General Rule of Thumb for Charging of Battery Bank

quique

I remember participating in a discussion about how many amps a pv array should produce based on the AH battery bank capacity.  Was it 5-10%?

BB.

5% is the minimum we recommend. Good if battery is charged during the day and used at night--For RV/Weekend/Seasonal cabin use.

10%, for some battery mfg. is the recommended minimum rate of charge. Supposed to be better at mixing electrolyte, and keeping plates pores from getting "clogged" (with sulfates?). Also, if you have mixed daytime loads, you don't worry that your 5% minimum is drawn near 0% charging when daytime loads are running.

13% rate of charge is the typical maximum for flooded cell lead acid batteries. Higher rates of charge can cause batteries to run hot. Also, can end up with batteries "fully charged" before noon -- And "wasting" money on an over-sized array. However, 20-25% rate of charge has been recommended for generator charging (fast charging, reduced number of hours on genset).

Note that our Rate of Charge numbers are based on the 20 Hour discharge rate. Other mfg. may make their percentage numbers different (based on 6 hour discharge rate, etc.). So, a 225 AH battery bank at 10% rate of charge would be ~22.5 amps charging current. Note, once battery is over ~80-85% state of charge, the charging current will naturally be reduced (battery will accept less than 10% charging current) over the last ~2-4 hours of the absorb charging cycle.

With solar panels being historically "cheap" these days--Trying for a 10% minimum rate of charge is usually recommended. RV's and Weekend cabins (and with generator backups)--5% may make more sense (limited roof space, less attraction to thieves, etc.).

-Bill