Review on Battery Monitor BM2 BM3 Bluetooth 4.0 Wireless Battery Tester for 12V Automotive Battery Load Testing, 🔋 Monitoring Automotive Charging Systems, and Cranking Analysis. A Digital Battery Analyzer Applicable for Android & IOS Devices by Douglas Hall

As you can see in the picture this meter is 0.056V different compared to a NIST calibrated meter. That may not sound like much, but if you calculate that every 10% of the Ah capacity of a typical 12V battery is represented by a change in open circuit voltage of about 0.10V, it means that the voltage accuracy of this device by almost 6 % differs from you. Ah. Capacity. Now suppose you have a 450Ah battery and that a 6% voltage error could mean up to 27Ah of distortion. Charging at 13.746V. As you can see, the monitor thinks the battery is charging when it isn't. I then ran a series of lead-acid battery discharge tests and found that the SoC's prediction differed from reality by up to 22%. This was a battery of known and verified Ah capacity that was charged to 100% SoC and then discharged at a rate of 20 hours to the known exact point of the SoC. When 50% of the tested Ah capacity was drawn from the battery, the BM2 showed an SoC of 72%. If you were to rely on it, you would be draining the battery well past the SoC's typical 50% point.