How to identify a burnt power resistor with unreadable color bands?

I can't make sense of the colour code on this resistor, anyone got any ideas?

Odd power resistor JPG 100k

Difficult one.  Looks like it has yellow and purple so  a multiple of 47.This looks like a board from a battery charger of some kind, would I be right?  If so it could be 4.7 ohms or 47 ohms.  Based on similar experiences I have had I would say 47 ohms.  If I am correct, the thing to do is tailor the resistance to the battery is is charging.  If it's (say) a 1 amp hour battery, tailor the resistance so that it charges at around 100mA (1/10 of the battery capacity) maximum.I'll take another punt here, it's a charger for a NiCd (Nickel Cadmium) or NiMH Nickel Metal Hydride) battery (now obsoleted by Lithium batteries).  You would often find chargers advertised as "Fast Chargers" with the ominous warning in the instructions "DO NOT Charge for more than 5 hours".  They were just a resistor, like this, and if you left the battery in for over that time the battery would be cactus.  Don't jsut replace it, change the resistor so it's charging at 1/10 the battery capacity and it won't matter if you leave it on for a day or longer.A possible reason for the resistor's demise is that one or more cells in the battery have shorted, so the resistor has to drop more than the usual voltage.  Check the battery - you should have more than 0.5V per cell if they are good.  Any that are not, give them (single cell only)a zap or two with a high current 12V source (eg a car battery) with a 20-50 watt headlamp bulb in series - this will run a current of 2-5A through the battery and burn off the cadmium whiskers that cause this short-circuting.  After you've done this the battery should read above 1.2V (put a meter across it while you do this if you can).Some time ago I designed a circuit to do this repeatedly - zap the cell with 3x capacity for 10 seconds, then discharge it to 1.1V, then zap again.  Half an hour of this and the cell would be ready for another yar's work normally.If this was not from a battery charger, let me know what it IS from and I'll make some more educated guesses!

You're right, it's from a charger but for a 6V 4Ah lead-acid powered lantern battery (Draper RHL185) belonging to a blog reader of mine. I've got one as well, but my board is slightly different and it's got a 3.9 ohm (2-3 watt est). on board.  All it does is trickle charge the SLA from a 9V mains adaptor, as the terminal voltage gradually rises the red led gradually fades out.The board is in a block that clips over the demounted battery holder. Strange one, I can't recognise the resistor colours! Could be the SLA has shorted a cell and taken the resistyor out.
Draper lantern

Hi Alan.  Well I was not too far off then.  I have a similar spotlight and have replaced the battery 3 times now.  My charger and battery are inside the spotlight body.   Lead Acids usually fail high resistance so I am surprised at the resistor burning out.If it's a SLAB then the resistor is probably 4.7 ohms but if it is likely to be left on charge for long times or permanently then maybe up the resistor to 6.8 or 8.2 ohms.  Get the multimeter out and measure the current - bear in mind the meter resistance will reduce what you read a bit from the actual value.I have a lot of rechargeable stuff I have made myself and I try to make everything rechargeable from 12V .  That way you can get a cigarette-lighter lead and recharge them in you car if you wish.  So I would be very tempted to put a socket on the body of the spotlight and build a charger in. Use a 7808 or 7809 regulator and from the 8/9V output take a series diode (1N400x) and a resistor that gives a reasonable charge rate on a flat battery (<6v) but tapers the charge off as the voltage rises (it will approach 7V).On a 7808, you have 8V output less the 0.6V of the diode = 7.4 V.  With a 3.9 ohm resistor you'll get around 360mA  charge at 6V, tapering off to around 100mA at 7V battery voltage.  A 7809 will give less difference between flat / charged currents. But a 5-cell NiCd battery will approach 7.5V at full charge, so I'd use a 7809 for NiCds.  Heatsink your regulator appropriately -  if your 12V supply is actually 14V from a car battery, it'll dissipate a good few watts at the highest charge current.  But it will thermal-shutdown if it gets too hot, so there's extra protection.This is a good way of charging Lead Acid and Ni-Cd batteries - I did a project using this technique herehttps://www.eetimes.com/author.asp?section_id=30&doc_id=1319957
Hope this has given you some ideas....

I tend not to leave SLABs on trickle charge but top them up once a month (ish). I had several batteries in lanterns and 12V car jump starters fail when they self-discharged over winter. I hoped to add a charger, as you say, to the body of the spotlight so I can charge it direct without removing the battery. Thanks for the ideas! That's a project for the winter holidays.

Alan... I'd agree, leaving them on trickle is not really necessary as long as you do top them up once a month or so (reminds me, I need to do mine .  But more important is to limit the charge so if you DO forget to take it off charge, it's not going to wreck the battery.

David, Alan,I usually put my batteries on a mains driven time switch of the programmable type so they get top up charged on a regular basis.

Crusty (Conrad) a fine idea - I have done this using one of the old motor driven timers so  that things get a charge for 1-2 hours a day.  I'd like to build a charging shelf with continuous and switched 12V and mains so you could plug anything in to charge or just keep up the charge.   Time, Time, Time.......I need more of it!

During winter months when my car battery gets hammered (night times, short journeys, slow speeds, heating, lighting full on etc) my car won't get used for quite a while so I use a CTEK electronic charger to trickle charge it when unattended. A special connector is rigged across the car battery terminals, then just lift the hood and plug it in. Also used for vintage cars and motor caravans etc during storage., and good for the motor mower battery, trickle charging it over winter.  Back to taking my Draper handlamp apart...
CTEK charger

I took apart my own Draper charger, it's slightly different from the burnt out one, defintely 3.9 ohms (guess 2 or 3 watts)
Draper charger pcb