Voltage drop through loose connections

When I had a new 50 amp Renogy DCDC charger installed in our Isuzu Dmax, I was keen to see how quickly it would charge our new lithium battery. However, with a bit of mucking around, I could only ever get it to put out about 23 amps from the alternator, plus solar on top of that. I had a quick look at the cabling, and couldn’t see how it would be undersized, but forgot about it as the DIY lithium battery is usually very happy and full without the alternator.

However, on a recent stay at Gym Beach, we camped underneath trees for two nights, and by the time we departed both the Dmax lithium battery and camper batteries were sitting at under 40%, and needed a decent charge. Upon arriving at our next camp though, I noticed that the Dmax battery had hardly gone up at all, which was strange.

New electrical box
Something wasn’t right with the charging

I set the phone up with the DC home app, and watched what would happen when you turned the car on and ran it, and virtually nothing (like less than an amp) was going into our lithium batteries from the starter one (alternator), which wasn’t right at all.

Renogy DCDC app history
The bluetooth and app makes seeing what is going on really easy

Again, up until now I’d not really bothered investigating the limited alternator charge too much, as the solar normally kept up easily. However, on an overcast day, it wasn’t doing much and I don’t like things that aren’t working as they are supposed to be, so started looking around with the multimeter.

I could see that the starter battery voltage measured by the DCDC was 13.1V, which was weird as I could see on our Ultragauge that it was actually 14.3V, and when I measured the voltage at the Anderson plug on the rear of the car it was over 14.2.

The Dmax was wired by an auto electrician, who ran a massive cable from the starter battery through a 150A fuse, to a big terminal box at the rear of the canopy, where it tee’s off. One smaller cable goes to the rear Anderson, and then the main cable continues out of the terminal box, into the canopy and directly to a positive lug inside the electrical box.

I took a measurement of the voltage of the positive stud, and was getting 12.9V, and even when the car was running and being revved it never went up, and that’s certainly not right. I stood there, completely puzzled as to how it can read 14.2V inside the terminal box, and then 12.9 less than 2 metres above it, with a giant cable. By now, my Dad had wandered over, and had joined the hunt. We confirmed there was a 1.2V drop in that cable, which made zero sense.

Whilst Dad was poking around in the electrical box, he let out an exclamation. It’s loose! I said really?! I went over and put a spanner on the nut, only to find it completely tight, and yet the main positive cable was clearly able to move up and down easily and wasn’t tight.

Upon closer inspection we noticed a nut in between the terminals, and with some further digging we realised that the main positive lug was put on, then a nut (which wasn’t tight) and then two other cables were hooked in, with another nut put over the top. The two outer cables were tight against the two nuts, but the main one wasn’t tight and was floating around.

Dmax loose connection
We finally found a loose connection, on this stud
Loose electrical connection
The outer nut was tight as, but the inner one was loose

I removed the two outer lugs, locked the first nut up, put the two other lugs back on with a spring washer and then the nut, and then we fired the car up. Almost immediately the charger started moving current across from the starter to the lithium; problem resolved.

At idle it was sitting at 23 amps, and when I disconnected the solar, and revved the motor it jumped up to 34 amps, which is the highest its ever been.

When you have the solar plugged in the 50 amp Renogy unit splits the input, so you can do a maximum of 25 amps from the alternator, and 25 from solar. On a good day, we’d probably do 25 + 13 or 14 from solar, making it 38 – 39 amps of input, which is pretty decent.

So, a bit of a learning for me, and I’m now familiar with how its all set up. I don’t think this was correct from the get-go, as the DCDC was never able to deliver the current it does now, but its obviously gotten worse as it was working originally, and then it stopped.

What’s funny is I got lucky when I measured the voltage, as I was using the stud to check the volts, and not one of the connections, and the main positive terminal wasn’t touching the stud properly.

Never mind, problem sorted, but a seriously good, practical reminder of how a lack of contact due to a loose connection can play havoc.

I suppose I should count myself lucky, as it would have been getting warm, and that’s never a good thing. We were woken by a 12V camper trailer fire a while back, and it was not a nice thing to see.

We’ve had loose connections on our auto resetting circuit breakers (now deleted) cause similar issues, so its not the first time, but not something I thought was possible with a tight outer nut!

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