Hi everyone,

I'm building a custom hexacopter and having trouble powering my SIYI MK15 Air Unit. I've replaced the original BEC, but the new one also appears to produce almost no output voltage. I'm trying to determine whether this is a BEC problem, a compatibility issue, or something related to my CUAV PMU Lite 2.

Hardware

- Flight controller: CUAV Pixhawk V6X

- Power module: CUAV PMU Lite 2, 5V version

- Battery: 6S LiPo, 22.2V nominal

- Radio system: SIYI MK15 Enterprise Combo

- New BEC/power module: SIYI 4S–18S module advertised for the HM30 Air Unit

- Connections: PMU Lite 2 battery output → XT90-to-XT60 adapter → BEC input → XT30 output → Air Unit

My original SIYI BEC appeared to have failed. It received approximately 22.2V, but its output measured around 0.2mV, so I ordered a replacement. Unfortunately, the new BEC appears to have the same problem.

Measurements and symptoms

1. The PMU Lite 2 battery output measures approximately 22.00V.

2. The XT90-to-XT60 adapter also measures approximately 22.00V.

3. The new BEC's XT30 output measures approximately 0.01V on my multimeter in DC voltage mode. I still need to verify the reading directly across the BEC input and repeat the output measurement with the Air Unit disconnected.

4. The Air Unit does not appear to power on, and I cannot establish the expected connection through QGroundControl on the MK15 monitor.

I noticed that many setup videos show the older CUAV PMU Lite 1, while my setup uses the newer PMU Lite 2. According to the specifications, the PMU Lite 2's battery output carries battery voltage, while its 5.2V regulated output is on a separate connector.

I don't want to assume the PMU Lite 2 is defective, but I'm wondering whether there is something important about the power path or connection that I'm missing.

One additional uncertainty is the Air Unit generation. My seller described the unit as an older MK15 Air Unit, but the physical unit does not clearly identify itself as either MK15 or HM30. The replacement module is advertised as a 4S–18S HM30 power module.

Questions

1. Should the PMU Lite 2 battery output work with an external SIYI 4S–18S power module in this configuration?

2. Could the newer HM30 power module be incompatible with an older MK15 Air Unit, even if the battery voltage is within the module's input range?

3. If approximately 22V reaches the BEC input but its XT30 output remains near 0V with the Air Unit disconnected, does that point to a faulty BEC or another possible issue?

4. What exact wiring or measurements would you check next to identify the fault?

I would appreciate advice from anyone who has used a SIYI MK15/HM30 with a CUAV PMU Lite 2. I can provide photos of the modules, labels, connectors, and wiring.

Thank you!

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