Short: If it is uses standard PV panels your system is probably a nominal 24V system with max power being produced at ~=3D 36V DC. You can safely connect the PV panels to the battery via a diode. You will get about 30%-40% of the charge current that and MPPT converter will provide without incurring the high power dissipation that basic calculation may lead you to expect. When loaded to a voltage well below the Vmp (maximum power voltage) the panel will look be close to being a current source and Vpanel will be Vout-Vdiode with no need to drop the "extra" voltage. An MPPT regulator that is as RFI-noisy as you describe probably violates your local RF regulatory requirements. Loading the panel when Vpanel falls below dusk levels will possibly stop your after-dusk RFI problems. ____________ Longer: There is no reason for an MPPT regulator to be inherently noisy - it's a matter of implementation and how much the supplier cares. The interference that you describe almost certainly contravenes regulatory requirements in most places on earth. That said, I have no practical experience of large-system MPPT interference issues and it may be that nobody cares and they all do it, but this seems unlikely. MPPT necessarily involves some form of switching regulator to accept energy at one voltage and output it at a lower voltage. There are a few systems which upconvert (including the portable systems using the Intivation boost converter IC that can accept one cell / 0.5V or 4 cell / 2V input - these are found in most cellphones that have integral solar charging capability and in the Wakawaka solar light) BUT almost all systems over a few watts will downconvert using a buck converter. A buck converter can be about as quiet as any smps going (except perhaps resonant types) but issues such as diode switching noise can be a problem at the high currents involved. It is also possible that there are "sub harmonic" effects such as are experienced with GSM systems where the characteristic loud audio break through signals are caused by a low frequency frame rate signal and not the main carrier. Re your systems nominal voltage rating: A 12V system usually is specified at 18V Max power with 36 PV cells and a 24V system is usually 36V max power with 72 cells so I'd expect that your 45 V_oc system is in fact a nominal 24 volt system. 45V OC / 72 cells =3D 0.625V/cell OC - which is higher tha= n I'd expect, but possible. At say 0.55V/cell OC that would be 45/0.55 =3D 81 cells which is a non standard value, but also possible. A 36V system will usually have 108 cells or 45/108 =3D 0.42V/cell OC which is definitely far too low. Regardless of why the MPPT system is causing the problem, it should be possible to bypass it using direct panel to battery connection (via a diode) if charging requirements are modest. As others have noted, your 45 V O/C system will be inefficiently used when direct connected. If it is a 24V system rated at 175W it will be specified at 36V max power and 175/36 =3D 4.9A I_max_power. If you connect the panels directly to the battery via a suitable diode you would expect about 5A in full sun, whereas the MPPT converter would provide about 12 or 13 amps (174W/12.5V x 90% efficient =3D 12.6A). So the ~=3D5A is about 35%-40% of what you'd get using MPPT. Curren= t using direct connection is slightly greater than Imp into the MPPT system as Isc is > Imp and as you load the panel beyond optimum current will asymptote towards Isc as Vout drops. Note that you can safely connect the PV panels to the battery via a diode without incurring the high power dissipation that basic calculation may lead you to expect. When loaded to a voltage well below the Vmp (maximum power voltage) the panel will look be close to being a current source and Vpanel will be Vout-Vdiode with no need to drop the "extra" voltage. You could easily have a system that switches between direct connection and MPPT operation as required - just switch in a diode from panel to battery as required, shorting out the MPPT controller. The MPPT controller MAY include an external diode (a good one will not) and you can connect on the load side of that, provided that the MPPT controller does not back feed from an internal voltage source (probably not). _________ The interesting action at 12.42 volts is probably caused by either - 1. the MPPT controller making a decision about battery max voltage and switching between boost and float modes. or 2. You say "about an hour after dusk" but the light conditions are not clear from this. The panel may be making a very small voltage and the MPPT controller may be valiantly trying to provide charge and this may be the point where the panel input energy is inadequate to maintain MPPT overhead and the system goes unstable as it seeks to optimise an input which is severely impacted by the attempt to do so. In normal operation a PV panel operates at close to constant voltage. At extremely low light levels Voltage rises from zero with light level and typically reaches something like operating voltage at 1000 lux if unloaded. This will vary with panel PV material. 1000 lux is 1% of full sun. A 36 V Vmp panel will output say 14v at about 400 lux, MAY have about 500 mW of power available and will probably tempt an MPPT converter to try to use it. Odds are it will simply load it down and drop out, and then start again. Check: What is panel voltage when post-dusk RFI mode occurs? What happens if you then completely block light to the panel or short the panel? If this is what is happening then this can be prevented by shorting or heavily loading the panel input (on the panel side of any battery isolation diode ! :-) ) shortly above the voltage level where it goes into its post-dusk RFI mode. This could be accomplished with two transistors, one modest load resistor and a few glue resistors. Arrange a circuit to load panel with say a 10 W resistor when Vpanel falls to near V_RFI. Determine suitable R by test. A slightly more sophisticated system which shorted the panel at Vmin and unshorted it at Isc > Imin would be easy enough and more effective. Easier/better still if possible is something that disables the MPPT when Vpanel is < V_RFI + safety margin. The above extra load could be activated by a light sensor independent of the system proper. A few PV cells would suffice. Beware differential aging die to different UV degradation affects on sensor and main panel. Russell McMahon --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .