What's a good Android app for keeping in touch with the whole damn crew on a job site, without having to yell across the yard?

Look, I need something that works, plain and simple. Slack, it gets the job done. We use it to coordinate deliveries, share blueprints, and even send pics of problems we find. Beats the hell out of yelling or trying to find someone on the phone. The channels are good for keeping things organized – one for plumbing, one for electrical, you get the idea. It's not perfect; the battery drain can be a real pain in the ass, especially when you're out on a site with limited charging. And the tutorial? Forget about it, it's useless. But once you figure it out, it's solid. Been using it for a year now, and it's actually rugged. Survived a couple of drops already. Rain doesn't stop it, either. Just wish it didn't suck up so much juice.