Why won't this battery charge? (not e-bike related)

[El_Steak]

My friend's brother has a landscaping business and he uses a Sokkia laser Level. The battery for this thing died and a replacement is 100$

I inspected the battery and found out it is a simple 4.8V pack made with 4 NiCd D cells. I bought some replacement cells for 4$ a pop, opened the pack and replaced the original cells.

If I take a voltage reading from the discharge leads on the pack, I get a proper reading of 5.05 volts. However, when I plug in the charger the light doesn't come on and nothing happens (no charging). Once after 10 minutes, a red light came on the charger for a few minutes and the pack actually discharged to around 4.9V . The charger didn't come on after that.

The charger connector has 3 pins instead of 2. The middle pin gets connected to the pack to the negative lead, but it first goes through a an electronic thingy (see pic below). Any idea what could be wrong ?

 

[TPA]

The label on the charger say the output is 2.4-10V, so I'm going to guess that it is the device that regulates the voltage to the value matching the pack to be charged.

Can you get a voltage reading on the charger while it is disconnected from the pack?
Try voltage before that circuit and after.

Inside the box is the transformer, It likely has an inductor as well that could be bad. If you find the transformer unit bad by checking output voltage and current, you can replace those cheaply like this one.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=744-1287-ND

You would use a similar circuit with either a resistor or diode to drop the voltage to around 5.8V to charge 4 nicads.

 

[Lessss]

Try replacing the electronic thingy.

 

[El_Steak]

The label on the charger say the output is 2.4-10V, so I'm going to guess that it is the device that regulates the voltage to the value matching the pack to be charged.

Can you get a voltage reading on the charger while it is disconnected from the pack?
Try voltage before that circuit and after.

The voltage directly from the charger with no battery connected is 2.4V.

You are right that the charger probaby adjusts its voltage to the number of cells it thinks there are to charge.I started with cells that were charged to 1.27V each so maybe the charger though they were full?

Anyways, I just discharged the cells to 1.05V using my iCharger 206 and replugged the charger. This time it kicked in and it seems to be working. The pack voltage is already at 4.5V and going up. I'll see where it will stops...

 

[heathyoung]

The electronic thingy is a thermister, chances are the charger does delta T.

 

[El_Steak]

Charger stopped at 4.75V, which seems fairly low (4 cells NiCd).

If it does Delta T instead of Delta V, maybe the temp characteristic of the new cells make the charger stop prematurely?