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paulm asked

What battery voltage should I auto connect / disconnect my inverter load ?

I'm sure this has probably been discussed before but cannot find a definitive answer...

I have a 650W off grid solar PV system with a Victron MPPT 100/20 charger, four 95Ah Exide AGM batteries linked as 2 x 24v pairs and a Victron 800W 240v inverter. I'm powering my computer servers, as they run 24/7, via a UPS that allows me to manually transfer the power connection from grid mains to PV power and back manually without interrruption. The computers are drawing ~250-300 Watts at present.

I'm building an automatic switch to handle the switching between grid mains and PV power, PV power having priority when available, else grid mains being connected. This has to be self contained due to its physical location.

I have an Arduino SBC monitoring the PV and battery voltages and currents, the plan being to use the Arduino to control the inverter remote on-off with a relay, hence causing the (remote) automatic power switch to connect the PV power to my computers when it's on and grid mains when it's off. I also have a Raspberry Pi running the Victron Venus OS.

I've been running the system for a couple of months, manually switching the power between grid mains and PV power based on the weather and data from the Arduino monitor and Venus OS.

I've trawled through endless notes and discussions and come to the conclusion that I should disable the Victron inverter when the battery voltage drops to ~24v on load, which seems to indicate ~50% battery discharge. I should re-enable the inverter when the battery voltage reaches ~26.0v on charge but off load. This appears to give me a sensible cycle between grid and solar power whilst attempting to maximise the life of the AGM batteries.

I'd appreciate any comments on this scheme from those much more knowledgable than me on this. I'm aware I can probably achieve this set up using standard Victron equipment but there's no fun in 'leggo electronics' :-)

Regards

PaulM



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JohnC answered ·

Hi @PaulM

Using V and a 'load' is a really coarse way to do this. You're really guessing, and your '26.0V on charge' is bound to be an insufficient charge to expect long life from the batts.

Grab one of the shunts, maybe a BMV with a relay, and you can do your switching from there using even SOC.

Also, if your batts are subject to varied temperatures, it gets nigh impossible to control with V.

You owe you batts a shunt. The day you find they've failed you'll know that for sure..

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paulm avatar image paulm commented ·

Many thanks for your comments.

I really can't justify spending any more money at present, so need to make use of what I already have available. I'm retired so have the time, and the interest, to deveop my own bits and pieces.

The batteries are in the basement of the house, which is pretty much at a constant temperature due to the thermal mass of the building :-)

You apear to be suggesting I should re-enable the inverter when the batteries are fully charged rather than when they reach 26v as the latter isn't a good way to determine the charge state ?

I already have a current shunt in the ground connection to the batteries, monitored by the Arduino. Is monitoring the battery charge current on no load a better way of determining when the batteries are fully charged ? I can see the change in battery voltage when the charger changes mode on the graphs, and the charger mode is available from Victron OS, so may be I can use that ? Failing that I could use the Arduino to track the state of charge of the battery as it has access to the battery voltage and current, are there any technical notes anywhere giving details of how to do this ?

Regards

PaulM


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JohnC avatar image JohnC ♦ paulm commented ·
@PaulM

26V is really a 'nothing' charge. Depending on how many A you're charging with, this can happen very early, way below a suitable SOC. Pb batts require a regular return to 100% or they'll sulphate up and die.

Ok, you have a shunt, great. You can tell if the batts are fully charged if they are holding their target charge V (let's say 14.4V) and won't accept more than a certain current. You'll need to determine this for youself, but as a guide, I know mine are 99.0% full when they won't accept more than ~3% (A-in vs Ah-capacity). My mppt then drops to Float and it finishes off at 13.6V (~0.6% A/Ah at that Float V gives me 100.0%).

The Victron shunt algorithm is quite complex, but you may be able to get a clue as to how low you're depleting them by counting the Ah discharged. But that's just a clue, putting that energy back isn't very efficient, and counting Ah back in will mislead you.

Try checking with the shunt and see how you go..

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