The front of the battery features four white LEDs that serve as battery indicators. The battery is fully charged when all four are lit -- from there, they'll drop out one by one as the juice runs out. Again, Fremo claims that'll take at least six hours of continuous playback.
My tests bear that claim out -- and then some, to put it mildly. After giving the base a full charge, I attached an Echo Dot, then unplugged it and left it playing at full volume. Six hours later, it was still going strong, with three of those four battery indicators still lit up.
I ended up taking the speaker home with me to continue my tests. It kept on streaming my Spotify tunes for another several hours, finally running dry at three in the morning after roughly 13 hours of continuous use (save for about 30 minutes on standby as I drove home from the office with it). Afterward, it recharged in about four and a half hours.
Both of those are terrific results, and as I tested the Evo out, taking my Echo Dot around the house to listen to podcasts while in the shower or some music while I cooked dinner, I could absolutely see the product's appeal.
That said, it's not as elegant an option as the Amazon Tap, which sits in a base to recharge. When you want to take it with you, you just pick it up and go, and when you're done, you just set it back in the base. You don't need to worry about the cable at all.
To that end, a higher-end version of the Evo or something similar with a separate charging base might be a product worth waiting for. You might also consider spending a little extra on a battery base that doubles as an enhanced speaker, given the Dot's relatively limited sound output. The Evo doesn't do anything to improve the Dot's volume or sound quality.
Still, I like the Evo, and with its impressive battery life and relatively low price, I think it merits a look from Echo Dot owners who are happy with their speaker's native sound output but who wish they could take it with them around the house.