Gulu ST5 Battery Tester Review: A Solid Tool for Checking 12V and 24V Batteries

Introduction

The Gulu ST5 battery tester is designed to test 12V and 24V vehicle batteries across cars, trucks, boats, and similar equipment. It is a fairly simple tool in the best sense: it does a focused job well, and it does not try to complicate things.

Right now it is on sale for around $70 on Amazon, down from a retail price of roughly $99, which puts it in affordable territory for a tool most vehicle owners will only reach for a few times a year. The build is a rubber over-molded case, and the unit is a little larger than some competitors, which actually helps when you need to balance it in an engine compartment while revving an engine or starting one up.

One detail that stands out immediately is the clamps. Many budget testers ship with small, fussy clamps; the ST5 comes with two very strong ones that grip a battery terminal confidently. There is also a QR code for the full instruction manual, though the menu is simple enough that most users may never need it.

Setup and First Impressions

Getting started takes almost no effort. The tester is powered directly by the battery under test, so there is nothing to charge or configure before use. Once connected to a functional truck, it immediately read 12.66 volts and presented the entire menu on one screen.

The menu covers three test types: battery, cranking, and charging. Before running a battery test, you select the battery type from a long list and set the rating system. Depending on your country, this may be CCA (cold cranking amps) or another standard. You then scroll to the battery’s rated value, anywhere from 100 to 2,000 CCA.

Battery Test: State of Health

The battery test is the core function and reports state of health and charge. On a healthy truck battery, the ST5 returned a green result: 98% health, 100% charge, 12.65 volts, and a measured 644 CCA against a rated value of around 650. That is a strong, credible reading.

The status bar color coding is a useful quick reference. Green indicates a good battery, while yellow flags a battery that is usable but past its prime. Results are also stored in a history menu, so you can scroll back through previous tests and compare readings over time, along with an about screen with additional device details.

Cranking Test

The cranking test analyzes voltage and crank time during engine start, giving extra insight into starter condition. After starting the truck, the ST5 recorded 470 milliseconds of crank time at 10.79 volts, which is a solid result and a good sign for the starter.

The only practical note: since the tester sits in the engine bay while you start the car from the cabin, having two people makes the process easier. One person to start the engine, one to watch the reading.

Charging Test

The charging test evaluates the alternator. It includes a ripple test, which measures extraneous AC voltage riding on the DC system. In this test, the ripple came back at a very low value, which is what you want to see. Excessive ripple can indicate alternator problems, and on newer vehicles with sensitive electronics, it is worth paying attention to.

The full charging routine involves starting the engine and revving it to around 2,500 RPM for a moment, then letting the tester process loaded, unloaded, and ripple readings. It adds a few steps compared to a battery test, but the alternator insight is worthwhile for anyone diagnosing charging issues.

Testing a Stored RV Battery

To see how the ST5 handles a weaker battery, we tested a flooded car battery that had been sitting in a box all summer in an RV, unconnected and untouched. After selecting the battery type (flooded, among other options) and setting 550 CCA, the test ran for a few seconds.

The result: 77% health, shown with a yellow status bar instead of green. That matches expectations for a battery in that condition, and the color shift from the earlier green reading makes the difference immediately obvious. The tester clearly distinguishes between a healthy battery and one that has aged or lost capacity, which is exactly what you need when deciding whether a stored battery is worth recharging or replacing.

Who Should Buy It

The Gulu ST5 makes sense for vehicle owners who want to check battery health before winter, diagnose slow cranking, or verify an alternator is charging properly. It is equally useful for boats, RVs, and trucks with 12V or 24V systems. At the current sale price of around $70, it is an easy recommendation for a home garage, and the strong clamps and rugged over-molded case mean it should hold up to real workshop use.

Conclusion

The Gulu ST5 battery tester does three useful jobs, battery health, cranking analysis, and charging/ripple testing, in a compact, well-built package with strong clamps and an intuitive menu. Its readings were sensible across both a healthy truck battery and a tired stored RV battery, and the color-coded results make interpretation effortless. At roughly $70 on sale, it is a practical addition to any DIY diagnostic toolkit.