replacing battery in a cordless telephone

[mdanh2002]

Hi all,

I was given a dead Siemens Gigaset A140 and its docking station for free. The phone did not seem to power on when placed in dock and connected to the 9V AC adapter. Within minutes I have identified the problem - the 540mAh 2.4V NiMH rechargeable battery pack (with 2 AAA) inside the handset is dead. It's not the smart battery type (the pack only has 2 wires) and all the docking station does is to pass the 9V from the AC adapter to the handset via 2 pins at the bottom of the unit.

I replaced it with my new pair of 600mAh rechargeable AAAs and the phone was able to power on and make calls.

However, now the battery indicator only shows 1 bar and the display reports "low battery" all the time and apparently attempts to charge the battery. I measure the voltage with a multimeter and it reads 2.43V (full) so I think the charging circuit needs to be recalibrated after the battery is replaced.

I have tried to leave it charging for 1 hour but the battery level did not increase and the battery became really hot. I have also tried a FACTORY RESET inside the phone menu but that did not help.

Thanks for any advice.

 

[audioguru]

The battery will measure 2.8V when fully charged and is still charging. It will get warm but it should not get hot.
I think the charger is defective and feeds AC instead of DC for charging.

 

[mdanh2002]

Hi audioguru. Thanks for your reply. I think the AC adapter is fine. I tried to leave the battery to drain totally until the unit can't power on and let the the unit charge it overnight. Now the battery level reads full and the pack measures 2.8V like what you suggested. The batteries were slightly hot after charging is done but are now no longer hot even with the unit in the docking station for some time - I think the charging circuit has sensed full battery and stop charging. So I hope my repair is done and I have a new working cordless phone now!

 

[audioguru]

Hi audioguru. Thanks for your reply. I think the AC adapter is fine.

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