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I have led headlight with 3 AAA batteries. I will rather have similar device use 1 AA battery, even if it will be more expensive. But seems there are no such devices. Why?

I've got the question after watching https://www.youtube.com/watch?v=4alV5LzHLE4 and later I've found Least expensive way to boost DC voltage where charge pump is also mentioned.

How expensive can converter from 1.5 to ~5V be if mass produced? How small by volume and weight? TIA

TL;DR

As I've understood from mentioned QA there are limitations to charge pump. Namely only 2x raise and some issues "under load". How much load is LED? I've just tried to use my headlight with only 2 AAA and light is much dimmer (I recall there is optimal voltage to LED); ok, producer may insert bigger LEDs to compensate for that (I don't recall efficiency drop is voltage is reduced, or is it? *). Or maybe solutions to raise from 1.5 to 5 are also cheap is mass produced?

* When searching for efficiency of LEDs found How can I efficiently drive an LED? where graphs are mostly vertical.

Now found https://www.candlepowerforums.com/threads/why-3x-aaa-batteries-in-stead-of-1x-aa-battery.364897/:

The 50 cents for a boost circuit might seem like nothing, but on a $3 flashlight that would wipe out any profit.

If true I will gladly pay $1 higher price knowing I'll save much more on batteries. Really almost nobody else will?

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    \$\begingroup\$ I'm not sure what this is all about. It's quite possible and there are already efficient ICs for boost circuits that accept AAA batteries: LM2621 for example. I suspect I'm just missing the real question, though. Things get a little more complex when using dynamos (like on bicycles.) But that's not your question. \$\endgroup\$
    – periblepsis
    Commented Nov 9, 2023 at 3:04
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    \$\begingroup\$ 3xAAA have bigger capacity than 1xAAA (last longer)and also no needs a converter. Thats two advantages. \$\endgroup\$
    – Michal Podmanický
    Commented Nov 9, 2023 at 3:27
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    \$\begingroup\$ I'd say the question falls into the Don't ask about category - electronics.stackexchange.com/tour \$\endgroup\$
    – MiNiMe
    Commented Nov 9, 2023 at 3:42
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    \$\begingroup\$ please focus your question ... you asked two questions that are somewhat on topic here ... How small by volume and weight? and How much load is LED?... they cannot be answered with one answer ... the other questions about price are off topic here \$\endgroup\$
    – jsotola
    Commented Nov 9, 2023 at 4:11
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    \$\begingroup\$ Because 3 AAA batteries are cheaper than 1 AA battery plus the DC-DC buck charger. \$\endgroup\$
    – Solar Mike
    Commented Nov 9, 2023 at 8:23

2 Answers 2

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White LEDs run at around 3V. the current will depend on the size of the LED. For example, 0.33A for a 1W LED, or 1A for a 3W LED.

It's certainly possible to boost the voltage from a 1.5V cell to 3V to run an LED, and some small torches do that. They tend to use a small LED that draws perhaps 20mA (0.02A). But if you want to double the voltage, then you'll also draw double the current from the battery. So 0.33A at 3V requires 0.66A at 1.5V, even if the boost converter is 100% efficient (and it won't be).

So not only would the headlight require a boost converter capable of delivering a reasonably large current, but it would run the AA cell flat rather too quickly.

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    \$\begingroup\$ Note (@Martian2020) that small batteries become less efficient when driving large currents, and the difference is relevant in this range. So the increased rate of drain isn't just a matter of using the battery up faster, but of wasting more of its capacity. \$\endgroup\$
    – Chris H
    Commented Nov 9, 2023 at 11:30
  • \$\begingroup\$ @Martian2020 what do you mean by "washing” batteries? And surely you mean “affect capacity” not “effect capacity”? \$\endgroup\$
    – Solar Mike
    Commented Nov 10, 2023 at 3:38
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    \$\begingroup\$ @Martian2020 I suggest you look at battery discharge curves for typical AA and AAA alkaline cells. And read up on internal resistance. A decent headtorch might draw 300mA from a series setup, so 900mA from a single cell with a voltage converter. Yes for these purposes AA and AAA are both small. \$\endgroup\$
    – Chris H
    Commented Nov 10, 2023 at 6:36
  • \$\begingroup\$ @SolarMike, typing in small letters etc. Beg you pardon. I replied to "wasting". \$\endgroup\$
    – Martian2020
    Commented Nov 10, 2023 at 9:49
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I offer an alternative solution here, which worked for me.

I have a head torch that I use on the bike, designed to run off 3xAA cells. It works just as well with 1.2V NiMH cells as it does with 1.5V alkaline cells, i.e. the input voltage can be taken to be 3.6--4.5V.

This has a battery box on the back of the headband, and I wanted to mount a rear light in that box.

So what I did was to use a single 14500 rechargeable lithium cell (3.7V), which has the same dimensions as an AA. I already had a suitable charger for lithium cells. Rewiring the battery box to work off a single cells was easy (just bypass some contacts) and the space saved by removing 2 AA cells gave me plenty of room for the PCB of a rear bike light, with holes drilled for the LEDs and a switch.

The energy density of lithium cells is better than of alkalines, so runtimes were more than adequate for my use case.

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  • \$\begingroup\$ Thanks. AA/AAA are more convenient though, I just don't like wasting money on 3 where IMO one could do. I have to buy AAA for about same price as AA. \$\endgroup\$
    – Martian2020
    Commented Nov 10, 2023 at 2:18
  • \$\begingroup\$ The thing is, you're not wasting money on 3 instead of 1. If you used one with a perfect booster, and there were no ill effects of drawing higher currents, you'd use exactly as many batteries, because you'd be changing them 3x as often. But the extra losses from your proposal mean you'll get through more batteries. Shop around and you're likely to find packs at the same price point, but more batteries per pack if they're AAA, so 6AAA at the same price as 4AA,for example. \$\endgroup\$
    – Chris H
    Commented Nov 10, 2023 at 6:41
  • \$\begingroup\$ Thank you. Now I see discharge curves is the answer - nominal capacity of AA is larger 2.5 times than AAA only at minimal current. If current via AA is twice current via AAA, capacity is only e.g. 1,5 times more or difference is even less if current goes up. Do you want to write that as an answer? If not, I think I will write myself crediting your comments. TIA P.S. I compared AA to AAA for Ikea ones at lygte-info.dk/review/batteries2012/…. \$\endgroup\$
    – Martian2020
    Commented Nov 10, 2023 at 9:46

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