question

hry avatar image
hry asked

Recommended low voltage shutdown for AGM batteries?

Hello all!

I have 3x 280Ah 12V batteries in parallel running into a Phoenix 12/800.

Any recommendations on a good setting for the low voltage shutdown?

I have read variously around the internet that lower than 11.5 – 12v is already too deep a DOD to maintain the batteries for a reasonable life. Oddly, the default setting for shutdown on the Phoenix is 9.6V. Isn't that way too low? According to my (dodgy) sources, the battery should still be putting out ~10V at effectively 100% DOD.

I have contacted the manufacturer for discharge curves / data but you know. General recs are most welcome.

Thanks

H

Phoenix Inverterlow battery warning
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1 Answer
snoobler avatar image
snoobler answered ·

Unfortunately, voltage to state of charge relationships only apply when AGM batteries have been completely dormant for several hours, i.e., just sitting there - no charge or discharge.


It's difficult to specify an arbitrary voltage because voltage drops under load.


Your 280Ah (840Ah) batteries are likely rated at a 20 hour discharge, i.e., 840Ah/20h = 42A. Anything above this means you have less than 840Ah.


Assuming you're pulling a 800W load, that's a 800W/12V = 67A load. The moment you unload it, the voltage will rebound to a higher level more indicative of SoC than when loaded. If your battery is at ~60% SoC, and you apply a max load do it, do you want it to shut off because you've specified too high a votlage cut off? Probably not.


The best solution is to get a BMV or smart shunt and track the true SoC. Get it down to 50% and hit it with a max load. After a few minutes, note the battery voltage and then set your cut-off to something a little below that.


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npm108 avatar image npm108 commented ·
What a fantastic insight, learnt something new thanks to your reply about voltage vs load.


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