Smart Shunt SOC stuck at 100% while battery discharging

DIY
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1

Newbie. Recent install of Smart Shunt while the bank was fully charged, bank disconnected from both loads and the charger. I did not do a “manual sync” because it already reported SOC = 100%. Bank is now connected to loads. And remains not connected to charger.

Has been in use for 1/2 day. It’s current voltage of 24.73V (its a 24V system) clearly indicates that real world SOC is well below 100%. But the shunt is still reporting 100%. That’s my problem. What did I do wrong?

The bank is four Concord Lifeline AGMs in good condition (I think). Each 210Ah. Wired up as a 24V system. Shunt’s Capacity = 840Ah.

The charger is a Skylla-i 24/100(3) 230V. The naming of its charging phases is slightly different than typical. They are:

  • Bulk = 28.4 V
  • Absorption = 28.4 V
  • Float = 27.4 V
  • Storage = 26.4 V

I see the charger at 26.33 to 26.4 V when it switches to Storage (with a target of 26.4 V). I see this both from the Shunt’s logic as well as from a separate Murata digital multimeter connected to the same physical shunt.

I see the charger switch to Storage (26.4 V target) in the range of a few amps of current (don’t remember exactly), as reported by the shunt’s logic and the Murata (which reads amps high). It typically takes overnight to achieve that.

There are two of these banks, on a single buss, with the loads and charger attached to that single buss. 0, 1 or 2 banks attached to the buss at a given point. Typical charging routine is to fully charge bank X while on the buss, once charged swap banks putting Y on the buss, with X now fully disconnected, and resting.

Shunt’s Charged Voltage set to 26.3V. This is slightly below the voltage at the “point of full charge”.

Shunt’s Tail Current set to 1% based on the FAQ recommendation. However, I see the charger switches to Storage (26.4 V target) in the range of a few amps. So perhaps I should lower the Tail Current to 0.5%? But in either case this seems pretty close to the “point of full charge”. Advice on that?

Screenshots of status and settings attached.

Battery Status
Battery Status960×2079 118 KB

Battery Settings
Battery Settings960×2079 169 KB

Battery Settings Misc
Battery Settings Misc960×2079 52.9 KB

The auxiliary input is setup for “temperature”. The retailer said that the shunt’s readings will flow to the Skylla-i. But there are 3 shunts in the system. How does the Skylla know which reading to use? Now that I’m more familiar with how this thing works, I’m in serious doubt about this flow of temp information.

2

Have a read of this link. Your charged voltage setting in the shunt is too low, it is below that actual battery voltage so the shunt keeps syncing to 100%. Also look at the second link about correct wiring.

https://community.victronenergy.com/t/shunt-battery-monitor-jumping-to-100-or-reporting-a-high-soc-when-the-volts-are-low-and-or-the-bms-has-disconnected/88

https://community.victronenergy.com/t/shunt-battery-monitor-not-reading-the-correct-soc-amps-missing-solar-decreasing-daily-or-other-misreading-problems/87

3

If you have (4) 12v 210AH batteries that have been connected in series/parallel to 24v, then you have 420AH, not 840.

4

The manual says “As soon as these 3 parameters have been met, the SmartShunt will set the state of charge value to 100%, thus synchronising the state of charge.” I interpreted that to mean “all at the same time”.
So actual voltage of 26.35 is > setting of 26.3, so that’s “true”
and actual current of plus 3 amps < setting of 8ish amps, so that’s “true”
What am I missing. My head is clearly broken.

5

Thank you! I wondered about that. Google’d it. Couldn’t find a clarification. Will fix it. And document it.

6

Your charged voltage may not be the issue that is causing your misreading, BUT, it is still best to increase it to the absorption voltage - 0.2V because that way you know the battery has had a good charge to get to 100%. If you have it set just below the storage voltage, then you draw a load that the charger can not keep up with and the battery discharges, then goes back to storage it will immediately say 100% even though the use from the battery has not been replenished. A lead acid battery is full when it is at 14+V and a tail current of a few percent. This is the most reliable indication that you are full. Then when you drop to storage if you do end up using some energy from the battery it is properly recorded and it does not resynchronise. This is why I linked the first FAQ.

The other cause of the reading remaining high is that you have not got the wiring correct and some of the loads are bypassing the shunt. The normal cause of this in boats and vehicles is someone has the Chassis or hull ground connected to the battery when it needs to be on the system side of the shunt. This is why I linked the second FAQ.

These are the two top reasons we see DIY installs with shunts reading badly.

7

Thank you!
The Skylla-i puts the bank thru these stages:

  • Bulk = 28.4 V
  • Absorption = 28.4 V
  • Float = 27.4 V
  • Storage = 26.4 V
    The Skylla switches from Bulk to Absorption at about 20-30 amps
    It then switches from Absorption to Float at around 10ish amps
    It then switches from Float to Standby at a few amps.
    With the correction to capacity (4 batteries in 1 bank, rated at 210Ah at 12V, 105Ah at 24V, so the shunt setting is now set to 420Ah), the 1% recommendation would be 4ish amps.

If the logic SOC sync in the shunt is the AND of the conditions, then it will never see >28V AND <4 amps.

I think you’re saying that the sync logic is not how I was reading it; instead it is this: First it hunts for voltage to exceed Charged Voltage for at least Detection Time minutes, and then once that has been achieved, then it no cares about voltage, and it is hunting for amps to go below Tail Current. And when that hunt succeeds, the sync is complete. Correct? Does Detection Time also apply in the 2nd hunt?

re “hull ground”. Hull ground goes to the 24V system’s negative buss, which is the system side of the shunt.

8

The above is wrong. 3 conditions have to be met simultaneously, the voltage above Charged Voltage AND amps below Tail Current AND both have been satisfied for the Detection Time. You understood it correctly before I came along!!

If your charger is going from absorption to float at 10A on a 420Ah battery bank (2.5%) then the batteries are not quite full but very very close. I do not know the Skylla chargers to comment on the settings to get a longer absorption. If you can not get a longer absorption then set the Shunt to charged voltage 28.6V and 3% tail current, it will then sync when at the cusp of being full and before it goes to float. It should not see any subsequent syncs at lower voltages. My boat has worked fine for 4 years set like this with no problems of misreading at all. You have to set the shunt to match your system and increasing tail current has fewer downsides than reducing the charged voltage.