I have directly connected a 255W solar panel (with Vmpp of 49.5V but can peak at 56V in full sun at mid-day) to the 36V battery set of my golf cart. This seems to have been working fine for months, as I am a light user of the golf cart (a few miles per day) in a sunny state. In other words, I have been able go for months without connecting to the grid. However, my research certainly leads me to believe that there is a risk of overcharging (if there isn't a good balance between my driving and the sun-hours, I pressume). My question is whether the overcharging would be due to excessive Amps or excessive Volts? (depending on the answer, I may fashion the correct protection).
Charging a 36V battery set with a 255W solar panel
batteriesbattery-chargingsolar cell
Best Answer
More info, e.g. battery specs. I would almost certainly guess the batteries can handle much more input current than the panel will ever be able to put out, though. You'd be concerned about overvoltage.
From this SO Answer (referring to a single 12V, but you can extend it here):
"The maximum is typically around 14.4V to 14.8V at 21C temperature. If the maximum is exceeded for minutes to hours, the battery will be permanently damaged. The damage occurs around the same time that flammable gas is generated and vented, so there is a risk of gas explosion near the battery.
Your setup would require constant monitoring, especially if the load is disconnected or turned off. Check the voltage every hour, and disconnect the solar panel as the voltage approaches the maximum.
To find the maximum voltage, check the manufacturer's label or datasheet for the battery." (Emphasis added)
Based on the three stages of 36V battery charging described here, it seems a reasonable maximum voltage for your setup might be 40.5V. I don't know your batteries though, and I am also assuming that you are using a lead-acid chemistry.