Wiring a Refrigerator

Hey all, so I recently acquired a refrigerator for my truck. It's a military unit used for transporting blood/plasma bags. What I like about it though is that it has an onboard battery supply that can run the unit for up to 48 hours with no other electrical input.

So I'm envisioning wiring it up so that when the truck is running the alternator is running the fridge and recharging the batteries. When the truck is off the fridge, having lost it's external power supply, will switch over to internal power.

I'd like to run a plug like an Anderson to the bed with some heavy gauge wire and connect the fridge to that. So where should I tap in to get a power source that's only hot when the key is in the "ON" position? I want it to be totally disconnected from the truck's starting battery when the key is off.

Can I run some kind of solenoid that will make a connection from the starting battery to the fridge plug when the key is on? I can't think of any other way than using a solenoid or heavy duty relay that is activated by the key on.

Power consumption is estimated at 40W for start up and 20W during "storage mode"

 

Subd for responses

 

Bump for ideas!

 

Playing around with this calculator (http://www.powerstream.com/Wire_Size.htm) it seems 12ga wire will suffice for my purpose. The highest draw from the fridge should be about 3.3A (40W at 12V).

Estimating the wire length at about 15 feet, voltage drop for 12ga wire should be only about 0.163V. So I think I shouldn't have to run more than 12ga wire.

Still not sure about the switching...

If the fridge only draws 3.3A that should mean I can just run a simple relay. How about this:



Terminal 30: Battery +
Terminal 87: To bed for fridge
Terminal 85: Ground
Terminal 86: Switched 12V, hot only when key is ON

This configuration should automatically complete the circuit whenever the key is ON and disconnect the circuit when the key of at ACC or OFF.

Thoughts?

 

Use a high enogh amp relay switched hot. Appropriate gauge wire to fridge batteries and it'll do the trick. Though there are probably better solutions.

 

Also how big is the fridge drawing only 3.3 amps. Is this running or startup draw. Initial startup should be well over 3 amps.

Someone correct if wrong

 

b. Estimated power consumption during COOL mode startup:
<40 W at room temperature (55 W at 40°C (104°F)) ambient
c. Estimated power consumption for COOL mode storage only:
<20 W at 40°C (104°F) ambient

Copied from the manual. Current = power / voltage.

Should draw 3.3A on start up and 1.67A to maintain.

 

You will be fine with that configuration, my Engle is very close to that spec and I can run my Engle on the aux battery for days so you be pleased with you setup.
There is no huge start up current spike as there is with the typical compressors.

Do you have a pic of the unit? I've never seen a military version.

 

Thanks for the advice!

Here's pics from the ebay listing I got it from







The fridge is made by a company called Acutemp in Dayton, Ohio. It's used to transport and store blood and plasma bags. It can operate as either a refrigerator or freezer. Capacity is 56L. The fridge is built into a Hardigg case with 10 latches holding the lid on. It's very burly. The main drawback is the size and weight. It's about 21" x 21" x 37" and weighs around 140 lbs empty. The benefits are that it has it's own battery supply so it can run for about 24 hours on it own in "cool" mode. Also the durability is supposed to be pretty good. It can accept AC or DC/Solar.

I'm liking that I don't have to run a dual battery setup on the truck. I don't really have a need for dual batteries except to run the fridge, so when the fridge is not in the vehicle I don't have any weight penalty from the dual system.

Actually the jury's still out as to whether this unit will work out or not. I plan on taking it on a roadtrip this fall so we'll see if the size and weight are a hindrance. Honestly I like the small size/weight of the Engel or ARB units. If you spend the night in a motel with no refrigerator you can easily bring in the one from the truck and plug it into the wall.

Some stuff pasted from the manual...

a. The HemaCool® is a conventional liquid/vapor-phase refrigerator with a number of special enhancements:
(1) HemaCool® employs vacuum insulation panels to decrease the power required to regulate temperature in a given
environment, to increase holdover time when energy sources are lost or disconnected, and to increase internal
temperature stability.
(2) In the COOL mode (4°C (39ºF) set point), the unit prevents payload freezing in sub-zero environments by
applying heat, rather than cooling.
(3) The unit's control system works with a number of different power sources without special adapters. This power
system includes internal batteries that allow the HemaCool® to operate for a time without external power (see
Operational Specifications, Paragraph 1-4).
(4) The HemaCool® includes integral monitoring and logging of payload compartment temperature and battery
charging state.
(5) The HemaCool's® internal design is arranged to accept baskets for holding blood bags (or fresh frozen plasma
bags when the unit is operating in freezer mode).
(6) Small internal fans help circulate cooling air, to maintain uniform temperature.
b. The HemaCool® is designed to preserve liquid or frozen blood products and medical supplies. HemaCool® provides
exceptional field storage through its combination of superior insulation technology and temperature controls, based
on microcontroller solid-state electronics.
c. The HemaCool® is portable and self-powered, utilizing internal batteries. The system will accept electrical power
from multiple sources, e.g., 12-28 VDC from batteries or other DC sources at the DC power input, or from a wide
range of grid power standards (any combination of 100-250 VAC and 50-60 Hz line frequencies) at the AC power
input.
d. If the unit is fully stocked with conditioned contents and no power is applied (unit off), it will take more than 2 hours
for the internal payload temperature to rise from 4°C (39°F) to 6°C (43°F) in an ambient temperature of 49°C
(120°F).
(1) The HemaCool® will perform in either the COOL or FREEZE mode.
(a) In the COOL mode (4°C (39°F)), the unit, provided its two internal 21-amp-hour batteries are properly
charged, will maintain a temperature range of 1°C (34°F) to 6°C (43°F) for more than 24 hours at 24°C
(75°F) ambient temperature.
(b) In FREEZE mode (-22°C (-7°F)), the unit, provided its two internal 21 amp-hour batteries are properly
charged, will keep blood products frozen for more than 8 hours, at an ambient temperature of 24°C (75°F).
Freeze mode operation does, however, reduce the battery-powered hold time. (See Operational
Specifications, Paragraph 1-4).
(2) For additional payload security, the HemaCool® is also equipped with an internal heater that will prevent a
refrigerated payload from freezing if the unit is in an environment that is below 1°C (34°F), but no lower than
-20°C (-4°F).
e. The standard default temperature scale setting for the HemaCool® is Celsius (°C).