-- Electrical Design Question --

Bit_Bucket_07 said:

If that's true, then something is badly wrong with one of his meters.

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Swap the meters and check again. I don't think it's the meters. It has to be the wiring and connectors. If you have connectors on the return (ground) as well, then check those too. High resistance grounds are not your friend.

Jeev said:

Swap the meters and check again. I don't think it's the meters. It has to be the wiring and connectors. If you have connectors on the return (ground) as well, then check those too. High resistance grounds are not your friend.

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I don't think it's the meters either. I think the OP measured 12 V with the load disconnected and 7 V with the load connected, because the battery is overloaded.

Voltage drop due to smaller than needed wire was the issue here.

The customer spec'd out a special wire that could handle the amps and they did the testing themselves in their office setup, but the length of run was causing the voltage to drop from 12V to 7V when loaded. So it was overlooked on the run of cable.

Esieli said:

...but the length of run was causing the voltage to drop from 12V to 7V when loaded. So it was overlooked on the run of cable.

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So it was the meters (metres) after all.


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Esieli said:

Voltage drop due to smaller than needed wire was the issue here.

The customer spec'd out a special wire that could handle the amps and they did the testing themselves in their office setup, but the length of run was causing the voltage to drop from 12V to 7V when loaded. So it was overlooked on the run of cable.

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Interesting. You had said before that increasing the wire size only made the problem worse.

Esieli said:

Thank you.

I tried increasing the size of the conductor and the number of conductors, but still didn't help it. It actually seemed like the more conductor I gave it, the more voltage drop I had.

I'm starting to think maybe it's the connectors I'm using.

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That must be some very high impedance wire that you were using. Even then, it doesn't make sense that using paralleled wires would exacerbate the voltage drop.

Are you actually measuring the battery voltage with the glow plugs connected
i would expect to see the battery voltage drop to about 7 to 10 volts quickly with that high of a load.

you will need a very large battery bank to maintain the full voltage for more than a few minutes under that load.

Think about how long can you run the starter motor before the battery goes dead
most starter motors pull about 100 to 200 amps while cranking the engine