Battery chargers vary a great deal in intelligence, the types of batteries they can charge, how they maintain batteries, and how long they take to charge. As batteries are such a big investment when it comes to off grid solar power systems, it’s important to get the right one for your off grid solar power system.
A general rule of thumb is to charge a battery at about 10% of its amp hour capacity. For example, and 80Ah deep cycle battery should be changed at a rate of about 8A.
Different types of batteries require different charging regimes. For example, SLA (sealed lead acid), AGM (absorbent glass matt) and gel batteries are traditionally charged at a lower voltage than flooded lead acid batteries. This varies depending on the manufacturer and you should always check what charging voltage is required before you buy batteries and a battery charger.
Types of battery chargers
A simple charger works by providing a constant DC power to the battery. A simple charger will not alter its output based on time or the charge on the battery. These type of battery chargers are usually cheap, but there is a trade-off in quality. Typically, a simple charger takes far longer to charge a battery, and a battery left in a simple charger for too long will be severely damaged by over-charging.
Timer based chargers operate much like a simple charger, but (as the name would suggest) they operate on a timer. The timing would be set for a particular battery and then left. However, if batteries of lower capacity were charged they would be then overcharged and if batteries of higher capacity were charged they would be only partly charged. Timer based chargers also have the drawback that charging batteries that were not fully discharged, even if those batteries were of the correct capacity for the particular timed charger, would result in over-charging.
Battery charger output current depends upon the battery’s state. An intelligent charger may monitor the battery’s voltage, temperature and/or time under charge to determine the optimum charge current at that instant. Charging is terminated when a combination of the voltage, temperature and/or time indicates that the battery is fully charged. In a sense, the batteries tell the charger when they are full.
Some chargers use pulse technology or pulse width modulationin which a pulse is fed to the battery. This DC pulse has a strictly controlled rise time, shape, pulse width, frequency and amplitude. This technology is said to work with any size, voltage, capacity or chemistry of batteries, including automotive and valve-regulated batteries. Most good quality chargers and charge controller use this kind of technology.
Selecting the right charger
As with any component of a solar power system, it’s always best to consult with a battery expert first before purchasing to ensure you’re getting a charger suited to your particular set up. While all chargers work well off a mains power supply, you’ll likely be using yours in conjunction with a generator, so check that the charger you’re interested in is up to the task – some just refuse to function properly in this application.