An excellent quality car charger will deliver at least 2.1 amperes (or 2,100 mA) per USB port. To be safe, you might opt for a car charger that offers a range of 2,100-2,400 mA. In general, HP needs at least 1,000 mA to be recharged at optimum speed. If you wish to recharge the tablet, normally the amperage needed is bigger. In addition to the output, also pay attention to the protocols it supports. For example, Quick Charge, Power Delivery, and so on. This sort of protocol makes battery charging quicker.