Low voltage when compressor is running

[Richard Kollmann]

Energy management of 12 volt Danfoss compressors is important enough that voltage drop is not good. The question was what voltage drop should you design for?, not what you could live with.
Danfoss application engineering spec sheet I am looking at recommends for a five amp 12 volt unit with 27 feet between compressor and battery buss to use number 8 gauge wire. Number 10 wire is listed for 17 ft. For 40 ft both positive and negative wires I would use number 6 gauge wire. I believe that those who follow manufacturer’s application instructions are rewarded with reliable and extended life units.

[senormechanico]

Originally Posted by 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?

Is the round trip run 40 feet or is it 40 feet from the batteries?
Is the 20 year old wire turning a dark color? If so, that's copper corrosion.
If there's one thing on a boat that needs nice fat wire, it's the fridge.
Oh, and one more, an anchor windlass !

[noelex 77]

Originally Posted by Richard Kollmann

. For 40 ft both positive and negative wires I would use number 6 gauge wire. I believe that those who follow manufacturer’s application instructions are rewarded with reliable and extended life units.[/FONT][/COLOR]

I agree this would be the correct choice.
The voltage drop in the wire will be 0.25v(@5A).
There will also be some voltage drop in the required fuse and switch (or CB) as well as all the multitude of connections, so expect a higher voltage drop than 0.25v in practice when its measured.

[teneicm]

Originally Posted by senormechanico

Is the round trip run 40 feet or is it 40 feet from the batteries?
Is the 20 year old wire turning a dark color? If so, that's copper corrosion.
If there's one thing on a boat that needs nice fat wire, it's the fridge.
Oh, and one more, an anchor windlass !

It's about 40 feet from the main distribution panel, so 80 feet there and back. It's a cat, so 6 feet down to the chase in the bilge, 6 feet forward, 18 feet across and then 10 feet back to the compressor.

The outer strands of the cable are quite dark. Any way to remove that corrosion?

[David M]

Originally Posted by teneicm

It's about 40 feet from the main distribution panel, so 80 feet there and back. It's a cat, so 6 feet down to the chase in the bilge, 6 feet forward, 18 feet across and then 10 feet back to the compressor.

The outer strands of the cable are quite dark. Any way to remove that corrosion?

I use a small stainless steel wire brush for removing corrosion on bare wire.

[teneicm]

Just bit the bullet and ran new 6 guage all the way. Not fun. Needed a lot of lube to get it through chases already crammed with wires. But it's in and today I'm hooking it up. Will let you know what sort of drop I see now... Thanks for all the input and help!

[teneicm]

6 gauge did the trick. Also went directly from the breaker to compressor and cut out the panel switches (which cuts out 3 connnection points). Humming along nicely now.... still have about a .5 volt drop from the battery measurement, but some of that is from the batteries to the main distribution bus.

Thanks for the help.

[noelex 77]

Originally Posted by teneicm

Humming along nicely now.... still have about a .5 volt drop from the battery measurement, but some of that is from the batteries to the main distribution bus.

Thanks for the help.

Well done in solving the problem.
The 0.5v should be fine. You need to remember the predicted 0.25V is for the wire only. There are lots of connections (wire to ring terminal, ring terminal to switch etc) in the system as well as voltage drop for the CB (or fuse) main battery switch etc. All these have some voltage drop.
The end result is if you measure less than 0.5v drop, the system is pretty good.

There are some proponents of connecting the fridge to the batteries. This would reduce your voltage drop to 0.35-0.4v, as well as provide a slightly more stable supply, but in my opinion conventional connection, or at least after the main battery switches is better.