Low battery, wrong SOC displayed and inverter in Sustain mode

[SvenR]

An ESS / grid backup system that worked normally until recently, now shows an alarm for low battery. That alarm seems correct, because the battery voltage is only about 48 V.

However, the SOC displayed by GX is about 86%, which must be wrong.

The inverter seems to be stuck in Sustain mode.

This system is far north, so the sun that helped charge the batteries until recently is almost absent now.

It is the first time that I experience this problem. Would be very thankful for advice.

My guess is that wrong SOC calculation is a root cause. The ESS is set up for Minimum SOC (unless grid fails): 85%

So perhaps the SOC calculation drifts and the ESS thinks SOC is 86%, but in reality the battery is almost empty.

Could this guess be right? What should I do first - can I reset the SOC calculation so the ESS understands it should charge the battery from the grid? But I suppose that I must also somhow make the inverter exit Sustain mode. If I have to wait for the sun to raise the battery voltage for that to happen, it will take about half a year!

The system has a lead-acid battery with 48 V nominal voltage, a SmartShunt, a MultiPlus-II, Victron solar charge regulators and a GX system. The MultiPlus-II shows that mains is present and I can see in VRM that it draws a little power from the grid (about the same as the load).

[lxonline (LX)]

@SvenR

The system has a lead-acid battery

You certainly should not be leaving these in an uncharged state it is so bad for them.

Sounds like it could be SOC drift which can happen if battteries haven’t been fully charged for a bit. Or triggered by the ess programming.

That being said sustain is part of battery life , in ESS is voltage triggered. It can be triggered by one of the dynamic cut offs or by

Sustain can be cleared by charging it up a bit above the sustain voltage set.

[SvenR]

Thank you for quick reply, LX. I agree that I should not leave the batteries uncharged for long. The problem about charging the batteries up now is that the sun is disappearing and will not come back until spring (the system is very far north) and the MultiPlus-II seems stuck in Sustain mode. I really want to make it exit Sustain mode now.

[pwfarnell]

The other thing is making sure that your SmartShunt is set up so it does not synchronise to 100% before the batteries are full. The easiest way is to set the charged voltage about 0.4V below your absorption voltage. If the charged voltage is too low then as solar production falls, the battery voltage can be increased enough to trigger as synchronisation and the current will be low.

See the linked FAQ.

Battery monitor jumping to 100% or reporting a high SOC with low Volts / the BMS shuts down

This topic covers the reported SOC being out of sync with the actual battery voltage or state. It covers BMV battery monitors, SmartShunts, Lynx BMSs and Lynx Shunts and will just be called shunt below. It is also assumed that the shunt is monitoring the domestic / house / cabin battery bank and not engine start / bow thruster / winch / caravan mover battery banks. The problems may be one or more of the following and is often linked to mobile applications with small solar systems. The SOC jum…

[SvenR]

Thank you pwfarnell for a helpful reply. We have changed the SmartShunt settings, which should improve the SOC calculation. Regardless, I wish the SmartShunt would be delivered in such a way that if it drifts, it the battery should not be emptied. We also temporarily set the AC Input 1 to Generator, which helped to make the MultiPlus charge the battery up properly and thus get out of Sustain mode. Now I’m testing to have the ESS set to Keep batteres charged. That is probably a safer choice for us than the “Optimized with battery life” that could have killed our battery bank.