any battery experts out there

Brendan,

If your battery is dropping to 8 volts, it is cactus. 10.8 volts is dead flat and anything that low means the battery is probably badly sulphated and will not hold a charge.
When fully charged you should have a reading across the battery terminals of 12.8 volts without a charging process applied and with the vehicle alternator supplying a charge, the measurement should be around 14.2 volts.

The only real way to test the battery is to fully charge it (if possible) on a 240v charger and then have it load tested. This will determine if the battery will hold a charge, but based on your symptoms, it is beyond recovery.
You haven't said what type of auxiliary battery you have, or what Ah capacity, but you really need a deep cycle battery to maintain your fridge. The D250S dual is a good choice to maintain your battery via the alternator and with a solar panel of sufficient capacity when stationary as well. For a 40l Engel, you need a battery of at least 80Ah or bigger.

Even though you have a D250S dc-dc charger, the battery should be charged via a 240v smart charger from time to time, to ensure sufficient time for a full multi-stage charging process is applied.

How is the Ah capacity (60Ah left in it) being measured?
This intrigues me, coupled with the 8 volt measurement you are getting.

Are you sure you are measuring the voltage correctly?
If it has been rundown to 8 volts it should be stuffed.
If it is, buy a new battery and a cheap under voltage relay to drop the load off at about 11.5 volts protecting the battery.

I don't fancy myself as an expert , but you can do some checks yourself .

Firstly how big is the solar panel.? If it is say 150watts you will only get a max of say 7amps over about 4 hours in winter. =28 amps with maybe 3 for another 2 hours=6 on a clear day. Total say 34 amps.
If the panel is dusty and not oriented towards the sun it will be less.

An Engel 40 litre will use about 1.5 to 2 amps per hour over 24 hours that is 36 to 48 amps.

Maybe you can check the voltage at 4PM and if it is much under 12.7 then you are not getting enough charge to maintain the fridge. Although a C-tek 250s can charge a maximum of 20 amps , it can only charge how much the panel charges.

You can buy a small meter from ABR Sidewinder which will readout the cumulative amps from your solar panel during the day along with instantaneous voltage.

You will have a much better idea of what needs to be done if you do the above checks, as asking your question as it is will just elicit guesses.

Regards Philip A

Brendan
I have a very similar setup, I have a Landcruiser ute as well as the Iveco. Aux battery, Ctek 250D, and a 135 watt solar panel, 80 litre fridge.
Ctek's are definitely sus in the heat, I had mine under the bonnet, in Karratha - very hot. It used to error on a hot day, I moved it to in the back (roof and canvas sides). So the aux battery, the Ctek, and the fridge is all in the back. It now works as endless power when camping, I cant flatten it.

Ctek is a pretty intelligent device, on the solar side, and it needs wiring correctly. Have you removed the solar regulator from the panel? Ctek has a MPPT regulator internally, so it cant be fed from a panel that is already regulated. You will lose a lot of charge potential if the incoming solar voltage is 12.8 volts, rather than 17 volt. Ctek steps down the 17 volts on the solar side, to a good charge voltage, and steps up the incoming from the alternator side, again, to a good charge voltage.

I have had this system in place for several years now, and have never had a charge issue.
I would-
check and see if the panel regulator has been removed
Locate the battery, the ctek in the rear, not under the bonnet, too hot
Upgrade all wiring to 6mm or more (fit fuses as per Ctek wiring diagram)
Replace battery

I believe that charge failures kills batteries early. People are fixated with installing the latest, greatest batteries, but the problem is a lack of charge. I can verify, that solar does absolutely nothing in a shed. Any good system needs the alternator, solar, and a 240 volt charger for when you are at home. Flatten a battery (even once) to 8 volts, and I would reckon that it has been damaged. If you don't use the rig every day, plug it into the 240 charger. Charging has to be constant, not a once -every - now - and - again thing
You have everything sized well, mine is even smaller on the solar. I wouldn't bother upsizing anything of the major components. My panel is 135 watts, and the fridge is an 80 lt National Luna. I live just South of Cairns now.
good luck

I think there have been a lot of issues addressed, but THE most important thing to understand is...there is no free lunch.

These DC to DC chargers are so often being sold as mirricle devices...they are nothing of the sort.

With corrent installation and reasonable expectations they are great things......but they won't make something from nuthin and they are not always the fastest or best way of charging a battery.
In fact in may situations they are many times slower than direct connection via a dumb relay to the alternator.

The other matter is that batteries take a finite time to charge regardless of the charger being used......a good 20 amp charger may start off puting in a good solid 20 amps....but after a fairly short time the current will drop and in the latter part of the charge cycle it may be putting in 5 amps or less.

So a half discharged 80 amp battery in reasonable condition will take at least 4 hours under favorable conditions to get to a fair state of charge.

There are arguments about getting fully charged and what actually constitutes "fully charged"

But before you even get that far you need to be putting in enough to replace what you have taken out pluss losses.

If you have a deeply discharged battery, particularly one with some age or abuse to it..it may take 6, 8 or much more hours to return to a reasonable state of charge...regardless of the capacity of the charger.

If you fail to return at least as much as you draw pluss losses, your battery is on a slippery slope and will get progressivly deeper and deeper discharged till it fails.
This syndrom is all to common in travelers.


Now this fridge.
If you are running an engel.......you need an undervoltage cut out...realy everybody needs an under voltage cut out......lots of batteries would be saved.

Oh and what temperature are you running this fridge at.

pluss 2C is considered reasonable for "fridge temperatures" if you are running it lower than that at " freezer temperatures", the power consumption will skyrocket.....if set at reasonale safe freezer temp of -15C, the motor will run pretty well continuously even in a coolish place.

If this fridge is in the north and in a closed vehicle...even at +2C..the motor may run continuousy.

People are very optomistic about fridge current demand....believe nothing other than maximum continuous current draw....or what you read from your own instruments.

It will be quite easy to argue that both the battery and solar panel capacity are marginal to say the least to keep up with what you are doing.....you better have it all running properly or it wont keep up.


Now this underbonnet thing.
The worst place for batteries in under the bonnet of a vehicle...some of us have no choice.
BUT it must be understood that battery life and performance is progressivly reduced above 25C.

If you have a battery charging system with temperature compensation..the sensor needs to be mounted on or near the battery.

In particular, many of the advantages of DC to DC chargers are lost if they are not mounted close to the battery

you may well be best served replacing the DC t DC charger with a simple voltage sensitive relay and plugging in the solar pannel with its inbuilt regulator when you stop.

On the matter of this solar pannel........there has to be only ONE solar regulator in the circuit...be sure that there is not one on the pannel and another being the ctec.

cheers