If a solar panel is not equipped with MPPT (maximum-power-point-tracker) technology, it will not perform at its highest efficiency when charging the RV / Caravan / Boat battery. Below are some facts that cause this inefficiency to happen.
• During the different stages of the RV / Caravan / Boat battery’s capacity, the battery requires a different voltage applied to the terminals to get itself fully charged.
• A solar panel produces a different output voltage under different conditions: intensity of the sunlight, ambient temperature, the rated output voltage level on the STC (standard Test Condition).
• The required voltage to fully charge the battery and the output voltage from the solar panel is usually not matched at any single time.
So what happens when you hook up a 70 watt solar panel to your battery? Unfortunately, what goes into the battery is not 70 watts. On a given situation, your panel puts out 3.69 amps. Your battery is setting at 12 volts under charge: 3.69 amps times 12 volts = 44.28 watts. You lose 25 watts. That 25 watts is not going anywhere – it just is not being produced because there is a poor match between the solar panel and your RV / Caravan / Boat battery. With a very low capacity battery, say 10.5 volts, it’s even worse – you could be losing as much as 35% which is 38 watts (3.69 amps times 10.5 volts = 38 watts).
Conclusion: Due to the constantly poor match of the required voltage to fully charge the battery and the output voltage from the solar panel, the non-MPPT panel usually will have trouble completely replenishing the RV / Caravan / Boat battery. What an RV / Caravan / Boat owner needs is a smart enough MPPT device to convert the solar panel output to the required voltage, to fully charge the battery and not throw away the panel generated power.