admiral Posted October 18, 2019 Share Posted October 18, 2019 The setup is like this: 48v bank of AGMs with only consumer / charger being the inverter (from the grid, for now) Venus GX doing the reporting No external battery monitoring The state of charge as reported by the venus console was 100 % - then we had our first bout of load shedding yesterday. The inverter was sitting in "storage mode" before the load shedding started. After 2 hours of load shedding the state of charge was reported as 95 % - very little loads running. When the grid returned the inverter switched right back into storage mode, and state of charge ended like this (about 24 hours later). I am curious as to how the multiplus does the state of charge calculation - does it have an internal shunt / hall sensor? I am not too worried about this right now - will see how far "out" the state of charge goes - but I am curious if I should perhaps change some settings on the inverter. Perhaps the charge efficiency (0.85 right now)? Quote Link to comment Share on other sites More sharing options...
___ Posted October 18, 2019 Share Posted October 18, 2019 49 minutes ago, admiral said: storage mode Battery voltage has to drop low enough before it will go back into Bulk (I think it has to drop 5V below the float voltage, or around 49V). With a large enough battery bank (or small enough loads) this often doesn't happen, and the inverter remains in the state it was before. 52 minutes ago, admiral said: I am not too worried about this right now - will see how far "out" the state of charge goes - but I am curious if I should perhaps change some settings on the inverter. Perhaps the charge efficiency (0.85 right now)? In my experience, 0.85 is pessimistic. You can increase it slightly. I've had one case where an ESS system was in KeepBatteriesCharged mode, using excess PV for loads... and the SOC would drop linearly to zero over about a week. The battery was held at the float voltage the entire time, so clearly the SOC was wrong. The reason for that was a charge efficiency that was too low. While the PV is used to power loads, tiny amounts of charge goes into- and is taken out of the battery over time. If the inverter gives too little credit for the charge current that is returned to it, it tends to drop out like that. admiral 1 Quote Link to comment Share on other sites More sharing options...
admiral Posted October 18, 2019 Author Share Posted October 18, 2019 Thanks, I will adjust the efficiency upwards at some time. Quote Link to comment Share on other sites More sharing options...
admiral Posted October 19, 2019 Author Share Posted October 19, 2019 I checked the efficiency setting today, and it was actually set to 0.8 - so I adjusted it to 0.9. I will see how that goes. I am curious as to how the inverter does the SOC calculation, and measurement - does it use a shunt or something else like a hall effect sensor? Quote Link to comment Share on other sites More sharing options...
___ Posted October 20, 2019 Share Posted October 20, 2019 23 hours ago, admiral said: I am curious as to how the inverter does the SOC calculation, and measurement - does it use a shunt or something else like a hall effect sensor? There is no sensor in the DC side. The DC current is estimated as whatever power comes in on the input side, minus whatever goes out on the output side. The remainder must be watts into the battery, so divide that by the battery voltage, and voila, you have your DC current. Well... a good estimate anyway. Typically fairly accurate above 2 ampere. After that it's just numerical integration over time to get amp-hours. Quote Link to comment Share on other sites More sharing options...
Coulomb Posted October 21, 2019 Share Posted October 21, 2019 8 hours ago, plonkster said: After that it's just numerical integration over time to get amp-hours. Eek. Integrating estimations. Does it seems to do a reasonable job, say at the end of a few days of rain/cloud were the SOC can't reset to 100%? Quote Link to comment Share on other sites More sharing options...
admiral Posted October 21, 2019 Author Share Posted October 21, 2019 Indeed, I am also curious as to how it will "reset" to 100 % like a BMV? Quote Link to comment Share on other sites More sharing options...
___ Posted October 21, 2019 Share Posted October 21, 2019 6 hours ago, Coulomb said: Eek. Integrating estimations. Indeed 6 hours ago, Coulomb said: Does it seems to do a reasonable job, say at the end of a few days of rain/cloud were the SOC can't reset to 100%? It's okay for AC-tied PV systems and UPSes, but honestly... for PV systems I'd advise installing a BMV. Much more sensitive, much more room for adjusting things (tail current, detection time, etc). The SOC determination can drift quite a bit, especially if the configured efficiency estimate (default=85%) is incorrect. With all that said... at least it has SOC tracking, and it's not much worse than the BMS in a BYD battery (which also goes all squivy if you do low power discharges for extended periods). 1 hour ago, admiral said: Indeed, I am also curious as to how it will "reset" to 100 % like a BMV? It doesn't reset like the BMV, so again... I tell people the 2.5k (ish) you spend on a BMV is absolutely worth it. Quote Link to comment Share on other sites More sharing options...
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