Two-Year-Old Tubular Battery Losing Backup: Battery, or the Ten-Metre Controller Cable?
A shop in a block town runs three 20 W LED tubes, a 40 W fan and a 12 V DC television, about 100 W total for six hours each evening. The system is three 150 Wp modules in parallel, a 30 A PWM charge controller, and a 12 V, 100 Ah flooded tubular battery. The owner insisted the controller be mounted in the front display area, ten metres of 6 mm2 single-core cable away from the battery. Peak charge current measured at the controller is 25 A.
After two years the owner reports backup has fallen from six hours to about one and a half hours. On inspection: electrolyte levels normal, no bulging, case 34 C, all six cells read specific gravity 1.150, dawn rest voltage 12.05 V. During a 25 A charge the controller display shows 14.4 V absorb but only stays there about 20 minutes before dropping to float 13.5 V, and the voltage measured at the battery posts is 13.2 V.
1. Calculate the voltage drop and explain the mechanism by which this installation undercharges a healthy battery. Explain why the uniform specific gravity matters to the diagnosis.
Read the worked answer
The arithmetic. Ten metres each way is a 20 m loop of 6 mm2 copper: R = 0.0175 x 20 / 6 = 0.058 ohm. At 25 A that is 1.46 V, which agrees with the measured 1.2 V (the remainder is lugs and terminals).
The mechanism. The controller has no idea what the battery is doing; it regulates the voltage at its own output terminals to 14.4 V. The battery therefore sees 14.4 minus I x R. At 25 A it sees about 13.2 V, well below the 14.4-14.8 V needed for absorption and gassing, so the plates never get a full charge. As the battery fills, current falls, the drop shrinks and battery voltage creeps up, but the controller's fixed absorption timer (here about 20 minutes) expires long before the battery is actually full. It then drops to float 13.5 V, and the battery sees only about 12.3 V at float, which is below its own resting voltage. Result: the battery is permanently part-charged.
Why uniform SG matters. All six cells are equally low at 1.150 and the rest voltage is 12.05 V. A shorted, reversed or dead cell would show one cell much lower than the rest and a depressed rest voltage of about 10.5 V. Uniform depression is the fingerprint of a whole-battery charging fault, i.e. chronic undercharge with sulphation, not a cell failure.
2. The owner says the battery is "dead" and wants a new one. What else in this installation will still make a new battery look weak, and how do you demonstrate it?
Read the worked answer
Replacing the battery alone leaves two faults in place: the long cable on the charge side and the same cable on the discharge side.
Charge side. A new battery will be undercharged exactly as the old one was. Within a year or two the owner will be back with the same complaint, and the warranty will be void because the failure is installation-caused, not a manufacturing defect.
Discharge side. At the evening load of about 8 A the drop is 8 x 0.058 = 0.47 V. The inverter's low-voltage disconnect is typically 10.8-11.0 V measured at its own terminals, so it will cut out when the battery is still at about 11.3-11.5 V, that is, with usable capacity left. Part of the reported "capacity loss" is therefore a measurement artefact created by the cable, and a capacity test done at the inverter will understate the battery.
How to demonstrate it. Put a DC clamp meter and a min/max voltmeter on the battery posts and a second meter on the controller battery terminals during a full charge cycle, and plot the difference against current. Then repeat on discharge. Show the owner the 1.2-1.4 V gap at 25 A and state what it costs: roughly 5% of charge energy wasted as heat in the cable and a battery that never reaches full charge.
Fix offered: relocate the controller to within 1.5 m of the battery with short 16 mm2 leads and a 40 A fuse at the battery positive, keeping only the array cable at the display position. If the display position must be kept, use 25 mm2 and a controller with remote voltage sensing. Do the capacity test after the cable fix and one full charge, not before.