How to convert charger input to USB3

[massop]

I have a Wahl battery operated shaver (Sterling 2 plus) thats very old but still probably the best one you can get.

Every few years the rechargeable battery inside dies so I open it up and change it when that happens.

The only downside to this shaver is the charge dock is HUGE and not very practical for when you travel so I wanted to have a go at trying to convert this to a USB3 port so that all I need is my cable.

My question to you is how can I do this? The shaver has no intelligence, just a battery, switch and motor, see below pics of it disassembled.

 

[hang-the-9]

Cut a USB cable, attach the two power wires from it to the battery charging plates, see if it charges.

 

[Alabalcho]

Before you do that (and potentially blow up the battery) - what are the charger' parameters? I see this is single 1.2Volts battery, and USB gives out 5Volts - quite a difference...

 

[massop]

Before you do that (and potentially blow up the battery) - what are the charger' parameters? I see this is single 1.2Volts battery, and USB gives out 5Volts - quite a difference...

Took me a while to find the output as its not on the charger but opening it up found this..

Looks like 4.5 Volts and 320mA

 

[hang-the-9]

Before you do that (and potentially blow up the battery) - what are the charger' parameters? I see this is single 1.2Volts battery, and USB gives out 5Volts - quite a difference...

I would find it very strange if USB power did anything to a cordless haircliper.

 

[massop]

It worked guys.

I ripped out the old contacts and butchered a USB extension lead. I rand the battery dry and tested charging this using my phone charger and it works perfectly.

Didnt think it will be so simple, finished product below

 

[sizzling]

I would find it very strange if USB power did anything to a cordless haircliper.

He’s right to check. If you ended up supplying 5v to the cell it would be dangerous.

 

[vov4ik_il]

Wow. So bad advise from a moderator...
OP, If you want that 1.2v cell to last for a few cycles and not fry the charger (potentially blow up the battery) get a small dc charging circuit.

 

[massop]

Wow. So bad advise from a moderator...
OP, If you want that 1.2v cell to last for a few cycles and not fry the charger (potentially blow up the battery) get a small dc charging circuit.

Thanks for the feedback,

I wish I knew this before finishing the job .

I am based in the UK so would be better to buy from UK eBay, could I use this?

 

[vov4ik_il]

Thanks for the feedback,

I wish I knew this before finishing the job .

I am based in the UK so would be better to buy from UK eBay, could I use this?

The one you linked is for Li-Ion cells (3.7v), it won’t fit. Those circuits come from China anyway, so you can order it anywhere
Look for NiMH or NiCad, 1S (one cell), CC/CV (constant current/constant voltage) charging circuit with 5v input.

 

[massop]

Thanks I made the order form your link.

I have a question though, the original charger shows its putting our 4.5v & 320mA and there was no power conversion boards between the charger a the actual battery.

Would I definitely need to install this board since the voltage the battery was receiving on the old charger is almost the same? (4.5v vs 5v)

 

[vov4ik_il]

the original charger shows its putting our 4.5v & 320mA and there was no power conversion boards between the charger a the actual battery.

Would I definitely need to install this board since the voltage the battery was receiving on the old charger is almost the same? (4.5v vs 5v)

It only reads 4.5v on the transformer, it is alternating current, not what the cell gets. What is on the back of the board?

 

[massop]

The board makes no mention of output voltage nor the charge base itself, see below

 

[vov4ik_il]

It is hard to see the markings on the front photo, but it looks like there is a rectifier (the small round black piece) and a resistor. On an open circuit, it would read 3-4v but with the battery attached much less than that (depends on the charge) with the excess turned into heat on the resistor and a charging current of .32A (<1.44w).
Your USB charger is 10-15W and a whole different animal, you do need a charging circuit.

 

[massop]

It is hard to see the markings on the front photo, but it looks like there is a rectifier (the small round black piece) and a resistor. On an open circuit, it would read 3-4v but with the battery attached much less than that (depends on the charge) with the excess turned into heat on the resistor and a charging current of .32A (<1.44w).
Your USB charger is 10-15W and a whole different animal, you do need a charging circuit.

I will definitely go with your advice,

I have ordered the board and am waiting for delivery.

For the sake of completeness here is a clearer image of the parts that are on the original charger board

 

[vov4ik_il]

I will definitely go with your advice,

I have ordered the board and am waiting for delivery.

For the sake of completeness here is a clearer image of the parts that are on the original charger board

Yep, diode bridge and what seems to be 12ohm resistor to limit the current. 1.5w charger with 2 color led. Simple and cheap.

 

[massop]

The one you linked is for Li-Ion cells (3.7v), it won’t fit. Those circuits come from China anyway, so you can order it anywhere
Look for NiMH or NiCad, 1S (one cell), CC/CV (constant current/constant voltage) charging circuit with 5v input.

The china one is still in transit and taking forever. i found a UK based on HERE , from your description this should work?

 

[vov4ik_il]

i found a UK based on HERE , from your description this should work?

Yes, it should