Can A Charge Controller Be Too Big?

Determining the size and type of charge controller to deploy in an off-grid solar power system is the last step in designing the system. Charge controller calculations use a 25% over capacity factor to cope with optimal power generation conditions.

Bigger is not better when it comes to charge controllers. The cost of a charge controller increases by 20% per 20A increase with no benefit to the efficiency of the system. The additional capacity factor of 1.25 over the desired charge current provides sufficient protection to the battery bank.

The process steps required to determine the correct size charge controller must be followed before purchasing any solar system components.

  1. An accurate assessment of the daily energy demand of the household needs to be done to define the Watt-hours per day needed.
  2. The battery capacity size to supply the Watt-hours needed per day needs to be determined.
  3. The size of the solar panels needed to generate the required power needs to be calculated. Watts needed per day.
  4. A solar charge controller must be selected to convert the solar power generated into the optimal charge current (A) and voltage (V).

Let’s look at how this process is followed to ensure that the solar system is designed for optimal efficiency and longevity.

Solar panel setup engineers controlling a solar system

How To Correctly Size A Charge Controller

The solar power produced in Watts must be converted to battery charge power in Watts in the most efficient manner to avoid conversion losses and charge the battery bank at the voltage (V) and current (A) specified by the battery manufacturer.

PWM Charge Controllers must always be sized such that the solar panel output voltage is balanced with the battery voltage requirements. If the charge controller charge amperage is too high, the charge voltage will be too low, and the battery will not charge.

For MPPT solar charge controllers, the optimal battery charge current must be matched with the output current of the charge controller. Power input from the solar panels in Watts must equal the battery charge power needed at the specified voltage and current.

Power (W) = Voltage (V) x Current (A)

The total solar power of the solar panels in (W) has to be brought in balance with the optimal battery charge voltage (V); therefore, we can calculate the required output current (A)of the charge controller.

Current (A) = Power (W) / Voltage (V)

This output current will define the maximum Amperage rating of the MPPT charge controller. If the maximum Amperage rating is higher than the optimal current needed, the charge controller can still be used but will be programmed to operate below the maximum Amperage.

This inefficient use of the capability of the charge controller will cost more money with no advantage to the system. Selecting the correctly sized charge controller will enable you to divert funds to invest in better-quality batteries, connecting cables, or solar panels.

Assume that you have determined that you will need two 200W solar panels to charge your 12V battery bank during the day. To determine what size MPPT charge controller to install we determine the maximum current potential as (200W+200W) = 400W / 12 V = 33A.

An MPPT charge controller of 30A will be required to protect the battery and optimally charge and protects the battery from overcharging.

Which Components To Oversize In An Off-Grid System

Needlessly spending money on an oversized charge controller is a waste of your project budget. The best areas to invest your funds are in the size of the solar array and the battery bank.

The size of the battery bank must be able to cope with the power demand of the appliances, lights, and electrical components of your household. Planning the battery bank to have surplus storage capacity is wise as the household power demand can expand with electric vehicle charging stations and more modern electric appliances.

Investing in surplus solar power capacity is also recommended as the array’s efficiency will vary during the day with the intensity of the sun or as weather conditions dictate. Adding some power generation capacity as a redundancy measure is advised.

Alternative modes of power generation that will augment the solar array are wind-power, hydro-power, or gas-powered generators. Investing in some redundancy power generation is strongly advised if you live in northern latitudes with long winters and unpredictable weather.

Additional solar arrays or modes of power generation will feed into dedicated charge controllers as their voltage, and current outputs will have to be individually matched to the needs of the battery bank.

Such separate power sources cannot be added to an existing charge controller but rather to dedicated charge controllers linked to the battery bank in parallel.

See also: What A Solar Charge Controller Does (Explained)

Where Size Matters On A Charge Controller

The primary sizing of a charge controller is to determine the current load on the battery bank at the optimal charge voltages. Once this determination is made, you can look at other aspects of the charge controller, such as its size and sturdiness.

The cable connections between the solar array and the battery bank to the charge controller must be very secure.

The connection terminals on a charge controller must be large enough to accept 6AWG or 8AWG cables to charge the battery bank. Some highly regarded and expensive brands of charge controllers often have connection terminals that are too small and flimsy.

The size of the PV cabling from the solar array is most often 10AWG, but the current load between the battery and the charge controller is likely to be up to 6AWG thick. The terminal connections must be large enough and strong enough to accept and tighten up the connection between the controller and the battery bank.

Make sure that the charge controller selected as an interface not only has the correct Amperage rating but is also fitted with large connection terminals. Undersized terminals or poor connections can be a source of heat buildup and potentially lead to an electrical fire.


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Elliot has 20+ years of experience in renewable technology, from conservation to efficient living. His passion is to help others achieve independent off-grid living.

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