LED torch battery performance

[slowworm]

I have just bought a Fenix LD25 2xAA torch and like many similar powerful LED torches it has various brightness settings. The LD25 has 180 lumens & 45 lumens with a quick twist of the top, plus other 'hidden' settings.

My question is, when the batteries run down a bit it seems to automatically switch from 180 to 45 lumens. This seems like a good idea but as I can't find anything in the instructions about it, does anyone else have a similar torch and does it behave in the same way?

Other than that I'm quite pleased so far with the torch, the runtime at 180 lumens is very short with alkaline batteries, which seems to be common, so I need to try it with some high quality Ni-MH rechargeables before I reach a conclusion. I'm impressed that the torch comes with spare seals and a spare switch cover.

 

[Manacles]

It sounds like a failsafe system, has to be better than being plunged straight into darkness.........

 

[nunzionuk]

Yeah, I have a Fenix, and its set to drop the Lumens as the battery life goes.

This is standard.

 

[ged]

Some high output devices will destroy themselves (thermal runaway) if they don't have an automatic cutout. The same goes for some Lithium-ion battery packs too.

The faster you discharge a battery, the less energy it can give you. So if you discharge a battery at its 'nominal' rate (whatever that is, say 20 hours for a lead-acid battery) and work out how many ampere-hours it gave you, then you'll find if you charge it up fully and discharge it at say twice the rate you will typically get less than 80% of the ampere-hours that it gave you at a lower discharge rate. This applies to every battery you can find, whether it's primary or rechargeable. So you will not get such good performance from batteries in the higher-output torches, full stop.

Some alkaline batteries from, shall we say, far eastern parts give you a lot less energy than some made closer to home -- even though they look identical. The differences can be astounding.

It's a well-known engineering principle, often expressed as "Life's a bitch".

Don't try recharging primary cells, even though there seem to be several chargers on the market which claim to be able to do that. They MIGHT NOT explode but they WILL leak and that WILL damage equipment.

 

[Rebel]

"Turbo" mode goes on mine when the batteries are low. However because Fenix lights have a regulated circuit they can continue to provide you with light from low batteries long after lesser torches give up. To prove this I took the batteries out of a cheaper Tesco LED torch that no longer shone and put them in my Fenix and I could still get a decent beam. Could be a lifesaver and this is one of the reasons they cost a bit extra.

When the batteries are about to completely give up your light will start flashing to give you warning.

 

[Rebel]

Hmm, that was my 1,000th post. Do I get a prize?

 

[johnnythefox]

thats a good safety feature

 

[offroadmick]

You'll find after a while with this light, that 45 lumens is more than enough for most camp and walking tasks.... 180 lumens is great to show your mates and sweep the woods if you hear a noise, but ruins your night vision when you turn it off...
As I understand it, the fenix lights and most other decent LED torches have a circiut to regulate the power to the led.... Say for instance your LED requires 800mAh to run at 180 lumens, the circuit supplys 800mAh exactly until the batteries run down to a point they cant maintain it, then it automaticly switches to a lower power level, like 450mHa with 45 lumens. With un-regulated lights, the output drops steadily down from max lumens until you can hardly see any light at all.... With the regulated, you get a good usable light until the batteries are flat, then nothing. I love my Fenix lights... My TK10 will flash in turbo mode if the cells are low to warn you. The PD20 and PD30 just step down in stages.
Hope that helps!

 

[slowworm]

Thanks for all the replies, they're very helpful. So it's a feature and one that will be useful. I've also decided that there's not much point on using the 180 lumens setting unless I'm searching for something at distance. Interestingly, while looking at high capacity rechargeables they stated they can last up to 7 times as long as good quality alkalines, and that seems to match up with my experience so far.