Thursday, Jun 06, 2013 at 20:15
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 | J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
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