Batteries/Chargers

You're mixing up two units.

Let's say your battery has a capacity of 2 Ah (ampere * hours) and your charger outputs 2A (Ampere). This means the charger would need 1 hour to fully charge your battery. Now take a 1A charger, this would take 2 hours to charge the same battery.

Ah is capacity, A is flow rate. As long as the voltages are correct, it'll charge eventually.

Ah == size of the bucket, A is the size of the hose you fill it with.

Great answers already, I'd like to add a few things: Lithium batteries like to be charged at 1C, meaning that if they have a capacity of 1.6 Ah they like to be charged at 1.6 A and will (theoretically) be fully charged in one hour (this is wrong in practice though). You can charge most of them at a higher rate (e.g. at 3.2 A), but they should be rated for it. If you don't better, stick to a lower amperage. Too low of an amperage should not be an issue but I'm not sure - it will take ages though.

The voltage has to fit! Batteries change their voltage over the course of charging/discharging. This is more pronounced in older NiMH or NiCdH (discard those) batteries but still relevant. A 12V charger can charge your batteries to 12V. Most chargers can handle different voltages and will usually select the correct voltage depending on the battery. This is important if you are charging LiIons or LiPos (two types of Lithium batteries), as overcharging them is NOT advised. They generally peak at around 4.2V per cell I think with a working charge (don't know the term) of 3.7V per cell. Some chargers require you to select the number of cells. Charging a 2-cell LiPo with 12-13 volt is a bad idea I wager and charging a 3-cell LiPo with 9.something volt won't work (it won't drain it, but the minimal voltage is higher than what the charger provides - current would like to flow the other way if It could).

TL,DR: If you can adjust the charging parameters manually or you are doing something the manual states you shouldn't be doing, educate yourself. If however you only plug in your phone (with a painfully small battery at 2 Ah) and only provide a 1.5A*5V=7.5W charger, you are perfectly fine.

so 2Ah really means "2 amp-hour". it measures how much of a charge it has- basically, you can draw 2 amps for one hour from the battery during ideal conditions (IE, it's not an old battery, and everything about it is perfect- perfect temp, perfectly isolated from vibrations. all sorts o things. We never see that in the real world, however.)

alternatively, you could draw 1 amp for 2 hours, or 4 amps for half an hour.

1.5 amps is the amount of charge it's providing to the battery, again, under ideal conditions (like temperature, etc.) a 1 amp charger would charge slower where a 2 amp charger would charge faster.

The important thing is that the voltages match- 12v or 18v tend to be the most common. if the charger has too much amperage, it's fine- the battery will only draw what it can use; too little and it'll just take forever to charge.

It means it's going to take at least 1h20 until the battery is fully charged.

Given it's the correct charger. Because these values don't say anything about that. It could also damage the battery or catch on fire if it's some random charger that isn't supposed to be used with that battery.