Parallel Battery Current?

[peps1]

If I have 2  batteries in parallel what happens regards the Amp ratting?

For this question im talking about LiMn 18650 batteries

Capacity: 3100mah
Max discharge current: 20A
Quick charge current: 3000mA (1C)
Quick discharge current: 2000mA
Cycle life: 300 times
Charge cut-off voltage:  4.2V ± 0.5V
Charge cut-off current: 5A
Discharge cut-off voltage: 3.0V
Discharge temperature range:-10°C—60°C

Is there a back of the envelope calculation?

[mikerj]

It will nominally double the capacity (mAh) and the maximum discharge current.  However to double the discharge current in practice you would need to ensure there is an equal current draw from both batteries.

[JesusCB]

batteries in series -> You add the voltage ( make sure that all the batteries have the same mAh)
batteries in parallel -> you add the current (make sure all the batteries have the same voltage )

[Ian.M]

+ whenever you connect batteries or individual cells in series or parallel, they should be the same make, type, and charging history.  Otherwise one weak cell can cause premature failure either by discharging the good cells(s) in a parallel circuit, or by becoming over-discharged and leaking or rupturing (or even worse) in a series circuit.

[PeterFW]

+ whenever you connect batteries or individual cells in series or parallel, they should be the same make, type, and charging history.

Lithium cells in parallel can be a bit tricky, without the proper knowlege i would suggest against wiring up lithium cells in parallel.
Even if the 18650 are protected cells, a series configuration on the other hand is fine as long as you use either protected cells or a proper battery protection circuit.

If you want the cells to be replaceable (swapping them out when they run flat) it is a tremendously terrible idea to wire them in parallel