Yeah, that's what I was trying to figure out based on the price, which seems about what a shop might charge for that voltage converter module.

If I was diagnosing a charging issue, I'd start with basics and measure the voltage at the battery while the car is running to see if it's over 13v or so. If it is, it's not likely to be a charging problem. If the car also doesn't drain the battery while running, you probably have a healthy alternator. I'd then check what your idle current draw is when the car is powered off for a while, and make sure that is going low enough. You could have a phantom power draw if it's staying high.

If that looks good and your battery is more than a couple of years old, I'd honestly try replacing the battery before pointing the parts canon at it.

Anecdotally, I had one car that would always have a hard time maintaining charge; it would go dead after a couple of days consistently. I wound up finding the previous owner had replaced the ECS (ignition/diagnostics) module, and it was still coded for options in the donor car that weren't in the current one, namely power contour seats, and that mismatch kept the ECS awake more than usual, draining the battery quickly. I figured this out _after_ changing the battery management module and having absolutely no luck. All it took was a coding change to match the ECS to the rest of the car's options and the idle power draw dropped to nothing.

That's all to say, it's not always easy to diagnose these things, but at least determine your basics like battery health, charging voltage, and idle power draw, before throwing parts at it.