A trickle charger is used to replace the self-discharge of a battery in storage, so that the battery doesn't "go dead". All batteries naturally self-discharge to some extent, meaning that they lose capacity and the voltage drops. The amount of self-discharge that will occur in a month depends mainly on the battery type and storage temperature.
The two most common types of lead-acid batteries are flooded batteries (the ones you refill) and sealed batteries like AGM or gel batteries that don't need refilling. Most starter batteries are the flooded type.
Trickle chargers are most commonly used with flooded batteries, because the rate of self-discharge of a flooded lead-acid battery can be as high as 30% in a month. This means that within a few months an unused flooded battery can "go dead". That can be a real problem if that battery is needed to start a vehicle.
Sealed AGM lead-acid batteries have a much lower rate of self-discharge. The loss in capacity will be about 3% of its capacity per month for a battery stored at room temperature.
At 50F (10C), the loss would be approximately 1.5% per month, and at 104F (40C) the loss would be around 10% per month. Because of the lower rate of self-discharge, sealed AGM batteries can be stored for 9 months or more without "going dead". A trickle charger would only really be necessary for an AGM battery when it will sit unused for very long periods.
Tips for calculating what size solar panel you'll need to trickle-charge your battery:
1. To start, you'll need to know the capacity of the battery you want to trickle-charge (usually listed on the side of the battery in "Ah" or "mAh", where 1 Ah equals 1000 mAh. If you can't find it there, try looking up the battery model online). Tip: Most car-sized starter batteries are about 100Ah
2. Next, calculate the self-discharge losses per month. If you will be storing a flooded battery in the summer, multiply the battery capacity, in Ah, times 30%. If you will be storing a flooded battery in the winter, multiply the battery capacity, in Ah, times 8%. Then, divide by 30 to find the average loss per day.
A 100 Ah flooded battery stored in the summer:
100 Ah * 0.3/30 = 1 Ah per day= daily self-discharge loss of the battery
3. Next, determine the number of full sun hours to expect at your location, based on the time of year. If you're in the US, you can use these solar insolation maps to approximate this.
Note: 1 kWh/m2/day is the same as 1 full-sun hour per day.
4. Finally, divide the total capacity loss per day from Step 2 by the total number of full-sun hours per day from Step 3 to get the desired current of the panel.
The 100 Ah battery from Example 1 loses 1 Ah per day. In NH in June, the total full-sun hours per day will be 5.25 hours average. 1/5.25 = .19A (190 mA).
For this case, a panel with a current output of at least 190 mA would be sufficient to maintain this battery in storage. Our 1.8W trickle charger produces 125 mA, and our 5W trickle charger produces 300 mA, so for this application the 5W charger would be the best choice. If you have a smaller battery, are storing it in the winter, and/or you are in a very sunny location, a 1.8W charger may be a better choice.
All prices are in USD.