Bulk Charge Current / Battery Rule of Thumb

My rough rule of thumb:

• 5% minimum rate of charge for Lead Acid battery--Weekend/seasonal power usage
• 10% rate of charge for Lead Acid Battery--Full time off grid (9+ months a year, some daytime loads)
• 13% rate of charge for Lead Acid Battery--Typically about the "cost effective" maximum charging current, plus roughly the maximum amount of current into a flooded cell lead acid battery without a Remote Battery Temperature Sensor to reduce the chances of thermal run-a-way during charging.

That is all based on the C/20 battery capacity (i.e., the current draw that will take a battery from 100% State of Charge to 0% SoC). You will find other "rates" (C/5, C/6, C/24, C/100, etc.). A C/100 "capacity AH rating" will be higher than the C/20 AH capacity rating for the same battery (the higher the discharge current, the lower the apparent battery capacity).

There are folks that will run 20-25% rate of charge--Typically with an AC Battery Charger (mains or genset powered).

Note that while a Lead Acid battery only needs around 2.5% to 5% rate of charge for "equalization" charging--If you "force" even 2.5% rate of charge into a 100% full battery, it can overheat and fail (or even catch more).

There is a lot more behind the rules of thumbs--But this is our "go to" numbers for a typical off grid solar power system. If you have "non-typical" power needs, we can discuss the details.

But, you are correct, one of the reasons that 10% rate of charge is "better" than 5% rate of charge... It is to "fast charge" the battery (you have several days of cloudy weather, the next day you have full sun and need to get all of the energy you can harvest. Discharge a battery to 50% state of charge, it will take around 12-14 hours of "sun" (about two long summer days) to get back to 100% state of charge. A 10% rate of charge can get you backup in (roughly) 7-8 hours (a bit more than 1 day of full sun).

Also, if you have loads during the day (household appliances, computers, TV, water pump, etc.), that uses energy from the solar panel to run those loads, and slows down the recharging of the battery bank. Having a larger array usually means less day to day energy management decisions (what loads I can turn on today with a bit of overcast).

Also, for larger deep cycle (tall case) and industrial flooded cell lead acid batteries--Some vendors recommend 10% minimum rate of charge.

Now, if you want to talk about AGM, LiFeO4, or other type batteries--These rules can be adjusted some.

-Bill