As noted in the July 2009 article, the May 2006 Lead-Acid Battery Zapper & Condition Checker has been a very popular project but since it was published a few shortcomings have become apparent. The metering circuit sometimes had a tendency to “lock up” on the 6V range and the current pulse loading circuit was sometimes unstable with 24V batteries, if the power switching MOSFETs were at the high end of their transconductance range.
Many readers also found the combination of the Battery Zapper & Condition Checker fairly tricky to assemble and disassemble because it was a bit of a shoe-horn job into the plastic case. In view of this, we recently decided to develop improved versions of both the Checker and the Zapper but to feature them as separate projects, to make them easier to build. As noted, the new Battery Zapper was presented in July and here we present the companion Battery Condition Checker.
How it works
The circuit of the new Battery Condition Checker is shown in Fig.1 and comprises two distinct parts: an upper section incorporating ICs1-3 and transistors Q1-Q7 and a lower section involving IC4, IC5 and LEDs 1-8. Essentially, the upper section is a pulsed current load which draws a sequence of three very short high-current pulses from the battery, after you press the CHECK pushbutton S1.
The lower section of the circuit is basically a sample-and-hold digital voltmeter which samples the battery voltage only during the last of the three current pulses and compares it with the battery’s no-load voltage. This indicates the battery’s condition by showing how much its terminal voltage droops under load. In effect, the heavy current pulses drawn from the battery enable us to measure its output impedance.
If the battery voltage doesn’t droop much at all, blue LED8 will light, indicating GOOD; if it droops by only a small amount, green LED7 lights (OK); if it droops more but not too much, green LED6 glows (FAIR). If it droops even more than this, either yellow LED5 (POOR) or red LED4 (FAIL) will glow, giving you an idea of how urgently the battery should be replaced. This assumes that you have just charged the battery, of course.
If none of the LEDs light, your battery is dead or flat. If charging and zapping does not fix it, it is beyond redemption.
In more detail, the heart of the pulsed current load section is IC2, a 4017B decade counter. This can count clock pulses from gate IC1d, which is configured as a relaxation oscillator running at about 66Hz. This oscillator only runs when pin 12 is high and this is controlled by a “run flipflop” comprising gates IC1a & IC1b.