Note that although Q1 is being used here as an on-off switch, it has an on-resistance of about 0.2W and therefore needs to be provided with a good heatsink because of the heat dissipation during charging.
We suggest a finned extrusion heatsink like the Jaycar HH-8566, which has a thermal resistance of 2.2°C/W. You’ll need to provide electrical insulation between the device tab and the heatsink though, because the Mosfet tab will be at positive battery potential.