Lots of options...
Power rectifier would be the easiest.
An Emerson R48-3000e3 can be had for $75.
Based of the informative info recently posted by @ChrisFullPower in another thread I'm intrigued in DIY'ing a charger from used rectifiers. A quick search on eBay for '48v rectifier' has made me realize it's definitely doable at costs much lower than off the shelf options... I'll start with a...
diysolarforum.com
You don't need to use any of the fancy programming features in the thread above, could just buy it for a pre-programmed voltage.
You could also pick a "cheap" AIO like the SRNE HYP, ASP, or HESP and use it for charging only if you wanted to have a additional solar charging power as well.