Quote Originally Posted by mikeml View Post
Did you measure your alternator voltage as well? This is important. As per Kalahari Safari's post quoting Ctek's reply, the CTEk will have switched off if the battery voltage did in fact draw the charging voltage below 13.1V. If it did do this, then their solution makes sense.

But I very much doubt that this is what happened. It is very unlikely that your alternator is too small, old or faulty. My original Defender 60A alternator voltage dropped to 12.9V when hot, and this would have dropped out the CTEK charging circuit. My replacement alternator sorted this. Your alternator should not drop below 13.5V if in good nick. This could also of course happen (volt drop) if your wiring from the primary battery to your CTEK is too thin.

Looking at your wiring, all looks fine at the CTEK chargers - I disagree with HobieDave - your smartpass has been connected correctly. You do not need to connect stuff to the bottom right terminal - you can run it straight off your battery. I would not mess with the wiring before determining your actual fault. I do however agree with him that it would be better for your second aux battery to be in close proximity of the first one. It simplifies and reduces your wiring, and removes the battery from the hot engine bay - batteries hate heat.

I read the CTEK manual again, and learnt something - in my vehicle I actually only need the Smartpass, not both units, as my alternator is a smart alternator, and I do not need the solar controller. Also, something I did not know is that the Smartpass keeps the cranking battery topped up if the aux batteries are charged. A very handy feature, explaining why my starting battery does not lose any charge even if standing for a few weeks.

But more importantly in your case, it states that "The D250SA optimises the charge voltage by increasing the charge voltage at temperatures below 25C/77F and reducing it at temperatures higher than 25C/77F. The functions is always active." Unfortunately it does not give more details. I assume that it gets this temperature from the temperature sensor, and not from a temperature sensor on it's circuit board.

This is not good news, IMO. I would rather sacrifice some battery life in return for charging up the batteries at a faster rate and knowing that they are fully charged. This also partially explains the problems that I had in the past.

The manual states the following about the temperature sensor in the Smartpass "SMARTPASS 120 protects the battery by switching off charging if the service battery temperature
rises too high
". It would be nice if they gave more details.

What would I do in your case?
1. Check the cable feed to your CTEK, and check the input voltage to your CTEK on a hot day with discharged batteries.
2. Trace your temperature sensors. They exit the chargers at the arrows
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3. Relocate them if necessary.
4. If you are in a jam, and the units shut down and don't charge, short out the 2 connectors between the units. This connects the aux batteries directly to the alternator/starter battery, and does not damage the chargers. I had a cole hersee relay connected across here which I could activate when necessary

Reading more about the temperature sensors, especially from the main charger, I think I would bypass it. I think some testing of the one on the Passthrough would be in order to see at what stage it switches the uit off
Thanks for this Mikemi - sorry I am rejoining the thread quite late this evening!
I did constantly check out my alternator meter during my trip and I never saw it drop below 13.5v so it seems that an underperforming alternator is unlikely to be the issue. My main/cranking battery was replaced a year ago and looks like it is in good shape.
I also had my auxiliary batteries tested and one replaced before my trip so the issue is unlikely to be defective aux batteries.
I think my non-standard setup does need a thorough recheck but I remain convinced that temperature is an issue with a Ctek in normal African summer conditions and your reference to the 25C threshold in the manual seems to confirm this.
Your suggestion about a Cole Hersee solenoid is interesting - will look into it!
Thanks for the clear reference to the temp sensors. I have found the wires coming from these points but I still have to trace where they go in a very congested area.
Thanks!