"I have external charger for my DSLR Nikon. It has output voltage 7.4V and current 0.9 A. Can I use it to charge BL-5J battery (3.7V) ? "

I wouldn't.
Grab a bl5j charger from china...(so many cheap on ebay..I have one and it works admirably..have a bp4l as well for my n810's )
or better as has been suggested...
Grab a universal battery charger (uboc)
then you know you won't be frying it....
not hoping you don't fry it...with something never really meant for your battery.

What may happen if I connect battery to such a charger (7.4 V) ?

https://youtu.be/v7ssUivM-eM

Like endsormeans posted. Or, on a more serious note, http://m.youtube.com/watch?v=-DcpANRFrI4.

Look, I know I said that all LiPo batteries are basically the same. The nominal voltage is about 3.7. Your camera charger is rated at twice that, so clearly your camera battery contains two LiPo elements in series inside. DO NOT USE THAT CHARGER for anything else!!!

This thread is already 2 days old and 3 pages long. You could have listened to the advice you received on the first day and the first page and been done with it by now.

Yup
what he said ^^^


And yup.
always use recommended devices / procedures
failure to do so can result in more than the loss of a battery or device,
failure to be vigilant can (in worst case) result in severe loss of property and life.
Smokey the Tech says....
"Only you can prevent battery fires..."
So yeah.
Use officially recommended devices to charge batteries.
Don't leave them unattended whilst charging.
(which means always keep one eye literally on that charging battery).
If you need to go out or leave the area ...even for a short bit...unplug the charger...
It saves explaining to next of kin what happened and who is at fault.

Hello everyone,
I had a similar case a while back, dead battery and phone not charging it. I think I solved the issue quite good, so I will share with you how I fixed it.
My N900 was in reboot loop, and the battery was almost dead (it does not keep the charge even today, it's quite old).
Since the OS is not able to start, the phone can't charge the battery.
Without the battery however, reflashing is impossible.
What I did:
- remove the phone battery
- get a 4.5 battery (http://www.consumablestore.co.uk/pro...pg&w=360&h=272)
- let the 4.5 battery discharge for about 3/4 hours, for example putting it in a flashlight (http://www.lighting-gallery.net/gall...artapetrix.jpg)
- measure the voltage after the discharge with a tester, discharge further until reaching 3.7 volts
- using small electrical leads (http://www.harborfreight.com/media/c...mage_16393.jpg) connect very carefully the battery to the connectors on the back of the phone - avoid short circuits placing insulating material e.g. a tiny paper piece or adhesive between metal parts
- connect the phone to a computer equipped with the flasher, and reflash the phone
- once the phone is up & running again, turn it off and put its battery back in place
- turn the phone back on and it should happily resume charging its own battery

Hope it will help someone!
M

Glad it worked for you.
Honestly though...
A TON of testing this method needs to be done.
I would never ever ever do this myself until it could be verified as safe for myself,
the building I'm in,
or the well being of my device,
And naturally would not condone in any way shape or form such methods until such time.
To reiterate test, test, and retest...
over a prolonged period of time to make sure there are no long term issues ..
show extensive results that it is safe then I'd feel comfortable trying this.

I would not be so harsh. You of course have to know what you are doing but with some basic background in electrical engineering, I see no reason why the 4.5V battery trick would not work. My only uncertainty would be about the sense pin, missing in maurochiado's solution, but I am happy to believe that the worst that can happen is the battery capacity measurement being bogus. Since this is only a temporary stop-gap solution, that would not bother me too much.

You should have seen the improvised solutions I've made in my time. When I was stuck in a hotel in the middle of nowhere, in a foreign country with different mains socket plugs, with no wires... I had to make do with rolled-up aluminium foil from chocolate wrappings I definitely would not recommend that to anyone who does not know exactly what s/he is doing.

ach ..
I'm all for experimenting...
I'm more concerned with scenarios popping up like what we've seen with new folks trying alternate chargers and alternate methods right off the bat.
I just don't want new folks to see experimental methods (especially really new ones) and attempt them as a permanent solution without understanding the risks involved.