When choosing a solar trickle charger, the most important considerations are the type, capacity, and anticipated rate of self-discharge of the battery which the charger will maintain. The current produced by solar trickle chargers varies, so choosing the right charger depends on matching the charger's current to the battery's self-discharge rate. Other factors to compare include design features, quality, and price.
Electronic devices like cameras, cell phones, and laptop computers all run on batteries that can be recharged using solar chargers. Solar trickle chargers are a special type of charger designed to maintain 6 or 12 volt lead-acid batteries, such as those used in motorcycles, boats, and automobiles. Three pieces of information are needed to calculate the minimum current a solar trickle charger must provide to maintain a battery at the proper charge. These are the capacity of the battery, the daily rate of self-discharge, and the average daily amount of sunlight received at the storage location.
Figuring out the capacity of the battery is the first step in calculating the amount of current a solar trickle charger must produce. Capacity is measured in ampere-hours (Ah), or sometimes milliampere-hours (mAh). Most car batteries have a capacity of about 100 Ah, or 100,000 mAh. If the capacity is not listed on the battery, it can usually be found in the specifications for the battery brand and model.
After the capacity has been determined, the self-discharge rate of the battery must be calculated. This rate depends on the type of battery as well as the temperature at which it is stored. Lead-acid batteries can be flooded — the type most often used in cars — or sealed, such as the AGM or gel batteries often used in RVs and boats. The type is important because flooded batteries self-discharge at a faster rate than sealed batteries.
The temperature at which the battery is stored is the next consideration. Warmer storage temperatures increase the rate of self-discharge. Flooded batteries lose about 8% of their capacity each month in winter and about 30% each month in summer. Sealed batteries lose about 1.5% of their capacity each month in winter and about 10% each month in summer. In summer, assuming a monthly loss of 30% capacity, a flooded 100 Ah battery would lose an average of 1.1 Ah of capacity per day.
Once the daily rate of self-discharge has been determined, the average daily amount of incoming solar radiation — or insolation — received at the storage location must be determined. Insolation varies by latitude and time of year. Solar insolation maps show the average daily insolation a location will receive depending on the time of year. New York City, for example, receives about five hours of full sunlight per day in July.
Both the daily rate of self-discharge and the average daily amount of insolation are needed to calculate the minimum current a solar trickle charger needs to maintain the battery. The current needed is equal to the daily rate of self-discharge divided by the average daily hours of insolation. A solar trickle charger would need to have a current of at least 0.22 A, or 220 mA, for example, to maintain the charge of a 100 Ah capacity battery in New York City in July. This is calculated as follows: 1.1 Ah daily rate of self-discharge divided by five full hours daily insolation equals 0.22 A, or 220 mA.
After determining the required current for the solar trickle charger, the purchaser should consider and compare the design features, quality, and price of available chargers. One convenient design feature is overcharge protection, which regulates the current so that the battery is not charged beyond its safe capacity. These types of chargers are also known as float chargers. If maximum portability is desired, consider purchasing a rollable charger, which is made of a thin film that can be rolled up for compact storage.
Chargers that will be used often should be reliable and durable. Consulting consumer product reviews can be helpful in determining the overall quality of available solar trickle chargers. The chargers under consideration should also be compared by price and by any available warranties or service plans.