No current from one battery tube



Hi everyone!

I always get the warning You are only drawing current from 1 battery tube. This may indicate a flooded tube, low battery charge, or a bad battery cell in cockpit. Battery tube 2 always shows no current at all in telemetry stream.

I already checked:

  • no water in tubes
  • batteries are fully charged (also switched between tubes)
  • measured voltage on DB-25 connector (Pin 5 and 6) - both tubes at ~9.8V
  • measured voltage on Controller Board 2.8 (Pin 5 and 6) - both tubes at ~9.8V

As I’m a electronics noob I don’t know where to go on.
Any idea is appreciated - Thanks in advance!


Your checklists sounds very comprehensive and I would hardly think that this is a hardware defect. Have you updated to the latest OpenROV image / which is your current software version? For updating to the latest software follow the steps in this thread.


Thanks for reply!
The latest stable version 30.0.3 is installed. How can I check raw data from the controller board at runtime? I’ll also check socket messages for “frontend issues”.


Hi Dennis:

You’ve made a great start at narrowing down where the problem is, now you just need to take it a couple more steps.

There seems to be voltage getting to the DB-25 connector, so if no current is showing up in telemetry, then either the blocking diode for that channel is bad, or the telemetry circuit that measures current is bad. To figure out which it is, remove the batteries from Battery Tube 1 (the good side), and try to power your ROV. If it powers up, check the telemetry for both channels, and see whether both are reading zero. If that’s the case, then the telemetry circuit is bad.

If the ROV does not power up, then there’s something wrong between the DB-25 connector, through the Channel 2 blocking diode, and onto the ROV battery bus.

Once you’ve narrowed the problem down, you can then decide what further action to take.



Hi Walt,

thanks for your input. The ROV doesn’t power up without Battery Tube 1.

I also measured voltage at D1 and D2 - both get 9.9V. I tried to figure out how to go on measuring voltage behind the diodes with assistance from the Controller 2.8 Schematic and Controller 2.8 Top Layout, but found no suitable point at these tiny parts.

Do you have any further guidance for me?
Thanks in advance!



Hi Dennis:

I’m glad you found the schematic, as I was going to lead you there next.

Battery Tube 2 feeds through only two components before it gets to the battery bus. These are blocking diode D2 and current shunt R2. The battery bus can be probed at the Aux Power Input pads, J11, which are on the edge of the board right next to the four sets of I/O headers. So here’s what I’d recommend.

But batteries in tube 2 only, and then use a voltmeter to trace things between the DB-25 connector and the battery bus. Hook one end of the voltmeter to ground (there are several marked places to pick up ground on the board), and then probe on each side of diode D2, each side of resistor R2, and then the positive side of J11. Somewhere in there you will stop seeing battery voltage, and then you will know where the problem lies.

Let us know what you find.



Hi Walt,

sorry for the delay, we performed some dives during last weekend. Today I measured voltage at the points you described in your last post. Unfortunately I got weird results. This time can’t see voltage at all on the bad battery tube… So I started the whole procedure from scratch:

  1. check each battery: 3.35 V
  2. check combined: ~10.0 V
  3. check at DB-25 (male): 0.0 V
  4. checked pass from + wire to DB-25 Pin 6: passed
  5. checked pass from - wire to DB-25 Pin 7: failed

I can imagine two causes - cable break or this part is broken. To check this I could only imagine to cut the wire between battery tube and DB-25 connector.

I measured voltage on all discussed points from battery tube 1 (good one):

DB25 male: 10.07 V
DB25 female: 10.07 V
D1: (-): 9.9 V / (+): 10.07 V
R1: (-): 9.89 V / (+): 9.89 V

I think these values are to be expected.


You should be able to do a continuity check with your multimeter between the spring terminal (-) and the DB-25 pin, and then the button terminal (+) and the DB-25pin. The spring terminal can be a little difficult to get to, but if you tape the probe to a stick you can use that to maneuver it in the tube. This will check for a break in the wire without cutting anything.


Hi Brian,

That’s what I already did:

So my conclusion is that either the cable is broken or this part.


The error was somewhere in the highlighted area.

We replaced the battery tube and everything is fine again. But I don’t understand why I could measure voltage on DB-25 from the defective tube on Friday and can’t measure anything on the controller board (D1, R1, Aux).

Thanks for your support!


Hi Dennis:

I suspect you had a cold solder joint somewhere.

The part you photographed is called a polyfuse. It acts like a fuse, but will reset itself automatically if power is removed and the part cools down. They’re pretty reliable, so I’m skeptical that the part itself was the issue.

If you’ve got some spare time on your hands, it might be interesting to dremel apart your old battery tube to try to discover where the problem was.