Honeywell ST6400C Programmer Battery Replacement

This may help members finding loss of program data after power loss or switching off. I have had the original central heating programmer since 1995 and no doubt there are similar units with battery back-up. The programmer [see image] unclips from the wall mount if needed to isolate electrically the central heating system and should retain all the programmed settings. The instructions state the data should last 4 days. At some time in the past I bought another unit (ca 2006) as I thought there was a problem with the original – there wasn’t, in fact, the backup battery had failed and timings were lost.

I assumed the battery might be replaceable if the plastic case could be opened! [A well known repair problem as with my previous message relating to a plastic case toaster on July 2016 and a Bosch electric whisk on July 2017! ]

Unclipping the unit from the wall back-plate the green-covered battery was obviously soldered to the internal PCB under another cover, which would need to be removed to get at the soldered joints – see second image. Looking at the programmer sides I could just make out two small plastic tongue clips aligned with one battery side. Careful easing and pressure with flat bladed screwdriver enable me to prise the case apart, finding in the process a third clip the other side of the 6 pins.

Having separated the case halves this now enabled me to see there were two PCBs; a large one on top of a small one carrying the battery. At this point I checked my Maplin catalogue to see if a suitable battery was available as I had found they stocked PCB Mounted Ni-MH batteries of various types when I replaced the batteries in my Remington shaver. They had exactly the same battery under part number BN24B!!

Replacing the battery is a reasonably easy job taking care not to damage the PCB tracks – the last image is the corroded battery . Assembling the unit and connecting to the mains nothing happened, the LCD display was blank. Disassembling once more I checked the battery and it gave 3.77VDC (with hindsight I should have double-checked when replacing!).

The slide switches can be seen in the first image. These have three fingers running over the PCB contacts. I wondered if the dis-assembly had perhaps somehow disturbed the contacts. I carefully cleaned the PCB stripes with alcohol and gently bent the fingers slightly to put a little more pressure on the PCB. On re-assembly and connecting the mains this time the LCD showed data! All was well. I left the unit on the mains charging the battery for a day and then disconnected. Checking every 24 hours I found the battery held the programmer data for at least 5 days; the user instructions suggest 4 days in case of power failure. Before assembling the case I made sure the plastic clips had a small amount of silicone grease applied.

Hope these notes will help forum members.

Rodney Fry