My first Arduino project was to build a battery capacity tester. I’ve got a box of rechargeable AA batteries, and it seams they’ve been less and less effective. Since most applications require 4 batteries, invariably one problem battery makes the rest of them look bad.

The Atmel ATMega328 microcontroller has 6 analog inputs with 10-bit A-to-D converters and a external AREF that allows you to define what voltage 0x3FF represents. In other words, it’ll give you ~1.4mV precision measuring 0-1.5V when given a 1.5V analog reference. Plenty accurate for a battery capacity measurement.

The principle is fairly simple. Apply a known load to a battery, record the voltage periodically while the battery discharges, stop recording at some point, and integrate to arrive at the area under the curve in order to derive the amp-hours delivered from the battery.

Enough theory, let’s see how it works. The UI starts with a helpful message:

This simple circuit protects an SLA battery from over-discharge by disconnecting the load when the terminal voltage drops below a preset level. In operation, a sample of the battery voltage is derived from the 22kΩ resistor and 20kΩ trimpot divider. This is applied to the non-inverting input (pin 3) of IC1, where it is compared with a reference voltage on the inverting input (pin 2). When the sampled battery voltage falls below the reference voltage, IC1’s output (pin 1) swings towards ground, switching Mosfet Q2 off and disconnecting the load from the battery. The reference voltage is derived from a 4.7V zener diode (ZD1), which is connected to ground via the collector-emitter circuit of Q1 (ie, when Q1 is on).

Low Voltage Cutout For 12V SLA Batteries.jpg
However, when the op amp’s output is driven low, Q1 is switched off, causing the non-inverting input to rise towards the full battery voltage. This greatly reinforces the switching action, latching the circuit in the "off" state until the battery is recharged and the reset switch (S1) pressed. The Mosfet used for Q2 should be selected to suit the intended application. The circuit could also drive a relay simply by connecting the coil across the "load" terminals. As is usual practice, a diode should be connected across the relay coil to limit back-EMF spikes.

In our article Make a Simple Battery Status Monitor we showed how a few components could be put together to make a very basic battery voltage monitor to give a visual indication of the charge state of a 12V lead acid battery.

LM3914 dot/bar display driver - to be used for a battery voltage monitor
Here we will show you how to make a more advanced battery monitor using 10 LEDs (or an LED bargraph display) and the LM3914 dot/bar display driver (pictured above) to show the battery voltage very accurately.

ACS712'nin çıkış gerilimi, hiçbir giriş ile birlikte 2.5V'dir, spesifikasyon ise 66 ila 185 mV / A çıkış duyarlılığını belirtir. Bu geniş bir aralıkta ve Arduino'nun 10-bit ADC'sindeki hatalar göz önüne alındığında, bu yaklaşık akım çıkışı veriyor. Çoğu uygulama için yeterince iyi.