacts before soldering, again to minimize exposure to soldering temperatures.

  4. jj_alukkas

    Distinguished Member

    Jan 8, 2009
    First option would be to get another manufacturer's battery with the same spec's and size.

    And if you can, you can use 3 AAA's without any problem. 1000mAH will only give you more talktime. If your cordless set is an old one, it uses the C/10 charging technique which would be 86mA charge current which would do no harm on a battery rated for 100mA charge. You simply would need a bit longer, thats all. However, if its a modern phone, it uses a microcoontroller based charging system and would do no harm as it uses a combination of NDV(Negative Delta Voltage at full charge), rate-of-temperature-increase (dT/dt), temperature sensing and timeout timers. The charger utilizes whatever comes first to terminate the fast-charge. So dont worry.

    The only thing you will need to worry arises when you use a lower capacity cell or a Nicd instead of NiMH.

    However, try if the 3 AAA's are fitting into your slot. Also when soldering, observe not to overheat NiMH cells.
  5. erotavlas

    Thread Starter New Member

    Aug 13, 2009
    thank you everyone for the advice !

    I went ahead and made the battery pack. It seems to work very well and it is charging as expected. It fits ok in the phone like the original. The hardest part was getting the solder to stick to the battery terminals. I was afraid to heat up the battery so instead I used small springs on each of the ends that were to be connected in series and soldered those together with a wire between them to make the contact, then I taped them up real tight against the battery. Kind of ghetto but it makes a good enough electrical contact.

    But I did have to solder the last two terminals +/- that connect to the phone because I ran out of springs :rolleyes: That was so hard I almost gave up but on my last try I managed to get it to stick just enough so I could solder the final wires to it. I used a glue gun and applied copious amounts of glue around the contact to ensure I didn't dislodge the solder again .... :) so sad I know but it worked....
  6. Søren

    Senior Member

    Sep 2, 2006

    To get a good solder connection, you need to sand the metal with fine emory paper or similar and then use a soldering iron of at least 50W with the temperature a bit higher than usual, to get a proper connection without heating the cell too long/much.
  7. Audioguru


    Dec 20, 2007
    I went to buy a new battery for my cordless phone and found a new cordless phone with a battery for much less cost than a new battery and about the same cost as a battery that I make myself.