Battery

The kinds: a 9 V battery, AA or AAA cells, NiMH packs, a CR2032 or CR2025 coin cell, an LIR2032, one or two 18650s, a LiPo pouch or a 2S or 3S pack, or a 12 V sealed lead-acid battery. Aliases: 9V battery, AA pack, AAA, coin cell, button cell, 18650, Li-ion, LiPo, 2S pack, 3S, NiMH, eneloop, SLA, lead acid, bateria.

The number on the label is not what a circuit gets. A battery is a voltage behind an internal resistance: fresh and unloaded it reads more than the label, and under load it sags by the current times that resistance. That is why a 9 V battery struggles with motors.

Pins

Pin	What it does
V+	The red lead.
V-	The black lead. Nothing connects it to ground but a wire.

Properties

kind is one of the kinds in the table below.

charge is 0 to 100 percent, in a straight line between a chemistry's empty and full points, a cell at a time: alkaline 0.8 V to 1.6 V (fresh), NiMH 1.0 V to 1.40 V, a lithium coin cell 2.0 V to 3.2 V, lithium-ion and LiPo 3.0 V to 4.2 V, and the lead-acid block 11.8 V to 12.8 V at rest.

kind	cells	fresh, no load	internal resistance
9v	six	9.6 V	1.7 ohm
aa2	two AA	3.2 V	0.4 ohm
aa3	three AA	4.8 V	0.6 ohm
aa4	four AA	6.4 V	0.8 ohm
18650	one 18650 Li-ion	4.2 V	0.04 ohm
18650x2	two 18650 in series	8.4 V	0.08 ohm
aa1	one AA	1.6 V	0.2 ohm
aa6	six AA (a 9 V pack)	9.6 V	1.2 ohm
aa8	eight AA (a 12 V pack)	12.8 V	1.6 ohm
aaa1	one AAA	1.6 V	0.25 ohm
aaa2	two AAA	3.2 V	0.5 ohm
nimh4	four NiMH AA	5.6 V	0.12 ohm
nimh6	six NiMH AA	8.4 V	0.18 ohm
cr2032	a CR2032 coin cell	3.2 V	10 ohm
cr2025	a CR2025 coin cell	3.2 V	10 ohm
lir2032	an LIR2032 rechargeable coin cell	4.2 V	0.6 ohm
lipo500	a 500 mAh LiPo pouch	4.2 V	0.18 ohm
lipo1000	a 1000 mAh LiPo pouch	4.2 V	0.10 ohm
lipo2000	a 2000 mAh LiPo pouch	4.2 V	0.07 ohm
lipo2s	a 2S RC pack, 7.4 V	8.4 V	0.02 ohm
lipo3s	a 3S RC pack, 11.1 V	12.6 V	0.03 ohm
sla12	a 12 V 7 Ah sealed lead-acid battery	12.8 V	0.023 ohm

While it runs

The readout under the battery is the voltage across its terminals, under whatever load the circuit puts on it.

What the model gets right

It is a source. The battery starts an analog solve the way the bench supply does, so resistors, LEDs, motors and the TB6612FNG's bridge all draw real current from it.

Internal resistance. Energizer's E91 sheet gives a fresh alkaline AA as 150 to 300 milliohms; the part uses 0.2 ohm a cell. The 9 V figure, 1.7 ohm, is an assumption in the range fresh 9 V batteries measure (Energizer's 522 sheet gives no number). Into 18 ohm, half an amp, a 9 V battery loses about 0.8 V inside itself.

Fresh and empty. A fresh alkaline cell reads about 1.6 V unloaded, and both sheets rate a cell down to 0.8 V.

18650 cells. Samsung's INR18650-25R sheet charges a cell to 4.20 V and gives its AC impedance as 18 milliohm or less. The part uses 0.04 ohm a cell, an assumption that adds the slower DC part of the drop and a holder's springs. That is why two 18650s run a robot car's motors where a 9 V battery gives up: two amps cost them about 0.16 V. The sheet discharges a cell to 2.5 V under load; resting, an emptied cell comes back to about 3.0 V, which is the part's 0 percent.

AAA. Energizer's E92 sheet gives the same 150 to 300 milliohms as the AA; the part uses 0.25 ohm, inside it, for the smaller cell.

NiMH. Panasonic's eneloop sheet: 1.2 V nominal, tested down to 1.0 V, about 25 milliohm at 1 kHz; the part uses 0.03 ohm a cell with a holder. Off the charger a cell rests at about 1.40 V, which is a measurement rather than a datasheet figure. Four NiMH cells give 4.8 V where four alkalines give 6, which is why a 5 V project runs on four of them.

Coin cells. Energizer's CR2032 and CR2025 sheets: 3.0 V, rated down to 2.0 V. A fresh one reads about 3.2 V unloaded (a measurement). The CR2032 sheet's pulse test starts near 10 ohm inside, rising past 100 ohm as it empties; the part uses the fresh 10 ohm. That is why a coin cell cannot run a servo or a Wi-Fi radio: 100 mA would cost it a volt inside itself. The LIR2032 is the rechargeable Li-ion one, 4.2 V full, 0.6 ohm or less.

LiPo. One-cell pouches charge to 4.2 V and stop at 3.0 V, like an 18650. Their sheets give no internal resistance; comparable cells' sheets, protection board included, give under 200 milliohm for 500 mAh, about 100 for 1200 mAh and about 70 for 2400 mAh, which is what the part uses. An RC pack is far stiffer: 0.01 ohm a cell is an assumption (new packs measure 3 to 10 milliohm a cell) with the leads added. The 2S and 3S packs draw their balance plug, but it has no pins: charging is not modeled.

Lead-acid. Yuasa's NP7-12 sheet gives 23 milliohm; its manual has a 12 V block at about 12.8 V at rest when full and 11.8 V empty. Under load it is rated down to 10.5 V.

What it does not model

The battery emptying while the circuit runs (a 2850 mAh AA would take hours of simulated time to notice), resistance rising as it empties, temperature, and recovery after a rest. A coin cell's resistance climbing past 100 ohm as it empties is not modeled either, nor its resistance rising with the current it is asked for. So a red LED wired straight across a CR2032, the "LED throwie", takes about 100 mA here and burns out, while on the bench the cell sags further and the LED survives. Give the LED a resistor, as you would anyway. Between full and empty charge is a straight line, which is an assumption: the real curve is flatter in the middle. It is there to show what a flat battery does to a circuit, not to predict run time.

Common mistakes

Running motors from a 9 V battery. Six small cells and 1.7 ohm inside: it sags as soon as a motor starts. Four AA cells do better, and two 18650s better still.

Feeding a full 2S pack, 8.4 V, to a part rated to 6.5 V such as the L9110S.

Forgetting to join the battery's V- to the board's ground, so the two halves of the circuit have no common reference. A module on its own battery that takes its signals from a board does nothing until that wire is there, here as on the bench, and the Checkup finds it (ground_not_shared).

A circuit that is all the battery's own needs no ground symbol: a battery, a resistor and an LED in a loop light up with no gnd part, as they do on the bench, and every voltage in it reads from the battery's V-. Ground it as soon as it shares a part with a board.

See it in action

TB6612 robot drive runs two motors from a four-AA pack. Open it at /templates.

The Battery level indicator reads an 18650 on A0; drag its charge and watch the percentage. The 7805 voltage regulator circuit runs from a 9 V battery until it sags below the dropout.

The Arduino battery level indicator guide explains that circuit: the cell straight on A0, volts to percent, and the divider a battery over 5 V needs.

Related

• The battery part page, with its pinout
• TP4056 charger
• Voltage regulator