Well, ya checked all the issue boxes!
If you were able to decipher what I did, either I have just the IpH's pump connected to the Power Center, OR, I have both the IC and IpH connected to it, but in their "normal" configuration. So you're going to attempt to have the IC connected to the PowerCenter, while at the same time also powering the IpH pump. I believe others here have done that, but I haven't personally. It should be OK, but, again, when my IC is online, and my IpH starts pumping, my IC is not producing, so the total amps would be lower. See what I mean? Something to watch out for. So...
You got some of it. Acid pump wired to a Buck converter, wired to a relay on the ET, wired to the IC power center output. But I also have a flow switch in series too, so three things have to be true before my acid pump gets power: the ET relay must be closed, there must be flow, and the pool circuit must be on.
If your Power Center is properly wired, it should only get power when the pool circuit is on. So that would cover that aspect. You don't have to have a flow sensor in the mix if you don't want to, but I wanted that extra safe guard. I can walk you through that if you want to add that. And you don't have to have a buck converter, either. Another here wired his up without one. The Power Center states a DC output of, like 20-38VDC, something like that, which is pretty vague. The IpH pump gets 24VDC, so I used the Buck Converter to better guarantee the IpH pump gets that 24, and not 38 or 20 or whatever. Just precautionary, as is the flow switch. So I went super safe, others just wire the pump to an ET relay and the Power Center and go with that.
Yes, the dosing is then controlled by scheduling the ET relay in one minute intervals. That works for me in the winter. If you need finer control, you could always adjust the dilution of the acid in the tank.
I forgot to mention, the other thing you give up without the IpH controller, is that it's programmed to dispense only for a set, short time. That's yet another of its safety features. With this alternate workaround, there is nothing doing that for you except your ET schedule. If it goes haywire, or if someone inadvertently presses the wrong button on the face of the ET, that could conceivably empty the tank into your pool. I think I wrote about that. If you dilute your acid 1:1, then at most you have a gallon and a half or so of acid in the tank, which won't destroy anything if that ever happens. Again, just making you aware of what you're getting into.
So lastly is the issue of dispensing acid through an active IC. Pentair suggests the injector go before the IC. I'm not sure what the ramifications are of installing it after the IC. Another here told us, but I've not forgotten, what the problem is with injecting acid into the IC while it's making chlorine. Mixing acid and chlorine is bad, so there's that. But I think it was also something to do with being bad for the IC's delicate plates. Whatever, it's not a good idea.
So you could try to solve this with scheduling, but as I mentioned, the ET severely limits what you can do. You're not going to be able to dispense a little acid every hour, for eight hours. And the cool thing about the IpH controller, is that it doesn't power down the IC while dispensing, it only suspends it, so that it can resume immediately after the IpH is done. You could try to schedule the IC to power down, then power on the IpH pump, then off, then power on the IC after that. But that'd be, what, three schedules or more, and then the IC would have to go through its entire startup procedure again. But that could work if you have enough schedules.
Or, you could schedule the filter pump to run, but not using the pool circuit, then pump acid first thing, then turn on the pool circuit so that the IC would start up after that. But then you couldn't really use the Power Center, because that should only power up with the pool circuit. You'd have to have a separate 24VDC power supply dedicated to the acid pump. Which is fine. You wouldn't need the Buck converter if you did it that way.
Sorry, just thinking out loud. I didn't have to solve for this, because the only time I can run my IpH pump via an ET relay, is when the IC is offline for the season, so coordinating the two is not necessary. You can see why I went to the lengths I did to keep the normal IpH/IC setup intact. That's what I meant by niceties. There are many here that promote the Stenner solution over the IntelllipH, because I think maybe they don't fully appreciate all that the IpH controller is doing behind the scenes like I do, in spite of its flaws...