Battery
A 9 V battery, a pack of AA cells or one or two 18650 cells, as a power source. Aliases: 9V battery, AA pack, 18650, Li-ion, 2S pack.
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 9v, aa2, aa3, aa4, 18650 or 18650x2.
charge is 0 to 100 percent. For the alkaline kinds 100 is a fresh cell at 1.6 V
and 0 is 0.8 V a cell, the point the datasheets call empty. For the 18650 kinds 100
is a full cell at 4.2 V and 0 is an emptied cell resting at 3.0 V.
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 |
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.
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. 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.
See it in action
TB6612 robot drive runs two motors from a four-AA pack. Open it at /templates.