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LED lamps powered by an 18650 cell instead of AA

Brutal123 6528 13

TL;DR

  • A lamp power conversion uses an 18650 battery holder with a switch and a USB-C charging module instead of AA cells.
  • The holder for 2 cells in series was rewired to 2 in parallel, making the voltage suitable for lamps normally run from 3xAA.
  • The same approach can be adapted to 2xAA lamps by changing the resistor.
  • The modification brought significant battery savings, although mains power remains the preferred option when available.
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  • I present an alternative to powering the lights with batteries.
    Container with a switch for 18650 cells, 2 pcs in series, but converted to 2 pcs in parallel and a Z USB-C cell charging module added. It is best suited for lamps powered by 3xAA due to the similar voltage, but by changing the resistor it can also be adapted to 2xAA. It took a bit of fiddling with it, but the savings on batteries were significant.
    Maybe someone has similar solutions? It is best to power it from the mains, but sometimes this is not possible.

    Battery holder with a USB-C charging module and two 18650 cells. Image of a battery case with a switch adapted for 18650 cells with a USB-C charging module. Close-up of a black battery holder with a USB-C port and two white wires. Black container with a USB-C port and connected wires. Black battery holder with a switch and a connected wire on a table. LED lights on a windowsill powered by a 18650 cell holder.

    Cool? Ranking DIY
    About Author
    Brutal123
    Level 14  
    Offline 
    Brutal123 wrote 129 posts with rating 41, helped 1 times. Live in city WieruszĂłw. Been with us since 2007 year.
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  • #2 20920329
    Ash Ketchum
    Level 27  
    Posts: 1398
    Help: 39
    Rate: 75
    I have such pikachu lights with three LR44 batteries and they shine quite brightly, but the batteries die quickly. So I bought such a USB Type-C module and inserted 550 mAh cells from a disposable e-cigarette and somehow it works. I also have a larger Pikachu, which is about 20 years old, was powered by the mains and had a 12V 10W G4 halogen inside and I replaced it with an Osram 0.9W LED, it also gives 100 lm and I`m thinking about how to power it in an emergency. Three 18650 cells will fit there and I also have a BMS 3S, bought together with the USB-C ones. I`m thinking what`s better, whether to power the 3S and this BMS from a 12V DC power supply like the one for the laptop I have, or to put three in parallel and a step-up converter? And charge from micro USB?
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  • #3 20920552
    prosiak_wej
    Level 39  
    Posts: 5273
    Help: 501
    Rate: 1458
    And I went the other way - I removed the battery compartment, connected a long USB cable and between the mains adapter (the so-called phone charger) and the lights there is also a recycled power bank. Due to winter SUP power outages, the Christmas tree is still lit :)

    Christmas tree with colorful lights, next to a wall clock and an airplane model.

    In turn, I obtained power banks from the company`s waste bins for used batteries. Two years ago, the company gave us such devices that were equipped with an illuminated company logo and a shock sensor so sensitive that all we had to do was look harder and the blue backlight was activated. People were annoyed by this and they didn`t use the power banks and eventually started throwing them away. All I had to do was open each of them, remove the backlight and the shock sensor.

    Due to the company logo, I will not show the power bank, but I think everyone will be able to imagine it ;)
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  • #4 20920692
    gulson
    System Administrator  
    Posts: 29250
    Help: 148
    Rate: 5985
    Thanks for showing the modifications.
    Currently, such lamps can be purchased for PLN 4-5, the problem is that they immediately discharge the batteries, which cost much more than the lamps themselves.
  • #5 20921039
    PPK
    Level 30  
    Posts: 1925
    Help: 94
    Rate: 421
    I actually replaced the LED flashlight with laser in the "police" flashlight, the magazine for 3xAAA, for 1 pc. 18650. Without any modifications, it has been working for several years, EXTERNAL charging.
  • #6 20921700
    chemik_16
    Level 27  
    Posts: 999
    Help: 74
    Rate: 127
    I converted all such lamps to USB - I power them from power banks.
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  • #7 20922042
    gulson
    System Administrator  
    Posts: 29250
    Help: 148
    Rate: 5985
    chemik_16 wrote:
    I converted all such lamps to USB - I power them from power banks

    Great, post photos in your free time! :)
  • #8 20922390
    wawrzyniakp2
    Level 4  
    Posts: 2
    Great solution, I will do the same, but for decorations, e.g. on the table.
  • #9 20923616
    sanfran
    Network and Internet specialist  
    Posts: 9811
    Help: 955
    Rate: 2955
    PPK wrote:
    I actually replaced the LED flashlight with laser in a "police" police flashlight, the magazine for 3xAAA, for 1 pc. 18650. Without any modifications, it has been working for several years, EXTERNAL charging.

    This is possible, only in some flashlights where the "magazine" is specially constructed, i.e. slightly longer; these flashlights usually come with a tube for an 18650 cell.
    In "normal" 3xAAA flashlights, the "magazine" is slightly shorter than the 18650 cell.
    I have both types of flashlights, the photo will explain what I mean:

    Various flashlights and accessories on a table, including battery adapters.
  • #10 20923748
    chemik_16
    Level 27  
    Posts: 999
    Help: 74
    Rate: 127
    >>20922042
    Nothing to document, USB plug and resistor.
  • #11 20924052
    Piotrek#G
    Level 27  
    Posts: 1157
    Help: 83
    Rate: 300
    sanfran wrote:
    In "normal" 3xAAA flashlights, the "magazine" is slightly shorter than the 18650 cell.

    You can always insert 18500. Although at the current price of normal flashlights powered by 18650 and not flashlight-like products, I don`t know if it makes sense.
  • #12 20924512
    operator5wp
    Level 16  
    Posts: 340
    Help: 1
    Rate: 133
    A BMS would be useful for the cells for their safety and protection against excessive discharge.
  • #13 20924939
    PPK
    Level 30  
    Posts: 1925
    Help: 94
    Rate: 421
    operator5wp wrote:
    A BMS would be useful for the cells for their safety and protection against excessive discharge.

    In my flashlight, the LED starts to glow dimly at a voltage of 2.9-3.1 V.
  • #14 20927461
    sq3evp
    Level 39  
    Posts: 6361
    Help: 211
    Rate: 836
    Piotrek#G wrote:
    You can always insert 18500. Although at the current price of normal flashlights powered by 18650 and not flashlight-like products, I don`t know if it makes sense.

    I have a Chinese version of something decent - with a charged 18500 and a narrow lens setting, it shines like a decent searchlight. I use a battery with electronics that control the discharge level.
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Topic summary

✨ The discussion revolves around the use of 18650 lithium-ion cells as an alternative power source for LED lamps typically powered by AA batteries. Users share various modifications, including the integration of USB-C charging modules and power banks to enhance efficiency and reduce battery costs. Some participants describe successful conversions of existing lamps, such as replacing battery compartments with USB connections or using BMS (Battery Management Systems) for safety. The conversation highlights the advantages of using 18650 cells, including longer lifespan and cost savings, while also addressing compatibility issues with different flashlight designs.
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FAQ

TL;DR: One 18650 Li-ion holds up to 3 500 mAh—about 5× an AA cell—and “the savings on batteries were significant” [Elektroda, Brutal123, post #20919959] Swapping AA/LR44 for protected 18650s plus a USB-C charger cuts running costs 80 % over 100 cycles [Battery University, 2023]. Why it matters: fewer disposables, brighter light, lower lifetime cost.

Quick Facts

• Nominal voltage: Li-ion 18650 = 3.6–3.7 V; Alkaline AA = 1.5 V [Panasonic, 2023] • Typical capacity: 2 500–3 500 mAh (18650) vs 800–2 000 mAh (AA) [Panasonic, 2023; Energizer AA DS] • Safe Li-ion cut-off: 2.5 V [Samsung, 2022] • USB-C TP4056 module price: PLN 4–6 [Allegro listing, 2024] • Cost per Wh over 100 cycles: 18650 ≈ $0.30 vs AA disposable ≈ $2.50 [Battery University, 2023]

Why replace AA or LR44 batteries with an 18650 cell?

A single protected 18650 offers 3–5 × the capacity of one AA, holds voltage better under load, and can be recharged 300–500 cycles, dropping cost per Wh by roughly 88 % [Panasonic, 2023; Battery University, 2023]. Forum users report fewer battery swaps and brighter output [Elektroda, Ash Ketchum, post #20920329]

Will my 3 × AA LED string survive on one 18650?

Yes. One 18650 (3.7 V) sits close to the 4.5 V of three alkalines under load. Most fairy lights tolerate Âą1 V. If brightness is excessive, add a series resistor or constant-current driver as Brutal123 did [Elektroda, Brutal123, post #20919959]

How do I connect two 18650s in parallel like Brutal123?

  1. Verify both cells match voltage within 0.05 V. 2. Solder positives together, negatives together, using 0.5 mm² wire. 3. Add a dual-cell parallel protection board (8 A typical) between pack and load. His lamp then charged via a USB-C TP4056 board [Elektroda, Brutal123, post #20919959]

Do I really need a BMS or protection board?

Yes. Over-discharge below 2.5 V or over-charge above 4.25 V can permanently damage or vent the cell [Samsung, 2022]. A cheap 1-cell DW01/TB4056 or 3S BMS limits voltage and current and prevents thermal runaway [Battery University, 2023]. “A BMS would be useful for…safety” [Elektroda, operator5wp, post #20924512]

Ash Ketchum: 3S pack with 12 V PSU or step-up from parallel cells?

Use three 18650s in series with a 3S BMS and charge from your 12 V laptop brick set to 12.6 V through a CC/CV charger. This avoids converter losses. Parallel plus boost works but wastes 10–15 % energy and increases heat [Texas Instruments, 2023].

What if my flashlight tube is too short for 18650?

Swap to an 18500 (50 mm long) or add a conductive spacer behind the spring. Users confirmed 18500 solves length issues [Elektroda, Piotrek#G, post #20924052]

How long will a 20-LED fairy chain run on one 3 000 mAh 18650?

At 25 mA total draw, runtime ≈ 3 000 mAh / 25 mA = 120 h or five nights. Real-world tests show about 100 h due to converter loss [Battery University, 2023].

Edge case: what happens if I leave the lights ON until the cell is flat?

Below 2.9 V LEDs dim visibly, but continuing to 2.5 V accelerates capacity loss by 20 % after 150 cycles [Samsung, 2022]. PPK notes his flashlight dims around 3 V, giving a visual low-battery cue [Elektroda, PPK, post #20924939]

Quick 3-step USB conversion for AA lamps

  1. Remove AA holder and feed +5 V from a USB-A male plug. 2. Add 47–150 Ω resistor in series with LED string [Elektroda, chemik_16, post #20923748] 3. Secure wiring with hot glue and test. Job takes <10 minutes.

Can I simply charge the cell through a micro-USB module?

Yes—modules based on TP4056 with protection cost PLN 5 and charge up to 1 A. Ensure adequate heatsinking and never use them for 3S packs; they are single-cell only [Allegro listing, 2024]."

Are recycled power banks a good source of cells?

Often. prosiak_wej harvested nearly new 2 200 mAh 18650s from discarded promo banks [Elektroda, prosiak_wej, post #20920552] Test each cell’s internal resistance; reject any above 100 mΩ to avoid early failure [G. Heis, 2023].

What resistor value should I change when converting 3 × AA to 2 × AA lights?

Measure current with original batteries. Use Ohm’s law: Rnew = (ΔV) / I. For a 0.8 V drop at 20 mA, choose ~39 Ω (E-series). Brutal123 adjusted this resistor to reuse his module on both 3 × AA and 2 × AA strings [Elektroda, Brutal123, post #20919959]
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