Low Voltage Battery Charging

[Alan de Enfield]

I have a 2.4v (2x 1.2v LiOn) battery powered hair trimmers, which is charged by a mains charger dropping down to 3.0v 800Ma DC.

I am looking to simplify the number of charger units I use and wondered if It is likely to cause damage / fire / explosion if I use a USB adapter / charger, which would give ~5v at up to 1 amp.

(Pillips shavers have a 'charging' USB lead with the correct fitting, and from what I can find on the internet appear to use the same battery set up - 2x 1.2v NiMh)

 

The alternative, and more cumbersome way is to use a USB 5v to 3.3v 800Ma adapter and make up a lead using one of these :

 

Edited January 28, 2018 by Alan de Enfield

[WotEver]

Are you sure they’re Li-Ion?  At 1.2V they sound more like NiMH or even NiCd. You shouldn’t exceed the 3V charge voltage whatever they are or they will overheat.

 

Edited January 28, 2018 by WotEver
Added a bit

[Alan de Enfield]

8 minutes ago, WotEver said:

Are you sure they’re Li-Ion?  At 1.2V they sound more like NiMH or even NiCd. You shouldn’t exceed the 3V charge voltage whatever they are or they will overheat.

 

I had originally posted NiMh but after reading the destructions found they were (it said) Lithium Ion.

So the 'way to go' is the £0.70 (inc postage & packing) USB voltage reducer and make up a lead - will they live with 3.3v ?

Thanks.

Edited January 28, 2018 by Alan de Enfield

[WotEver]

I honestly don’t know. The usual advice for NiMH is 1.4V to 1.5V per cell. 1.65V is a lot more than that and I suspect the Batteries would overheat but as I say, I don’t ‘know ‘. 

[Alan de Enfield]

6 minutes ago, WotEver said:

I honestly don’t know. The usual advice for NiMH is 1.4V to 1.5V per cell. 1.65V is a lot more than that and I suspect the Batteries would overheat but as I say, I don’t ‘know ‘. 

Thanks, I have a spare battery, so I'll order the 70p 3.3v adapter and see what happens.

[forsberg]

Probably not Li/Ion with that voltage, the disposal instuctions can be generic. The question at hand is where the charge circuit is located, in the mains adapter or in the trimmer?

If in the Trimmer it might work if the charger circuit can handle 3.3V, if in the adapter you will fry the batteries or perhaps blow the dc/dc converter.

Have you measured the output from the mains adapter?

Edited January 29, 2018 by forsberg
grr spelling

[Alan de Enfield]

27 minutes ago, forsberg said:

Probably not Li/Ion with that voltage, the disposal instuctions can be generic. The question at hand is where the charge circuit is located, in the mains adapter or in the trimmer?

If in the Trimmer it might work if the charger circuit can handle 3.3V, if in the adapter you will fry the batteries or perhaps blow the dc/dc converter.

Have you measured the output from the mains adapter?

Good question

Charger is a 'plug in the wall' (230v) type.

No-Load output of the mains adapter is 4.48v (just measured it)

Label on mains adapter states "3.0v ...... 800mA"

Battery pack 'very slightly' warm to the touch after several hours charging.

Battery pack showing 2.85 volts 5 minutes after taking off charge.

[Johny London]

Are there not other dc to dc converters available? It is something I have been meaning to get around to - sorting out several items that could be running stepped down from 12v rather than from the 240v.

Can you not get adjustable ones so you can custom set? I'd go that rout if I were you.

Edited January 29, 2018 by Johny London

[Alan de Enfield]

15 minutes ago, Johny London said:

Are there not other dc to dc converters available? It is something I have been meaning to get around to - sorting out several items that could be running stepped down from 12v rather than from the 240v.

Can you not get adjustable ones so you can custom set? I'd go that rout if I were you.

Yes I could, but I want to try and get as much running off USB sockets,

You are 'right' - It does not make sense to take 12v DC, convert it up to 230v AC (with all of the attendant losses) then take it back down to 3 volts (with all of the attendant losses) and as I already have several USB / 5v electronic goodies, I am trying (where possible) to standardise.

My Phone, VHF radio, Watch, GPS, and other bits & bobs all charge from a USB.

 

[Stephen Jeavons]

I'm not familiar with the device you have there but I somehow doubt you can push 5V into a 5V OUTPUT as you have it and expect 3.3V from the other output  (the third option you have shown). Electronic regulators don't tend to work in reverse.

Stephen

Edited January 29, 2018 by Stephen Jeavons

[Alan de Enfield]

16 minutes ago, Stephen Jeavons said:

I'm not familiar with the device you have there but I somehow doubt you can push 5V into a 5V OUTPUT as you have it and expect 3.3V from the other output  (the third option you have shown). Electronic regulators don't tend to work in reverse.

Stephen

Thanks for that - I had seen that, but not given it any thought, as all I want is to plug it into a USB socket and get the 3v out, if it does that I'll be happy.

(Pic was taken from the website selling the gizmo)

[WotEver]

1 hour ago, Stephen Jeavons said:

I'm not familiar with the device you have there but I somehow doubt you can push 5V into a 5V OUTPUT as you have it and expect 3.3V from the other output  (the third option you have shown). Electronic regulators don't tend to work in reverse.

Stephen

All that the third diagram demonstrates is that the 5V output is directly wired from the USB input. It’s simply showing that you can either use the USB connector or the +5V connections to supply the device with 5V. 

1 hour ago, Alan de Enfield said:

Thanks for that - I had seen that, but not given it any thought, as all I want is to plug it into a USB socket and get the 3v out, if it does that I'll be happy.

(Pic was taken from the website selling the gizmo)

Yup, there’s nothing wrong with it. 

[Stephen Jeavons]

Yes, I stand corrected. There is only one regulator evident so that must be the 3.3V one. The 5V output is clearly the same point as the USB input.

Stephen

[forsberg]

3 hours ago, Alan de Enfield said:

Good question

Charger is a 'plug in the wall' (230v) type.

No-Load output of the mains adapter is 4.48v (just measured it)

Label on mains adapter states "3.0v ...... 800mA"

Battery pack 'very slightly' warm to the touch after several hours charging.

Battery pack showing 2.85 volts 5 minutes after taking off charge.

Well that rules out that it is a fixed 3V adapter. Still no clue as to wether it is a charger or not unless it specifically says that on the adapter (such statemantes are not to be trusted anyway).
So 3.3V will most likley not work well. Either the battery will not get fully charged or not charged at all.
The only way to be sure is to dismantle and locate the charge regulating components, photos would do.

Also to consider is what kind of USB- sockets you use, some will only deliver 100mA, and some share the maximum current.

[Alan de Enfield]

6 minutes ago, forsberg said:

Well that rules out that it is a fixed 3V adapter. Still no clue as to wether it is a charger or not unless it specifically says that on the adapter (such statemantes are not to be trusted anyway).
So 3.3V will most likley not work well. Either the battery will not get fully charged or not charged at all.
The only way to be sure is to dismantle and locate the charge regulating components, photos would do.

Also to consider is what kind of USB- sockets you use, some will only deliver 100mA, and some share the maximum current.

The charger (mains adapter ?) is a sealed up 'black box about a 2" cube, and as it presently works Ok so I don't really want to break it open.

The USB supplies I have are 1 amp and 2.1 amp, so I reckon that the 1 amp should be OK.

Thanks for you help, if you think of anything else just let me know.

eg - if you think 3.3v won't charge - would a straight 5v USB cable to the battery pack work or go 'bang' ?

I've got the 5v to 3.3v board on order (4 weeks) and will report back when I have it, wired it up and tried it.

Edited January 29, 2018 by Alan de Enfield

[forsberg]

24 minutes ago, Alan de Enfield said:

eg - if you think 3.3v won't charge - would a straight 5v USB cable to the battery pack work or go 'bang' ?

That depends on where the regulator are, is it difficult to open the trimmer too? If the regulator is in the trimmer and of the simple kind 0.5V extra might harm the batteries. but if you put a diode that drops .5V in series you should be ok. If there are no regulator in the trimmer i would expect some magic smoke after a while.

To charge batteries you need a regulator that limits the initial current and protects against overcharge. First one's easy the second as complicated as you like to make it..

[WotEver]

5 hours ago, forsberg said:

So 3.3V will most likley not work well. Either the battery will not get fully charged or not charged at all.

I disagree. It will charge the batteries very well, possibly too well and make them go bang. The little mains brick sounds like it has very simple regulation so it floats high with no load. As soon as a load is applied it’ll regulate to 3V. 10% over-voltage will possibly increase the charge current to a high enough level as to damage the batteries. Or maybe it won’t; Alan’s test might give us the answer.