Battery charger burns out - Is it preventable?

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Kart31

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I have a charger for a 9.6V NiCd battery pack. I'm now on my third one. I opened up number 2 to find a burnt circuit board and fried resistor.

Using the new third one as a reference. I found the circuit.

11VDC Wall Wart Pos -> 1N4009 Diode -> 15 ohm 2W resistor -> Battery Pos -> Battery Neg -> Wall Wart Neg

I don't know if the wall wart is regulated. The 15 ohm resistor could be 3 watts, definitely more than 1 and not 5. Appears to be a metal film or metal oxide resistor.

Is it as simple as increasing the resistor wattage and locating it even further from the circuit? I was thinking of replacing the stock resistor with a 5 or 10W sandstone or metal cased (the gold colored one with the screw holes).

All thoughts (other than changing battery chemistry, LOL) welcome.

TIA.
 
When I used Ni-Cad batteries 28 years ago they frequently became shorted inside with metal slivers that developed. Your charger circuit is so simple that of course it burns up.

I recommend using Ni-MH battery cells (same voltage as the Ni-Cads) and a proper charging circuit for them (it properly limits the charging current then detects a full charge and turns off).
Another benefit of using Ni-MH cells is that they have a much higher capacity so each charge will power the load for a longer time.
 
That is all good information. This is the stock charger for the pack. Is a simple solution to preventing the resistor from burning out using a higher wattage resistor? Part of me says yes (the resistor wattage is under rated), part say no (I need to find a new or adapt another charger to this base).
 
Nigel Goodwin said:
Yes, but it would then probably destroy something else instead.
Click to expand...
This appears to be a long term issue. Such that the charger fails after repeated use. If it was a single use and dead, that would be a different situation begging a much different solution.
 
As long as there's nothing wrong with the battery (as AG suggested) a much larger resistor will probably sort the problem, but bear in mind whatever size the resistor is, it's still dissipating the exact same amount of heat, and while it may survive it will get pretty hot. You mentioned the metal cased ones, with screw holes, that's the sort of thing you need, and bolt it a decent size heatsink (or piece of metal) to remove some of the heat from it.
 
Buying a better quality charger would be a great idea, the existing one is the crudest nastiest one they can make. I've seen a lot of them over the years, and they always get far too hot.
 
The smart charger will let you know soon when the Ni-Cad battery has failed, then you can replace the battery with Ni-MH.
 
audioguru said:
The smart charger will let you know soon when the Ni-Cad battery has failed, then you can replace the battery with Ni-MH.
Click to expand...
Please get off that kick. I posted in the OP, changing chemistry is not an option.

Edit. Wasn't an option as they did not charge with the same capacity as the NiCd in THIS charger. Perhaps I will dig out the batteries and try with the new charger.
 
Ni-Cad is an old technology with batteries shorting with metallic slivers which might be why your chargers burned out. Cadmium is very toxic and is banned in many countries. A Ni-Cad battery develops a "memory" resulting in a very low capacity. Even when not used, a charge does not last long.

A modern Ni-MH battery does not have these problems and has more capacity. They are sold everywhere today. An unused charge is guaranteed to last for 1 year.
Since the original charger is not a proper charger circuit then it should not be used.
 

As usual you're only giving one side of the argument, NiCd have significant advantages for many applocations, will provide higher current (you don't see NiMh in cordless drills) and survive better on permanent tickle charge (again, you don't see NiMh in emergency lights).

Both have their reasons for use, and aren't simply interchangeable - and for anything remotely modern of course Li-Ion have pretty well taken over.
 
I have a cordless drill and haircutter that both have dead Ni-Cad batteries in them. I don't use them much and they work fine when plugged in so I did not update their batteries.

I found two portable vacuum cleaners. The Ni-Cad batteries on both of them were shorted. Instead of zapping away the shorts I replace the batteries with old 18650 Lithium cells from thrown away laptops. I have many 18650 cells waiting to be used in a new project.

"Emergency lights"? I recently replaced my smoke detectors. Each one uses TWO 9V alkaline batteries. I wondered if the second battery was in case the first battery failed, but no, one battery is for the smoke detector circuit and beeper and the second battery is for the emergency light to light up the smoke.
 

Do you live in a bubble?, perhaps Canada is a bit backwards? - but in the UK workplaces and businesses have had to have emergency lighting for finding exits in case of a fire for many decades - and they always used NiCd or Lead Acid, never NiMh, and presumably may now use Li-Ion?.
 
I have never looked inside an emergency light.
I see that ebay sells Ni-Cad batteries for emergency lights in many voltages and sizes. Many of them are not available.
Amazon wrongly says they sell Ni-MH batteries for emergency lights but looking closely the battery says Ni-Cad. They are also not available.
 

RS Components list 46 different NiCd battery packs, some of which are specific sizes for Emergency Lights, they also list 8 D size NiCd's, but 9 NiMh.
 
Nigel Goodwin said:
... and survive better on permanent tickle charge (again, you don't see NiMh in emergency lights).
Click to expand...

Huh? I had read that it was bad choice to trickle charge NiCd. The last fire system group I worked with said the batteries are actually on a short cycle and the light runs part of the day from the battery to partially drain it then enter a recharge cycle. Not discharged even close to what the user of a tool or electric shaver would discharge, but the fact is there is some discharge and not a constant permanent 'top off' charge.
 

There are many millions of NiCd batteries worldwide on trickle charge at this very moment, in emergency lights etc.

'Perhaps' some VERY expensive ones might do what you suggest (but I've never seen or heard of one), but generally they are extremely crude devices as NiCd are fairly happy with a gentle trickle charge permanently.
 
There are several factors involved with maintaining NiCd cells in optimum condition.

They can be left on a low trickle charge indefinitely without any actual harm. However if they are never discharged, the plates gradually undergo cold crystallisation, which increases the internal resistance in discharge.
That's the so-called memory effect - they can appear to be flat when used on a high current load, when discharged below the normal cycle depth.

A full discharge cycle about once a month prevents that happening, or at least getting too bad.

For low current loads like memory backup, the higher internal resistance has no effect - they still retain full capacity & will often work for several decades on permanent trickle charge.

And as AG mentioned, if left flat or even just unused with no charge for long periods, the cold crystallisation effect can create needle spikes that pierce the cell separators and short out the cells.
 
rjenkinsgb said:
For low current loads like memory backup, the higher internal resistance has no effect - they still retain full capacity & will often work for several decades on permanent trickle charge.
Click to expand...

I would very much disagree with that, as pretty all NiCd's used for that purpose failed within a relatively short time, I've replaced hundreds of them, as have all TV Engineers - and as have Computer engineers, where they were sometimes used for BIOS backup (soon replaced by non-rechargeable there as well).

Quite a few manufacturers brought out modifications, where you replaced the NiCd with a non-rechargeable battery and removed the charging resistor, the live time of the battery was actually considerably longer than the life of the NiCd's.

Another attempt was using Super Capacitors - these promptly failed as well - I've replaced lot's of those in VCR's.
 
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