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Pylontechs and Axpert Off Grid


Bush Man
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Dear All

 

The current setup is as follows:

Completely off grid, in the bush, no Eskom, no Wifi.

12x250 Watt panels, 4 strings of 3 each

Microcare 4 string combiner unit

2016 Model Axpert/Mecer SOL-I-AX-5M 4kw 5kva Inverter

8 Omnipower 180 AGM batteries (actual rating 150Ah), 2 strings of 4.

Very light on power, fridge and LED lights only, with the occasional microwave or a hair dryer use for short duration. Calculated night time consumption with fat built in is 3kwh. 

The place is used as a weekend place by different people, so there is no one there to monitor and keep an eye on the system. 

The batteries have served well for 5 years, but it is time to look at replacement. After much reading on the forum and elsewhere, the current choice is for 2 x Pylontech UP5000 4.8kwh batteries, with a combined capacity of 9.6kwh. 

 

Now for the one remaining question, specifically relating to the Axpert’s cut-out voltage setting off grid.

The Axpert inverter is known for inaccurate SOC readings. It would typically drop the voltage reading by around 2V under load, and then recover.  With a 4V range between full (52V) and empty (48V), the system easily temporarily dips below 47V under load. With AGM batteries the solution was simple – set the Axpert cut-out voltage lower to provide for the dip.

 The Pylontech’s internal BMS will, however cut out the battery when this happens.  It can be manually reset, but one doesn’t want unfamiliar people fiddling with the batteries in the dark.

 The normal go-to for a more accurate SOC reading by the inverter would be Centurion Solar’s ICC Raspberry Pi.

 

However, according to both Centurion Solar and this Pylontech guide this will be of no use in an off grid scenario.  I believe it works only for the back-to-grid setting in a grid tied scenario, and not for the cut-out setting in an off grid scenario. 

In addition, this model inverter will not communicate with the Pylontech BMS through the comms cable available from Pylontech.

 

That leaves the Axpert’s fluctuating voltage reading through the battery cables as the only possible communication between the inverter and the batteries with regard to SOC. 

The only way to deal with it that I know of would be to set the Axpert’s cutout voltage at, say 47.5V, so that the Axpert cuts out before the batteries do.  Switching on the microwave while the batteries are still at, say 49.4V, the inverter voltage reading will drop to below 47.5V and the inverter will cut out.  This, however, will mean that a huge part of the available battery capacity (everything below 49.4V in this example) will be lost / unusable.

 

Is there any other way to have more of the batteries’ capacity available with this inverter in an off grid scenario?

 

Regards

Bush Man

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4 hours ago, Bush Man said:

The only way to deal with it that I know of would be to set the Axpert’s cutout voltage at, say 47.5V, so that the Axpert cuts out before the batteries do.  Switching on the microwave while the batteries are still at, say 49.4V, the inverter voltage reading will drop to below 47.5V and the inverter will cut out.  This, however, will mean that a huge part of the available battery capacity (everything below 49.4V in this example) will be lost / unusable.

The Pylontechs should be a little stiffer in voltage (less voltage sag for the same load). You could reflash the inverter with the LFP flavour of patched firmware version 73.00e. Among other features, it has KettleKomp™, which compensates for the internal resistance of the battery. There is a little setting up (covered in the manual), not too hard. It improves the SOC reading from dreadful to merely poor. Or you could install something like a Victron BMV to provide a more accurate SOC reading, if that's important.

I don't know the Pylontechs, but I thought that they could handle a reasonable overload for a short period of time. Adding up all these slight improvements, things might not be too bad.

 

Edited by Coulomb
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  • 4 weeks later...

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