Solar Charge Controller

So, you have heard the term Solar Charge Controllers and have been part curious and part unclear about the terms, benefits and cost variations in them. Well, lets try and understand them all in a gradual easy to understand manner.

MPPT Solar Charge Controllers
MPPT Solar Charge Controllers

A solar charge controller, sits between the solar photo voltaic (SPV) panel and the battery. Its primary job is to deliver the power received from the SPV panel for storage into the battery for utilization later. We know, batteries need careful control of voltage and currents for their effective operation and life. By now, it is evident that we can’t simply connect the batteries to the panel if we are to expect performance and life from it.


Category-1 ON-OFF Charge Regulators:

In an ON-OFF Charge regulator, the battery voltage is monitored, and solar panel is connected to the battery when the battery is not full (determined by battery voltage). If the battery voltage exceeds the threshold / limit, the panels are disconnected from the battery to protect it from over charging. Most ON-OFF charge regulators also have a load switch, which disconnects the load if the battery falls below the “Battery Empty” limit.

Typically, this is the cheapest Solar Charge Regulators around.

Pros Cons
 Over voltage and Low voltage cut off  No battery charging (Constant Voltage, Constant current)
 Cheap solution  Inefficient, does not provide adequate charging, needs larger panel size


Category-2 PWM Charge Regulators:

PWM charge regulators are a significant improvement over the ON-OFF Charge regulators; in that they throttle current supplied to the battery when a battery set regulation point (based on the battery chemistry) is reached. This reduces the harmful gassing effect of the battery. We can say PWM charge regulators (should) have the ability to monitor the battery current and battery voltage. They control the current presented to the battery and good PWM Charge Regulators (should) implement a CC = Constant Current and CV = Constant Voltage charging algorithms.

Pros Cons
Implements Battery CC, CV charging algorithm Does not optimize for panel characteristics
Reduces gassing effect on the battery and improve charging efficacy over ON-OFF CC Subjects the battery to switching spikes, that reduces the life for the battery*


Category-3 Maximum Power Point Tracking (MPPT) Charge Regulators:

So far in the charge regulators, we discussed, we ignored the solar panel characteristics and assumed, it will be delivering the all the power generated for the given lighting and temperature conditions. In fact, nothing could be further from reality.

Panels have a mind of their own! so to speak…

When harnessing power from the panel, neither maximum voltage nor maximum current have any significance. Its always about power i.e., P = V x I.

Power at that instant… so it becomes P(t) = V(t) x I(t)

Putting the words of MPPT together,

1. Maximum power point: The instantaneous values of voltage and current that yield the maximum instantaneous power at given conditions. (Power is calculated and not measured like current and voltage)

2. Tracking – the activity carried out to identify/locate “Maximum Power Point” of the panel at the given irradiance and temperature conditions.

So, we have located the maximum power point alright, but if it can’t be used it to charge the battery, it is of no practical use. We need a “power transfer” mechanism, to transfer this maximum power into the battery. Typically, panel voltage is chosen to be higher than the battery (bank) voltage. So, the panel voltage has to be lowered to that of the battery bank. Since the power into and Power out has to be same (Assuming zero conversion loss), the current going into the battery will be higher than the current delivered from the panel.

There are 2 important efficiency parameters that are to be considered.

Tracking Efficiency:

How quickly and accurately does the tracker react change in irradiance and temperature conditions?

This number is generally around 97% to 99%. This only means you can detect the peak power point quickly – and this is important. But not sufficient to decided on the charge controller.

Conversion Efficiency:

How much power is transferred from the panel to the battery or load? (What is the conversion or converter loss?). This number ranges from 90% to 97%. This number is a even more crucial. If this efficiency is < 85% it is a bad controller and you should not consider buying it.

Pros Cons
Implements Battery Constant Voltage and Constant Current charging algorithm  Expensive compared to the ON-OFF or PWM chargers.
Reduces gassing effect on the battery and improve charging efficacy over ON-OFF CC
Inductor or Transformer at the output side of the CC reduces the current spikes and presents a smooth current, extending the life of the battery.  –
Ensures specified panel life as cells are not stressed by operating at the Peak Power point

GreenTronics Design Labs makes high quality, efficient and reliable MPPT controllers. You can view the range of products here