Vehicle Charging Through Solar Regulator

The Ctek D250S is not suitable for vehicles with variable voltage regulators.
I just went through this when installing a DC-DC in the tray of my Dmax. I had to go with a Redarc 1240-LV, but the Redarc 1225-LV is also suitable.

John

Couple of things in this thread.

1. I am curious about (but not disputing) the problem referred to by evaredy regaring "variable voltage regulators" and why that would upset the Ctek alternator input (evaredy did not elaborate but I assume the Ctek was dropping in and out of charging ??)

2. I can feed as low as about 12V from a power supply into the solar input of the Ctek (which is MPPT as far as I know isn't it ??) and it will cut in and charge the batteries - I have not tested how much current it will take through the solar input (as in it max's at 20A on the alternator input but not sure if same applies to the solar input).

If it is MPPT will it not just derive as much power from the input as it is designed to do within it's designed limits of voltage and current or do I have the wrong theory about MPPT .... accepting also that not all MPPT may operate exactly the same.

Would solar not at times drop as low as 12V due to environmental conditions and the MPPT controller still generate as much as it can from the voltage available ??

Ken,

Let me say at the outset that I've no personal experience with the Ctek. It's highly regarded and seems a good bit of technology combining a number functions in a very effective way.

1) The variable voltage alternators being fitted to some recent vehicles drop the charging voltage down low when the engine is idling to reduce engine load, hence fuel consumption and emissions. The Ctek D250S is supposed to drop out if the alternator voltage drops below 12.8V for 10 seconds.

2) The Ctek D250S does include an MPPT controller. I'm very surprised that it will provide any charging current with only 12V on the solar input terminals. I doubt that it's a significant current. It may cut in, but unless it somehow cross feeds into the dc-dc charger, the low "solar" voltage (12V) simply isn't high enough to provide any charging. To provide charging that 12V needs to be increased, and that is not the function of most MPPT controllers. I don't have a D250S, so hope someone else may be able to shed some light on this one.

"If it is MPPT will it not just derive as much power from the input as it is designed to do within it's designed limits of voltage and current or do I have the wrong theory about MPPT .... accepting also that not all MPPT may operate exactly the same." Ken, those design limits are unknowns! Generally, an MPPT controller accepts the energy from the panel and loads the panel (ie draws current) sufficient to pull the panel voltage down to a level where the power from the panel (voltage x current) is maximised. (This occurs for most "12V" panels at about 17-18V.) It then converts this voltage and current to suit the battery's requirements. (Typically 14.4V with a corresponding increase in current. It is this ability to trade the excess voltage for extra current that makes MPPT so attractive.) With very rare exceptions, solar controllers will always need to convert the voltage DOWN and consequently lack the ability to convert upwards. Converting up (as a dc-dc charger does) uses similar techniques, but the components are arranged differently. The D250S is an interesting combination - a down converter for solar and an up converter for the alternator, no doubt sharing as much circuitry as possible. Maybe I'm overlooking something. Could someone who has one and has done the measurements please comment - can the box increase a low voltage on the solar input to a charging voltage on the output?

Ken your last question is a good one! The controller will maintain the load on the panel at such a level that the panel operates at the voltage where it delivers maximum power. This means that for most "12V" panels the panel should be delivering current at about 18V. The critical fact is that a solar panel should be thought of as a source of current, not as a source of voltage. Unloaded (zero current) the panel voltage will typically be about 22-24V, whether in full sunlight or moonlight. The current it can deliver though is very different in sunlight from what it is in moonlight! The MPPT controller's first job is to draw sufficient current to load the panel voltage down to the panel's power optimum at about 18V. It's second function is to take this energy and convert it meet the battery's requirements.

Hope that makes sense!

Cheers

John

Thanks John - good info and explanation - I know it digresses a bit from the original query but these things get me curious, and finding much info on design and operation of various makes and models can be hard to find.

I guess an MPPT controller is primarily designed for solar and mulling this over in my mind after I posted it I started to consider that yes the solar panel is pretty much a constant current source over a range of voltage until it hits it's "knee point" around 17 - 18 V and also that lack of sun only reduces current output and not voltage. So until there was no current available, the MPPT would always be operating at around the 17V to 18V at it's input with whatever current was available in the conditions.

As you say, putting any other kind of supply on the solar input and what happens as a result woudl purely be up to whatever the particular MPPT design let it do. I hve been digging around for some circuit diagrams for MPPT and have seen some that are simply a variable regulator that would never work with less voltage in than you want out.

One day I might be able to get around to experimenting more with the Ctek to satisfy my curiosity about how it does actually behave under those conditions. I won't swear to it, but I am pretty sure a quick check I did once on the D250S showed the output to battery at a higher voltage to what I had on the solar input, which would tend to indicate it is actually doing a DC-DC conversion on the solar input. But I need to do some more serious testing before I can claim that.

If you want to know more about the Ctec D250S may I suggest you give Battery +Business - Warriewood NSW a call. They were referred to me so I went in there last week and their service and knowledge on this stuff was the best I've experienced so far. 02 9970 6999