Hi Paul,
Where you see a pair of sockets (solar and grid) with a dotted line to a device or circuit, that's just a manual change-over socket. Nothing more than a 13A plug that can be moved between two 13A sockets. Avoids having any "dodgy" switches or relays with different phase mains on it which can cause magnetic recoil in transformers if you switch too quickly between sources. By using a 13A plug/socket it also makes a convenient 3 pole switch that is guaranteed to take more than 1 second to change over so there's no risk of a magnetic recoil surge from the load.
The two controllers co-operate just fine. They are programmed for the same voltage set points and both have the same remote temperature readings for compensation. Mostly, they both put out current into the battery but in the final stages of charging, the weaker one (by reason of PV input or output capacity) will sometimes defer to the other one.
So, if the battery and loads are only demanding 5A and both controllers are receiving at least 5A of PV input, one might throttle back to 0A (but not go negative) and the other will sit at 5A. If neither can deliver the full 5A, then they'll do what they can (e.g. 3+2) as they both want to maintain the same voltage.
If one goes into float before the other then the one that is trying to "float" at a lower voltage will just sit at 0A (again, it won't go negative) until the other one gives up "absorbing" and then they'll both sit at float level. When I used flooded batteries I used to set just one controller to do equalise charges as that way you didn't get too many equalise charges in a month (it was programmed on a 28 day cycle or after a >70% discharge). They didn't communicate though so you couldn't tell one to not do an EQ because the other had already done one, so I disabled EQ charges on the other.
It's not an issue with my current gel batteries as they don't need equalisation charges anyway.
A slow cooker might be harder to power than you think. A typical slow cooker is about 350W and might have to run for 4 hours. That's a lot of battery juice (if the PV can't keep up). That's about 64Ah at 24V.
By contrast, I often use a mini toaster oven (450W for 5-10 mins) or a kettle (950W for 3 mins) or a automatic egg steam boiler (350W for 10 mins) or the rice cooker (350W for 20 mins)... None uses more than 5Ah so even if it's cloudy or early in the morning, I can use them and the PV will put it back quite quickly.
Only on really clear blue sky days can I consider the immersion heater over to solar power (650W for 2-3 hours). The battery has to be in a high state of charge and I have to keep an eye out for cloudy weather coming so as to make sure that the PV is taking most (if not all) of the strain with the battery just filling in the gaps.
Putting a PWM controller on the same PV input as the MPPT of the Soldin might wig it out. I had problems with one experiment where I had too much voltage on the input of my SunSaver with amorphous panels in triplets (the open circuit voltage got too close to the 75V limit of the controller). So I devised a voltage limiter to apply a load across the PV when it was swept by the MPPT sweep. This clamping of the PV open circuit voltage to about 65V confused the hell out of the MPPT software and it started doing weird things (locking on to non-maximum power points at odd voltages), so I gave up and went back to just pairs of panels.
A simple PWM controller (like the 12/24V ProStar) can't handle the high input voltages of a Soladin PV string. Only MPPT controllers are designed to work at higher PV voltages (75V to 150V). I believe the Soladin needs at least a 48-60V nominal array to work. A 48V ProStar would do though (if using a 48V nominal array) but then you'd need a 48V battery bank and a 48V inverter to do something useful (like run a slow cooker) from it. Those cheap recycled 3kW UPS inverters you sometimes see on eBay often run from 48V.
Trouble is, the ProStar controllers only use PWM throttling when in their absorption or float phase. In the bulk phase they connect the PV to the battery directly and so try to push as much current as the PV can deliver into it (maybe with PWM current limiting if you overload the input - the ProStar is rated for up to 25% overload). If that doesn't wig out the Soladin, I don't know what would
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