Q:
Hi Sean, NABCEP includes the following practice question in the NABCEP PV Installation Professionals Resource Guide:
The battery bank for a backup utility-interactive PV System is located in a harsh environment with temp and humidity extremes. The system does not have a provision for temp compensation. What is the MOST LIKELY result on the battery state of charge?
c.) Overcharged in cold weather and undercharged in hot weather. d.) Undercharged in cold weather and overcharged in hot weather.
The correct answer is d according to NABCEP. I thought hot batteries charge less efficiently. Why in this case does that not lead to undercharging?
A:
When it is cold, chemical reactions slow down.
When it is cold, batteries take more voltage to charge.
When it is cold, there is less available energy in a battery.
When it is cold, batteries self discharge less (why grandma keeps them in the freezer).
When it is hot, everything is opposite.
To get specific with the question, if you are charging at a voltage meant for a medium temperature, when it is cold, you will have less than optimal voltage and when it is hot, you will have more than optimal voltage.
You may notice in this situation that your batteries may require regular addition of distilled water in the summer, since the overcharging will cause the battery to off-gas.
You may also have a greater tendency to get led sulfate crystals on the plates in the batteries in the winter, since you are not reaching a full charge.
Equalization, a controlled overcharge can help cure the batteries from lead sulfate crystal buildup.
Thanks for the good question!!
Sean White