I second Jake's recommendations. I also used PW's, 580 and 580II's until I replaced them all with the Flashpoint system. Instead of fumbling with 4 "AA" batteries with Canon speedlites, you use a single square Li-ion battery cell that yields 650 flash pops at full power (I tested it) and recycles faster and at longer duration. With built-in receiver, no more having to attach any receivers to 580's, either.
A couple years ago, Powerex battery company contacted me out of the blue and asked me to do another round of battery tests after 10 years since I posted my first battery test on this forum. Apparently, their revenue went up significantly after the test results posted here. I did follow through with their request for the updated test, but I did so half heartedly knowing that, with these new Li-ion batteries that offer nearly triple the power of "AA" batteries, I really didn't see much of future for "AA" batteries. I told Powerex as much and suggested that they need to look beyond "AA" format.
I now have 3 Flashpoint Zoom Li-on R2 TTL flash units, all working perfectly for the past several years. Since then, they came up with a round headed version, and it's currently on Black Friday sale for just $.35 more than the one Jake linked to. Given the price, I'm really tempted to get one:
https://www.adorama.com …ca.html?CategoryID=249442
One perhaps significant advantage of the round headed version is that there are all kinds of flash head attachments that you can get without the adapter (rectangular to round head).