This article is based on the Victron Smart Shunt used with LiFePO4 batteries, however a lot of it will apply to other brands of shunts too. If you’re unsure if your device is working properly or not, and this article doesn’t help confirm it, get in touch with whoever you purchased it from, or who installed it.
DIY’ers you’re often on your own when it comes to support, so online forums are often your best bet. Just be sure to vet who you take advice from, there’s a lot of misinformation online.
Shunts are very simple devices, and also very reliable and accurate providing they are set up correctly. However the interpretation of the readings can lead people astray and that’s where digital circuits help reduce those interpretation errors, providing they are set up correctly.
Settings – All shunts will have some settings that need to be customised for your system. “System” in this case means the batteries, chargers and to some extent the loads too, especially if these are interconnected.
The most common incorrect setting is the “Charged Voltage”. For most systems, set the Charged Voltage to 0.2v below your Charger Bulk/Absorption voltage. So if your chargers (Solar, DC-DC and Mains) are set to 14.2v, set the Charged Voltage to 14.0v. This is the industry accepted normal setting.
There are some other settings such as…
Discharge floor (suggested 0%) , Tail current (suggested 3%), Charged detection time (suggested 3m), and Peukert exponent (suggested 1.03) that all have effects too, and these are to be tailored to the individual.
All these settings are starting points that can normally be used to get a system going, and they can be tweaked as required providing you understand the effect.
Resolution, Accuracy and Current Threshold – Resolution is how much detail you get. For the Victron Smart Shunt you will see a Resolution or 0.1A, so if you are drawing 1.44A, it will show 1.4A, but if you draw 1.45A, it will show 1.5A. This is pretty good as far as shunts go. More Resolution tends to be unnecessary and results in the numbers changing too often.
Accuracy is generally not a big issue, but across different brands it’s common to see up to 5% difference in current readings.
Current threshold is a factor for people who are regularly drawing very low currents, such as LED lights and accessory standby power. For the Victron Smart Shunt, it’s default is 0.1A so if you’re drawing 0.04A, it won’t register it. It’s not a big issue for most systems but some shunts (especially those inbuilt to BMS’s can have up to 2.5A Current Threshold (PowerPaul batteries use 0.7A for this setting).
Power interruptions – For the Victron Smart Shunt, if power is interrupted to it (such as a battery BMS shutdown), it can either turn back on with the same SOC, or set to 100% (this is a setting you can change). Generally you should have it set to “Keep SOC”.
However if you view the “Consumed Ah”, you may notice this figure can come back wrong after a disconnect event. Victron are aware of this but they consider it to not be an issue, however it’s a software bug that should be fixed… come on Victron, get to it!
Battery capacity – Battery capacity needs to be set for a shunt to report proper SOC. You can either go by the battery capacity written on your batteries (add them all up), or you can do a test to find the actual battery capacity and use that figure. Battery capacity also declines over time, so a 100Ah battery from 2 years ago may have lost anywhere between 1-10% depending on how it’s been used. If you find your system is shutting down earlier than expected, and the SOC is showing 10% when it shuts down, you may consider dropping the battery capacity by 10%.
Time – Over time, your shunt will lose accuracy if it has not reached the 100% reset trigger point. This is because batteries have a natural self-discharge that the shunt does not account for. If the battery has discharged 50% over a year or so, the shunt may still show 100% SOC which can lead to very inaccurate readings. So when planning a trip away, always get the van ready a few days prior to setting out, charge everything to 100% SOC and give the system a full test to ensure everything is working correctly. This will help alert you to potential problems (such as corrosion or mice chewing cables) or electrical items that may have failed during storage.
Wiring errors – This is still a very common issue for DIY’ers and even some “professional” installers. We often see chargers or other items connected directly to a battery negative terminal, and this will cause erroneous readings because the shunt cannot measure this charge or discharge. Look at your battery Negative terminal, and your Shunt B- terminal, there should be just one wire between the two. If there’s more than one, there could be a wiring error.
These are the most common things we consider when helping people fault find, but it’s not exhaustive. If there’s something else you think we have missed, feel free to leave a detailed comment below so you can help others.
Hi Paul could you comment on the apparent loss of BMS SOC accuracy with parallel joined batteries.
It’s effectively the same thing. Various combinations of these things can drift a shunt positive or negative.
Earlier this year I installed 2x100ah lifepo4 batteries, Victron 30A battery charger and 300a smart shunt. Took the van away for a few nights running purely on battery power, and everything seemed to be working correctly, apart from with everything turned off there was still a small drain on the battery which I assumed was either the solar controller (Victron) which was disconnected from the array, and/or the battery charger.
When I got home, I double checked my wiring, and calibrated the shunt. No discharge showing at all. I dropped the SOC to 62% by leaving lights on for a couple of days. After about three weeks the shunt was still telling me that SOC was 62%, but I turned some lights on and the batteries were too flat to light them up. Why is the shunt not telling me the correct situation?
Sounds suspiciously like a wiring error, but could also be wrong settings. Victron gear is excellent, but it’s not simple to set up properly. Seek professional help.