Fnirsi HRM-10 Battery Internal Resistance Tester

Now that everything is becoming battery powered, the need for tools to test and repair batteries is rising. I already reviewed the Fnirsi SWM-10 spot welder for assembling and repairing battery packs, this time we will examine the Fnirsi HRM-10 High-Precision Internal Resistance Tester. With this little tool, you can check the quality of your batteries.

The Fnirsi HRM-10 Looks Nice

The HRM-10 is a rather stylish handheld device the size of a multimeter. The device feels solid and heavy while its design is sober black and light blue, like most other Fnirsi tools I have tried. After switching the unit on (you may have to charge its battery first) you’ll discover a crisp graphic color display showing two values: voltage and resistance. A 5-key keypad below it controls the tester.
 

On the top side is a round 4-way connector to which you attach the test lead. This is called a Kelvin Plus test lead, and it has two clamps at the other end. It is fixed to the tester with a ring that you screw on it, so it won’t come off easily.

Updating the Firmware of the Fnirsi HRM-10

On the right side is an (unprotected) USB-C connector for charging the HRM-10 and for retrieving data. It can also be used for upgrading the firmware. The Fnirsi website has firmware V0.3 available for download, but my test unit runs V0.4, so I didn’t try this function. The software version of the tester can be found on the ‘Factory default’ page in the ‘System settings’ menu.

Setting the Date and Time Is Complicated

When switched on, the Fnirsi HRM-10 also shows the date and time. As it is probably wrong, it is a good idea to adjust it. You do this in the ‘System settings’ menu I mentioned before. Doing this is surprisingly unintuitive. Firstly, this is done digit by digit. Once you know this, you understand that the up/down icons above and beneath the box indicate the digit that can be changed, but it is confusing anyway. Even more confusing is that up is down and down is up. ‘Why bother?’ you might wonder but setting the time is useful for the Pass/Fail test mode, as the time and date are recorded together with the test results.

A Word on the Kelvin Test Leads

Before you start testing batteries, it is interesting to read the user manual carefully. The reason is that it hides an important detail in the ‘Precautions’ section concerning the Kelvin Plus test leads. It says that the clamps must always be open when testing a battery. For reliable measurements, the jaws should not be in direct contact with each other. When clamping them on a battery with protruding contacts, this will automatically be the case. However, when the battery does not have such contacts (e.g. the negative side of an AA cell), this is not easy to achieve. And don’t think that it doesn’t matter, because it does. Pass/Fail testing for instance does not work with closed clamps. Why didn’t Fnirsi write this (in big fat characters) in the operating instructions?

Accuracy

Using the Fnirsi HRM-10 is easy enough, especially if you leave it in auto-ranging mode. Just clamp it on a battery and read the results. This takes us to their precision. The voltage value has four decimals, which is a lot. Connecting the tester to a bench power supply adjusted for 5 V gave a reading that differed by only 100 µV from the value displayed by my Fluke 189 multimeter. True, the latter has not been calibrated since I bought it some ten years ago, but I still trust it.

The resistance value has two decimals in milliohm mode, suggesting an accuracy of 10 µΩ. I happen to own a 5 mΩ, 0.1% resistor, and after connecting it, the HRM-10 showed a value oscillating between 5.07 mΩ and 5.08 mΩ. This is a bit on the ‘high’ side if we may believe the manufacturer of the resistor (Vishay Precision Group). However, in ‘Resistance calibration’ mode, it showed a value toggling between 4.99 mΩ and 5.00 mΩ, which is spot on.
 

These two tests convinced me of the accuracy of the HRM-10, and I decided to refrain from fiddling with the calibrations. I assume Fnirsi has better tools than me for doing this.

Is the Fnirsi HRM-10 Too Accurate?

Now, as we all know, with great accuracy comes great responsibility. Interpreting readings gets complicated. When working with such tiny resistance values, contact resistance becomes important too. Therefore, where and how you place the leads will influence the result. And how good are these leads anyway? I wonder if fewer decimals wouldn’t have been better.

Pass/Fail Testing

The Fnirsi HRM-10 also has a Pass/Fail test function intended for quickly comparing cells. After setting the voltage and resistance upper and lower thresholds for a good battery, it will tell you if the battery under test passes (green light) or fails (red light). The results are stored in a table that can be visualized later (‘History records’) or downloaded as a CSV file over the USB port. The instrument allows for eight test presets; however, it is unfortunate that the thresholds or the number of the preset are not recorded.
 

Conclusion

The Fnirsi HRM-10 Internal Resistance Tester is an easy-to-use accurate device for testing the quality of batteries. It is also a good voltmeter and milliohm meter, which is interesting as most multimeters cannot go below 100 mΩ or so. The Pass/Fail test mode will be practical for people needing to sort (large quantities of) batteries. Mechanically the HRM-10 is excellent with a stand for placing it upright and a hole for hanging it somewhere. The device simply inspires quality and trust.