Fully Charge your Batteries for Max Life

As you may have noticed, several of our articles state to always fully recharge your batteries.  In this article, we are going to explain why this process is critical to battery maintenance.   

During normal discharging, soft lead sulfate crystals form on the lead plates inside a lead-acid battery.  As the battery is recharged, these soft lead sulfate crystals are removed from the lead plates.  If a battery is left in a discharged condition or simply being undercharged, the soft lead sulfate crystallizes into hard lead sulfate.  Unfortunately, hard lead sulfate cannot be removed during recharging.  Thus, the surface area of the lead plates becomes reduced.

The storage capacity of a battery is based on the available surface area of the lead plates.  As that surface area reduces, so does the storage capacity.  Eventually, the surface area can become so reduced, that a battery will no longer accept a charge, rendering it lifeless.

Failing to fully recharge a lead acid battery is estimated to cause approximately 85% of deep cycle lead-acid battery failures.  When it comes to battery maintenance, this is a priority.

As always, take care of your off-grid array and it will serve you well for years.

The Power of Batteries

When it comes to batteries, a common problem is a misunderstanding of the actual power available from the battery.  Unfortunately, clever marketing can perpetuate misleading information.  Before you invest in an off grid array let’s take a minute to discuss the real power of batteries.

All batteries are generally rated by their maximum capacity.  For example, a battery with a voltage of 12 and an ampere-hour (Ah) rating of 100 will be listed as having 1.2kWh (kilowatt-hours) worth of capacity.  This is the accepted practice to standardize the way in which batteries are rated.  However, this rating doesn’t mean that this is the capacity available for use.

Depending on the battery type we choose, our battery will not be able to produce its maximum rated capacity.  Let’s use a lead-acid battery for off-grid solar as an example.  Remember from our earlier article, “Top 5 Battery Mistakes” we do not want to go below 50% depth of discharge. We do this in order to insure our batteries last as long as possible. Therefore, using our battery example above, using a 50% depth of discharge we have roughly 600Wh (watt-hours) worth of capacity available for use.

Here is where most off-grid designs fall short.  Even though we aren’t overtaxing the batteries on paper, we have left out a very important detail.  That detail is how we plan on consuming the power from the battery.  It is true that our battery from above has a rated capacity of 1.2kWh.  But, what we must realize is that the rating is based on a C/20 rate or 20 hour charge rate.  In simple terms, this means that we have 1.2kWh if we consume that capacity over the course of 20 hours. That gives us roughly 60Wh worth of power each hour for 20 hours.

This nuance of batteries generally catches people off guard.  If we consume more than 60Wh each hour, the available capacity of the battery reduces.  We have to remember that batteries use chemical reactions to produce and store energy.  When we speed up the chemical process, we reduce the efficiency of the reactions.  This means our battery’s true capacity will be reduced during higher power draws.  If we consume our power over the course of 10 hours, the battery capacity will reduce down to around 90% of its rated capacity.  In our case, our 1.2kWh battery is now a 1.1kWh battery.

The biggest change we will see in a battery’s capacity is when we consume the bulk of our power in a single hour.  When a battery is required to give the bulk of its energy stores in one hour, its capacity will reduce to around 60% of its stated rating.  This is a significant change in the battery’s capacity.  The same 1.2kWh battery from above is now an 864Wh battery.  Remember, we still do not want to consume more than 50% of the available battery capacity.  Therefore, we have now effectively dropped our consumable power from 600Wh down to 432Wh.

By better understanding how batteries capacity is affected by our energy consumption, we can prolong the life of the battery.  Most early battery failures are the result of over-discharge.  Take care of your batteries and they will reward you with long lasting life.

To MPPT or to not MPPT, that is the Question

Do I need a MPPT charge controller for my system?  This is a question that comes up for any off-grid solar array.  And it’s an important question not just for your wallet, but your system as well. 

Remember, the whole goal of the array is to charge your battery bank.  Therefore, we want to harvest as much amperage from the array to fully charge our battery bank.  It doesn’t matter if you do this with or without an MPPT charge controller.  What matters is that your batteries always come back to a full charge as well as never dip below 70% depth of discharge.

So how does an MPPT help in the above goal is the real question?  First of all, what is an MPPT charge controller?  A maximum power point tracking (MPPT) charge controller is a device that can change its internal resistance in order to output the maximum possible power from your solar array.  It does this through algorithms used by a DC to DC converter.  In simple terms, this means that the charge controller can take any excess voltage your array produces and convert that into a higher amperage output.  And again, our goal is to harvest as much amperage from the array to fill our battery banks amp-hours.

In any off-grid design, the array voltage is always higher than the battery bank nominal voltage.  This allows for the charge controller to provide enough voltage for the different charging cycles.  For example, a 24 volt battery bank will require around 28 volts for the absorption charging cycle.  Also, the voltage of the array has to be increased to account for voltage drop and temperature change as well.  With this in mind, there is always excess voltage to be used for the MPPT charge controller to carry out its function.

Why wouldn’t I choose an MPPT charge controller?  Right now they sound awesome.  And they are awesome.  In reality, any off-grid design can be done without them though.  Every design simply has to pair the modules with the battery bank.  In areas or at times with less sunlight hours, the array size and battery bank size just has to be increased.  However, there will be a point where the costs of increasing the array size and battery bank exceed the cost of an MPPT charge controller.  In general, if you are in an area or time of year that has limited direct sunlight hours (2-3 hours/day), an MPPT charge controller can really benefit you.

Take your time planning your off-grid array.  In the end you will save money and have a great performing array that last a long time.