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.