MPPT Load Current Monitor Error when no solar

[aidan (Aidan Hedley)]

I am busy lab testing a Blue Solar MPPT 75/10 and have found an error in the load current / power indications.

Using variable electronic test load and power supply to simulate solar panel input.

If I set load current to one amp, and the solar is providing most of the power all is well and good with the VEDirect output figures.

But as the solar drops and battery current increases to discharge of ~0.5 Amps a 100mA error is introduced in the load current.

Example: With electronic load current set to a constant 1 Amp.

| Solar V| Solar I | Batt V | Batt I | Load I | Load W |

| 18.64 | 0.5 | 12.65 | -0.2 | 1.0 | 13 |
| 18.77 | 0.2 | 12.46 | -0.7 | 0.9 | 11 |

I can guess that the VEDirect load monitor value may be derived from just physical solar and battery current shunt measurements and a constant rounding error has been introduced.

I can accept it is not a big error especially as it is constant so small proportion of higher load currents.

For info : MPPT 75/10 Serial HQ1924TUUH9 Firmware v1.64 (latest)

[MikeD (Mike)]

The resolution of the current measurements in the MPPT’s is not all that great. This will lead to rounding errors and small offsets, due the the 1/2 LSB +/-1 LSB resolution of the A/D conversion process. I’m not sure how many bits of resolution the 75/10 has, but would not expect it to be any greater than 10 bits. 8 bits would give a count of 256, so scaling for 10A plus some headroom would give a resolution of about 50mA, so an error of 100mA is quite reasonable.

[aidan (Aidan Hedley)]

Mike, Thanks for your reply and I yes I can accept that one should no expect too much. Just thought it worth noting.