Voltage drop under load, calculating batteries required

[ns31]

I recently switched on my 11kw mpp solar off grid solution which has 26x 405w panels and 2x us5000 batteries.

The system works great, but when running from battery power and a heavy load us being pulled (e.g. 8kw), the system switches to utility.

So it seems 10kw batteries has too much voltage drop to handle large loads. Is this correct? If so, what's the best way to calculate battery requirements? Ideally the batteries would handle 11kw loads

 

[ABJ]

You don't have enough batteries, is the quick answer.

The inverter can draw 228Ah max for 11kW @ 55V the power drops off with lower voltages, at 48V you may only get 9.5kW max when running just on batteries.

Your two batteries maybe able to discharge 200Ah max ???
Adding another will get you 300Ah total discharge ( if they are rated @ 100Ah discharge )

 

[SeaGal]

Agree with above, apart from the fact you won't be running anywhere near 55V.

You need to bear in mind that Pylontech use the unusual (=cheapskate) format of only 15 cells, rather than the more common 16 cell architecture - therefore your voltages will be 3.2-3.3V lower than most systems and thus requiring more Amps to give the same number of Watts.

At 3.2V per cell you will be running the dc side at 48V. The US5000 have a recommended discharge rate of 50A, with max of 100A. So absolute max at 48V is 9.kW.

But assuming your Pylontechs are configured to run at the recommended 50A rather than max discharge rate, you will only get 2.4kW 4.8kW from each of them (= total 4.8kW).

I'd recommend another 2 US5000's to sensibly handle up to 9.6kW or another 3 to handle the 11kW loads.

Also bear in mind that even with five us5000's, running at 90% DOD, you will only be able to power 11kW for just shy of 2 hours.

 

[ns31]

Great info, thanks.

Any thoughts on better batteries than pylontech? It's prob too late for me now but I'd be interested to know similar brands with 16 cell

 

[SeaGal]

Great info, thanks.

Any thoughts on better batteries than pylontech? It's prob too late for me now but I'd be interested to know similar brands with 16 cell

I DIY'd mine, but as you're in the UK, the Fogstar ones seem to be a popular choice...

Fogstar Energy Server Rack Battery 48V (5.12kWh)

UK-designed Fogstar Energy SR51.2V Lithium (LiFePO4) Server Rack Battery with heating, pre-loaded inverter protocols, LCD touch screen, 16 Grade A EVE Cells, a DC Rated Breaker and an 8 year warranty.

www.fogstar.co.uk