Battery Packs for gaming laptops/high power laptops

I have a Lenovo Y50-70 and I want to buy an exteal battery pack to charge it when I'm outside to extend the battery life, I want something like a full day charge (8 to 10 hours on would be fine, I will not have the PC on all the day, sometimes I will tu it off). The charger of the laptop claims an output of 20v/6.75A (135W), but most of the battery packs I found are of 20v and 4.5 (3A, 4A too (so 90W as max) [in Amazon.com]. So I want to know: 1. if those battery packs can charge my PC 2. if the battery pack can't give me all the power I need at a certain moment because of the amps (when my PC is on), Can the battery pack "slow down" the rate of discharge of the laptop? 3. Will the battery pack charge the PC while off? (at least at a lower rate than the wall charger)

I think laptops works like smartphones (but I'm not sure), for example, a phone with a charger of 5v/2.0A, can work with other chargers of 5v and 1.5, 1, 0.8A even 0.5A (obviously it will charge at a much lower rate, but it will charge!) some phones (like mine, an Asus Zenfone 2, with an 18W charger [9v/2A with QC, and 5v/2A when it is not using QC]) can use 5v/0.5A chargers but only if they are off, if I tu it on it will not show the "charge icon" (I think because the discharge rate is too much more than the charge rate [another question: 4. but, Will it slow down the discharge rate, at least, a little bit?]), but if I use a 5v/1A charger my phone will charge (at a lower rate) even if it is on.

Will my laptop charge with a battery pack like my phone do (only off, or when it's on too)?

Thanks in advance for your answers, I'm not an expert in electricity/electronics, if you find some error in my thoughts, just let me know, and explain me why it is not correct (if you want to give me a complex answer, do it), I want to lea. Also, I'm not a native English speaker, I'm leaing, so sorry in advance if you find a mistake.

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