Starter Issue - Starter clicks, PMU shuts down

PMU
1

Hello everyone,

I’m running into a problem with my setup and would appreciate some advice:

  • ECU Master PMU16

  • Starter motor is powered through 1x 25A Output

  • Also tested with 2x 25A outputs in parallel

  • Activation is done via a CAN-keypad

  • When I try to start, the starter just clicks

  • At the same time, the PMU shuts down completely (I have a video of this happening)

  • In the data logs I can’t see anything, because the PMU turns off during the start attempt

Has anyone experienced something similar? Could it be an inrush current issue?

Thanks in advance!

On the Video the Status LED looks red, but it’s orange

2

Hi,

When you say ‘starter,’ do you mean the starter solenoid or the actual starter? Even a small starter can easily draw over 100 A.

Overall, it appears that the device resets due to a voltage drop. If the supply voltage at the PMU drops below 6 V, it will switch off. The loads will turn off, the voltage will rise back, and the device will switch on again.

3

Hi, thanks a lot for your reply – and sorry, I should have mentioned this more clearly in my first post.

  • The PMU output is connected to the starter relay, not directly to the starter motor.

  • I measured the supply voltage during cranking – there is no significant voltage drop, only about 0.5 V.

  • At the moment the starter solenoid clicks, the PMU still shows around 12 V supply voltage.

  • If I run the starter through an external relay, the PMU stays on without any issues.

  • I’ve also tried different ground points, but the problem remains the same.

So it doesn’t really look like a supply voltage issue. Do you maybe have another idea what could be causing the PMU to shut down?

Voltage Drop

4

You can’t measure voltage drop like that with a multimeter. It’s too slow. You would need an oscilloscope.

Record a PMU log with the starter activation attempts and post the project and log files here.

5

Ok, i’ll use an oscilloscope.
I can’t really record a log file, because the PMU shuts down. You’ll not see anything…

You’re right, the voltage drops below 6V!

Here’s the scope, log and project file

IMG_20250916_134059510
IMG_20250916_1340595101920×2550 242 KB

20250916_1341.voltage drop.pmulog (23.4 KB)

Caterham#16.16-09-2025.pmu (6.3 KB)

Maybe Battery or Plus Wire?

6

Did you measure that on the PMU post?
If yes, measure the same thing on the battery terminals.

We will know if the voltage drop is on the wires or the battery itself.

7

i measured on the pmu post.
This is measured on the battery itself

IMG_20250916_162206262
IMG_20250916_1622062621920×2550 236 KB

IMG_20250916_160432846
IMG_20250916_1604328461920×2550 284 KB

Unfortunately i don’t have a battery test with wich i can test LiFePO4. I will charge the battery and take the measurements again.

Or do you have any suggestions?

8

Since the voltage drop occurs directly at the battery terminals, it indicates a problem with the battery.

Either discharged or faulty.
If you can, try a different battery for testing.

9

Different Battery

IMG_20250916_144753976
IMG_20250916_1447539761920×2550 264 KB

7,5 Ah LiFePO4 Recharged

IMG_20250917_090810082
IMG_20250917_0908100821920×2550 262 KB

So does that mean the battery is simply at fault?

Why does it start even with uncharged battery when the starter motor is operated via an external relay?
My assessment: The current for the starter does not flow through the wiring to the PMU and therefore does not cause an additional voltage drop. Is that correct?

Thank you very much!!

10

That’s possible, or the battery is too small for the starter.

Because the relay/starter is not interrupted when the voltage drops.

If PMU controls the starter relay:

  • You try to start the engine,
  • PMU enables the output and switches on the starter relay,
  • Starter current starts flowing and battery voltage drops,
  • When the voltage is too low, PMU switches off,
  • If PMU switches off, so does the starter relay output,
  • Starter is switched off, voltage rises, and PMU switches back on.

When you used an external relay, without PMU, to start the engine, your PMU still reset. But the reset PMU didn’t switch off the starter.

With the voltage drop you have on a charged battery, the PMU should stay on, but it might be very close to resetting. A bit more discharged battery, or even a cold starter, can put it over the threshold of not starting up.