Low Voltage transformer question.

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Bellow

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Perplexed.
I have a Snap-On Tools LED strip light but not its designated charger. The battery in the light is 3.7V. So, I find a small transformer that has written on it that its output Voltage is 3.7V, solder on the appropriate jack and then measure the output (on two separate meters) as 10V.
How can this be, and dare I plug it into the light (which will need a recharge very soon)?
TIA.
 
You're thinking power supply rather than charger. A charger needs to be able to provide more than the nominal voltage of a battery to be able to charge it (hence why a car alternator provides around 14v).

I would really stress caution here though. 3.7v implies this is a Li-ion pack, they do not react well to being mischarged. Do you know if there is power management for the charging circuit inside the light or not?
 
More than like the psu you are looking at is AC in but out to the jack would be DC, you'll know that from the range you're using on the meter.

A no load psu will be at a higher output voltage, the load pulling down the output voltage.
A 3.7v cell to run the LED would perhaps charge at 4.2v. It may have the 18650 rechargable cell within.
Perhaps your psu will be good,but I wouldn't want to say for definite. If the charge voltage is excessive it could well rupture the cell and even dramatically.
If you can remove the rechargeable sell there are cheap chargers available, but more often they aren't easily removable.
 
You're thinking power supply rather than charger. A charger needs to be able to provide more than the nominal voltage of a battery to be able to charge it (hence why a car alternator provides around 14v).

So 10V is to be expected from a 'rated at' 3.7V charger?

I would really stress caution here though. 3.7v implies this is a Li-ion pack, they do not react well to being mischarged. Do you know if there is power management for the charging circuit inside the light or not?

Single (18650?) cell and I assumed Li-ion. Not sure about charging circuit as I only partially dismantled it. From top to bottom, charge point and switch; battery; LED. No external charge indicators. If there is a charging circuit, it is at the LED end - plausible? Or lacking a a charging circuit?

edit PS. Charger is a Nokia - been for a phone at some point.
 
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I've a charger that charges 18650 cells.
The no load voltage is 4.19v.
At 10v I wouldn't want to use the Nokia.
 
You always need more than a cells nominal voltage to drive a charging current so the advice of 4.2 volts seem in the right ball park for a 3.7 volt cell.

The main differences in charging a Li-ion battery is the charging voltage must be regulated so it can't exceed the desired level. 10 volts for example would be disastrous. If 4.2 is the correct voltage then it on no account be exceeded.

If a small dc PSU is used and it's a simple transformer and diode construction, it's not going to be have a regulated voltage output and won't be suitable. m80 alluded to the fact that transformers have what's called regulation. The no load voltage is always higher than the rated load voltage. For Li-ion charging there needs to be an electronic circuit to control the voltage to a fixed level and even then the charger should be disconnected when the battery is fully charged and that's not easy to determine. Proper Li-ion chargers will stop charging when the charge current falls to a certain point as they must not be left on float charge. The only option would be to physically monitor the charge current with a meter. All in all lithium -ion charging is tricky so I would probably play safe by reducing the charge voltage and accept a longer charge time perhaps experimenting with 4.0 or 4.1 volts and see how long it takes to charge.
 
What are the chances that the light unit has charging control? If I open it up again, will I recognise it if it's there? Or could I mistake any electronic components for the LED as being charging control?

I do have a proper charger for Li-ion batteries - but 18650 cells are the longest it will accept. The cell in the light cannot be 18650 as the 3rd and 4th digits denote length and it is around double the length of of a proper 18650.

Would a better option be to find a way of connecting a fly lead between my Li-ion charger and the light's charging port?
 
The cell in the light cannot be 18650 as the 3rd and 4th digits denote length and it is around double the length of of a proper 18650.

I didn't know that about the length,
is it possible that yours is 2 cells bonded as 1?

If there are 2 cells, and if your torch is rated at 3.7v then the guess is that they are charged in parallel. So the fly lead should work.
Seems like some faff though.

If this is a work light you might like this
Waterproof Portable COB LED Work Light USB
Rechargeable Garage Camping Spot Lamp | eBay


It only needs one of the four 18650 cells and runs for 2 hours on the one.
I'm keeping one for the van and one for the boat now.
Actually I've a different but similar type for the van also, as I like the idea of the strobing red / blue to prevent being run into if I'm stuck on a dark road at night.
Loads of light output.
 
Opened it up again and, 2 x 18650 (shrinked wrapped end to end) with two wires (black and red) emerging that connect to a circuit board behind the LED strip. On the battery is written '18650; 3.7V; 4000mAh, and NF10'. Another 4 wires connect to the board form the charge port/switch end.
On the circuit board are what I think are micro chips. 6 of them are marked '3R3', the other 2 '2R2'. Is this a charging circuit?
 
Opened it up again and, 2 x 18650 (shrinked wrapped end to end) with two wires (black and red) emerging that connect to a circuit board behind the LED strip. On the battery is written '18650; 3.7V; 4000mAh, and NF10'. Another 4 wires connect to the board form the charge port/switch end.
On the circuit board are what I think are micro chips. 6 of them are marked '3R3', the other 2 '2R2'. Is this a charging circuit?

It's beyond my knowledge to be sure, but it sounds like it.
Only other thing I could think of is if the lamp has dim and strobe functions.
 
3R3 and 2R2 would normally denote 3.3 ohms and 2.2 ohm resistors. While they may not look like the familiar resistors with leads, if they have only two connections they could be surface mounted resistors.
 
Have you a 5v USB lead with the correct DC connector and just use any USB outlet to charge it. That's what I do on my magnifier/soldering lamp.:dk:i
It hasn't blown up yet...unless I'm confusing the smoke with that of soldering!
 
Have you a 5v USB lead with the correct DC connector and just use any USB outlet to charge it. That's what I do on my magnifier/soldering lamp.:dk:i
It hasn't blown up yet...unless I'm confusing the smoke with that of soldering!

Nope - but I could make one if I knew which wires within the USB side to use. Did you buy your lead or make it?
 
Have you a 5v USB lead with the correct DC connector and just use any USB outlet to charge it.

I found just such a lead (for an external CD/DVD player) and it seems to be working.
Thanks for that, and thanks to everyone else who stopped me connecting 10V to it and ending up with a face-full of exploded Li-ion battery.

One more question - charge time?
Snap-On say it was supplied with a ''5 hour charger'' and I know from using single 18650s that they take a bit of time to charge. If the two in the torch are wired in parallel, will that take longer than a single 18650? Is 5 hours about right? Anything I can look (feel) for to gauge when they are fully charged? I'm charging from a 240V USB charger - as opposed to lap top.
 

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