Even "directly in parallel with the batteries" isn't really directly in parallel with the batteries, thanks to wiring resistances. The capacitor should have the closest and most direct connection to the load, then this pair should be connected to the battery via wiring which gives you some control of the current drawn from the battery.
When capacitors are connected together in parallel the total or equivalent capacitance, CT in the circuit is equal to the sum of all the individual capacitors added together. This is because the top plate of capacitor, C1 is connected to the top plate of C2 which is connected to the top plate of C3 and so on.
If we place a capacitor in parallel with a lamp, when the battery is removed, the capacitor will begin to power the lamp. It slowly dims as the capacitor discharges. If we use two capacitors, we can power the lamp for longer. Let’s say capacitor one is ten microfarads and capacitor two is 220 microfarads. How do we calculate the total capacitance?
We can also define the total capacitance of the parallel circuit from the total stored coulomb charge using the Q = CV equation for charge on a capacitors plates. The total charge QT stored on all the plates equals the sum of the individual stored charges on each capacitor therefore,
It’s worth pointing out that many people accidentally connect batteries of different voltages in parallel every day. For example: If you mix brands even of the same labelled voltage – you can experience problems. Due to different manufacturing processes, the exact voltages of batteries from different producers can vary slightly.
The basic concept is that when connecting in parallel, you add the amp hour ratings of the batteries together, but the voltage remains the same. For example: two 6 volt 4.5 Ah batteries wired in parallel are capable of providing 6 volt 9 amp hours (4.5 Ah + 4.5 Ah).