Low voltage when compressor is running

[teneicm]

Hello-

I have an isotherm refrigeration system that is not turning on at night and I'm pretty certain it's because of the low battery cut-off that prevents the untit from operating if the voltage drops below 10.4 volts. My question is this: What is the 'normal' voltage drop that I should expect to see when the unit is running? When the unit is off, and I check the connection at the compressor, I register the full house battery voltage (usually 12.4 - 12.6 V, so pretty much fully charged). When the unit is running, I register a voltage 1.8V LESS- so about 10.8V when the batteries are at full charge of 12.6. Is this normal? Or should I expect to see the the full voltage even if the unit is running?

Thanks- Matt

[noelex 77]

That voltage drop is way too high.

It indicates you have a bad connection, wiring that is too thin, or both.

Generally marine wiring should be done so that there is less than 3% (0.36v). Some people will accept 5%(0.6v).

A good general rule is to engineer the system with wire that gives less than 3% voltage drop but after all the connections circuit breakers etc less than 0.5v should be measured.

Whatever standard you choose 1.8v is way too high.

You can use a multimeter to trace where the voltage drop is occurring by breaking the connections down and measuring the voltage drop in each section.

Start by measuring from the positive pole of the compressor to the positive pole of the battery, then do the negative. The circuit can be further broken down, until you find the culprit.
Often a defective connection will reveal it self by feeling hot when the current is flowing so feel all the connections as well.

[teneicm]

Thanks- That makes sense... but you can only do the troubleshooting when the compressor is running, right? Because there's no voltage drop when the unit is off, only when it's running.

Is the voltage drop a function of the current? eg, the more current the compressor is pulling the greater the voltage drop that will be seen?

[noelex 77]

Yes spot on.

[Fishspearit]

Originally Posted by teneicm

Thanks- That makes sense... but you can only do the troubleshooting when the compressor is running, right? Because there's no voltage drop when the unit is off, only when it's running.

Is the voltage drop a function of the current? eg, the more current the compressor is pulling the greater the voltage drop that will be seen?

No, you don't need to check it with the compressor running. The voltage drop is the current mulitplied by the resistance in the circuit.

You can check for the high resistance connection by using your ohmmeter and measuring the ohms between the battery and the refrigerator(both positive and negative side). Just make sure you are disconnected from the battery and there is no voltage in the circuit when you are measuring the resistance, or you will blow your meter fuse.

Another possibility is that you have a bad battery, or too small of a battery, which is allowing the voltage to drop when a load is applied.

[noelex 77]

Originally Posted by Fishspearit

You can check for the high resistance connection by using your ohmmeter and measuring the ohms between the battery and the refrigerator(both positive and negative side).

The problem with doing this is that you need to accurately measure low resistances, which is quite difficult to do
The resistance on either side will only be 0.18 Ohms ( assuming the compressor is drawing 5A).

[sailorboy1]

How much current is your unit drawing? The voltage drop could also be from a current higher that the wire size was based on and maybe the issue is the compressor itself.
Otherwise, and more likely, you are just looking for a bad wire/connection! Just replace the wire instead of screwing around looking for the bad part of it.

[David M]

The more accurate method is to measure when the current is being drawn although both methods work.

I agree that the wire is either under gauged or there is high resistance wire termination (or both). A third possibility is that your battery(s) are not rated for that much current draw without there being a significant voltage drop at the battery.

[Richard Kollmann]

This Isotherm unit is drawing anywhere from 3 to 6 amps depending on model compressor and compressor speed. In most cases when battery voltage reaches 10.5 volts compressor electronic control module will stop compressor. On a compressor start up at 5 amps any resistance in wiring can cause a low voltage spike that will prevent compressor from starting. These low voltage micro second spikes can not be seen with a volt meter. All modules that have a D terminal can identify low voltage problem if troubleshooting LED is installed. A voltage drop of ½ volt when unit is running is too much. Best solution is to run new 15 amp fused positive and negative properly sized wires direct from battery busses to module avoiding branch panels. A 20 amp or greater off on switch in power wire is also required.

[noelex 77]

Originally Posted by Richard Kollmann

A voltage drop of ½ volt when unit is running is too much.

What voltage drop should we be designing for?

[sailvayu]

Voltage drop should be no more than 10% better to shoot for 3%

[Richard Kollmann]

"What voltage drop should we be designing for?"

Design for Zero volt drop when compressor is running. If present voltage drop is 1/2 volt replace wiring with proper size larger wire

[noelex 77]

Originally Posted by Richard Kollmann

"What voltage drop should we be designing for?"

Design for Zero volt drop when compressor is running. If present voltage drop is 1/2 volt replace wiring with proper size larger wire

Richard, I respect your detailed knowledge on refrigerator based systems but zero voltage drop is impossible, it would require an infinitely thick wire.
My boat is too small for that

[teneicm]

OK- So I'm seeing a voltage drop of .6 volt on both the positive and negative side from the panel to the compressor. The existing wire appears to be at least 12 gauge (maybe 10), but is 20 years old and has a run of about 40 feet. I'm planning to buy some new wire to see if that wil help. What's the proper size (guage) for this purpose?

[onestepcsy37]

just checked my adler barbour install manual. sad news for you. for a run of 36 feet or greater they recommend size 4 wire (your read that right, #4 wire). they also recommend wiring directly to the battery with a 15amp circuit breaker in line, regardless of the length or size of wire, just like fridge guru richard kollman recommends.

12 guage wire is suitable for runs of 10 feet or less. i have a 15 foot run and use the next larger size -#10.

and, they cite all the problems you describe as what to expect if the proper size wiring is not used. you may need to move your batteries or your fridge or install another set of batteries just for the fridge....