Blocking diodes

[john8750]

Should blocking diodes be needed for paralleling cells?
My open circuit voltage at the CC is 20 volts. Connected, the voltage drops to 13.1, now matter how many batteries are connected. Voltage drop across the diodes?
I have 4 100 watt cells at this time, and 3 300 AH batteries. Max charging current is about 22 amps.
I'm new here.

Thanks,
John Smith

[Mike90250]

Blocking diodes prevent the battery bank from back-feeding the panel at night. Most charge controllers have this function built in. You need to read a primer about Bypass Diodes that go into a PV panel to prevent total power loss with a small amount of shade.

[inetdog]

Should blocking diodes be needed for paralleling cells?
My open circuit voltage at the CC is 20 volts. Connected, the voltage drops to 13.1, now matter how many batteries are connected. Voltage drop across the diodes?
I have 4 100 watt cells at this time, and 3 300 AH batteries. Max charging current is about 22 amps.
I'm new here.

Thanks,
John Smith

The CC is usually meant to remain connected to the batteries at all times. If it is a PWM CC, then as it is charging the batteries the panel voltage will drop to near the battery voltage until the CC gets towards the end of Absorb. What you see is that you are never getting the bank out of Bulk.
400 watts of panel and a PMW CC is nowhere near enough to properly charge 900 AH of battery. You need to use a generator or grid-powered charger to recover from this condition before your batteries are damaged.

Or else try connecting only one battery at a time to the CC and not putting any load on them until they have all recovered.

[john8750]

Thanks for the info men. I need to check the batteries, dont know the actual condition. They were free to me and were never installed, but old and obsolete.
The CC says they are at 100 % but when I place a 230 watt load on inverter, battery goes to about 60 percent right away. Need to charge with charger to check conditions. I agree that the battery needs much more current to charge. Will have 1KW ASAP, money wise.
So I do not need a blocking diode for each cell? Bypass diodes are installed.
Also, I jumped the CC and connected cells directly to the batteries and still got a charge voltage of 13.1 V. Open circuit is 20V. I'm thinking maybe a sulfated cell in the battery causing a low internal resistance.
Time to test batteries with load and charge tests.
Thanks again.

John Smith