Scary power issue with the mercury, but what did happen exectly and why?

  • In the last flight of my big jet there was an issue that i cannot fully explain. I use the mercury with 2x 2s 3150mAh LiOn cells (rated for 30Amps). In the first 2 minutes of the flight i got a low battery warning on the TX (from one of the RX batteries) but i thought the trigger level was a bit to high and it was only for 1 second so i continued the demo. 2 minutes later i deployed the flaps on a bit faster then normal speed and got the low battery warning again, but the plane started a dive also from which i could not recover by pulling up, meaning there was not enough power in the elevator servo's (normally with flaps it stays level). This scared the ehll out of me and removing the flaps gave me elevator control again. I landed quickly without any issues.

    Then i looked at the logs (sadly the RX voltage was not logged) and looked at the battery voltages. Battery 1 was a nice level line that showed 7.0-7.1V for the entire flight. Battery 2 showed about 6.9V with a large dip to 6.3V the same time i lost the elevators. So it was clear battery 2 was the issue.... but why did i loose the power to the servo's? Both batteries charged the same amount back in (2200mAh) so batteries were used through the regulators. How do both regulators work? Are the outputs in parrallel? Why was battery 1 still at a good level but the output voltage not? Could regulator 1 be overloaded and loosing voltage when regulator 2 had not enough from the battery? Normally the PB heatsinks are cold after a flight.

  • Hello,

    I assume that the voltage droped quickly in peaks, so the elevator stopped (maybe other servos too). The telemetry doesn´t measure such fast drops.

    The regulators are not parallel- they are seperated by a diode. The reason why you have two different woltages is because every regulator works independent- little different output voltages makes this more or less discharge of one battery.

    I don´t like the 3150 batteries- they have a high capacity, but are also high ohmic. Also if you are flying at a 7V level, the batteries are nearly empty. In this range they have additional higher ohmic outputs which lets the voltage drop much more.

  • Yes, i will change them for LiPo's

    So you think the power drop was due to the high load but for battery 1 the low value was missed do to the resolution of the telemetry data? This data updates very quickly normally.

    The main issue is that both batteries charge the same amount and report the same internal resistance. The no load voltage is also the same.

  • Richard,

    Can you clarify how it works?

    If one of the batteries is “bad” or maybe I charged only one but not the other, will the unit draw all needed power for all ports from “the good one”?. Or are the regulators feeding separately the servo ports and if so which are tge 2 “blocks”?

  • Hello,

    Due to the seepreted regulators- there are always slight differences in the battery discharge. This means that one battery is maybe discharged 10% more than the other one. That´s normal. In your case 6,9V (Batt2) to 7.0V (Batt1)

    gbrod : in a case of a bad battery it draws the power only from the good one of course.

  • How is the voltage from the SBUS receiver ports in regards to the servo voltage? Is that the same? I noticed after the issue flight that the SBUS receiver reported a voltage of 5.9V when i moved the controls quickly. Sadly i did not log that value. Since both batteries behave the same i will replace the receiver batteries because they cannot handle the load when they are getting empty. I will review the other logs to check if see any related voltage drops in 1 battery.

  • Small correction for my first port: After looking better at the log data both batteries performed the same and both dropped to 6.4V at the same time. Sadly i did not log the servo voltage before so i don't know what the servo (receiver) voltage was at that time. I remember it should be 0.4V below the battery voltage correct?

    My Mercury is set to 7.4V but my receiver only reports 6.8V (with 8.3V battery voltage) so i guess that is not accurate.... i will check the PB output with a good DMM.