Battery charging

Cariboocoot wrote: »

You could, but it would be problematic.

The first problem would be the need to build two totally isolated 24 Volt charging systems. Keeping the negatives separate from one another is the biggest challenge; sometimes they are tied to the cases, et cetera. The negative of one will be connected to the positive of the other at the midpoint in the battery bank. This all sounds simple, but in practical terms it isn't so easy.

The other issue would be the potential for one half of the battery bank to be charged differently from the other. One charge controller will have one 'take' on the Voltage, set points, and temperature while the other may have another. So one half gets 28.8 Absorb and the other might get 29.5 or some such. This is a problem that will get worse with age, as equalizing will be nearly impossible from two separate charge points.

boB wrote: »

We've done this quite a few times. As coot said, you have to be careful of the cases (grounds) possibly but for MidNite or Outback MPPT controllers, the limiting factor
there is simply the internal MOVs used for lightning and surge protection had to be pulled out for very high PV voltages because they are basically in series
and tied to the chassis (ground). So it depends on the controller.

The top half PV array cannot be grounded on its negative or positive lines so it needs to be floating relative to the bottom half controller.

You also cannot ground the top half controller's negative line and you also have to watch out for communications lines as they are usually
referenced to battery negative... At least the top half charge controller has to be watched out for. Ethernet, if available, is normally
isolated so it would be OK to reference to ground.

As for the voltage adjustments, that should be OK as each half of the battery bank would be charged to the same voltage (hopefully)
if the Absorb and Float voltages are set the same.

So it can and has been done. You just have to watch out for these types of things.

At Outback, people with 120V battery banks would split them up between two (2) 60V halves and use two MX60 charge controllers.
The internal MOVs would start conducting somewhere around 200 volts so the two PV arrays together, referenced to ground would
be close to that voltage and so the top half MX60 had to have its PV positive MOV clipped out. That was the red disc MOV on the PCB.
Other charge controllers may be similar, especially and Outback and Midnite Classic but I'm not sure about other controllers...

Merry Christmas !

boB