highlandbishop avatar image
highlandbishop asked

Smartshunt Configuration


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 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 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.

SmartShuntsmart solar set-up help
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3 Answers
Alexandra avatar image
Alexandra answered ·


Since you have a battery of unknown condition and unkown charge history, that was 105AH new, it would be prudent to assume (especially if it is more tha one year old) that there no longer 105AH of capacity there.

Set your dischrage floor to 50% as you really should not be drawing it under that. 12v is basically discharged on lead acid. Yes you can discharge to a power voltage but generally you are damaging it at that point.

SOC is not determined by voltage but rather by how many amps over a specific time hence the term Amp Hour or AH.

On lead acid, if it was not charged fully through the correct length of absorption at the coreect voltaged charge to reverse the chemistry, then has not had a good start point and also will not have the full AH of charge. Hence the weird voltage for 85%. This combined with the fact that the second hand battery does not have the full capacity of 105Ah means it is is going to be interesting to work out the true SOC of your battery.

Q1. Having the charged detection voltage lower and charge detection time and tail current not set correctly means the SOC might synchronise too early.

What charge voltages are you using on the mppt? And how is it compared to the shubt settings? Maybe if you share some of them we can help you tweak it to get it to work better.

Under a 1Amp load your battery voltage should not drop that much if the battery is in good condition.

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highlandbishop avatar image highlandbishop commented ·

Hey @Alexandra, Thanks for taking the time to consider my issue. I'll attach the screengrabs of my shunt and MPPT configs.


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Kevin Windrem avatar image
Kevin Windrem answered ·

SOC is calculated from current in/out of the battery. Make sure ALL connections besides the battery negative are on the system side of the shunt so it sees all charging and discharging currents.

There is a good explanation of that mechanism in the manual:

The "charged" voltage of the shunt is one part of the trigger to reset SOC to 100%. The other part is the tail current. SOC resets to 100% when battery voltage is above the "charged" voltage and when the current into the battery is less than the tail current.

As you say, battery voltage is a poor indicator of battery state of charge unless the battery is in a true resting state for some time. which is generally not possible in an RV. For this reason, a battery monitor that bases state of charge on current in and out of the battery is needed.

SOC should not be dropping unless a load is detected by the shunt. One common problem is that the shunt is connected backwards. That would mean a charging current would be counted as a discharge and visa versa.

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highlandbishop avatar image highlandbishop commented ·
Hi @Kevin Windrem , Thanks - everything is connected as designed and spec'd. I think the issue is a) battery age/chemistry/history and b) configs for the shunt.
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kevgermany avatar image
kevgermany answered ·

You have a very high charge efficiency factor, 85% more normal. For an old lead acid this may be too high.

Normal absorption is 14.4V

Shunt will only show state of charge change if it detects current. Showing SOC drop while charging with small load indicates load greater than charge current, or a wiring fault, like charge current bypassing the shunt. This usually from extra connections to battery neg, or a reverse wired shunt.There should only be a single neg connection to the battery, the shunt. All other negs to the load side of the shunt

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