Electronics, Lighting, & Gadgets - Dimmer lights with rechargable batteries?

Does the use of rechargable NiMh batteries cause some lights to be less bright compared to using alkaline or lithium batteries? I have read a couple comments like this including a customer review of the Princeton Tec Eos light, where he claimed a longer runtime with rechargables but reduced brightness.

Is this the case, and if so, why? I'm mainly referring to lights that use AA or AAA batteries.

Thanks!

It's my understanding that rechargeable batteries run at a lower voltage than alkaline batteries but have a longer run time. If the light that you are running is directly driven off the batteries then yes it would be dimmer. However, if your light has a voltage regulator (eg Fenix lights) the light will output a given voltage no matter what the input voltage is so they won't be any dimmer.

It's my understanding that rechargeable batteries run at a lower voltage than alkaline batteries but have a longer run time. If the light that you are running is directly driven off the batteries then yes it would be dimmer. However, if your light has a voltage regulator (eg Fenix lights) the light will output a given voltage no matter what the input voltage is so they won't be any dimmer.

That's my understanding as well....

A NiMH is 1.2v nominal vs 1.5v for an alkaline(ie thats whats written on the battery)
But as batteries are used the voltage drops. NiMH are actually delivering around 1.2v for most of their useful charge.

see here for some fancy graphs and stuff:
http://www.eznec.com/Amateur/1.5_vs_1.2_Volt_Batteries.pdf

If the alkalines are wired in series to give 3.0, 4.5, 6.0 volts or whatever just divide the total alkaline voltage by 1.2 to get the number of NiMH batteries to get the same voltage. As in 3 common AA batteries giving 4.5 volts could be replaced by 4 NiMH batteries giving 4.8 volts. The slight over-voltage will improve light output with a slight loss of bulb life.

When I switched my LD1000 from alkaline to NiMH I noticed that the intensity was lower. Eventually, I put a regulator in my battery pack and now run from my 14.8v Li-Ion battery. This way I can set the voltage I want to run. In the case of the LD1000 I run 3.2v so it is always very bright.

That is correct... in general fresh alkalines will be brighter than NiMH rechargables. But the NiMH will keep their brightness and soon outshine alkalines. If your lights run for 30 hours, after 15hrs they will be brighter on NiMH (maybe after 5hrs even).

That is correct... in general fresh alkalines will be brighter than NiMH rechargables. But the NiMH will keep their brightness and soon outshine alkalines. If your lights run for 30 hours, after 15hrs they will be brighter on NiMH (maybe after 5hrs even).

That's not the complete story however. Freshly charged NiMH will provide more consistent power for longer but not many of us use a light for 15 hours straight. That 15 hours might happen over 15 days. A rechargeable in an application like that is not the best choice because it will self discharge significantly over that 15 day period. They'll experience significant self-discharge in a few days.

Alkalines, on the other hand, have an insignificant self discharge. For applications such as taillights, where the demand cycle and current draw are fairly small, alkalines are a good choice. Most taillights have to be disassembled to access the batteries and that makes rechargeables inconvenient for that application. I put fresh alkalines in my taillights at the start of light season and seldom have use the battery up over the 6 months or so of usage...even when using them on steady mode.

That was true until sanyo developed the eneloops. NiMH with very low self discharge.

That was true until sanyo developed the eneloops. NiMH with very low self discharge.

The Eneloop looks interesting but the cost factor would be somewhat high. From what I have seen and the number of lights I run, it would be rather cost prohibitive to make the switch. I use around 5 taillights on my bike using a mixture of AA and AAA batteries. At $10 per 4 pack for , that's going to be 3 packs...1 AAA and 2AA...and a charger (another $30) or around $70 outlay. Plus I have 4 bikes with rear lights on them that I leave on so that's another 2 packs of AA. Total outlay for 24 batteries (I'd actually have to buy more then I need) would be around $90. A 4 pack of alkaline AA from Radio Shack cost me $4 and replacing the batteries once a year would cost me $24...even less if I buy a larger pack of 40. It'd take 4 years for me to break even on the rechargeables. Hardly worth the investment.

I may look into them for my cameras however.

Im running 24 AAs(two front LED systems using 12AAs each, road and MTB) and 6AAAs for blinkies. Cost a fortune. Got half of them just before the eneloops arrived in NZ. Damn.

Anyhooo...
Have setup my el600 cateye blinkies in the shed, one with new energiser alkalines and the other with NiMH. Camera taking a picture every hour so should have a good comparison to post tomorrow. Interestingly the NiMH was initially brighter(maybe because it was fresh off the charger?).

I can't wait to see the results of your test.

I have started buying the Rayovac Hybrid rechargables which are also slow to self-discharge (or so they claim).

OK, a day later.
The AAA Alkalines are going strong still ... the NiMH are pretty much dead. Im surprised.
Unfortunately I set camera to take 1 photo per minute instead of per hour so no pretty pictures to post. Damn.

An alternative to the Eneloops that are a decent amount cheaper are GP ReCyko (http://www.gprecyko.com/en/index.html). They come recommended from Dan's Data (http://www.dansdata.com) which is a pretty reliable site. They can be found on Ebay amongst other places.