When do I need a charge controller to charge a battery with a solar panel?
This is a very common question and one that is absolutely crucial.
In most cases you will need some type of a charge controller to safely charge a battery pack. This prevents overcharging and reduction in the battery life of the system.
Battery technologies such as Lithium Ion, Lithium Iron Phosphate, Nickel Metal Hydride, or Nickel Cadmium always require a charge controller to safely recharge the battery pack. Lead acid batteries are the exception to the rule. If you are trying to charge a lead acid battery quickly or using a large solar module you will want to have a charge controller to keep the battery from overcharging and drying the electrolyte in the battery.
If you are using a solar module for a low current maintenance charging this can be done safely without a charge controller as long as the solar panel output is <1% of the battery capacity. For example, if you have a 12V, 100Ah battery this could be safely trickle charged with a panel capable of supplying up to 1A at 13.4V.
By the nature of how solar will cycle on and off each day as the sun rises and falls not all charge controllers will be safe for lead acid or AGM batteries if solar is used.
Most notably, chargers using a three stage charging algorithm will slowly dry out the battery once it has reached a full charge. The reason for this is that when the solar starts producing power each morning the charger will start at stage 1. It will quickly switch to stage 2 since the battery voltage is high indicating a close to full charge. During stage 2 the charger will top off the battery and balance the cells by taking the battery voltage up to 14.3-14.6V, which will take a few hours. After that is achieved the charger will move to stage 3 which is a float voltage maintenance charge at 13.4V with a trickle of current.
It is the high voltage cell balancing that occurs in stage two that causes the battery to dry out and reduces its lifetime. Three stage chargers work very well when they are connected to the grid and only take the battery through stage 2 once and then hold it in stage 3. It is the repeated cell balancing in stage two that occurs due to the daily on-off cycling of the solar that destroys the battery.
Charge controllers aren’t just something that is nice to know about. They could very easily be a part of your system and one that you must utilize to avoid damaging your battery.
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