PIC-controlled SLA battery capacity meter

Sealed lead acid (SLA) batteries are ideal for wide variety of applications and they are maintenance-free. With correct charging and use, they can last for many years on standby and hundreds of discharge cycles.

However, if they are deeply discharged, over-charged or left discharged, their life can be greatly reduced. And even with proper care, all SLA batteries eventually need replacing but how do you work out when? You could try a number of load cycles to estimate the battery’s capacity but unless this is carefully controlled, you can very easily end up with a completely discharged battery which will never recover.

The solution is this battery capacity meter built around a PIC16-F877A microcontroller. The circuit monitors the battery under test and terminates the discharge at a user-defined voltage. It also computes the battery capacity in amp-hours (Ah) as it goes. It allows the discharge rate to be varied between 0.25A and 2A to cater for different battery sizes. Even when the battery voltage drops to a level where it cannot deliver the set current into the load, the circuit constantly monitors the current so it can correctly calculate the battery capacity.

In operation, the PIC measures the battery voltage using the internal ADC at pin 2 via a potential divider. The PIC produces a range of analog control voltages between 0.25V and 2V using a resistor ladder (pins 15-18 & 23-26). The output voltage of this ladder is varied by grounding it at various PIC pins.

The control voltage is fed to the non-inverting input of op amp IC2 which controls power FET Q4. This controls the current flowing through Lamp1. The voltage across Q4’s 1W source resistor is fed to the inverting input of IC2 to provide precise current control. A second ADC at pin 5 measures this voltage and therefore the current.

The PIC keeps track of time using integral interrupts and it drives the LCD panel to show the cumulative battery capacity, the instantaneous current, battery voltage and elapsed time. Once the voltage drops to the threshold value, the discharge is terminated and measurements are displayed. An audible beep indicates the end of test.

The load is one of four different wattage 12V lamps, selected to suit the required discharge current and so minimise the power dissipated in Q4. Settings are stored in the PIC’s EEPROM for subsequent recall. The user is prompted via the LCD interface, with control via two pushbuttons for selecting discharge settings and modes.

The software will be available on the SILICON CHIP website at www.siliconchip.com.au

David Mitchell

Figtree, NSW.