Winter Battery Maintenance

Winter Battery Maintenance

Fall is here!  That means it’s time to think about winterizing our cottages.  Every year we are asked about what to do with those battery banks.  Should you take them out, or leave them in place for the winter?

Taking batteries out and maintaining them over the winter in a nice, warm garage is certainly the best thing you could do to preserve your battery bank investment.  However, for many that option just isn’t practical or realistic.   In cases where you need to leave them behind, this bit of information may help you rest easier knowing that you did everything you could to keep them safe.

The two destructive elements affecting battery health over the winter are temperature and state of charge.   The chart right below shows the relationship between temperature and battery state of charge as they relate to batteries freezing.  We added a vertical, dotted black line to indicate the 60% minimum state of charge recommended by Trojan Battery Company to prevent freezing.  We also added a vertical, dotted red line to indicate the minimum state of charge recommended at any time for a wet lead acid battery bank to ensure long battery bank life.

Let’s look at ways to mitigate their impact:

  1.   Ensure your battery bank is fully charged before you leave.
  2.   If your batteries are wet lead acid, ensure that the water levels are topped up and perform an equalization charge cycle.
  3.   Turn your inverter (and any other electrical loads) off.
  4. Protect your battery bank from wind chill factors.   Wind chill has been known to impact the freezing point of a battery bank.   If your battery bank is external to the cottage (underneath or beside), make sure that the battery box itself is well insulated.

Should you leave your solar on during the winter?

Most people leave their solar system on during the winter to keep their batteries healthy, but there are situations where keeping your solar system on can actually do more harm than good.   The advantage to keeping your solar array active during the winter are obvious.  The solar array will keep trickle charging the battery bank and therefore keep it fully charged and active during the winter.  The disadvantage is that your solar controller can become a parasitic load to the battery bank if your solar panels become too heavily laden with snow over the winter that they stop charging the battery bank.

Let’s look at a this in detail;

According to the Trojan Battery Company, a battery bank will self-discharge in the amount of 5-15% per month, with rates decreasing (closer to 5%) as the temperature decreases and the batteries become less active internally.   Therefore, let’s see how that self-discharge could impact a battery bank:

The chart to the right shows the impact of battery self discharge at rates of 5, 10, and 15% per month.  As the chart shows, a 5% self discharge rate is quite acceptable over the course of the winter without the addition of solar charging, but 15% is not acceptable since the battery bank will drop below 60% SOC in just over 3 months.

Now, just for fun, let’s add the impact of a small, parasitic load (such as a solar charge controller when the solar panels are 100% blocked from the sun):

According to the datasheets of two popular solar controllers, their stand-by power consumption ranges from 1W to 4W.  We assume that load to be continuous, so that translates into anywhere from 24 w/h per day to 96 w/h per day consumption.  The impact of that consumption will vary depending on battery bank voltage and size.  First, let’s translate our daily consumption (in watt/hours) to monthly consumption (in amp/hours) for the three most common battery bank voltages:

12V Battery Bank:

  • 1W power consumption = 24 w/h/d x 30 (days) = 720 w/h/m / 12V = 60 a/h per month
  • 4W power consumption = 96 w/h/d x 30 (days) = 2880 w/h/m / 12V = 240 a/h per month

24V Battery Bank:

  • 1W power consumption = 24 w/h/d x 30 (days) = 720 w/h/m / 24V = 30 a/h per month
  • 4W power consumption = 96 w/h/d x 30 (days) = 2880 w/h/m / 24V = 120 a/h per month

48V Battery Bank:

  • 1W power consumption = 24 w/h/d x 30 (days) = 720 w/h/m / 48V = 15 a/h per month
  • 4W power consumption = 96 w/h/d x 30 (days) = 2880 w/h/m / 48V = 60 a/h per month

Now, let’s express these values as a percentage of the “usable” battery bank storage for some common battery bank sizes:

12V Golf Cart (GC) battery bank – 225 a/h rated capacity:

  • 1W power consumption = 60 a/h / 225 a/h = 26 %
  • 4W power consumption = 240 a/h / 225 a/h = 106 %

24V Floor Scrubber (L16) battery bank – 370 a/h rated capacity:

  • 1W power consumption = 30 a/h / 370 a/h = 8 %
  • 4W power consumption = 120 a/h / 370 a/h = 32 %

48V Floor Scrubber (L16) battery bank – 370 a/h rated capacity:

  • 1W power consumption = 15 a/h / 370 a/h = 4 %
  • 4W power consumption = 60 a/h / 370 a/h = 16 %

Conclusions:

As you can see from the above numbers, the potential impact of a 1 to 4 Watt parasitic load is not to be ignored.  In fact, if we combine the natural battery self-discharge rate with the impact of any parasitic load then the health of almost any battery bank will drop below 60% SOC (state of charge) within 1-3 months.

Therefore, we can say that we would definitely want to turn our solar off if our solar panel installation area (such as a low sloped roof) is known to accumulate and keep snow all winter and we can not visit our cabin/cottage location at all for 6 months.  We would also keep our solar off if the stand-by power consumption is too high relative to the battery voltage and size (as shown above).

Right now, you might be thinking that you should always turn your solar off just to be safe, but there are cases where that isn’t a good idea.  If your solar panels are still actively charging the battery bank (even at a reduced rate), then it makes more sense to leave your solar on.   Remember that solar panels actually produce more energy in cold temperatures, and the reflection of snow around (not on) the panels will also increase their production.

Unfortunately, there isn’t a “one size fits all” answer.  Your location, installation area, solar controller stand-by power consumption and battery bank size all play a role in how well your battery bank can stay healthy through the winter.   We hope that this bit of information can help you make the right decision to protect your investment.  As always, if you have any questions, please let us know!

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