Rechargeable batteries and chargers

[angelmaz11]

Well, in need for more information and advice...

I need to get a rechargeable battery and I know the new nickel metal hydride has just come out (very light by the way). The thing is, are they reliable? Does anyone own one?
I know they've got this really nice small stick you can buy with the charger, which tells it to stop when it's fully charged. Problem is, they are so small, they're bound to be stolen?! Aren't they?
I just don't want to waste my money...

Any thoughts??

Thanks.
Marion

 

[Stu]

When you say rechargeable, what type exactly? Off the shelf AA types or a Speleotech?

 

[angelmaz11]

speleotech I think. Sorry don't really know what Im talking about as I'm still learning. I've got an oldham, which works perfectly and with that I've got one of these simple battery packs which take a large 4.5V battery. I thought rechargeable ones are better...or can I just use the 4.5V? How long do they last?

 

[Stu]

Still not sure what set up you have. Oldham? Is this an off the shelf lamp or a home build? Assume the battery is a 4.5v flat, with terminal tabs (0ne longer than the other) on the top. If it is you can buy plastic "pods" which means it's possible to put in AA etc type batteries. Problem with most rechargeable's is that they are usually 1.2v. So three batteries would be 3.6v (underpowered) or four would be 4.8v (and slightly over powered). Four may not fit into your battery holder case.

Bit of guessing here on my part as I don't know what you have exactly.

As for longevity...

Not a simple answer unfortunately. There are some threads on the forum about this, try a search. Basically it will involve how much capacity your battery has (amp hours), what bulb/s you have in your lamp and how much they draw from the battery. If for example you have a 4v bulb drawing 1 amp per hour and the battery is 5 amp hours 5 / 1 = 5 hours capacity (the bulb will fade before this though). This example would produce 4 watts. If the bulb drew 0.5 amp hours it would be 5 / 0.5 = 10 hours but the bilb output would be 2 watts. You have no control over the bulb drawing a current other than changing the bulb! This is why a lot of lamps have a main and a pilot. The rating of the bulb will be printed on the side.

 

[potholer]

Regarding capacity, with 4.5V alkaline packs, the *nominal* capacity is something like 5500-6000mAh.
In practice, pulling a large current from an alkaline battery (such as powering a halogen bulb for main beam) will reduce the runtime rather more than a calculation based on the nominal capacity might suggest

Apologies for the long URL, but the charts on:
http://www.candlepowerforums.com/ubbthreads/showflat.php?Cat=&Board=UBB6&Number=744320&sb=5&o=365
make interesting reading - pulling 1A out of AA alkalines tends to give ~1/4 of the runtime (~1/2 as much total power delivered) when compared to pulling 0.5A. Even at 0.5A, the actual amp-hours extracted are only ~60% of the nominal figure.

A rechargeable pack should give much nearer the predicted life (and voltage) at decent current drains. In practice, at high drain, the voltage of a 4.5V alkaline pack will quickly drop to near 1.2V/cell, so the voltage difference between alkaline and NiMH may be significant.

On the subject of S-T nickel packs, see:
http://ukcaving.com/board/viewtopic.php?t=848
(Though bear in mind that people with problems are likelier to post than those without.)

Angelmaz, is your current battery case an S-T one, or [something like] a Petzl battery case. (Does it have a plug on the outside, or does the cable go right inside it?)

 

[angelmaz11]

Thanks for the info. My light is basically a bulb in the middle with built in 7 LED and reflector. So really it runs on the LEDs unless I want to look at something in detail or far away. It's got a cable which runs from the light out into a plug. It's what all my mates use, but I need some source of power and was wondering what difference there is between alkaline battery and rechargeable ie. Nickel/cadmium?

Yes I think the battery case is S-T and definitely has a socket.

 

[potholer]

I spotted a typo in my previous post. It should have said "...so the voltage difference between alkaline and NiMH may *not* be significant."

I don't know what current the 7-LED reflectors pull, so it's hard to estimate battery life. I *think* the S-T Nickel pack is ~4500mAh, so in practice one would probably give roughly the same life as a 4.5V pack, if fully charged

Regarding the possibilities of gear 'disappearing', I suppose it depends on where you cave.

 

[Stu]

Isn't it about 20 - 25mA's per bulb so 7 is 140 - 175mA's? This seems about right when compared to my 2000mA AA's.

 

[potholer]

It's probably something like that, but I don't know what the voltage vs. current relationship may be with varying battery states (would the LEDs be overdriven on a fresh 4.5V pack?).

 

[Stu]

Just. From what I can gather (and I think this has been covered but cannot be arsed to find it!) you can run an LED at an unlimited voltage. It'll go bang of course but at more reasonable levels...say 4.5V there will be a % reduction in LED life. IIRC it may be 10%.

 

[potholer]

The problem with just thinking about voltage and LEDs is the voltage/current curve for LEDs is far from a simple straight line to the origin. Below a lower threshold voltage, little current will be consumed, but above that voltage, the current drawn will increase rapidly.

For example, a 3W luxeon will typically pull little current below 3V, but between 3V and 3.8V, the current drawn will increase from near zero up to 1Amp, and would carry on increasing at a similar rate with increasing voltage if the LED didn't burn out first.

If you stuck a (properly heatsunk) 3W luxeon directly across 3xAlkaline batteries, (nominally 4.5V), it might well work OK, since the current it would start to pull would depress the battery voltage well below 4.5V, and possibly into a range where the LED was operating within its ratings. If you tried the same thing with 4x1.2V NiMH cells, the cells would be capable of suplying a high current without dropping much voltage, and the LED could rapidly end up dead.
Unless you know your power source isn't capable of supplying an excessive voltage at a current the LED can't cope with, you do need some kind of current-limiting circuitry, even if it's only a resistor.

There are various mechanisms that affect LED life - the passage of current can cause defects to slowly form in the crystal lattice even at normal currents, and such ageing will affect devices at least proportionately at higher currents (if running at 50% beyond rated current, the lifetime will reduce by at least 33%, and quite possibly more).
All other things being equal, LEDs will also age more rapidly if run at higher temperatures, and running at higher currents will usually result in a hotter LED.
Running significantly beyond spec certainly won't improve an LED's life, and may well reduce it quite dramatically.