Saturday, May 18, 2013 at 14:03
Hi Ken,
To answer basically, just because the Voltage cut-out inside the fridge is set at 9.6 Volts does not mean the battery will be discharged to this level, as even on a
well setup system the battery will normally only run down to around 10.5 Volts but on a lot of systems the fridge will stop
well before that Voltage is reached (12.0 Volts or higher as below).
Of the two options you have given I would suggest option 2 is important for a lot of people, that don't mind how or why, they just want to plug the fridge in and it run for as long as possible.
To answer in more detail;
The lowest Voltage cut-out a Danfoss 101N0210 or 101N0220 electronic box can be set to is 9.6 Volts on the 12 Volt side. This is the standard electronic box that is used with the Danfoss BD35 and BD50 Compressors. The default cut-out is 10.4 Volts.
Evakool originally used the default of 10.4 Volts but changed the cut-out to 9.6 Volts around 2005.
There is around 0.2 Volt drop through the cabling inside an Evakool fridge (from the electronic box to the 2 pin power outlet) and around 0.2 Volt drop through the standard cigarette cable. This Voltage drop is based on a number of factors including the cabling being in factory condition and assuming an average Amperage load (the Amperage draw based on many different factors).
So if the Voltage cut-out was 10.4 Volts at the electronic box and there was a 0.4 Voltage drop to the end of the cigarette lead then fridge will run until the Voltage drops to 10.8 Volts at the end of the cigarette lead.
Assuming another 0.5 Voltage drop through the vehicles cabling then the fridge will stop when the battery has dropped to 11.3 Volts.
Depending on a few factors this could indicate a lead acid battery that has around 20 - 30% capacity left. It could be argued that this is still too low a state of charge to run a battery down to but as long as a high quality battery is used and the battery is recharged soon after reaching this point it should still have a long life.
Unfortunately we at Evakool, as have many other companies / people, have found out that most vehicles in standard form (and some that have been modified) are not close to only having only 0.5 Voltage drop when running a fridge.
This means if the Voltage cut-out was set to 10.4 Volts, assuming a cut-out point around 10.8 Volts at the end of the cigarette lead and a 2 Volt drop through the vehicle’s wiring then the fridge would stop when the battery was at 12.8 Volts. This would mean the fridge would stop working soon after the vehicle's engine was turned off.
If on the same system the Voltage cut-out inside the fridge was changed from 10.4 to 9.6 Volts then the fridge would continue to run until the battery was around 12.0 Volts. This would give a much longer run time.
So, as shown above, depending on different vehicles / setups the same Voltage cut-out can mean the battery discharges to very different points.
So setting the Voltage cut-out has to be a careful consideration but in most circumstances it is very important not to set the cut-out too high.
Thanks.
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