Rechargeable Batteries in BT Synergy Cordless Phones

We’ve had our set of three BT Synergy 5500 cordless DECT handsets since 2009, and after about 3 years of heavy use, the rechargeable batteries appear to have come to the end of their life.

The manual with this BT handset does state that performance will deteriorate over time, and normal Nickel-metal hydride batteries, like the ones used in this phone, typically can only survive about 1000 full charges. Oddly, it seems that the handset we use least, has fared the worst, presumably as it almost never gets run down to flat and fully recharged.

BT Synergy 5500 DECT cordless

BT Synergy 5500 DECT cordless

Replacing the batteries in the Synergy 5500

The act of replacing the batteries is very straightforward, so no need to go into much detail here. For us, the hard bit was finding the replacements….

BT Synergy 5500 DECT cordless battery compartment

BT Synergy 5500 DECT cordless battery compartment

The two batteries originally supplied with each handsets are as follows:

  • Type: 1.2 Volt AAA Ni-MH
  • Rating: 550mAh
  • Manufacturer: Kaifeng New Energy Co. Ltd (China)
  • Standard charge: 16 hrs at 55mA
Kaifeng 550mAh AAA battery

Kaifeng 550mAh AAA battery

Possibly rather naively, we assumed that it was a simple case of popping out to a high street electrical shop such as Currys, and buying a replacement set of rechargeable batteries. That’s exactly what we did – opting for a set of Energizer 700mAh batteries. These worked perfectly in one of the two handsets be tried, but not in the third identical handset. We also tried a higher capacity 850mAh battery, but this failed to work in any phone, despite being fully charged.

AAA Energizer battery

AAA Energizer battery – not quite a success for us

After consulting the manual and doing some checking online, it seems that BT cordless phones are very fussy about what types of batteries can be used. We put in a call to BT’s Synergy Helpline on
0800 145 6789 to be told that many rechargeable batteries don’t work and it’s important to buy the correct ones that are designed for the phones and with the correct power rating. Apparently only batteries rated between 550 and 750mAh will work – and they strongly recommended buying the correct replacements otherwise the phone may not work.

In the end, we were sold three sets of replacement batteries for £15.98 (inc postage and packing), with an estimated 5 days delivery. (Price as of May 2013)

Since ordering, we’ve found an online shop selling batteries suitable for the BT Synergy 5500 – sell packs of 750mAh with claim to be compatible. Price for 6 batteries is £10.20 including postage and packing – a saving of £5.78 on BT’s price.

We also looked on Currys Partmaster, normally a great place to buy spares, but at the time of checking – no stock. It;s worth having a look though: Partmaster Cordless Phone Batteries

It pays to shop around.

This entry was posted in Phones.

30 Responses to Rechargeable Batteries in BT Synergy Cordless Phones

  1. richard says:

    Thanks for posting this. Did exactly the same as you, trying other replacements etc which dont work. Went to buyabattery, for exact match, and cheaper than BT. All 4 phones working fine now.

  2. diane says:

    it happened the same to me. I bought Panasonic and when I popped them in, they wouldn’t work. I tried putting one of the Kaifeng batteries together with a Panasonic one and, hey presto, the phone was on again. I tried swapping the position of the batteries, and the phone went off. That meant that the panasonic batteries worked for the phone but only on one of the sides of the batteries holder. I compared the Kaifeng and Panasonic side to side and the Panasonic have a longer body and shorter positive terminal. This meant that a little plastic holder on the positive terminal inside the battery holder was impeding the positive tip on the Panasonic battery from contacting the positive terminal on the phone. I just nipped the little plastic guard with scissors and the batteries fit like a glove. Check inside the battery holder before going to buy new BT batteries, but be careful when nipping off the little plastic guard. It worked for me.

    • Jackie says:

      thanks for this great tip, was about to buy another set of phones as the rechargeable batteries didn’t work!!

      • Mick Kelly says:

        Thanks Diane!

        Just got my 900mAh batteries to work by 30 seconds with the craft knife.

        Much obliged.

    • Ray BENNETT says:

      Just tried this appears to be working , best advice yet

    • ceno says:

      I know it’s been literally *years* since you posted this but I just had the exact same problem with my TELEFUNKEN phone (Colombo) which came with the green Kaifeng batteries shown in the article.

      I also couldn’t for the life of me figure out why no other brands would work until I stumbled upon your comment. And sure enough, my phone also had those little plastic guards, preventing the usage of other batteries than the one originally provided (with the slightly longer node).

      So thanks again for this tip and BOOO to all the manufacturers who do this 🙁

  3. Steve says:

    Hi Diane, a great steer thanks.
    My spare 700mAh no brand rechargeables worked a treat once I’d removed the little plastic shrouds around the +ve ends. Cunning old BT! Feels good.

  4. Mark Stothart says:

    Great tip. Nipped and 7dayshop batteries now working. Result. Thanks.

  5. Bethan says:

    Thanks so much, as above I snapped off that tiny plastic bar & ay, I’m now using non-rechargeable batteries!!


  6. Andrea says:

    Thanks so much for taking the time to give advice – very helpful

  7. Phil says:

    Do I need to charge up new batteries before installing into phone?

  8. Valerie jones says:

    I have had four set for four years now they run out after 15mins so I have ordered new phones. But would like to keep these as silver what should I do now please

  9. Disco Jones says:

    Hmmmmm, I don’t get these phones one bit. Some Duracell regular batteries didn’t work at all, just says, no batteries, but the LED light is on. I went back to the shop to complain about them, and we tested duracell batteries from his TV controller and they worked.

    Then, I came home and tried some random batteries out of my tv controller and they work. Very obnoxious and annoying. Leave it to BT to sells these crappy phones.

  10. Phill says:

    When everyone had the battery low problem did it also make the main base unit to also go crazy? i’m not sure if its the battery or the phone itself! the display on the base unit flickers and when the phone goes you can’t answer it. Did that happen to anyone else?

  11. eric johnston says:

    just purchased batteries and they didn’t work at all, so after reading the above comments, decided to put a small bit of rolled up tin foil onto each positive terminal in the phone, as i had no small scissors to hand. This enabled the batteries to make contact and now the phone works again. Thanks all

  12. Ed W says:

    Our home phone batteries started to fail and I am so glad to have found this website. I had bought a pack of rechargeable batteries only to find they did not fit due to the shoulders in the moulding. I didn’t go straight in but looked for advise first. Now have three up anf running phones without the irritating beeps telling me the batteries are duff! Thanks a million!

  13. Oliver Harrison says:

    Just found this page and very glad I am too. Instead of removing the plastic bars I bent the metal terminal out a few mm with a small screwdriver. Job done! THANKS

  14. Elaine H says:

    Hi – I just wanted to second the solution posed by Oliver Harrison, above. I have the same phone, with the very same issue. I purchased Varta Rechargeable batteries, and when I inserted them, the phone wouldn’t turn on (annoying, as they cost €15 for four!). However, after pushing out the positive terminals by just a fraction, and re-inserting these batteries, the phone is now working!

  15. Gavin Moore says:


  16. John Fairhurst says:


  17. Mick Hancock says:



  18. Would the batteries mentioned in the above posts work for a Synergy 6500 set of cordless telephones. Be pleased if anyone could advise me as three of my four phones run out of charge very quickly and I need to replace the batteries.


  19. Sona Patel says:

    Thank you so much for taking the time, brilliant advice!

  20. Arthur frost says:

    I bought a pack 4 x 550mAh Lloytron batteries on ebay. Recommended for BT graphite. They work a treat no problems and they are only £3.75/pack

  21. Faheem says:


  22. Jane hay says:


  23. ShaVerZ says:

    I’ve purchased many Rechargeable batteries and couldn’t understand why they would never charge nor work at all. Just found out that the scammers at BT have designed the phone in a way that you cannot use other branded battaries! Used a short knife and cut the small plastic either side of the positive end as this was blocking a connection! Now all my battaries work (Duracell).

  24. Richard says:

    Previous set of replacement Philips rechargeable batteries fitted/worked fine for several years, but I was bit confused with the recent replacement Duracell rechargeables as one phone worked ok but the other three phones were ‘dead’.

    Found this forum and Diane’s post … used a craft knife to pare away the plastic tabs either side of both +ve terminals and Duracell batteries now work in all four phones.

    Well done Diane … I should have figured this myself being a retired electronics engineer!! Never say it’s too late to learn something!!

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