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Description:

This Tech Tip discusses the use of today’s 60 cell PV modules with Morningstar’s solar controllers. It explains why 60 cell modules are not well adapted for meeting the nominal array voltage requirements of PWM controllers. It also shows how Morningstar’s MPPT controllers can be used with great success at higher input voltages.

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All Morningstar PWM type controllers have a PWM switching frequency of ~300Hz.

Some Morningstar PWM controllers have a feature which will reduce the switching frequency to ~1Hz (called on/off charging). This on/off charging virtually eliminates switching noise (both radiated and conducted) and can be used when experiencing noise problems with 300Hz switching operation.

Current models with this on/off charging option are:

– SunSaver Generation 3

– SunSaver Duo

– ProStar PWM

– TriStar PWM

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Most Morningstar controllers will take up to 5 minutes to realize that the solar input voltage is no longer present before transitioning into the ‘night’ state. The controller performs periodic day/nigh checks (about once every 5 minutes). Depending upon where the controller is in this cycle when the input is removed, it may take anywhere from 0-5min for the controller to turn off its Charging LED and go into the ‘night’ state.

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Morningstar controllers have been designed to mount vertically. This is the orientation in which the heatsinks are most effective. Mounting the controller horizontally will reduce its passive cooling efficiency, but the risk of overheating will be determined by the ambient temperature and the power level at which the controller will be operating.

All efforts should be made to mount the controller vertically.

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Frequently Asked Questions about PWM battery charging are addressed.

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Here’s a brief explanation on how to calculate the temperature compensated adjustments our controllers make to their charging voltages based on the ambient temperature changes.

The charging voltages listed in the controller operator’s manual are specified at a 25C standard test condition ambient temperature with a -.030V (@ 12V battery) adjustment per degree C change in temperature above or below the 25C reference. The (-) in front of the .030V indicates that voltage adjustment is inverse to the change in temperature. In other words, if the temperature increases above 25C, the controller decreases it’s charging voltage by -.030V per degree C rise above 25C and vice versa. An accurate measurement of the ambient temperature is required for calculating the the controller’s temperature compensated adjustment.

Example:

An ambient temperature of 35C = +10C increase above the 25C base temp = 10 x -.030V compensated adjustment = temperature compensated -.300V decrease in charging voltage.
Therefore, a 14.4V charging voltage specified in the manual @ 25C would be reduced to 14.1V @ 35C or increased to 14.7V @ 15C.

The compensation rate is doubled to .060V/C for 24Vnominal battery systems and quadrupled to .120V/C for 48V nominal battery systems.

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Can I use a power source other than a PV panel to charge my batteries?

We do not recommend using a DC power supply with any of our PWM controllers. While they appear to be similar to PV modules in function, DC power supplies have considerably more output capacitance. Connecting a DC power supply to our PWM controllers may cause excessive heating and premature failure. MPPT controllers can be used for this purpose without issue. In addition, AC powered battery chargers are often the best option for systems requiring AC battery charging.

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How do I calculate the effect of temperature compensation on the regulation voltage?

Refer to the accompanying document download, which includes compensation curves for most Morningstar Controllers, or refer to temperature compensation coefficient in the specifications section of the Operator’s Manual.

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Do Morningstar Controllers regulate according to SOC (State of Charge) calculations?

No. Morningstar Controllers regulate according to battery voltage setpoints. Because battery health and other factors affect charging over time, using voltage setpoints is a way to naturally track these changes and affords the user a complete understanding of the controllers operation. SOC calculations and algorithms may not be precise under all conditions and may leave the user with a “fuzzy” understanding of the workings of the unit.

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Can I charge separate battery banks with one Morningstar charge controller?

Yes.  Refer to the document, “Charging Isolated Battery Banks – 1 Controller”.

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Can I wire multiple controllers in parallel off the same PV array to charge different batteries?

Yes. Refer to the document, “Charging Isolated Battery Banks”. This can ONLY be done with PWM controllers. MPPT controllers cannot be used in this configuration.

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I sometimes hear a buzzing noise during charging. Is something wrong?

No. The buzzing sound is mechanical resonance created by the switching of current through the circuit board and solid-state devices. This is normal and not indicative of a problem.

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A higher voltage solar array (e.g. 24V nominal) can only be used with a lower voltage battery bank (e.g. 12V nominal) if the controller used is an MPPT controller.

PWM controllers CANNOT be used to charge a lower-voltage battery bank with a higher-voltage solar array. Please use an MPPT style controller if this is required.

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Our controllers are designed to charge lead-acid batteries which have different charging requirements than NiMh. Please DO NOT use our products with NiMh batteries, it is dangerous to do so.

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The internal switches of a controller, called MOSFETs or power transistors, switch current on and off to control solar current and load current. Because these components are in the power path, the controller will no longer be able to switch current if they fail.

Power transistors can fail for the following reasons:

• Over-voltage: lightning or other high voltage transients on the PV power cables.
• Over-current: too much PV for the controller or a short in the PV line pulls too much currentout of the battery through the controller.
• Extreme temperatures during charging.
• A premature failure of the component. The transistors are solid state devices that are manufactured in huge volumes with extremely low failure rates. However, no electrical components are 100% defect free and can pass test and then fail after a short period of time.
• Old age

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We do not recommend wiring inverters to the load terminals of our products because of current in-rush on start up. Inverters typically have large amounts of input capacitance (for input filtering). When power is first applied, these capacitors can draw very large currents in the 100’s of amps, albeit for a short period of time. These current surges can stress our power transistors and will trip our short circuit protections. The inverter may start eventually after several attempts to reconnect from the short circuit condition, but this mode of operation is not normal. For this reason, we do not recommend wiring inverters directly. Many inverters have their own LVD circuitry and will not allow the battery to discharge too low. However, if the customer requires load control and LVD functionality, they can switch the inverter through a relay wired in series between the battery and inverter.

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PWM is an acronym for Pulse Width Modulation. When the battery is recharged to regulation voltage, the controller will begin limiting the amount of current into the battery so that the regulation voltage is maintained but not surpassed. The method of regulating the current, referred to as PWM, pulses current into the battery with pulses of a varying width. Wider pulses allow a greater percentage of the input current to flow into the battery, narrower pulses restrict current to a lower percentage. Refer to the accompanying download,  “Why PWM?”, for more technical and detailed information concerning PWM charging.

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The Load connection allows the user to wire DC devices such as fans, bulbs, or communication equipment directly to the controller. This allows the controller to disconnect the loads if necessary for protection of the controller, battery, or the loads themselves. This connection is optional and, as an alternative, the system loads can be wired directly to the battery bank.

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Yes. There can be as many Morningstar controllers as required in parallel on the same battery bank. For best performance, set each controller to as close a charge setting as possible. Since the controllers will not have direct communication with each other, it is not unusual to see the controllers transition between charging stages at different times. This is generally not a concern.

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The SunLight controller will have a -2V load voltage measurement when in the LVD state with no load physically connected to the controller.  This is only a residual voltage that will be pulled to 0V when a load is connected.  It is not an actual -2V supply to the load.

This also applies to the SunLight controller during its normal daytime charging state.

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The SunLight has delays before charging begins and at dusk when charging stops. The SunLight uses the solar panel voltage to determine when dusk and dawn occurs. For this reason, the SunLight must be certain that dusk or dawn is actually occurring. The solar input  voltage is sampled and averaged. This process may take up to 10 minutes.

If the panel is connected, it may take up to 10 minutes before charging begins. If the panel and battery has been connected for more than 10 minutes and the SunLight is still not charging, refer to the SunLight testing document in the Related Documents list.

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Pulse Width Modulation (PWM) charging may cause interference in sensitive loads in the system during charging. The SunSaver switches current at 300 Hz. PWM switching noise can sometimes be heard in the controller itself, the wiring or wiring connections, or the system loads. AM radios and CB radios are especially prone to PWM interference. Noise in the controller or wiring is caused by mechanical resonance when current is switched through the circuits. There is little that can be done to remedy this issue. However, noise/interference in the system loads can usually be reduced or eliminated by the following:

1) Minimize cable runs between components

2) Twist power pairs(+/-) to reduce radiated noise

3) Good system grounding

4) Add capacitance across load power(at load input, 22000uf or more)

5) Add a line filter. some have had success with car audio filters that eliminate alternator whine. If these measures do not eliminate or reduce interference to acceptable levels, the controller can be modified to reduce the switching frequency. The 3rd generation SunSaver has a wire loop under the faceplate. If this loop is cut, the controller will reduce the switching frequency to ~1 Hz. Refer to the SunSaver Operator’s Manual for more information.

6) Some Morningstar controllers allow for operation in a ‘low-noise’ state. Check your controller documentation for details.

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When two conductors (a wire and screw terminal for instance) are mated, there is a resistance at that connection. Usually this contact resistance is very low and poses no threat. However, when connections become loose, oxidation builds on the surfaces, or corrosion accumulates, the resistance at the connection can increase to a dangerous level.

When current passes through resistance, power is dissipated in the form of heat. If the value of the resistance goes up, so does the power dissipated (for the same amount of current). When a connection has high resistance, the heat generated at the connection can melt surrounding material such as plastics and actually catch fire.

Resistive connections can occur on any electronic device that carries significant amounts of power. It is important to periodically check the tightness of the connections and inspect for corrosion as suggested in the Maintenance section of the Operator’s Manual. Along with periodically checking the connections, dielectric grease can be applied to the terminals to avoid corrosion build-up on the conductor surfaces. Mobile installations tend to be the most susceptible to developing resisitve connections because motion works cables loose from their terminals, however, resistive connections can occur on stationary installations as well.

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All Morningstar controllers will automatically prevent reverse discharge of the battery back through the solar array during the night. No external diodes on the input of the controller are required.

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Other charging sources can be used in parallel with the Morningstar charge controller; you do not need to isolate the controller from the battery. For best performance when using other charging sources, ensure the charging source and Morningstar controller are set to charge to the same (or close) voltage. If the supplemental charging source is set to a higher charging voltage than the Morningstar controller, the Morningstar controller may go into a fault state temporarily when the battery voltage rises higher than the controller’s setpoint. The controller will automatically recover from this state when the battery voltage drops to a lower level.