You phone doesn't instantly heat your house in seconds, it just plays Candy Crush and it still needs power constantly to stop it discharging throughout the day otherwise you would have a 6 month standby time.
Bad analogy. It's using power all the time to remain connected to the network, that's why it needs recharging more regularly.
A better analogy would be a Car battery. Over a very long period, it may lose it;s charge but you can charge it and store it for a month or more before using it without consequence. The Cell should work the same way. When it's discharged into the shields, it should draw power to recharge but once it's fully recharged, there is no need for any power draw.
I just can't understand the 5 second delay. It should be charged and ready to use from the start, not have to charge up to be used. What's the point in having something that needs charging when the power to do that could have been diverted to the shields anyway? I thought the idea was to charge them up while you're not drawing full power, like when you have weapons retracted and not in combat.
I have a battery booster for my car. If my battery dies and I can't start the car, I connect the battery booster to give my car the power it needs to start. I don't run off to find a charging point to charge my battery booster first, if I did, I may as well disconnect the car battery and put that on charge instead. The whole idea of having it is because it's already pre-charged. It might need a little top up every few weeks but it's constantly on full charge and ready to use, it's not on constant charge while not in use, it only gets a recharge after it's been used once.
Shield cells should work the same way as my battery booster or it really makes them a bit pointless.