SmartShunt Nonsense

[highlandbishop]

Hello!

At Christmas, I purchased a 200W solar array (Renogy), a SmartSolar 100/20 MPPT and a SmartShunt for our RV. I am having a real challenge configuring the Shunt...it seems like a little bit of hocus pocus to me. I am currently bench-testing before installing in the trailer come the spring.

Battery specs: 12V Lead-Acid (maintenance-free w/ vents) 105aH. History is unknown, Suspect it has seen a few 0% draw-downs. Came to me used. Small battery/small trailer - when this batt is dead, plan to upgrade.

Notes: battery can be charged from shore power or off vehicle starter battery/alternator when installed in trailer

Question 1: Some people seem to think that the 'Charged Voltage' spec on the shunt should be 0.2 or 0.3 V below the Absorption charge (for solar applications) contrary to Victron's recommendation of 0.2 to 0.3V below Float voltage. Does this not start counting the SOC down early?

Question 2:: With a .77A load on the battery in full bright sunlight, the SmartShunt is starting to countdown the SOC. Charged voltage currently config'd to 12.9. If the solar/MPPT is harvesting enough energy (it very much is), to keep the battery charged, why is the shunt counting SOC down?

Question 3: Can someone confirm if there's a calculation for figuring out how much SOC is in the battery based on voltage UNDER LOAD...it seems that resting voltage and voltage under load are two different numbers and trying to get the capacity based on resting voltage is futility in a trailer (must be such-and-such-temperature for such-and-such time, etc.). I had the battery on around a 1amp 12V load overnight and my batt showed 12.17V (under load) in the morning...but the SOC showed 85%...there's a mismatch here. Just trying to figure out what I should set my voltage alarm to.

Forgive the novel here, but my google searches haven't yielded much luck. I've cross-posted with the Victron community. Would like to get this figured out before warmer weather hits, so we can hit the road with everything buttoned up.

 

[mikefitz]

The value for Ah battery needs be entered into the smart shunt, as your battery is suspect the actual capacity will be lower than 105 Ah. I will suggest a technique to estimate the capacity later.
Q1. Victron suggested 'charged voltage' applies where the charger has an ideal charge profile and is applied continuously to the battery for a long period. For example, a AC charger , charging set to 10 to15 amps needs to be connected to a 100 Ah battery for 12 hours.
Where the charging is Intermittant and perhaps with a varying charge current, its possible the unit will sync to 100% well before its actually reached that SOC. By adopting a 'charged voltage' value slightly lower than absorbtion voltage there is a higher chance the battery will be at that level .
In your situation charging from shore power overnight its probable the battery will be fully charged.
If you drive for a few hours and have no shore power, the battery may not be fully charged but will have exceeded the low 'charged voltage' value.

Q2. Provided the current from the solar charger is higher than the load current then the SOC will not reduce . Where the performance is not as expected the usual issue is incorrect connections. All negative cables in the system connect to the output side of the shunt. The battery negative connects to the Input side of the shunt.

Q3. Battery voltage under load and related SOC are not easy to estimate, very roughly with a light load, 50% SOC is around 12.0 volts. Once you have the smart shunt operating correctly this will give you the information you need.

Setting up your chargers, 14.4 volts absorbtion, 13.8 volts float, absorbtion period 2 hours.
Note once you have discharged to 50% it will take at least 5 hours to fully charge.

Setingup the Smart Shunt.
Battery capacity 100Ah this may need modifying
Charged voltage 14.2V or 0.2 volts lower than absorbtion volts
Discharge floor 50% uses this for time to go
Tail current 2%
Peukert 1.25
Charge efficiency 90%
Current threshold 0.1
Time to go 3m

The Smart Shunt will auto sync to 100% when these three conditions are met:
the voltage is higher than the 'charged voltage'
and the current is lower then 'tail current'
and there conditions have existed for more than 'time to go'

With a lead acid battery there is a need to put more energy in than you get out. The efficiency will vary with type of battery, depth of discharge before charging, charge current, age of battery, temperature. Start with 90% and see how things go.
Puerkert constant is used to calculate 'time to go' as the effective battery capacity reduces as the load current increases.

Tail current is a useful indicator that the battery is charged. For a new lead acid battery this would be around 1% of capacity, 1 amp per 100Ah of battery. As the battery ages this may not be the case and the battery 'bottoms out' at a higher value, thus 2% is a useful starting value. If under test the battery never drops to below this low value under the absorption charge voltage then it could be increased, but a high current at the end of the charge process suggests the battery near the end of its service life.
One issue with using tail current as a technique of indicating a 'full' battery is that when charging from solar, variable solar conditions will cause the charge current to fall, perhaps causing the Smart Shunt to sync early.

The battery capacity value entered is needed to calculate the SOC and 'time to go to a 50% discharge'. In practice a lead acid battery will gradually lose capacity over time and if ill treated can lose significant capacity in a few months.
Unless the battery is new its difficult to estimate the actual capacity.

Estimating battery capacity. (100Ah battery)
Once the Smart Shunt is operational charge the battery to 'full'.
Load the battery with a few amps load, ideally 5 amps per 100Ah.
Set the Smart Shunt to indicate amp hours discharged.
Keep the test going till the volts fall to 12.0 volts and remove load
Check the voltage after a few hours, it will bounce up.
Add the load again if its higher than 12.10 volts.
Repeat the load and rest until the voltage is stable without load at 12.0 to 12.1 volts.
The Ah discharged will be 50% approximately of battery capacity.

Mike

 

[highlandbishop]

The value for Ah battery needs be entered into the smart shunt, as your battery is suspect the actual capacity will be lower than 105 Ah. I will suggest a technique to estimate the capacity later.
Q1. Victron suggested 'charged voltage' applies where the charger has an ideal charge profile and is applied continuously to the battery for a long period. For example, a AC charger , charging set to 10 to15 amps needs to be connected to a 100 Ah battery for 12 hours.
Where the charging is Intermittant and perhaps with a varying charge current, its possible the unit will sync to 100% well before its actually reached that SOC. By adopting a 'charged voltage' value slightly lower than absorbtion voltage there is a higher chance the battery will be at that level .
In your situation charging from shore power overnight its probable the battery will be fully charged.
If you drive for a few hours and have no shore power, the battery may not be fully charged but will have exceeded the low 'charged voltage' value.

Q2. Provided the current from the solar charger is higher than the load current then the SOC will not reduce . Where the performance is not as expected the usual issue is incorrect connections. All negative cables in the system connect to the output side of the shunt. The battery negative connects to the Input side of the shunt.

Q3. Battery voltage under load and related SOC are not easy to estimate, very roughly with a light load, 50% SOC is around 12.0 volts. Once you have the smart shunt operating correctly this will give you the information you need.

Setting up your chargers, 14.4 volts absorbtion, 13.8 volts float, absorbtion period 2 hours.
Note once you have discharged to 50% it will take at least 5 hours to fully charge.

Setingup the Smart Shunt.
Battery capacity 100Ah this may need modifying
Charged voltage 14.2V or 0.2 volts lower than absorbtion volts
Discharge floor 50% uses this for time to go
Tail current 2%
Peukert 1.25
Charge efficiency 90%
Current threshold 0.1
Time to go 3m

Will add more shortly

Mike

Hi Mike, This is incredibly helpful. I have adjusted to your config and will test. Unfortunately, I've lost meaningful sunlight, so will see what tomorrow yields. I've brought my AH down to 95...though I am skeptical that it's even that high.

 

[highlandbishop]

The value for Ah battery needs be entered into the smart shunt, as your battery is suspect the actual capacity will be lower than 105 Ah. I will suggest a technique to estimate the capacity later.
Q1. Victron suggested 'charged voltage' applies where the charger has an ideal charge profile and is applied continuously to the battery for a long period. For example, a AC charger , charging set to 10 to15 amps needs to be connected to a 100 Ah battery for 12 hours.
Where the charging is Intermittant and perhaps with a varying charge current, its possible the unit will sync to 100% well before its actually reached that SOC. By adopting a 'charged voltage' value slightly lower than absorbtion voltage there is a higher chance the battery will be at that level .
In your situation charging from shore power overnight its probable the battery will be fully charged.
If you drive for a few hours and have no shore power, the battery may not be fully charged but will have exceeded the low 'charged voltage' value.

Q2. Provided the current from the solar charger is higher than the load current then the SOC will not reduce . Where the performance is not as expected the usual issue is incorrect connections. All negative cables in the system connect to the output side of the shunt. The battery negative connects to the Input side of the shunt.

Q3. Battery voltage under load and related SOC are not easy to estimate, very roughly with a light load, 50% SOC is around 12.0 volts. Once you have the smart shunt operating correctly this will give you the information you need.

Setting up your chargers, 14.4 volts absorbtion, 13.8 volts float, absorbtion period 2 hours.
Note once you have discharged to 50% it will take at least 5 hours to fully charge.

Setingup the Smart Shunt.
Battery capacity 100Ah this may need modifying
Charged voltage 14.2V or 0.2 volts lower than absorbtion volts
Discharge floor 50% uses this for time to go
Tail current 2%
Peukert 1.25
Charge efficiency 90%
Current threshold 0.1
Time to go 3m

The Smart Shunt will auto sync to 100% when these three conditions are met:
the voltage is higher than the 'charged voltage'
and the current is lower then 'tail current'
and there conditions have existed for more than 'time to go'

With a lead acid battery there is a need to put more energy in than you get out. The efficiency will vary with type of battery, depth of discharge before charging, charge current, age of battery, temperature. Start with 90% and see how things go.
Puerkert constant is used to calculate 'time to go' as the effective battery capacity reduces as the load current increases.

Tail current is a useful indicator that the battery is charged. For a new lead acid battery this would be around 1% of capacity, 1 amp per 100Ah of battery. As the battery ages this may not be the case and the battery 'bottoms out' at a higher value, thus 2% is a useful starting value. If under test the battery never drops to below this low value under the absorption charge voltage then it could be increased, but a high current at the end of the charge process suggests the battery near the end of its service life.
One issue with using tail current as a technique of indicating a 'full' battery is that when charging from solar, variable solar conditions will cause the charge current to fall, perhaps causing the Smart Shunt to sync early.

The battery capacity value entered is needed to calculate the SOC and 'time to go to a 50% discharge'. In practice a lead acid battery will gradually lose capacity over time and if ill treated can lose significant capacity in a few months.
Unless the battery is new its difficult to estimate the actual capacity.

Estimating battery capacity. (100Ah battery)
Once the Smart Shunt is operational charge the battery to 'full'.
Load the battery with a few amps load, ideally 5 amps per 100Ah.
Set the Smart Shunt to indicate amp hours discharged.
Keep the test going till the volts fall to 12.0 volts and remove load
Check the voltage after a few hours, it will bounce up.
Add the load again if its higher than 12.10 volts.
Repeat the load and rest until the voltage is stable without load at 12.0 to 12.1 volts.
The Ah discharged will be 50% approximately of battery capacity.

Mike

Hi Mike,

Just following up. I did the ampacity test and that was really helpful. Will see what the new shunt settings do.

Thanks again!