I was wondering since the voltage of my laptop charge is too high to charge one battery, it occurred to me if it could be possible to make a parallel or series arrangement to get my 2 12V batteries charged.
NO you cannot charge your 12V AGM battery directly with a 19V Laptop power supply. You need a battery charger.
- The laptop power supply IS NOT a battery charger, it is a fixed voltage SMPS power supply. The charger for the laptop batteries are inside the laptop.
- AGM batteries are indeed Lead acid batteries. So your fully charged state is about 13.8 - 14.5V.
- Putting two batteries in parallel does not change the voltage required to charge them it simply increases the current capability (increased Ah rating).
- Putting two batteries in series may require up to 27.6V to fully charge (though you need a battery charger and NOT a power supply).
Buy a battery charger, or build a charger to use your 19V power supply.
If you are building a charger then you need three things:
- Terminal voltage control
- Current control
- Float, trickle or On/Off charge control
There are several Chinese/Ebay charger modules (example - XH-M603) available that might suit your purpose. They take a wide range of input voltages.
Be aware if you intend to use your laptop 19V power supply that it is only capable of providing about 64W, so this limits your 12V charging to somewhat less than this. You might only be able to provide about 50W maximum to charge the batteries, giving about 3 - 3.5A of charge current as you approach fully charged.
Not recommended at all. Voltage level is only the tip of the iceberg here. A charging circuit measures the current and voltage that goes into the battery, and charges the battery depending on its type in many different ways in different states of the battery. For example, when a battery is depleted, it is charged with a very small current to protect it. Then, it acts more like a current source until the battery is close to being full. Only after then, it can be said that the charger is a voltage source.
Some battery chargers take the temperature of the cells into consideration.
Even if it was not a terrible idea, say, we crash landed on an island and have to operate the radio, 19V source would only charge them up very very slightly, small enough to dismiss it as a charger.
It is easily possible if you locate the feedback point in the regulator and add a pot and series R to shunt Vfb in order to raise to 24V