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# 2 x AA Battery Holder

An open holder with two AA alkaline cells in series that supplies a nominal 3 V for LEDs, 3 V logic and a Pico's VSYS.

Source page: https://www.tinkered.ai/components/battery-2xaa

## In short

A 2 x AA holder puts two 1.5 V alkaline cells in series for a nominal 3 V (Energizer E91 datasheet, p. 1), on a red + lead and a black - lead. It runs LEDs, 3 V logic and a Raspberry Pi Pico through VSYS, which accepts 1.8 V to 5.5 V (Raspberry Pi Pico datasheet, p. 8), but it is too low for a 5 V Arduino's 5V or VIN pin. Energizer rates its cells to 0.8 V each at end of life, so the pack falls from 3 V toward 1.6 V as it drains (E91 datasheet, p. 1).

The 2 x AA battery holder is the simplest way to run a small circuit away from USB. Two AA cells sit in series in an open plastic holder, and two leads bring the pack out: red for + and black for -. With alkaline cells such as the Energizer E91, each rated 1.5 V nominal (Energizer E91 datasheet, p. 1), the pack supplies 3 V nominal.

An alkaline cell's voltage falls steadily as it discharges. Energizer rates the E91's service time down to a 0.8 V end point, and its internal resistance is 150 to 300 mΩ when fresh (E91 datasheet, p. 1). So a 2 x AA pack sags with both age and load: the heavier the current, the sooner a circuit with a minimum voltage stops. The cells work from -18 °C to 55 °C and keep for 10 years at 21 °C on the shelf (p. 1).

Three volts suits loads that need no regulation: LEDs with a resistor, 3 V logic, and boards with a wide input. A Raspberry Pi Pico's VSYS accepts 1.8 V to 5.5 V and feeds an on-board buck-boost converter (Raspberry Pi Pico datasheet, p. 8), so it keeps running as the cells drain. A 5 V Arduino needs more: power it from USB, a 9 V source on VIN, or a boost converter.

## Agent rules

Follow these when you wire this part or write code for it.

- Do: A pack that powers motors or servos while a board drives their signals shares its GND with the board (black lead and board GND on the same rail).
- Never: No 4xAA on a Pico's VSYS, a 3V3 pin or a board's 5V pin.
- Never: No 18650 on any 3V3 pin, nor straight on an ESP32 DevKit's VIN (boost it to 5 V first).
- Never: No reversed leads: red is always +.

## Pinout

| Pin | Function | Type |
| --- | --- | --- |
| VCC | Red lead: Positive (+) | power out |
| GND | Black lead: Negative (−) | power out |

## Specifications

| Spec | Value | Source |
| --- | --- | --- |
| Pack voltage | 3 V nominal (2 × 1.5 V in series) | Energizer E91 datasheet, p. 1 |
| Cell chemistry (E91) | Alkaline, zinc-manganese dioxide (Zn/MnO2), ANSI-15A / IEC-LR6 | Energizer E91 datasheet, p. 1 |
| End-of-life point used by Energizer | 0.8 V per cell (1.6 V for the pack) | Energizer E91 datasheet, p. 1 |
| Internal resistance per cell | 150 to 300 mΩ fresh | Energizer E91 datasheet, p. 1 |
| Operating temperature | -18 °C to 55 °C | Energizer E91 datasheet, p. 1 |
| Shelf life | 10 years at 21 °C | Energizer E91 datasheet, p. 1 |
| Cell size | 50.5 mm long max, 14.5 mm diameter max; 23.0 g typical | Energizer E91 datasheet, p. 1 |
| Holder size | 59.5 × 32.9 × 16 mm, 150 mm leads | Tinkered registry |
| Leads | Red = + (VCC), black = - (GND) | Tinkered registry |
| Supply voltage | 3 V | Tinkered component registry |
| Dimensions | 59.5 × 32.9 × 16 mm | Tinkered component registry |
| Weight | 54 g | Tinkered component registry |

## Circuit notes

- Red is +, black is -. Connect black to the circuit's ground and red to the supply input, never the other way round.
- A Raspberry Pi Pico takes the pack on VSYS, rated 1.8 V to 5.5 V (Raspberry Pi Pico datasheet, p. 8); the board's buck-boost converter makes its 3.3 V from it. Do not wire the pack to a 3V3 pin of a board that has its own regulator; feed its supply input instead.
- A 5 V Arduino UNO or Nano cannot run from this pack: their VIN pins expect a higher voltage for the on-board regulator, and 3 V on the 5V pin is below what a 5 V board needs. Use USB, a 9 V source on VIN, or a boost converter.
- LEDs still need a series resistor. Size it for a fresh pack, so the LED never sees too much current.
- When the pack powers motors or servos and a board drives their signals, connect the pack's black lead to the board's GND.
- The pack's voltage drops under load and as the cells drain, down to the 0.8 V per cell Energizer uses as end of life (E91 datasheet, p. 1). Choose loads that tolerate the range, or regulate.
- Energizer's E91 sheet rates the AA cell down to 0.8 V and gives no maximum current.

## Common mistakes

- **Expecting it to run a 5 V Arduino.** Two alkaline cells make 3 V nominal (Energizer E91 datasheet, p. 1). That is below what an UNO's VIN regulator or 5V pin needs, so the board resets or does nothing. Use USB, a 9 V source or a boost converter.
- **Reversing the leads.** Red is + and black is -. Reversed, the pack pushes current backwards through the board's input and can damage it. Check polarity before connecting.
- **Driving motors or servos from AAs and the logic from the same weak pack.** Alkaline cells have 150 to 300 mΩ of internal resistance each when fresh (E91 datasheet, p. 1), so a motor's inrush pulls the pack voltage down and resets a board on the same supply. Give motors their own supply, and share only the ground.
- **Mixing old and new cells.** In series, a weaker cell is driven harder by the fresher one and can leak. Replace both cells together, with the same type and brand.
- **Charging alkaline cells.** The E91 is a primary alkaline cell (E91 datasheet, p. 1), not a rechargeable one. Never put it on a charger, a TP4056 included, which is for a single Li-ion cell only.

## Questions

### What voltage does a 2 x AA battery pack give?

3 V nominal: two 1.5 V alkaline cells in series (Energizer E91 datasheet, p. 1). The pack falls as the cells drain, toward the 0.8 V per cell, 1.6 V for the pack, that Energizer uses as end of life (p. 1).

### Can I power a Raspberry Pi Pico with two AA batteries?

Yes. Connect red to VSYS and black to GND. VSYS accepts 1.8 V to 5.5 V and feeds the Pico's buck-boost converter (Raspberry Pi Pico datasheet, p. 8), so the board keeps its 3.3 V rail as the cells drain.

### Can two AA batteries power an Arduino UNO or an ESP32?

Not well. 3 V (Energizer E91 datasheet, p. 1) is below what an UNO's VIN or 5V pin needs, and an ESP32 DevKit's VIN feeds an AMS1117 that needs about 1.1 V above its 3.3 V output (AMS1117 datasheet, p. 3). Use USB, a higher-voltage pack or a boost converter for those boards.

### Why does my circuit stop working while the batteries still show voltage?

Alkaline cells sag under load: each has 150 to 300 mΩ of internal resistance when fresh, and more as it drains (Energizer E91 datasheet, p. 1). A motor or a radio burst can pull the pack below what the board needs even when a meter with no load reads close to 3 V. Fresh cells, a separate motor supply or a board with a wide input such as a Pico's VSYS help.

### 2 x AA vs one 18650: which should I use?

Two AAs give 3 V nominal (Energizer E91 datasheet, p. 1), are sold everywhere and need no charger. An 18650 gives 3.6 V nominal and 4.2 V full, is rechargeable and delivers far more current, LG's INR18650 MJ1 up to 10 A (LG MJ1 specification, p. 4), but needs a Li-ion charger and protection. Both run a Pico's VSYS.

### Can I use rechargeable AA cells in the holder?

Yes, NiMH AAs fit the same holder, but each runs at a lower voltage than an alkaline cell, so the pack gives less than 3 V. A Pico's VSYS still accepts it down to 1.8 V (Raspberry Pi Pico datasheet, p. 8); check the minimum input of anything else on the pack.

## Related parts

- [9V Battery](https://www.tinkered.ai/components/battery-9v.md): same job: a power source
- [18650 Li-ion Cell + Holder](https://www.tinkered.ai/components/battery-18650.md): same job: a power source
- [Arduino Uno](https://www.tinkered.ai/components/arduino-uno.md): the board its circuit is wired to

## Try it

Tinkered can design a circuit with the 2 x AA Battery Holder, draw its schematic and check it. Free to start at https://www.tinkered.ai.