Did I just outsmarted a smart shunt??

[Archerite]

This might be a really stupid idea....which is why I am asking here if it makes sense.

Last week I finally installed a victron smart shunt into my solar setup. It's an experimental toy at the moment only used to charge USB powerbanks....which I use to recharge my phones/tablets when it's convenient to me and not when the sun decides to show up this time of year. I only have two tiny 7.2Ah batteries in parallel in this system so that's not even enough to keep a Raspberry Pi running more than 1-2 days! I use that Rpi to upload the stats to Victron and connect remotely when not in range of the bluetooth.

Offcourse I know SLA batteries should not be discharged below 50% of their rated capacity, and it's even better to stay above 80%. I have done my research. But what I do not like is that the display of "remaining percentage" is not smart enough to show what I can actually use! I mean...just figure it out and let me "See" it go to 0%!

What I have done now is instead of setting my battery capacity to 14.4Ah (or 14Ah as decimals do not work) I just set it to 7Ah now! Meaning I see a range of 100% all the way down to 0% without actually discharging the battery itself below it's 50% rated capacity! At least....that's my theory!

Now is there any technical reason or limitation why this should not be done? I know I am just fooling myself, but to me this makes a lot more sense actually.

 

[rmaddy]

The big question is what does the smart shunt show if you run the batteries down to an actual SOC of 40%, for example? Does it still show 0 or does it show -10%? If it shows 0 for all for SOCs below an actual SOC of 50% then it's a bad idea because then you don't know what is really happening. I know I'd much rather know the real value. When it get's to 50% I know it's done. If I see 45% then I really know I screwed up. But if, with your proposed setup, it I see it is at 0, I don't know if it just got to 0 or if it's been 0 for two hours and it's really at 30% and I'm screwed.

 

[Archerite]

The big question is what does the smart shunt show if you run the batteries down to an actual SOC of 40%, for example? Does it still show 0 or does it show -10%? If it shows 0 for all for SOCs below an actual SOC of 50% then it's a bad idea because then you don't know what is really happening. I know I'd much rather know the real value. When it get's to 50% I know it's done. If I see 45% then I really know I screwed up. But if, with your proposed setup, it I see it is at 0, I don't know if it just got to 0 or if it's been 0 for two hours and it's really at 30% and I'm screwed.

That's a really good point I had not thought off! I just had my android phone and tablet in mind when I thought about this....when they hit 0% they shutdown immediately. But before that happens you get a warning at 15% and at 5% the brightness goes down. Offcourse this does not happen automatically with a solar battery bank unless you build it that way.

I did test it quickly by lowering the "capacity" even more down to 3Ah...since I already used 3.4Ah today. And it seems to stay at 0% and does not go negative....probably because that should not be possible with real batteries! So you are right, for a real system you might need to know the actual value. For my experimental setup it might be "ok" to do this, but going below 0% is indeed an issue to keep in mind. And that is a perfectly valid reason why this is not a recommended thing to do.

 

[RCinFLA]

You can set battery capacity on a columb counter monitor to any number you like. It just uses that number to add or subtract its cumulative incremental charge and discharge current vs time tally from. When monitor reaches its reset voltage you set for full charge voltage it zeros out its cumulative summation. If charging continues and battery voltage rise a bit above reset voltage the monitor should just keep resetting to 100% until battery drop below full recharge reset voltage point.

Some 'smart' monitors compute and replace the battery AH number based on rapid voltage drop on battery near full discharge and max charge voltage setting. If you never allow battery to fully deplete, the monitor should never replace the AH capacity value. I am not a fan of using automatic battery capacity calculations as high load current spikes can sometimes be mis-interpreted as battery nearing full discharge.

In your experimentation realize the shunt resistance is likely appropriate for a larger amperage useage. For low current the shunt output voltage is very low, micro-volts, and the noise floor of monitor input op amp will impact monitor accuracy. Monitors have a very low frequency low pass filter, like a Hz or two, to average out the 120 Hz ripple current from inverter. Average is used rather than RMS because average better represents AH useage from battery.

If you do not charge enough to reach monitor reset voltage the cumulation just keeps running its established tally. Errors will cumulate over time so not allowing monitor to reset periodically will result in growing error in its % state of charge readout.

 

[Archerite]

You can set battery capacity on a columb counter monitor to any number you like. It just uses that number to add or subtract its cumulative incremental charge and discharge current vs time tally from. When monitor reaches its reset voltage you set for full charge voltage it zeros out its cumulative summation. If charging continues and battery voltage rise a bit above reset voltage the monitor should just keep resetting to 100% until battery drop below full recharge reset voltage point.

That sounds more like how I was thinking it worked! It just count's the amps going through and with the setting for voltage and capacity it calculates a remaining percentage. Then it might still be possible to do it this way right?

In your experimentation realize the shunt resistance is likely appropriate for a larger amperage useage. For low current the shunt output voltage is very low, micro-volts, and the noise floor of monitor input op amp will impact monitor accuracy. Monitors have a very low frequency low pass filter, like a Hz or two, to average out the 120 Hz ripple current from inverter. Average is used rather than RMS because average better represents AH useage from battery.

I am hoping that for the price of the Victron Smart Shunt they include high quality and very precise components. There are many tiny and minimal load like the MPPT and some LED's that it does seem to monitor and add up to the total. But it's too low to show a live usage in the app. Still a good thing to keep in mind that it's not an exact measurement....but at least far more accurate then guessing based on the voltage alone.

If you do not charge enough to reach monitor reset voltage the cumulation just keeps running its established tally. Errors will cumulate over time so not allowing monitor to reset periodically will result in growing error in its % state of charge readout.

That is entirely up to the sun how much it reaches that fully charged state, hahaha. But I know what you mean and I did read about the drift in the values if it does not reset often enough. Like I said before this is only in the solar setup for about a week now and this time of year not a lot of sunshine. But when it's cloudy for the day based on forecast data I do shutdown all the loads on the MPPT and let it charge up to 100%....or when it reaches float before I had the shunt installed.
Only then I enable the loads and that's what I did arround 12:00 today when it was at 100%. After about 4 hours with a 40 watt load and some of it coming directly from solar it now shows 35% of usable capacity left (with my new settings). ?