In theory it is OK to connect them in parallel with two conditions: Each battery must be in a state where it can be voltage charged. This is fine for lead acid batteries unless they are very run down. Very discharged lead-acid batteries have to be charged with fixed current until they get to a minimum voltage, then they can be voltage charged.
This reference design showcases a lead-acid battery charging solution. The solution uses the MP2659, a highly integrated switching charger designed for portable devices with 3-cell to 6-cell series Li-ion or Li-polymer battery packs. Figure 1 shows a block diagram for a highly integrated switching charger for lead-acid batteries.
Each battery must be in a state where it can be voltage charged. This is fine for lead acid batteries unless they are very run down. Very discharged lead-acid batteries have to be charged with fixed current until they get to a minimum voltage, then they can be voltage charged. The power supply is capable of maintaining the fixed float voltage.
However, there are few chips on the market that are designed specifically for applications that charge lead-acid batteries. This reference design showcases a lead-acid battery charging solution.
No, it is not recommended to use a single charger to charge two batteries in parallel. Each battery should be connected to an individual charger or charging circuit to ensure safe and effective charging. How should I connect the batteries in parallel for charging? To connect two batteries in parallel for charging, you need to:
Very discharged lead-acid batteries have to be charged with fixed current until they get to a minimum voltage, then they can be voltage charged. The power supply is capable of maintaining the fixed float voltage. In practise, I think it's a good idea to put at least a diode in series with each battery just because stuff happens.