Shunt / battery monitor not reading the correct SOC/ Amps, missing solar, decreasing daily or other misreading problems

This topic covers the SOC / Amps reported by 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

  1. The SOC decreases daily despite the solar charger or other charger(s) working.
  2. The SOC increasing when discharging and falling when charging.
  3. The SOC not decreasing when discharging.
  4. The Amps measured by the shunt are lower than those reported by other loads or chargers.
  5. The Amps reported shows a large negative value when starting the engine.

Most of these problems are caused by incorrect wiring. There are three simple rules here.

  1. There must be only one connection to the battery bank negative and that must be the battery terminal on the shunt.
  2. There must be only one connection to the battery terminal on the shunt and that must be the battery bank negative.
  3. All other negative connections MUST be connected to the system / load terminal of the shunt. Newer models label this “System”, older shunts label this as “Load”.

Point 3 includes (but is not limited to) all other negative connections from, starter batteries, other battery banks, chassis ground, hull ground, engine blocks, alternators, DC to DC chargers, Inverters, solar charge controllers, AC chargers, domestic loads, monitoring devices such as the Cerbo GX, split charge or battery combiner relays, battery protects.

The shunts work by measuring the current flowing into or out of the battery over time and adding this onto or subtracting it from the charge within the battery. If there is any negative cable attached direct to the battery (other than the shunt) then the current in that wire cannot be measured.

One of the most misunderstood connections is the chassis ground, especially on van conversions or RVs and this is often left on the domestic battery. Some Van and RV fitters do this when they should know better. If the chassis ground is connected to the battery, then current from the alternator can flow direct to the battery through the chassis and chassis ground cable and bypass the shunt.

For clarity, if you have a battery with a separate BMS, the BMS is part of the battery and is located between the cells and the shunt then if the BMS disconnects the shunt does not act as an additional drain on the cells after disconnection.

If there is not a wiring issue then some of the symptoms, especially reducing daily SOC may also be caused by incorrect settings. The two most likely settings that can result in SOC drift are the “Peukert exponent” and the “Charge efficiency factor”, but these do not affect the measurement of the amps.

Typical values for lithium batteries are 1.05 and 99%. For lead acid batteries the range is wider, from 1.25 and 80% to 1.15 and 98% with the values needing to be optimised for your own batteries and usage pattern. The most likely cause of drift is if lithium settings are used for lead acid and vice versa. The manual describes these in more detail.

If you have a very low permanent current drain on a system that spends a lot of time out of use, such as a boat or RV then the current threshold could be reduced, current below this value is ignored.

The final issue may be an incorrect zero reading resulting in a small mismatch in actual use and measured use that accumulates over time. This can be addressed by performing a zero current calibration as per the manual.

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