AAA NiMH Battery voltage problem with BT DECT phone. - Page 1
Forum section: electronics / beginners

author: brian_mk
author_rank: Regular Contributor
timestamp: February 22, 2022, 01:00:54 pm
content: I have an old-ish twin handset DECT phone (BT Stratus 1500) The handsets use AAA NiMH batteries.I'm currently using fairly new (a few months old) Lloytron 1100 mAh cells.One of the handsets produces a very distorted ring tone from the speaker when making out going calls.The sound is also often very distorted during the initial seconds/minutes of a conversation.After a while the distortion disappears and the sound becomes normal.The other handset does not exhibit this problem.I wondered if there could be a problem with the batteries.I tried temporarily replacing the rechargeable cells with a couple of 1.5V alkaline cells (making sure I didn't put the handset on charge).The distortion problem went away.If I measure the voltage of each cell with no load using a DVM, I get 1.7V on the distorting handset.If I do the same on the other handset I get 1.65V.This seems unusually high.The voltage does not change much under load when the cells are plugged into the handset and it is switched on.It would appear that the handsets have a problem when fitted with batteries above the 1.2V -> 1.5V you might expect from a NiMH cell.My questions are...1) Has anyone else encountered a similar problem?2) How unusual is it for a NiMH cell to produce a voltage as high as 1.7V? Is it only Lloytron cells that have this problem?3) What's the best solution? Try to find alternative NiMH batteries with a lower voltage?

author: brian_mk
author_rank: Regular Contributor
timestamp: February 22, 2022, 08:07:41 pm
content: I suspect the handset uses components that are designed to operate with a 3V power supply.The pair of NiMH cells I'm using give up to 3.4V when first switched on after sitting in the charger cradle.I might try adding a parallel connected pair of back to back schottky diodes in series with the battery to reduce the supply voltage.

author: Gyro
author_rank: Super Contributor
timestamp: February 22, 2022, 08:15:35 pm
content: It sounds as if there may be a fault in the charging circuit on that particular handset. You wouldn't normally expect NiMH batteries to reach 1.7V on discharge. You might get near 1.7V while charging at temperatures below 0'C but not at room temperature, and not under load.

author: brian_mk
author_rank: Regular Contributor
timestamp: March 18, 2024, 08:08:17 am
content: An old topic but this is a follow up question for anyone who has experience with the old BT Stratus 1500 DECT phones.I've clung on to these dreadful phones because I like the ability to have unique ringtones for numbers in the phonebook.I had a lot of problems with one of the two handsets.I would often fail to charge when left on the cradle, despite the charging LED being on.When the batteries ran low it would beep an alarm signal. Annoyingly, this often happened in the middle of the night, waking me up.The handset would also frequently lock up with all buttons non functional. This requires the batteries to be pulled to initiate a reset.I put up with this for some time but eventually decided to buy another used handset from fleabay.Guess what? It suffered similar charging and lock up problems.I thought the problem may lie with the cradle rather than the handset so I took the charging cradle apart.It appears to use a simple trickle charger with very few components inside.I checked a few basic things like the charging current and voltage.I noticed that the charging current appeared to fluctuate intermittently.This was not visible on the charging LED - it remained steady.The nickel plated spring fingers used to make contact with the handset looked clean but the spring pressure was poor.I bent the fingers slightly to maintain a better contact.That seems to have cured the charging problem.However there is another issue:-I had a spare charging cradle so decided to add my old handset to create a triple handset system.I first made the same adjustment to the spring fingers in the spare cradle.It seemed to work ok and the handset remained charged.The problem now is that with three handsets, the answer machine in the main base unit no longer responds to incoming calls.Instead the phones continue to ring until eventually the Vonage VOIP voicemail system takes the call.If I remove the third handset, the answer machine responds to incoming calls as expected.These phones are supposed to work with three handsets.Any suggestions?

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 18, 2024, 10:44:25 am
content: I never get over 1.5V from a NiMH cell even with no load. In fact a fully charged would read about 1.4 or so. I wonder how you can get 1.7V

author: brian_mk
author_rank: Regular Contributor
timestamp: March 18, 2024, 11:22:14 am
content: That was an earlier strange anomaly that went away.It only happend with brand new Lloytron cells. I don't have an explanation.Now the Lloytron cells have aged, they read a more sensible 1.2V when charged.