Charging a battery: Multi-stage charging

But before we dive directly into Chapter 3: Functions and features of a solar charge controller, we’d better take a look at necessary information about charging a battery.

If you are already quite familiar with this piece of information, you could jump to chapter 3 from here.

Pour water into cup

Pour water into a cup

2.1 Brief interpretation

Imagine pouring water into a cup – at the beginning, you will pour at a faster rate; when the cup is close to full, water flow slows down so that the water will not overflow from the cup. On the contrary, if you keep pouring water at a faster rate, it’s hard for you to stop the flow in time at the end, and water will overflow from that cup.

Charging solar battery

Charging solar battery

The same theory applies to charging a battery:

  • When the battery is low, the charge controller delivers lots of energy for a quick charge
  • When the battery is close to full, it slows the charger by regulating its voltage and current.
  • When the battery is full, it sends only a trickle of power to keep a full charge.

This is the so-called multi-stage charging.

2.2 Example: 3-4 Stages

Set Points:

In order to make sure you can easily understand the following content, which refers to an example of multi-stage charging (3-4 Stages), let’s firstly explain the jargon “set points.”

In brief,

the solar charge controller is set to change its charging rate at specific voltages, which are called the set points.

Set points are usually temperature compensated, and we will discuss this topic after the example of multi-stage charging.

Now, let’s go through the example in detail

The following is an example from MorningStar, which has 4 stages of charging.

MorningStar 4 stages of charging

Source: MorningStar, 4 stages of charging

2.2.1 Stage 1: Bulk Charge

At this stage, the battery bank is low, and its voltage is lower than the absorption voltage set-point. So, the solar charge controller will send as much available solar energy as possible to the battery bank for recharging.

2.2.2 Stage 2: Absorption Charge

When its voltage reaches the absorption voltage set-point, the output voltage of the solar charge controller will keep a relatively constant value. Steady voltage input prevents a battery bank from over-heating and excessive gassing. Commonly, the battery bank could be fully charged at this stage.

2.2.3 Stage 3: Float charge

As we know, the battery bank is fully charged at the absorption stage, and a fully charged battery cannot convert solar energy into chemical energy anymore. Further power from the charge controller will only be turned into heating and gassing, as it is overcharging.

Trickle from tap

Trickle from tap

The float stage is designed to prevent the battery bank from long-term overcharging. At this stage, the charge controller will reduce the charging voltage and deliver a very small amount of power, like trickles, so as to maintain the battery bank and preclude further heating and gassing

2.2.4 Stage 4: Equalization charge

The equalize charge uses a higher voltage than that of absorption charging, so as to level all the cells in a battery bank. As we know, batteries in series or/and parallels constitute a battery bank. If some cells in the battery bank are not fully recharged, this stage will make them all fully recharged and complete all the battery chemical reactions.

Boiling water

Boiling water

Since it follows stage 3 (when the battery bank is fully recharged), when we raise the voltage and send more power to the batteries, the electrolytes will look like they are boiling. In actuality, it is not hot; it is hydrogen generated from the electrolytes, producing a lot of bubbles. These bubbles stir the electrolytes.

Stirring the electrolytes regularly in this way is essential to a flooded battery bank.

We can consider it a periodic overcharge, but it is beneficial (sometimes essential) to certain batteries, such as flooded batteries and not sealed batteries, like AGM and Gel.

Commonly you could find in battery specifications how long the equalization charge should last, and then set the parameter in the charge controller accordingly.

2.3 Why flooded battery banks need equalization

In short,

to preclude the sulfation of a lead-acid battery.

chemical reaction of discharging

The chemical reaction of discharging

The chemical reactions of battery discharging generate soft lead sulfate crystals, which usually are attached to the surface of the plates. If the battery keeps working in this kind of condition, as time passes by, the soft sulfate crystals will multiply and become even harder and harder, making them pretty difficult to convert back to soft ones, or even further activate materials that were a part of the electrolyte.

The sulfation of lead-acid batteries is the scourge of a battery failure. This issue is common in long-term, undercharged battery banks.

If charged completely, the soft sulfate crystals can be converted back to active materials, but a solar battery is seldom fully recharged, especially in a not well-designed solar PV system, where either the solar panel is too small or the battery bank is oversized.

Sulfation of lead-acid battery

Sulfation of lead-acid battery

Only a periodic overcharge at high voltage can solve this problem; namely, equalization charging, which works at high voltage, generates bubbles and stirs the electrolyte. That’s why stage 4 is essential to a flooded battery bank. In many off-grid solar systems, we usually use a generator + charger to equalize the flooded solar battery periodically, according to the battery specification.

2.4 Control set points vs. temperature

Since absorption set-point (stage 2), float set-point (stage 3) and equalization set-point (stage 4) all can be compensated for temperature if there is a temperature sensor, we would like to spare some words for this little topic.

In some advanced charge controllers, multi-stage charging set-points fluctuate with the battery’s temperature. This is called a “temperature compensation” feature.

The controller has a temperature sensor, and when the battery temperature is low, the set point will be raised, and vice versa – it will adjust accordingly once the temperature gets higher.

Temperature sensor probe

Temperature sensor probe

Some controllers have built-in temperature sensors, so they must be installed in proximity to the battery to detect the temperature. Others may have a temperature probe that should be attached to the battery directly; a cable will connect it to the controller to report battery temperature.

If your batteries are applied to a situation where temperature fluctuation is larger than 15 degrees Celsius every day, adopting a controller with temperature compensation is preferable.

2.5 Control set points vs. battery type

When we come to battery type, another article about solar batteries is recommended.

Most solar power systems adopt a deep-cycle, lead-acid battery, of which there are 2 types: flooded type and sealed type. A lead-acid flooded battery is not only economical, but also prevalent in the market.

Various solar battery types

Various solar battery types

Battery types also affect the design of set-points for solar charge controllers; modern controllers have the feature to allow you to select the battery types before connecting to a solar power system.

2.6 Determining the ideal set points

Finally, we come to the theory about determining the ideal set points. Frankly speaking, it is more about equilibrium between quick charging and maintenance trickle charging. The user of a solar power system should take various factors into consideration, such as ambient temperature, solar intensity, battery type and even home appliance loads.

It is necessary to only cope with the top 1 or 2 factors; that is enough in most cases.