Ignoramus5722 wrote:

We do not have the compressor on at all times. We turn it on in the
morning or when it is necessary.

Sounds like you have a bad leak if the system won't hold air overnight.

But when we turn the compressor on in the morning, when we need air, I
do not want to wait extra 15 minutes.

Filling the extra tank through a throttle valve and delivering the stored
air through a check valve will do what you want, but the energy efficiency
will be around 50%. It's the same story as charging a battery through a
resistor. A manually-operated valve would be better if its operation can
be timed to miss the demand peaks.

If your peak loads are sufficiently infrequent (once in ten compressor
cycles or some such) the resistive reservoir charging might be useful. For
my part I'd try to make the system hold pressure when the shop's closed.
Compresed air is basically money.

Hope this helps,

bob prohaska