Replacing an AC Ni-Cad battery charger with a DC battery charger

[Dustbuster]

Hi. I've just bought an old handheld vacuum cleaner that uses a 9 VAC 5 watt power supply to recharge its built in 3.6 volt Ni-Cad battery. Trouble is the power supply was made for 110/120 volts mains power and I need a 220/240 volt power supply but can't find an 9 VAC anywhere, only DC power supplys. Could a DC power supply be used instead?

The vacuum cleaner doesn't seem to have any kind of circuitry in it, just a diode on the negative battery terminal. Thanks!

 

[d123]

Hi,

I think the diode will be reverse polarity protection.

No idea if you can use DC instead of AC there but I doubt so, normally devices are pretty explicit about permitted supply formats. However, if you can open the 9VAC supply it may turn out to be a 9VAC to ~12VDC but possibly unlikely if it says AC.

No desire to put together a clunky, heavy 220VAC to 9VAC transformer-based supply of your own? (...after inspecting the 9VAC one to get a good idea of what comprises it.)

If you can, why not open the supply and post a photo here? It would be helpful.

- - - Updated - - -

Interesting reading:

 

[BradtheRad]

Is the vacuum called Dustbuster by some chance? I saw two units where the nicad batteries started to fail after a few years. (Apparently a normal lifetime.)

The diode permits current through the battery in only one direction.

The inexpensive charger sends a few hundred mA through the batteries. It's the kind that you plug in overnight. If you're careless you forget to unplug it the next day.

A 'smart' charger stops charging batteries when they are full. It's also wise to limit charging current, according to the type of cell, and its amp-hour rating.

 

[Dustbuster]

Thanks for the replies. Yes, it's a Dustbuster

I feel a bit nervous about opening the psu as it's a sealed unit, though I've just found a 9 VAC 300ma power supply that might do the job. Do you think that 300ma will be enough and would the batteries explode or catch fire if left to charge for too long?

 

[Kajunbee]

Thanks for the replies. Yes, it's a Dustbuster

I feel a bit nervous about opening the psu as it's a sealed unit, though I've just found a 9 VAC 300ma power supply that might do the job. Do you think that 300ma will be enough and would the batteries explode or catch fire if left to charge for too long?

They make 240 to 120 volt plug in converters. They are used when traveling to a foreign country. You could then use your 120 volt charger.

 

[BradtheRad]

9 VAC 300ma power supply that might do the job. Do you think that 300ma will be enough and would the batteries explode or catch fire if left to charge for too long?

DC is needed to charge batteries. AC must be rectified by a diode (or else four in a full-wave bridge).
Subtracting battery volts from power supply volts makes it hard to predict charge rate. Expect 100-200 mA average. Suitable to charge C or D nicads for a day. They'll get warm but can be held in your hand. The aim is to stop charging when they're full.

 

[c_mitra]

I feel a bit nervous about opening the psu as it's a sealed unit, though I've just found a 9 VAC 300ma power supply that might do the job.

If you notice a diode in the circuit, you can safely apply the 9VAC and it will run. The batteries must have died (they usually live for 1-2 years) but do not remove them, they will just act like a capacitor. If you feel that the full power is not coming, you need to put a big capacitor after the rectifier.

 

[Dustbuster]

I think I'll give the 300ma power supply a try. If it doesn't recharge the battery then I'll try using it to directly power the motor. I never thought about using it as a corded vacuum!

 

[BradtheRad]

Expect the vacuum to need a DC power supply, at a few Amperes to run.

 

[Dustbuster]

I'll have to experiment with a few power supplies if the AC one doesn't work. I did notice that when I connected 2 x 1.5 volt AA batteries to the motor in series it would turn, albeit quite slowly, though when I tried connecting a 6 volt DC 1 amp power supply nothing really happened apart from a quiet ticking sound when the wires were first connected. At first I thought that the voltage might be too high but maybe it was the amps that were too low.

 

[c_mitra]

At first I thought that the voltage might be too high but maybe it was the amps that were too low...

It is really strange that the motor runs with 2 AA battery but not with a 6V 1A power supply.

Make sure that the polarity is correct; two AA cells cannot deliver 1A on a continuous basis for more than a - say 10-20 mins - perhaps.

But you are right that the amps are perhaps too low. Vacuum cleaner motor has to run fast, very fast to be really effective.

 

[Dustbuster]

I connected up the 9 VAC 300ma transformer and left the vacuum cleaner charging for 8 hours. Works perfectly! The battery in the vacuum cleaner barely gets warm which is exactly what I was hoping for.

The instructions that came with the vacuum cleaner say that the original 500ma charger would take 6 hours to charge the battery to 60% and 16 hours to charge to 100%. Not sure why it would take so much longer to do the extra 40%? I reckon the 8 hour charge with the 300ma charger will have charged the battery to around 65%. That's probably enough for now. I don't want the old girl getting hot and bothered

 

[c_mitra]

say that the original 500ma charger would take 6 hours to charge the battery to 60% and 16 hours to charge to 100%

I guess you want to know why this is so. The charging is like an exponential process (more like a capacitor but there are small differences).

Roughly speaking, if the battery is charged to 62% in 1 hour, it will charge 90% in 2 hours and 95% in 3 hours and 99% in 4 hours. (the numbers are rough but you can see any table)

For all practical purposes, you should charge the battery to 70-90% for full charge and should not drain it below 10-20%. Just a rule of thumb.

Both trickle charge and deep discharge should be avoided.

 

[Dustbuster]

Thanks. Just thought I'd post an update. I decided to buy another of these cordless vacuum cleaners that uses exactly the same 3.6 volt Ni-Cad battery and I noticed that the power supply with this one is 4.4 volts DC, not 9 volts AC like the previous one I bought.

The charging dock is exactly the same for both cleaners. Both have a diode on one wire inside the dock. Even the charging time is quoted as being the same for both. Does anyone know how 2 different voltages can have exactly the same charging ability or why the DC version has a diode on one wire?

Thanks

 

[BradtheRad]

Is it possible the 4.4 VDC supply is in fact rectified 9VAC? So it goes through one diode (or it goes through a full diode bridge, 4 diodes)? And so its output voltage adopts the battery voltage as soon as you connect it.
No smoothing capcitor is needed when you're charging batteries.

Such 'wall-wart' power supplies can give a voltage spec on the housing but show a much different reading when we use an ordinary multimeter. And when we attach a load things are unpredictable yet.
If unregulated the voltage can drop severely as we apply a heavier load.

On the other hand, if your 4.4VDC supply is regulated, then it will reduce current to the batteries as they reach full charge. This is an improvement, and it should lengthen useful life of the batteries.
Because it is typical for us human beings to abuse batteries by overly discharging them, then overly charging them.

 

[Dustbuster]

I never thought of that. The power supply doesn't say if it's regulated so it probably is a rectified 9VAC supply. Thanks!

 

[Dustbuster]

Would a 9VAC 670ma power supply have too much current for 3.6 volt Ni-Cad batteries?

 

[betwixt]

670mA is it's capability, not what it actually produces. It's the maximum you can draw before potential damage occurs but you can draw anything less than that. The amount drawn depends on the load (your battery) although as it seems to be an unregulated supply you should expect the voltage to be a little higher if unloaded and drop a little lower under full load.

The most likely reason you see two two different voltage ratings is because of the way they are measured. When it says DC it isn't the same kind of DC as you would see across a battery. I know that sounds crazy but DC just refers to the polarity, not the stability of the voltage. If as you state, the supply passes through a diode, it implies the DC is produced by blocking the passage of half the AC cycle. AC alternates polarity with approximately half the time spent with + and - one way around and the other half of the time with them swapped over, the diode just lets one polarity pass through and blocks the other completely. That means if you measure the voltage with a normal meter, you only see about half the voltage, by blocking one polarity the average has dropped to 50%. The peaks of the voltage are still the same though, they are just less frequent so if you use the peaks as the measurement you get 9V and average of 4.5V.

Brian.

 

[Dustbuster]

Thanks for the info. I think I'll give the 670ma AC power supply a try. So in theory, the batteries will only receive 335ma after the diode blocks half the input?