T&T: Low voltage and Danfoss compressors

[Candy Chapman and Gary Bell]

>>Why would any trawler's battery voltage ever get this low in the
>>first place?  That figure, 9.6 volts represents a pretty dead
>>battery!
>  
>

I am not saying the battery voltage got that low, but I did say that 
the wires to the fridges were too small, so a relatively low voltage 
at the battery, plus bad wiring, equals a very low voltage as seen by 
the electronic units.  Since the back of the fridges were relatively 
inaccessible, I could not confirm the voltage drop, but I am sure it was there

I have corrected the wiring and, while I had the fridges out, 
inserted jumpers.  I hope the problem is solved!

Best,

Steve


I disagree entirely Steve.  Most likely the voltage plunged much lower 
and the cutoff on the compressor system behaved just as it was intended, 
saving the compressor and/or its control circuit from serious damage. 

What's missing in this discussion, according to Mister Science anyway, 
is:  first, the assumption that a battery puts out a constant voltage , 
regardless of the current drawn; second that a load can vary 
dramatically in its current demand; and third that as the voltage at the 
load diminishes, the current involved increases in inverse proportion.  
Mister Science is a real stickler about enforcement of Ohm's Law. 

You all are absolutely right about the likely presence of unintended 
resistance in degraded connections and inadequate guage of wire.  
Perhaps with fresh connections and a well charged battery the original 
wiring was adequate.  I second Bob Faircloth's previous suggestion of 
adding a generous margin in wire guage, robust connectors (and proper 
circuit breakers/fuses as well) for carrying extra current, to account 
for natural degradation.   Particularly for any circuit with a 
significant motor, and most particularly for any motor that starts with 
a substantial load.  Like a compressor (cooling or of course a shop air 
compressor), or a somewhat larger pump -- see my second point below. 

Now, for my first point.  The voltage that your panel meter, or a modern 
handheld meter (with a standardized internal resistance of twenty 
million ohms) is the Maximum voltage that the battery can produce 
(ignoring small changes for temperature), under virtually no load (by 
virtue of the huge internal resistance of the meter, of course).   A 
healthy battery with a full charge will provide considerable current 
with only a small proportional drop in voltage.  Discharge graphs of 
batteries, plotting output voltage against current drawn plainly show 
that, and often they show curves for different states of charge, or 
different starting voltages.  The technical term used is 'Internal 
Resistance' of the battery.  They also show that a battery in poor state 
of charge, particularly an ailing battery (perhaps with significant 
sulphation, plate or electrolyte degradation, damaged internal 
connections, etc.) will show a much more precipitous drop in output 
voltage as load current increases.  Higher 'Internal Resistance.'  In 
short, your battery could not maintain the open circuit voltage as the 
current drawn by the load increased. 

Point two.  Most, if not all loads (electric consumers aboard our boats) 
demonstrate a need for more current when they start than when they have 
fully started and are at normal operating condition (normal temperature 
or water/air/refrigerant flow, etc.).  Technical terms for this phenomon 
are 'Starting Surge and Inrush Current).   The examples I mentioned 
above all share substantial Starting Surges.  Inverters are rated, 
mostly for refrigeration needs by the way) for normal output (say 2KW) 
and surge current (maybe 3KW for so many seconds).  Fuses and circuit 
breakers react to current, not voltage (normally).  Fuses are designated 
to account for 'Fast Blow,' generally used to protect semiconductor 
loads (let's skip over the reasons here, email me if you want to know 
the details -- and 'Slow Blow' for loads with substantial surges.   
Different terms are generally used for rating circuit breakers, 
accompanied by details of how much time it takes to trip them at 
different currents.  For surge current loads, select breakers with 
longer times, or increase the current rating to tolerate the surge 
current, not the normal draw for if you have a 'fast breaking' breaker 
rated just for the normal operating current you will have lots of events 
that trip your breaker, and you may not realize the reason.  Also, only 
the most expensive current meters, with a peak recording function that 
reacts fast enough, can adequately measure the peak current for most 
starting surges.  My professional clamp current meter takes about half a 
second to measure and calculate a current.  Only my high end Fluke 
digital meter has a peak reading function that will properly detect the 
brief current and voltage changes involved.  To be sure, the ordinary 
circuit breaker is generally slow enough acting to make this last point 
about measuring academic, but sometimes not in the presence of a longish 
surge.   Again, in short your refrigerator draws many more amps starting 
up than it does moments later when its juices are flowing properly. 

My third point.  We will have NO violations of Ohm's Law Here!  The law 
sez:  current is equal to the voltage divided by the resistance.   So, 
in my first point, as the battery voltage diminishes, keeping the load 
resistance constant for the sake of argument here, the current just 
naturally increases.  In my second point, disregarding any change in the 
voltage when the resistance is lower the current must be higher.   Bob 
Faircloth's issue of increased resistance in the wiring serves in the 
same way to depress the voltage at the load, so it requires more 
current.  Note that my second point and Bob's issues bring on increased 
current drawn from the battery, and that my first point indicates that 
the battery reacts with lower voltage, producing a cascade of 
reinforcing influences.  These are brought to a halt when the voltage 
passes below the lower limit on the compressor, even briefly -- or a 
fuse/breaker at the source or elsewhere reacts to the increased current. 

Conclusion:  Mister Science sez:  First provide the rated voltage and 
current capability to the reefer; then look at other issues, like 
degraded cooling from dirt, faulty fans, poor air circulation; after 
that look for a tripped breaker or blown fuse on the compressor; and 
least likely a bum compressor (unless it was damaged after the power 
went low -- not particularly likely).

Gary Bell