replace a 9v power outage backup sorce

[dave9]

I wouldn't even bother with a charging circuit. Instead, get two 9V NiMH batteries, determine how long they last, then swap one with the other charged battery within that interval... charged externally, assuming the memory is retained as long as the unit has mains power, not completely dependent on the 9V for memory in that powered state. If it is dependent, just solder on a 2nd 9V connector then connect the new battery before removing the old - but don't dilly dally, the freshly charged battery will be discharging into the drained one, so you could use a switch or isolation diodes but really you just need to promptly disconnect the drained battery... 9V (PP3 form factor) can't produce a lot of current so that won't be a problem like it would with a higher capacity battery.

 

[dave9]

As I said previously, it seems out of the ordinary to use a 9v memory backup.
Usually one find a 3v lithium coin battery doing the job.

Not necessarily, I've owned several LED clocks that use one for backup, as well as a weather alert radio.

 

[Bluejets]

I still think the "replace on ones birthday" is the simplest answer.

 

[Bluejets]

Not necessarily, I've owned several LED clocks that use one for backup, as well as a weather alert radio.

Common design then.......

 

[tuneup]

Less than 300 kB per image usually works well.

here are the resized photos. they are the front and back of the piggyback power board. the other board has the power distributions for the individual valves and the control for the system.
With the unit active, power on, there is almost no voltage at the 9v power connector, 120 mv.
There is no power from the 9v supply to common. the 9v seems to be isolated from the rest of the power.

 

[Bluejets]

Now they're too small to make anything out.

 

[dave9]

Yeah this is the worst possible forum to have a file size or resolution limit. Far better to embed a thumbnail pic for those who don't have the bandwidth, that links to the order of magnitude larger image. If it's not a 6MP or larger image, it's an insult that wastes our time. Even a phone that wasn't made obsolete when 3G was/is phased out, can do that.

Link to a good HQ pic!!

 

[Harald Kapp]

With the unit active, power on, there is almost no voltage at the 9v power connector, 120 mv.

That is to be expected. Standard batteries are not rechargeable. Trying to charge them can destroy them.

There is no power from the 9v supply to common. the 9v seems to be isolated from the rest of the power.

I'd expect at least "-" of the battery to be connected to common of the circuit. Check the traces I marked. Red for "+", blue for "-":

 

[Tha fios agaibh]

Why should the controller have 2 backup batteries? a 3 V lithium and a 9 V block.

On my older controller, I can remove the 9v battery without losing its program. But if left out too long its lost.

I don't see a second battery in the photos so I suspect there's an electrolytic cap that sustains voltage while swapping 9v batteries.

If this cap is bad, the program will be instantly lost if AC power is lost.

I would also check the 9v battery voltage before and after it's installed to see if the circuit is bringing it down.

 

[tuneup]

That is to be expected. Standard batteries are not rechargeable. Trying to charge them can destroy them.

I'd expect at least "-" of the battery to be connected to common of the circuit. Check the traces I marked. Red for "+", blue for "-":
View attachment 51681

here are the traces that I can find. blue and red traces originate at 24vac lugs. orange circle = fuse , green = 100uf 6v cap.(one side to red trace and the other to blue trace), lime = what I think is a diode, it has a clear casing with two copper colored rings inside with a very small separation between them. in this separation is a very small black material. and at one end is a black ring that appears to be painted on the outside of the casing ., black circles = transistors. grey circle= a regular looking diode. The two purple circles are the 2 large 12 ohm 5 watt resistors. the blue trace is connection to the black 9v battery connection and the red trace to the red 9v battery connection.
yellow connectors are the 2 24vac connections and the dark blue connector is the common connector.

 

[tuneup]

here are the traces that I can find. blue and red traces originate at 24vac lugs. orange circle = fuse , green = 100uf 6v cap.(one side to red trace and the other to blue trace), lime = what I think is a diode, it has a clear casing with two copper colored rings inside with a very small separation between them. in this separation is a very small black material. and at one end is a black ring that appears to be painted on the outside of the casing ., black circles = transistors. grey circle= a regular looking diode. The two purple circles are the 2 large 12 ohm 5 watt resistors. the blue trace is connection to the black 9v battery connection and the red trace to the red 9v battery connection.
yellow connectors are the 2 24vac connections and the dark blue connector is the common connector.
the RED trace on the photo is connected to the RED battery lead + and the BLUE trace is connected to the BLACK battery connector lead negative -.
this seems backwards to me since the red trace is connected to the system "common
" lug and also to the battery positive post, but I am not an engineer.