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Battery discharge,voltage levels, and load question


viceroy
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Hi all,

 

Now that my system is in and being used, I've been trying to tweak it for best performance, efficiencies and of course balancing usage of the batteries against longevity.

 

For those who haven't seem my system, I have the following:

 

- 2x 4kW (8kW total) Axpert MKS inverters with integrated MPPT charger

- 12x 265W SolaireDirect panels (These were supposed to be Rensola 250W, but when on the roof getting a closer look, I saw the change)

- 12x 120Ah 12V AGM Vision HF batteries.

 

Anyway, I've got the inverters set that solar output takes priority over grid except if one of these two conditions is met:

- Solar not available.

- Battery Voltage reaches my programmed minimum voltage (set at 1V increments which is disappointing as I would have preferred 0.1V increments) currently to 49V

 

Charging priority is what I've been fiddling with but currently, the batteries are charged by a combination of solar & grid using constant voltage method (56V absorb, 54V float) up to 30A (specified by battery documentation, although I've seen other battery documentation that says any amperage providing its constant voltage)

 

Now my real question:

 

-  So I've set the minimum battery threshold to 49V so when the batteries reach that level, the system automatically switches to grid bypass until the batteries are fully charged. This switchover takes place when the batteries are under load, so the 49V under load could be 50.x V under no load (ie, when the load suddenly stops and switches to grid. How exactly should I be setting this so that the batteries don't really discharge below 60%? Should the value be the load value or the no load value?

 

I really hope the above makes sense.

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  • 1 month later...

Difficult question Viceroy.

Firstly, how is the battries divided between the 2 invertors?

12 / 2 = 6 battries

6 x 12V = 72V !!!

Or did you do 3 strings of 4 battries and combine that to both invertors?

If doing strings, please make sure each string is individually fused as well.

Now if one battery lets go, it does not take the complete bank down with it!!!

 

Anyway, without knowing what your draw on the battries is, it is difficult to advise you.

What you can do is disconnect the PV input, set your charger amps to ZERO and run your system from battries.

Once the battries goes to 49V and the invertor switch back to grid, to what voltage does the battries recover to?

That voltage you can use to see to what level your battries have been discharged to.

48.5V under no load should be about 50% discharged, so you have some room to breath.

 

Ideally you only looking for a single charge/discharge cycle per day.

If you cycling 3 or more times per day your battries will not last!!!

You then iether have to reduce the load, increase PV capacity or increase your battery bank size.

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