Hello All, I want to know if I can connect 2 x Must 3kW inverters to the 24V battery bank? The two won't be paralleled and will supply seperate circuits (1 is for the lights and fridges the other for the geyser). They will each have 2 or 3 x 550W solar panels to charge the batteries in parallel. What can go wrong?

7 hours ago, ErichK said:

can connect 2 x Must 3kW inverters to the 24V battery bank?

Yes, you can. The loads must he separate if you don't have paralleling boards (for other readers).

7 hours ago, ErichK said:

What can go wrong?

You could overload your battery, depending on many factors. 24 V is already low, so that one inverter can generally tax a battery to the max on its own.  The geyser is likely far too high a load for one battery, let alone a battery with other duties.

Otherwise, not much.

Edited February 9, 20233 yr by Coulomb

2 hours ago, Coulomb said:

Yes, you can. The loads must he separate if you don't have paralleling boards (for other readers).

You could overload your battery, depending on many factors. 24 V is already low, so that one inverter can generally tax a battery to the max on its own.  The geyser is likely far too high a load for one battery, let alone a battery with other duties.

Otherwise, not much.

Thanks Coulomb, the batteries are Hubble S100 12V lithiums with a 1C charge/discharge rating. The geyser element will be replaced with a 2kW element. That equals about 83A at 24V. It is high, I know. There will be 3kW of solar on the roof charging via 2 x 60A mppt's and the geyser will only run during day time and one hour in the morning. My maths is very basic and I hope I'm not missing some other dynamic that will influence the performance.

2 hours ago, ErichK said:

That equals about 83A at 24V.

More like 90 A with losses. You might need another string of battery modules.

3 hours ago, ErichK said:

The geyser element will be replaced with a 2kW element. That equals about 83A at 24V. It is high, I know. There will be 3kW of solar on the roof charging via 2 x 60A mppt's and the geyser will only run during day time and one hour in the morning.

You would be better off with installing a separate direct water heating panel for the geyser.

I wouldn't think drawing from a battery bank in parallel should be a problem, but hitting it with two chargers in parallel might cause an issue if they don't know about each other and try to get "smart" with the charge curve.

If you can get away with it, charge with one charger and disable the other or use one charger for PV and the other for utility charging.

It might also be more efficient to put a DC element in the geyser and drive it directly from the panels but it is still not nearly as efficient as a direct solar water heater if that is an option. You get 1kw equivalent per square meter on a water heater, but a 1kw PV is probably 2 or 3 sqm or more roof space.

In my own setup I use the geyser (with a standard element) as an "excess energy battery". I monitor the PV supply and the load and if there is more supply than load, the excess is pumped into the geyser. This goes on the whole day, little by little, until it is hot. It is called an energy diverter... look at emonCMS energy diverter for ideas.

The algorithm also switches the geyser off if there is other demand to not overload the inverter. It's a bit finicky at times but interesting.

 

Parallel charging actually is a problem: I have seen this over and over again - since the chargers do not have a 100% perfect calibration, one charger will stop charging, prompting the other to open the throttle even wider - from the POV of the 2nd charger, the battery grew much easier to charge in a few ms, prompting it to go right through the CV ceiling, then throtteling back apruptly, falling below float voltage and repeating this cycle again and again. In german this is called "Pendelladung" (something like "pendulum charging") and it ruins the batteries quite fast.

I solve it by switching off one of the PV strings via an automotive (EV) relais and an arduino once the battery voltage hits a threshold.

Thanks everyone for the valuable feedback. I see there are too many risks with this setup. I'm going back to the drawing board.

On 2023/02/09 at 7:45 PM, RudShep said:

I wouldn't think drawing from a battery bank in parallel should be a problem, but hitting it with two chargers in parallel might cause an issue if they don't know about each other and try to get "smart" with the charge curve.

If you can get away with it, charge with one charger and disable the other or use one charger for PV and the other for utility charging.

It might also be more efficient to put a DC element in the geyser and drive it directly from the panels but it is still not nearly as efficient as a direct solar water heater if that is an option. You get 1kw equivalent per square meter on a water heater, but a 1kw PV is probably 2 or 3 sqm or more roof space.

In my own setup I use the geyser (with a standard element) as an "excess energy battery". I monitor the PV supply and the load and if there is more supply than load, the excess is pumped into the geyser. This goes on the whole day, little by little, until it is hot. It is called an energy diverter... look at emonCMS energy diverter for ideas.

The algorithm also switches the geyser off if there is other demand to not overload the inverter. It's a bit finicky at times but interesting.

 

Hi RudShep

 I am looking for a solar PV diverter solution in South Africa. You appear to have one. I installed solar PV when I lived in France and I bought a diverter from a chap on the web who constructed them. Essentially, it was a modified electricity meter on his circuit breaker board. All unused electricity for the PV panels was directed to the geyser, and only once the geyser was up to temperature did it export to the grid. For example, if only 350W was unused, it directed the 350W to the geyser. It did not have to wait for 2kW (or whatever the element rating was) to be unused before switching the immersion on. He has all the instructions on the web on how to do it. It looks easy if you can do the electronics and program the flash memory with the code he supplies. I bought a fully assembled unit and it worked perfectly. Essentially, I am looking for the South African equivalent. I can link to the website with all the instructions.

Thanks

Peter