Questions:

1) Am I correct to assume the batteries are being charged relying only in that 30ohm resistor to limit the current?
2) Is 12V a correct voltage to charge the batteries? The power supply that was given to me does not have the Hoover brand, might have been used incorrectly.
3)Should I make a small constant current charger with an LM317 instead?
4)How can I try these batteries for health?

The vacuum was given to me and I was told it holds no charge. It came with a 12V, 1A DC power supply that was being used to charge it.

I opened it up and the 3x AAA measure close to 3.6V at the terminals, although they go down close to Zero when I turn the switch ON.

There is a visible burn mark near the resistor. It looks like a 2W and measures 30ohm.

I tried to interpret the circuit and simulated it on Falstad. There is something odd in the way the LED is mounted on the pcb but it lights up when the 12V power supply is plugged in (the LED is OFF when the switch is changed to turn the motor ON).

http://tinyurl.com/hd4e24h (there is a diagram attached)

 

If this is based on a 12 volt power supply, The 30 ohm resistor limits the charging current to 1/4 amp if the AA batteries are healthy. It also dissipates 1.875 watts while charging healthy batteries. When the batteries fail, the resistor over-heats.

4)How can I try these batteries for health?

Charge them up and see if

they go down close to Zero when I turn the switch ON.

You might also check to see if the 30 ohm battery charging resistor has been overheated.

Should I make a small constant current charger with an LM317 instead?

The original charging method is terrible. Building a constant current circuit would be essentially the same thing, bad. NiMH batteries don't like being treated like that. They need a charger that knows when to stop.

In my humble opinion, this was a bad design in the first place. The batteries were doomed the day it was born and the motor can just about suck a fly off a sugar cube...but only if the fly doesn't struggle. Now the circuit board is toast. It's worth what you paid for it. If you invest $50 worth of parts and labor...buy new batteries and spend a few hours driving around getting parts and rebuilding the circuit board...it will still be a terrible design.

Play with it if that amuses you. Other than that, this machine is the worst possible quality that won't actually burst into flames.

 

If this is based on a 12 volt power supply, The 30 ohm resistor limits the charging current to 1/4 amp if the AA batteries are healthy. It also dissipates 1.875 watts while charging healthy batteries. When the batteries fail, the resistor over-heats.


Charge them up and see if

You might also check to see if the 30 ohm battery charging resistor has been overheated.


The original charging method is terrible. Building a constant current circuit would be essentially the same thing, bad. NiMH batteries don't like being treated like that. They need a charger that knows when to stop.

In my humble opinion, this was a bad design in the first place. The batteries were doomed the day it was born and the motor can just about suck a fly off a sugar cube...but only if the fly doesn't struggle. Now the circuit board is toast. It's worth what you paid for it. If you invest $50 worth of parts and labor...buy new batteries and spend a few hours driving around getting parts and rebuilding the circuit board...it will still be a terrible design.

Play with it if that amuses you. Other than that, this machine is the worst possible quality that won't actually burst into flames.

Should I just mount a 3x AAA holder on the outside and recharge the new batteries in a regular charger?

I own one of these:

https://www.manualslib.com/manual/89561/Kompernass-Kh-980.html

http://budgetlightforum.com/node/29740

 

What I would do is change the resistor to 150 ohms 1 watt and put it back together with new batteries, but that's because I don't want to fuss with a NiMH charger and taking all the batteries apart every time I want to charge them.
The complete lack of any circuit to stop charging when the batteries are full is still a killer, but cutting the charging current down by a factor of 5 will make them last longer...or you can do it your way. It's just inconvenient.

 

It's ok by me, let the charger keep the batteries in shape. Thanks for the tips.

 

Your led is drawn backwards, it lights up when on charge, and gets its power from the voltage dropped across the 30 ohm resistor, i would bin the batteries and replace with new ones, the charge current will be from 275mA at discharge to 250mA at fully charged.


Your circuit is like this..

 

I dont think the higher wattage resistor is in series in my circuit. Is it possible it isn't?

PS: Liverpool doing well this year, I still believe...

 

Well i think it is, so take better pics and prove it.

 

As I read the words and traced out the schematics, I found no discrepancies or contradictions except the Falstad circuit shows a 360M resistor which apparently doesn't exist.

There is something odd in the way the LED is mounted on the pcb but it lights up when the 12V power supply is plugged in (the LED is OFF when the switch is changed to turn the motor ON).

That sentence tells the story. The schematic is compatible with those words.

I think Dave made a mistake.
Whether he did or not, the results are irrelevant. Drawing the LED backwards doesn't change the fact that it is still connected to the circuit board and it still works. The physical structure has not been destroyed. The repairs do not involve the LED.

 

I read some more about charging NiMH batteries and now I understand relying solely on a resistor to drop the current is half way to ruin the batteries.
However, I cut-off the battery pack and charge them with my charger individually. It cycles and discharges them before recharging. I put them back on a 3x AAA holder, plugged them again to the vacuum cleaner but no luck.
Then I tried three 2300mah rechargeable Varta batteries with am 8C discharge rate according to the datasheet. did not work either.
Them I picked up a 12V, 4A regulated power supply and connected it to a PWM motor controller (12V-24V, 8A). I expected to at least be able to rotate the motor slowly but all I got was a oscillating sound and twisting the shaft didnt help getting it started. I picked another 7.2V motor I had in my parts box and that rotated slowly once the voltage reached around 7-8V.

The motor presents continuity between poles and a small resistance. I wonder if it needs a particularly high current to jumpstart, and the batteries I tried were no good. This hand held vacuum cleaner is rated at 35W.

I feel embarrassed because my ignorance is showing: it's a very simple circuit but I can't see what I may be missing (low power motors i've played with in the past never presented any difficulty to operate).

 

The good thing about NiMH batteries is that they are good at high amps in short bursts. 35W/3.6V = 9.7 amps. I was all the time trying not to say, "Ni-Cad" because they survive the over-charging better, but they can't provide that start surge the motor wants. Have you tried connecting the (3) AA pack directly to the motor? If it won't run then, it simply won't run.

 

I have tried connection (3) AA fresh alkaline batteries and the motor runs, the NiMH I tried before probably werent in good shape. I will buy some fresh rechargeable ones and report on my findings.