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 ±1 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?
- 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?
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
- 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]
Generated by the language model.
Comments
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... [Read more]
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.... [Read more]
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. [Read more]
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 chargi... [Read more]
I converted all such lamps to USB - I power them from power banks. [Read more]
Great, post photos in your free time! :) [Read more]
Great solution, I will do the same, but for decorations, e.g. on the table. [Read more]
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,... [Read more]
Nothing to document, USB plug and resistor. [Read more]
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. [Read more]
A BMS would be useful for the cells for their safety and protection against excessive discharge. [Read more]
In my flashlight, the LED starts to glow dimly at a voltage of 2.9-3.1 V. [Read more]
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. [Read more]