Shunts & Sensors

Keeping tabs on battery condition is essential to ensure both the safe operation of the battery as well as to help prolong it’s operational life.

Shunts are used to meter the battery – they monitor all power flowing into and out of the battery and provide a very accurate State of Charge (SOC) readout.

This SOC condition can then be used to trigger a range of ‘remediation actions’, perhaps starting and stopping a generator or sending an alarm message to the system operator.

A few different sensors can be installed on a battery – the most common are temperature and voltage sensors. These are used by the charging system(s) to adapt their output to properly look after the battery as the temperature changes.

Neither are ‘critical’ to the core functionality of the solar system, however including them will contribute greatly to increasing the life span of the battery.

1) Where should the Shunt be installed?

The role of the Shunt is to monitor ALL power flows into and out of the battery. As such, it needs to be positioned as close as possible to the battery, and nothing should be installed between it and the battery (other than the battery protective fuse).

2) Can Shunt(s) be installed elsewhere?

Absolutely. The shunt is a monitor, so secondary ones can be installed on any particular circuit that you want to keep an eye on. Just don’t forget they’re secondaries – the primary should always be the one watching the battery.

3) What else can Shunts do?

That depends on the other hardware in the system, and what else it’s connected to.

Shunts are usually used to trigger ‘something that can save the battery’ when it sees the SOC fall below a pre-set threshold. The actual ‘trigger’ mechanism is usually a relay contact, either located on the body of the Shunt itself or off the regulator/inverter it’s reporting its SOC info to (in some cases it can be both).

The most coming ‘actions’ the Shunt triggers are:

• Starting (and then stopping) and auto-start generator
• A coms message (SNMP, email, SMS, etc) to alert an operator or monitoring system of the battery condition
• A partial shutdown of the load (e.g. when ‘essential’ and ‘not-essential’ functions are segregated into separate circuits, the non-essential circuit may be temporarily turned off to lessen the load on the battery till it recovers to a healthier level).

4) What to be aware of with Shunts?

Always use heave cable for the shunt connection, especially the primary that handles the entire maximum charge/discharge current going through the whole battery bank.

Ensuring you use appropriate gauge cable and recommended termination methods and tensions is essential to minimising potentially system losses and faults.

5) What about Lithium batteries? Do they need the Shunt?

All Lithium batteries are guarded by a BMS (Battery Monitoring System).

• For some Lithium batteries the BMS is very simple and doesn’t offer any means for the user to view or use it’s information. In these cases a Shunt is recommended if you want to be able to view the battery state and trigger ‘other actions’ based on the state..
• Other Lithium batteries offer a Bluetooth interface to the BMS, allowing the user to view the battery information while physically nearby. If you want the ‘trigger something’ functionality you will need to install a Shunt, regardless of the information available via Bluetooth.
• Still other Lithium batteries add a BMU (Battery Management Unit) on top of the internal BMS’s to allow active communications connectivity between the battery and the inverter/regulator system. These types usually do not require the external Shunt as the information transferred via the BMS/BMU should be much more detailed. The regulator/inverter relays cay then be used to ‘trigger something’ based on the status information provided by the BMS/BMU.

6) Why should I use a voltage sense line/device?

The inverter/regulator/load is already connected to the battery so it seems odd to install a sense line/device just to measure battery voltage.

However, readings from the main ‘power’ connections to the battery can be clouded by cable impedance and resistance, especially during times of high charge or discharge currents, so a separate sense line that is never ‘under load’ is very useful for accurately measuring the battery voltage.

This can be particularly important in cases with undersized batteries and high start-up load demand. In this scenario, the battery can temporarily dip to an undervoltage level which could cause the inverter or LVD device to disconnect the load unnecessarily. A separate sense line on the battery voltage can help prevent this ‘nuisance tripping’.

 7) Why should I use a battery temperature sensor?

Batteries behave differently at different temperatures. Charge voltages in particular need to be adjusted as the temperature changes.

Charging a battery at the wrong voltage will result in early failure (have you ever seen a battery with swollen sides – that was probably caused by over-voltage charging!)

Most regulators and other charging devices will monitor their own onboard temperature and can use that to ‘guess’ the battery temperature. Installing a dedicated battery temperature sensor takes the guesswork out of the equation!

Ensuring the charging devices have an accurate read on the battery temperature will allow them to adjust their charging voltages and currents to suit the actual battery condition.

This can greatly extend the service life of a battery!

 8) Pros’ & Con’s of lug vs stick-on temp sensors?

Most manufacturers offer either (or both) lugged temperature sensors (i.e. an M6/8/10 eyelet lug that is directly bolted to the battery terminal) or a stick-on unit that uses a thermal adhesive at attach to the side of a battery case.

The lugged sensors are often most accurate as the lugs are extremely conductive and are directly connected internally to the conductive plates within the battery.

The stick-on sensors are often very convenient, particularly in battery systems which already have multiple lugs connected on the battery terminals. It’s important to ensure the battery case is clean before affixing the adhesive (i.e. alcohol or disinfectant wipes are a good idea!).

9) What about Lithium batteries? Do they need the voltage/temperature sensors?

Lithium chemistries behave differently from lead-acid. They often do not require any temperature compensation within a fair range of ‘normal’ temperatures. However, they usually have very hard and fast temperature floors and ceilings, outside of which the BMS may either limit the charging or discharging current or outright turn the battery off to protect it.

For Lithium batteries with the BMS/BMU communications to the charging system, an extra temperature sensor is not required as the coms between the equipment includes the battery temperature condition.

For Lithium batteries with the ‘no interface’ BMS, or the ‘local Bluetooth only’ BMS, an external temperature sensor is a good idea. It will not prevent the BMS from intervening to protect the battery from an extreme temperature event, but it will allow the regulator/inverter system to include behaviour adjustments at those extreme temperature times, as well as allow the use of the monitoring system to alert the operator of the ‘going out of bounds’ status – it won’t stop the battery from going offline, but at least the operator will know why!

10) Do I need multiple voltage/temperature sense lines/devices?

Depending on how the various components of the charging system integrate with each other, one single ‘master’ set of sensors may be all you need. On the other hand, the hardware you use may require each device to have a dedicated sensor….

Check your manufacturer’s guides and recommendations!