NiMH vs. Alkaline Runtime Discrepancy in Fenix L1P

As I understand, Ni-MH batteries should provide superior runtime compared to alkaline (see charts on http://www.flashlightreviews.com/reviews/fenix_l1p.htm). In my own L1P, I performed an informal test between Ni-MH (Titanium 2600 AA battery charged in Titanium Tg-700 charger) versus an alkaline (Rayovac). I found the runtime of the alkaline to be significantly greater.

While I had no quantitative way to do this test, I did it by periodically comparing the visual brightness of fully charged batteries by eye to the ones being tested. I also checked the charge level over time in a battery tester.

What I estimated to be a 50% brightness level was achieved in 2 hours and 45 minutes with the alkaline battery and 2 hours in the Ni-MH. While that is, of course, is a rough guess, I feel fairly certain that the alkaline held its charge better than the Ni-MH. This is just the opposite of what I would have expected. What gives?

Before running the test, my battery charger showed the alkaline to be fully charged (far end of the green scale), while every fully charged Ni-MH battery I have tested shows it to be only 2/3 in the green scale. Could it be that my battery charger cannot fully charge these batteries?

Thanks for any explanations.

- Greg

Take a look at the battery charger comparison. Here

The TG-700 doesn't seem to fully charge the cells.

Jay T said:

Take a look at the battery charger comparison. Here

The TG-700 doesn't seem to fully charge the cells.

Click to expand...

Thanks Jay T. According to that post, if I leave the batteries in the charger for at least 5 hours after turning green it should take care of most of the problem. I will do that and re-test. Thanks again.

- Greg

I'll take a guess that what you interpret to be a 50% decrease in lumens is actually much greater due to the non-linear response of the eye to brightness. An alkaline powered L1P will have a much longer output "tail" than a Nimh powered one, such that the L1P might still be putting out 20% of its original lumens while the NiMh cell long before exhausted itself.

If you used a light meter, you might find two things:

1. The area under the light output curve will be much larger with the Nimh than the alkaline.
2. The drop to 50% output comes much sooner with the alkaline than your eyes are telling you, and the NiMh is still going strong for quite a bit thereafter.

Unless you are using a light meter it will be nearly impossible for you to accurately determine what the 50% output level is.

wasBlinded said:

I'll take a guess that what you interpret to be a 50% decrease in lumens is actually much greater due to the non-linear response of the eye to brightness. An alkaline powered L1P will have a much longer output "tail" than a Nimh powered one, such that the L1P might still be putting out 20% of its original lumens while the NiMh cell long before exhausted itself.

If you used a light meter, you might find two things:

1. The area under the light output curve will be much larger with the Nimh than the alkaline.
2. The drop to 50% output comes much sooner with the alkaline than your eyes are telling you, and the NiMh is still going strong for quite a bit thereafter.

Click to expand...

Thanks for your responses. Is it normal then that a battery tester will show a new alkaline battery as fully charged when the needle is at the far end of green scale (needle pegged to the right), while a NiMH cell is fully charged when the needle is only 50% in the green scale? That is what I am experiencing. I am using Titanium 2600 AA batteries, charged in a Titanium Tg-700 charger. I am leaving the Nimh cells in the charger for about 20 hours after they are fully charged and the green light is showing.

- Greg

Not sure what tester you are using, however there are several factors possibly at play here....
The available energy in an alkaline is typically higher than in a NiMH rechargeable, all else being equal, although it's getting closer or equal (an Energizer AA alkaline is 2850, the rechargeables you used are 2600, for example). However, the alkalines are 1.5V versus 1.2V nominal for the voltage (maybe why your battery tester is showing "low" on the NIMH probably, it's based on a 1.5V "good" reading I'd imagine).
NiMH are much lower resistance than alkalines, so you'll get to use more of their energy, "faster" than the alkalines, as someone said, more light output in the life of the NiMH, than the alkalines. In other words, the alkaline will start bright and rapidly and continuously drop in output, voltage and current will drop as resistance increases, which leads to longer runtimes, but at reduced output. NiMH will provide a strong steady current until they are nearly depleted at which point they'll run out of energy fast and go "dark.
Put another way, I don't know the specifics of that light, but pretend for arguments sake it's designed to operate at a max of 1 amp of output. Turn both batteries on and at first you get 1A out of both... after 5 minutes, you are still getting 1A out of the NiMH, but only .8A out of alkalines... after 30 minutes, getting 1A-N and .7A-alk. ... after 60 minutes, getting .9A-N and .4A-alk... after 90 minutes getting .8A-N and .3A-AA... and after 2 hours, the NiMH rapidly goes out, while the alkaline is putting out 0.1A of output... in that time, the NiMH was giving full output nearly the entire time, while the alkaline was steadily dropping, becoming less and less useful. It's still running, but useless at this point other than maybe to find the light if you drop it in the dark. It'll keep dimly dying for a while longer.
Basically the NiMH lets you get all the power "out" of the battery, supporting a higher amperage output constantly until the batteries are nearly done. Alkalines quickly stop you from getting the desired output (100% amperage) due to resistance, but as a result hold onto their "remaining" power longer.
Kind of like 2 equal buckets of water hanging on a pole, poke a small hole in one and a large hole in the other, you'll get the same amount of water out of each, one just gives less, longer, the other gives more, shorter.
Keep in mind also, alkaline ratings (like 2850 for energizer AAs) are based on fairly low current discharge rates (0.2C for instance) compared to NiMH (0.5-1.0C typically)... this means that a 2850-alkaline is based on a discharge rate of around 570mA, a 2600 NiMH might be based on a rate of 1.3A, the alkaline total amp-hour rating falls fast when exceeding the 0.2C rate.

Thanks Christexan. Since NiMH AAs are based on 1.2 volts versus 1.5 volts for alkaline, it may explain why the NiMH never shows to be fully charged in my battery tester.

I was aware of the principle that NiMH batteries get their energy out "faster". It was only because I observed the opposite (visually/qualitatively) in my Fenix L1P that I submitted this post.

Since I do not have a light meter, what I plan to do next is visually compare the brightness of a NiHM versus an alkaline, each allowed to discharge for exactly 1:30 min and then visually compare the relative brightness. Then allow them to continue to discharge for another hour (to 2:30) and compare the relative brightness again. Based on the charts shown in http://www.flashlightreviews.com/reviews/fenix_l1p.htm, I would expect the NiHM to be noticeably brighter than the alkaline at 1:30 and noticeably less bright than the alkaline at 2:30. If this is not what I experience, I will report back.

Thanks again.

- Greg