Deficit Cycling equals Dead Batteries

In the last article, we mentioned the term deficit cycling.  But what is deficit cycling?  And more important, how does it affect you and your solar array?

Deficit cycling refers to a situation where the consumption of the batteries stored energy exceeds the energy production of the solar module.  This will cause the batteries capacity to slowly drain down towards zero.  Once that happens, you have a dead battery on your hands.

Here is how it works.  When you first get your battery, it is boosted to full capacity.  Let’s say this battery capacity is 200Ah for this example.  At a 50% Depth of Discharge, your available battery capacity for use is 100Ah.  With that in mind, you go ahead and use 100Ah.  Now, lets assume your solar module can only replace 90Ah worth of energy in a day.  This leaves your battery at a 10% deficit for that day (based on Depth of Discharge only).  The next day, you use 100Ah of battery capacity.  Again, the solar module can only replace 90Ah.  At the end of the day, your battery is at a 20% deficit.  In two more cycles, your batteries will be at a full 40% deficit.  A few more beyond that and the battery will begin to drastically reduce in performance and reliability.

An easy way to think about deficit cycling is like your bank account.  If you withdraw more funds than you are putting in, eventually you will have a zero balance.  The problem with batteries is that a zero balance means you finished off the battery and will need a new one.