Graham replied:

I thought a deep cycle battery was designed to handle almost flattening before recharging, hence the name. This aside ,your fridge should cutout long before the battery is drained sufficiently to cause damage to itself.

Mark replied:

Gary,

Most 12v fridges have a built in cutout that comes in around 10.5v and I'm pretty sure the latest Engel does. I bought an inline cutout for around $30 from KMart for my old Supakool fridge as it did not have a built in cutout, it worked ok but I suspect sometimes it cut in a bit early. You can buy a voltage guage to keep an eye on voltage but I just carry a small voltmeter with me when camping and check the voltage at the end of the day. An 85amp Alternator should charge the battery in around 1hr, I have found with my car though that the regulator limits the charge if the battery is really low to reduce the strain on the alternator, another good reason for not draining the battery completely.

Jeremy replied:

Piranha have a couple of low voltage cutout with a battery monitor built in which are quite good ! it is set at 10.5 volts but there is another one that cuts out at 11.2 volts and i think is adjustable ! The new engle does not have a low voltage cut out !..... in there wisdom they took it out ???? you are right about keeping deep cycle batteries at about 30% especially calcium batteries, lead acid deepies will take that lower end of charge better but modern alternators tend to boil them !, all batteries will take charge better when they are over 30% ! as for how long they take to charge depends on your system !, if you have a solenoid system with no voltage sensing your batteries will be equilizing coz the alternator is voltage sensed ! and there for only getting up to maybe 60 or 70% or even as low as 50 %! ,a voltage sensed battery managment system will fix that ! and should fully charge it between 1 and 6 hours ! many factors can make a big diference ! but if you keep it above 30 % you will have a lot better system and charging time will be kept to a minimum ! The cut out work through a relay some internal some you put in your self when the given volts are reached it cuts power supply !
Good Luck ! There is no easy answers !

Patrick Kearney replied:

Hi Gary,

I obtained the following info from a reader recently and i thought it may help you with your question.

"I then monitor the health of my battery by a digital multimeter (bout $28 at any electronics shop or even maybe a Supercheap - you must get a digital readout model if you get one). The voltage of a battery lets you know the percentage amp hour capacity of the battery used – which explains why when 80% of the battery is used the voltage is too low to generally operate a 12 volt applicance. The specs are this:
State of Charge 12 Volt battery reading
100% 12.7
90% 12.5
80% 12.42
70% 12.32
60% 12.20
50% 12.06
40% 11.9
30% 11.75
20% 11.58"

Simple advice to follow to avoid problems.

porl replied:

Hey, i think i may have been the reader Patrick was talking about. For the full 5c worth go to the overlander.com.au website and go to the forums and in the general stuff you will see pretty close to the top, 2nd from the top this morning, a thread with about 19 replies- i think there are bits there about 12v systems for everyone to learn something, me included. Note my typographical error in that when i said when the mulitmeter reads 11.58 volts the battery has 80% left, obvious mistake it means there is only 20% left which means you've used 80%. No fear with your issue though i think, 30% is better but 20% is the standard and your fridge does have a low voltage cut out which will stop the fridge running when there is probably more than 20% left, and in any event it is a general rule of thumb that your 12v appliance cannot run on a voltage less than about 11.60volts so it cannot drain the battery to zero even if it wanted to. That said you can run a battery to zero if you don't have blocking diodes to prevent trickle drain even if the fridge is off just by curernt cirulating through the system, but that is not going to be really a problem in a 77 amp hr battery as your food will go off and your beer be warm way before that happens.

porl replied:

having read above in a bit more detail i have to take back what i said about the Engel, i don't have one i just presumed, - i can't believe they took out the low voltage cut out, whatever.