Fine tuning battery charge settings

batteryOne spring, several years ago, we were going to be away from home for a few months on a seminar trip.  In preparation for the trip, I pondered how best to prepare my battery bank for such a long period away from my watchful eye.  I had good reason for giving this some thought, as a previous multi-month trip had left my batteries dangerously low on water and I didn't want to repeat that experience!

There were two main concerns I had about leaving the batteries home alone for months, and gaining an understanding of these has helped me in fine tuning my charging settings:

  • Self discharge
  • Water usage


Self Discharge

Most common batteries will self discharge over time, especially lead-acid batteries such as I am using.  The rate of self-discharge can vary from less than 1% per day with newer batteries to 3%+ per day with old, well worn industrial batteries.  Mine is fairly new industrial type battery (basically a forklift battery), so it would fall into the 1% per day range.  When at home or gone for a week or two this would never be a concern, but being gone for three months would leave barely 10% in the battery--at best.  The lowest my battery should EVER get is 20% and to get more longevity, I try to not let it fall below 50%.  So 10% is NOT an option!  That means I need to let my solar panels charge the battery while I am gone.  They will keep it topped off so that when I return, the battery will be full.  Simple solution!

If I had power lines nearby and chose to be hooked up, the charger on my inverter could easily handle this.  My Magnum MS series inverter has a "Full Charge (battery saver) mode" which keeps the battery topped off without excessive charging.  Another method useful for locations with no grid power would be using an inverter's AGS (automatic generator start) function to start the generator, charge the batteries, and turn off the generator once the battery gets down to a certain point.  This option, while neat and useful, is less desirable in my opinion due to potential complications (what if generator doesn't start, generator malfunctions, etc).  The solar option is the best fit for me.


Water Usage

Lead-acid batteries do use water, especially when left sitting for extended times with no loads consuming power and solar panels pouring electricity into them day after day.  So a certain amount of water usage is normal, but there are things we can do to fine tune the settings so they don't use more water than needed.  One of these would be adjusting the absorption set point and absorption time.  To understand this we first need a little theory.

When charging our batteries with either the solar panels or a generator, any decent charge controller (for solar) or charger (for generator) will have three different stages.

  • Bulk
  • Absorption
  • Float

The bulk stage is where the charger is throwing everything it has at those batteries, trying to raise the voltage as quickly as possible and charge the batteries.  Once the batteries reach a certain set point, called the absorption set point, our charger switches into absorption stage.  In this stage, the charger maintains battery voltage at that set point for a period of time (usually 2+ hours) and in order to do this, it needs to begin slowly backing off on the amount of power it is pouring into the batteries.  Once the end of the absorption stage is nearing, it will have had to back off quite a bit on the charging taking place and at the end it finally switches to the float stage where it maintains a much lower for a period of time (often 2 hours).

Okay, now we understand what the absorption set point and absorption time are.  Battery manufacturers who cater to renewable energy often provide a voltage or range of voltage recommended for the absorption set point as well as a recommended time frame for the absorption stage.  My battery manufacturer recommends an absorption set point of 14.4 to 14.8 (for 12 volt systems--double it for 24 volts or multiply by 4 for 48 volts).  So I started out at 14.8 and set my absorption time to the recommended 2 hours.  All seemed fine until I left on my long trip and returned to batteries that were quite underwatered.  After some research and talking with the manufacturer, I discovered the concept of dropping the absorption set point by 0.1 volts until water consumption is under control.  We want the set point to be as high as possible without using an excessive amount of water, so it may take some trial and error to discover what works best for your setup.  The other factor to consider is that we still need out battery to be fully charged and lowering the absorption set point is going to potentially impact that.  So it is important to check the specific gravity of each cell in the battery with a hydrometer after taking your adjusted settings through a complete trial run (contact battery manufacturer for specs on what this should read when fully charged).  If you find that the battery is not fully charging with the new settings, you have two options:  raise the absorption set point voltage or increase the absorption time.  Since we have already found the higher voltage to use an excessive amount of water, we will try lengthening the absorption time until we end up with a fully charged battery.  If that still doesn't do the job then you may have to raise the set point voltage and just realize that your battery uses more water than most.  But we really want to avoid that because every time we lose electrolyte from the battery and have to add distilled water, we are watering down the electrolyte.  So try lengthening the absorption time first.


  • John

    July 7, 2015

    Thanks Nick, this has been a concern of mine for when I get my system completed and will be in Florida for about 1/2 the year. Have you checked into the automatic watering systems? I see you don't want to add any more water than necessary but it appears an automatic watering system would be a good back-up.

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