>>28599569
USB charger for power tools batteries, they make them for a number of major brands. Can charge the battery from any USB output (slow though, like 5V 2.4A or 3A max) and can function as a power bank (probably the same 10-15W but enough to charge the phone) or output unregulated battery voltage (not tested yet).
Beats any OEM charger: it's more compact, doesn't need mains so I can use it in the car, costs like $15, has a battery SoC indicator. Going to order more of them.