Charging your iPhones and iPads every day with a stable charging system is critical to the success of putting phones in the field. There a 1000 different charging solutions for iPhones and iPads, but there are some trusted vendors that clearly better than others.
At a minimum, you need a modern charging hub. Some charging hubs are only chargers, others are chargers and act as actual USB Hubs that you can use to talk to the iPhones with. If you have 1 to 10 phones, you'll probably be happy with a charging hub. If you have more than that, you'll probably want a Charging Sync Hub to help you manage and update the phones.
Anker manufactures a line of charging hubs that are made of premium components and have the intelligent charging ports built into them to maximize the charge. Any 'PowerPort' charger by Anker is great. They can be bought from Amazon. A 10 port charging hub will cost roughly $40.
At the high end of charging / data sync, we use a Cambrionix PowerPad15s. Can charge and sync up to 15 phones at the same time, and it has the ability to monitor the charge status while it's charging. This is particularly useful when you have phones that aren't taking a full charge and you're trying to figure out which ones they are and how much power they are taking.
Quality Lightening / USB cables are critical to charging. Don't be fooled by cheap cables that don't meet the voltage or certification requirements. When you buy wires to charge your phones, please make sure that they are MFi certified, or made by a quality manufacturer such as Anker. Original cables from Apple are the best, but we've have good luck with Cables from AmazonBasics and Anker.
If an iPhone has decent cellular coverage, and the GPS is set to a Timed Update every 10 minutes, an iPhone can easily record 500 to 800 transactions with a single charge. Poor cellular signal forces the iPhone to try communicating with our servers over and over and chews through battery. Turning the precision of the GPS up also chews through battery. We highly recommend having some backup battery systems on hand.
Like all power related accessories, don't buy the cheapest battery systems. A good battery pack from Anker costs roughly $20.
Apple makes what is called a 'Smart Case' for the iPhone 6 and 7. They cost roughly $100 and work fairly well. The phone is slipped into the case and the case is put on the charger. We have experienced some minor issues with either the case getting charged and not the iPhone, or vice versa. This was solved by pulling the iPhone out and putting the iPhone back in the Smart Case.
To check the health of your iPhone / iPad battery, go to 'Settings' / 'Battery' and then select 'Battery Health'. It will give you a percent life of the battery. Anything over 90% is ok. Anything less than that and you really should look at having the battery replaced. Last time we checked you could take it to any Apple Store and have it done for $30.