A portable charger is reliant on a power source like an electrical main to stay charged. Power points are non-existent so you will need to bring a portable charger for Kilimanjaro. So, once it’s depleted of energy it won’t be possible to recharge it until the end of the trek. This also accounts for your other devices.
It’s worth bringing a portable charger if it can hold enough energy to power all of your devices for the duration of your trip to Kilimanjaro. However, keep in mind that batteries get depleted much faster than usual in cold circumstances. Therefore, a solar charger may be the best and most reliable choice.