If you put diodes in between parallel batteries to prevent one from charging or discharging another then you are preventing them from maintaining a balance. Voltage wants to run from high to low until equal  . The more the difference  , the higher the current flow. So if one battery has a tendency to drain faster  , its voltage will drop slowing flow from it to load  ,while battery with higher voltage will have increased current going to load as well as lower voltage battery  ,increasing proportionately in attempt to stay balanced. Different internal resistances might prevent this balance from happening with large loads so it would be important to use batteries with the same or near same "c rating" and to make sure that you never exceed the sum of the 2 batteries c rating. C rating determines the rate at which current of a battery can safely be discharged. Example : 4c rated 5ah battery would allow 4 x 5 = 20amps continous current.

If you put diodes in between parallel batteries to prevent one from charging or discharging another then you are preventing them from maintaining a balance. Voltage wants to run from high to low until equal. The more the difference, the higher the current flow. So if one battery has a tendency to drain faster, its voltage will drop slowing flow from it to load, while the battery with higher voltage will have increased current going to load as well as lower voltage battery, increasing proportionately in an attempt to stay balanced. Different internal resistances might prevent this balance from happening with large loads so it would be important to use batteries with the same or near the same "c rating" and to make sure that you never exceed the sum of the 2 batteries c rating.

C rating determines the rate at which the current of a battery can safely be discharged. Example: 4c rated 5ah battery would allow 4 x 5 = 20amps continuous current.