Why do rechargeables work better in regulated LEDs than primaries?

Was on www.flashlightreviews.com looking at the output and runtimes for LED flashlights and in almost all cases, the rechargeables allow longer runtimes, greater output and flatline regulation.

Why is this so?

In the case of the Fenix L2T, using AA Alkalines gives a pretty ordinary runtime graph with output constantly decreasing. But using NiMh rechargeables it doubles the run time at constant maximum output!

By using Energizer AA Lithiums in the L2T, will the runtimes and outputs be the same as using NiMh rechargeables?

You're refering mainly to NiMH AA and AAAs, right?

The problem with alkalines is that they can't sustain pumping their juice very well. I'm sure there's a compicated technical reason regarding amp draws, capacities, and internal resistance, but I'm not the one to explain that to you (although I would like very much to have it explained to me). Suffice to say that NiMHs, despite having lower voltage and lower capacity (although they're fast catching up) than alkalines, they can sustain the high-power sucking that high-drain devices like flashlights (and cameras and such) far better than an alkaline. Alkalines however will be better in low drain devices that don't get used very much like TV remotes because they have a much slower self-discharge rate (a NiMH, left sitting around, will drain itself over time).

A lithium AA or AAA will be even better than an NiMH in terms of runtime/power in a high drain device, with practically no self-discharge. The tradeoff is cost.

Fusion_m8, let me be the 1st (edit: 2nd ) to attempt an answer, since I'm in your timezone!

I'm definitely no expert, but it's got something to do with the fact that even if an alkaline and NiMH have the same capacity eg 2500mAh, the alkaline battery has a higher internal resistance under the high current drain which the average light pulls. This means that its voltage "sags" under the load, rapidly dropping to something lower than the nominal 1.5V. NiMH have much lower internal resistance and thus continue to deliver the same approximate voltage (1.25V) for longer - only really dropping when they largely run out of charge.

Of course I could be completely wrong and no doubt someone more knowledgable can fill you in better.

I know that primary Lithium batteries like CR123's and CR2's have practically no self-discharge BUT what about rechargable CR123's and CR2's??? I know if I leave my phone (turned off) alone for about 2 to 3 days fully charged, when I turn the phone back on, I dont have a fully charged battery so whats the deal there??

Fusion_m8 said:

...the rechargeables allow longer runtimes, greater output and flatline regulation...Why is this so?...

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If you compare the datasheets from an Energizer alkaline AA, NimH AA and e2 lithium AA, the reason will be clear.

Despite having good amp-hour capacity at lower power outputs, an alkaline battery cannot supply the power at high levels without adversely affecting capacity.

Note especially the discharge curves for each battery type, and the discharge rate at which the rated battery capacity is achieved. For alkaline AA rated amp-hour capacity is achieved at a 25 milliamp discharge rate. For NiMH AA, its capacity is achieved at 500 milliamp discharge rate, 20 times the alkaline cell. Lithium primary batteries typically can supply much more output current than alkaline without hurting capacity too much, but rechargeable lithium ion batteries can usually supply even more.

Re performance of Energizer AA lithiums in L2T vs NiMH (assuming 2500 mAH NiMH), I'd expect the lithiums would be roughly comparable, but don't know for sure. However except for infrequent use, AA lithiums would be an expensive way to go.

Alkaline AA: http://data.energizer.com/PDFs/X91.pdf
NiMH AA: http://data.energizer.com/PDFs/nh15-2500.pdf
Lithium AA: http://data.energizer.com/PDFs/l91.pdf

GrooveRite said:

I know that primary Lithium batteries like CR123's and CR2's have practically no self-discharge BUT what about rechargable CR123's and CR2's??? I know if I leave my phone (turned off) alone for about 2 to 3 days fully charged, when I turn the phone back on, I dont have a fully charged battery so whats the deal there??

Click to expand...

Rechargeable lithium ion batteries normally have very low self discharge, albeit a little higher than lithium primary cells. I suspect there's something wrong with your phone, or the battery is worn out or defective. Even a NiCd battery doesn't self-discharge in 2-3 days.

gadgetnerd said:

Fusion_m8, let me be the 1st (edit: 2nd ) to attempt an answer, since I'm in your timezone!

I'm definitely no expert, but it's got something to do with the fact that even if an alkaline and NiMH have the same capacity eg 2500mAh, the alkaline battery has a higher internal resistance under the high current drain which the average light pulls. This means that its voltage "sags" under the load, rapidly dropping to something lower than the nominal 1.5V. NiMH have much lower internal resistance and thus continue to deliver the same approximate voltage (1.25V) for longer - only really dropping when they largely run out of charge.

Of course I could be completely wrong and no doubt someone more knowledgable can fill you in better.

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Also since the Alkaline Batteries have a higher internal resistance they would generate more heat............

Thehappyman

But it appears on flashlightreviews that most flashlights using Lithium primaries have a higher lumen output and/or longer runtimes than the same flashlight using Ni-Mh rechargeables... why is that so?

Do Ni-Mh batteries have higher internal resistance to Lithiums?

It depends. If the current draw is only 10mA or so, alkaline provides superior usable capacity.

Fusion_m8 said:

But it appears on flashlightreviews that most flashlights using Lithium primaries have a higher lumen output and/or longer runtimes than the same flashlight using Ni-Mh rechargeables... why is that so?...

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First, for a valid comparison you have to use the same number and size of batteries, e.g, 2x AA NiMH vs 2x AA Lithium. You can't compare a CR123 lithium primary to an AA NiMH.

Secondly, AA-powered LED lights tend to be lower-powered devices, because they're designed mostly for alkaline primaries. You can sometimes use lithium and NiMH AAs in those lights. But at lower current drain, a lithium AA has more amp hr capacity than 2500 mAH NiMH AA.

Only at very high current drain do NiMH AAs have more capacity than lithium AAs. This isn't easy to see from the Energizer fact sheets, as the provided graphs use different construction. However on the e2 lithium AA datasheet (see above links), if you extend the logarithmic constant performance graph to the right, it's obvious the lithium AA has less capacity than the 2500 mAH NiMH AA at very high current drain.

Yet another factor is the varying capacity of AA NiMH batteries. They vary from 1700 mAH to 2500 mAH (and more).

A non-regulated light would have higher lumen output on lithium AAs than NiNH AAs because the cell voltage is higher (1.5 vs 1.2 v). A regulated light (depending on type of regulation) should have the same brightness, but runtime may vary between alkaline AA, lithium AA and NiMH AA depending on current drain and battery capacity of NiMH version.