SDG #290 Kingbolen BM550 Lead-Acid Car Battery Tester Review

The Kingbolen BM550 is a budget clamp-on battery tester aimed at home mechanics rather than workshops, and this review covers both how it performs in use and what is actually inside it.

Operation is deliberately simple. The unit draws all of its power from the battery under test, so there are no internal cells to go flat in a toolbox. You select the pack voltage (6V, 12V or 24V, covering mopeds, cars and trucks), enter the rated cold cranking amps printed on the battery label, and the tester returns a measured CCA figure along with state of health, state of charge, internal resistance and resting terminal voltage. An important caveat is highlighted during testing: the pass/fail verdict is relative to the CCA spec you enter, so a small battery judged against a 500A rating will be flagged bad even though nothing is wrong with it. Reading the internal resistance and voltage figures directly is often more informative than trusting the green “good” result.

The teardown is the more interesting half. Despite the low price, the clamps carry four conductors, giving a genuine four wire Kelvin measurement that removes lead and contact resistance from the result. Inside there is a switching pre-regulator (sized for 24V systems), an AMS1117 for the 3.3V rail, a low-cost Foshan SWM181 microcontroller with a 12-bit ADC, a gate driver and MOSFET pulsing an 11 ohm load resistor, and a de-branded quad op-amp conditioning the measurement.

There is no cranking or charging system test and no data connectivity, so this is strictly a static battery check, but the measurement approach is more honest than the price suggests.