Lithium Battery State-of-Charge Monitor from Micro Instruments

Lithium Battery State-of-Charge Monitor from Micro Instruments.

October 15, 2024
Johan Koekemoer
Technical Tips & Guides
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As LiFePO4 batteries have a longer lifecycle than conventional Lead Acid / Gel batteries, chances are that you have a Lithium battery bank that is connected to an inverter that was initially configured with a max cut-off voltage and minimum cut-off voltage, but no visibility into the actual state of charge of the battery bank.

The Micro Instruments Lithium Battery SOC V2 monitor is a device that allows you to:

1. Set the minimum and maximum Voltage of your connected Lithium battery bank

2. Monitor the SOC of your battery bank through SNMP

3. Allows for an additional dry-contact relay to be connected to either activate or deactivate at a specific SOC% to signal the start/stop of connected generators

As with most Lithium batteries, the Uniross 12V 50Ah (UR-BATLiFePO4-12-50)has a built-in BMS (Battery Management System) to prevent from overcharging and over discharging but has no SOC% reading or screen.

Lithium batteries does not operate on the same 50% DoD (Depth-of-Discharge) as conventional Lead Acid / Gel batteries. They allow for an 80% DoD before the BMS will stop the flow of energy to protect the cells.

The Voltage discharge curve below of a Lithium battery show that there is a nominal Voltage difference between the Depth of Discharge percentages, which makes it difficult to calculate the SOC of your battery bank solely based on the Voltage readings.

Voltage Discharge Curve

Added to the SOC V2 unit is a contactless non-invasive DC current sensor which is clipped around the positive battery wire. This type of current senor might have negligible 3 ~ 5% variable reading but makes installation much easier when compared to a wired in shunt sensor.

The SOC V2 monitor is compatible with 12V, 24V and 48V Lithium Batteries.

How it works:

The SOC V2 monitor measures the current flow in and out of the battery to determine the SOC of the battery with a bi-directional current sensor. The SOC first needs to be configured with the following information, relative to the battery connected:

1. Battery capacity: enter the battery capacity in Ah (Amp Hour rating)

2. End charge Voltage: the maximum end charge voltage as specified on the battery – also the maximum cut-off charge Voltage

3. Tail current: The battery is considered fully charged once the charge current has dropped below the “tail current” setting. This value is set in an Amps value and is usually around 2.8% of the battery total capacity. For example, a 100Ah battery would be set to 2.8Amps and a 50Ah battery would be set to 1.4Amps.

4. As soon as the SOC V.2 detects that the voltage of the battery has reached the Set “end charge voltage” parameter and the current has dropped below the “tail current” parameter, the battery monitor will set the SOC to 100%.

System connection and wiring

Visit our website or download the Datasheet for further information on the Micro Instruments SOC V2 Monitor or get in touch with our Sales Team at 012 657 0960 or sales@miro.co.za