Voltage drop

[bromleybysea]

Can someone please explain how I can check for voltage drop under load in a circuit. The power supply to my fridge shows battery voltage less about 5% but even when the batteries are fully charged the fridge won't start. If I run a lead from another source closer to the switch panel it will run. I have a digital multimeter but I'm no expert!

 

[Birdseye]

Do you even need to measure it? What you have already done is to show that the current wiring is inadequate so you need to rewire the feed to the fridge. Even if you could measure voltage drop ( one meter terminal at the battery +ve the other at the fridge posiitive feed ) it wont tell you much without the fridge working.

Dont overcomplicate. Just rewire.

 

[KompetentKrew]

I don't disagree with what @Birdseye says, but probably a good way to get you started with your multimeter would be to put the black probe on the battery negative and the red on the positive. What does it say? Is your battery fully charged? Do you have shore power?

 

[Sandy]

YouTube is your friend

 

[William_H]

Difficulty for OP is that if the fridge won't start using standard wiring then it will be difficult to check where voltage drop is. Start by bypassing or adding to earth negative return from battery negative to fridge negative. If you can get a start then check drop on positive line. Put positive probe on battery positive terminal. Then measure voltage drop at various points along the positive line till you get to fridge. So voltage drop could be at battery terminal, isolation switch, wiring to switch board , switch or circuit breaker /fuse or wire itself.
Try bypassing wiring pos terminal to positive of fridge. When fridge is running check in the same way the volt drop of the negative wire. ie to switch board then to fridge.
Volt drop may be caused by wire too light but also can be caused by faulty switching and fusing. Or could be a cumulative effect from all the above. ol'will

 

[bromleybysea]

Thanks guys. All helpful. I have new house batteries on shore charger so fully charged. So I'll be wriggling around in tight spaces today checking connections!

 

[colind3782]

If it worked before with the existing wire it is probably a bad connection rather than undersized wiring

 

[andsarkit]

The fridge compressor motor will momentarily draw about 5 times the normal running current as it starts. The wiring has to be sized for the starting current or the extra volt drop will prevent the motor from starting and it will remain in a stalled condition and possibly be damaged. Once the motor is running it generates a back EMF which limits the current to the normal value.
As colind3782 above, if it worked before, check all the connections.

 

[Refueler]

If it worked before with the existing wire it is probably a bad connection rather than undersized wiring

Exactly ....

Suggest before re-wiring .... disconnect all connections - give them all a good clean up and so metal to metal contact is bright ... reconnect and try again. If it now works ..... smear of vaseline on connections. Its also worth - if it does work ... to check temperature on the leads powering the fridge ... if warmer than surroundings - means they are too small diameter for the load.

Not unusual with starter motors on cars / boats etc. - contact terminal joint gets pitted / corroded ... engine won't start etc.

 

[Boater Sam]

Rule of thumb for 12v voltage drop with a fridge.

Measure the length of the cable between battery and fridge.

You need 1mm square of copper wire size for every metre.

So if the distance is 6 metres you need 6 square mm cable, If 7 to 10 metres, 10mm etc.

 

[colind3782]

Of course, check that the battery is actually fully charged. How do I know this? MY fridge stopped working and I was scratching my head for a while until I notice the battery voltmeter was showing 11.5 volts! Although we were on shore power, "someone" had accidentally knocked the charger switch to off!

 

[bromleybysea]

All very helpful. I did as advised and checked/cleaned all connections and as a last resort opened up the distributor panel and found the "culprit" was a faulty panel switch. Happily showing full battery voltage when tested, except when turned on under load. Swapped it for a spare and enjoyed a cold beer an hour later.

 

[thinwater]

Actually, I'd be more inclined to suspect a high-resistance connection then undersized wire. What Refueler said.

 

[Refueler]

All very helpful. I did as advised and checked/cleaned all connections and as a last resort opened up the distributor panel and found the "culprit" was a faulty panel switch. Happily showing full battery voltage when tested, except when turned on under load. Swapped it for a spare and enjoyed a cold beer an hour later.

Magic .....

I just took delivery today of 3 pontoons + a Vevor (Waeco style) portable fridge box ...... looking fwd to cold beers on board !