smallBMS NG 3. Installation 3.1. Important warning The shutdown by the BMS due to low cell voltage should always be used as a last resort to be safe at all times. We recommend not letting it get that far and instead either switching off the system automatically after a defined state of charge via the BMS discharge floor limit so that there is always sufficient reserve capacity in the battery or using the BMS remote on/off port as a system on/off switch. Lithium batteries are expensive and can be damaged due to over-discharge or overcharge.
Damage due to over-discharge can occur if small loads (such as alarm systems, relays, standby currents of certain loads, back current drain of battery chargers, or charge regulators) slowly discharge the battery when the system is not in use. In case of any doubt about possible residual current draw, isolate the battery by opening the battery switch, pulling the battery fuse(s) or disconnecting the battery plus when the system is not in use. A residual discharge current is especially dangerous if the system has been discharged completely and a low cell voltage shutdown has occurred.
After shutdown due to low cell voltage, a capacity reserve of approximately 1Ah per 100Ah battery capacity is left in the battery. The battery will be damaged if the remaining capacity reserve is drawn from the battery; for example, a residual current of just 10mA can damage a 200Ah battery if the system is left discharged for more than 8 days. If a low cell voltage disconnect has occurred, immediate action (recharge the battery) is required. 3.2. Things to consider 3.2.1. Controlling DC loads via Load disconnect To prevent deep discharge, DC loads must be switched off or disconnected when there is a risk of cell undervoltage.
The Load disconnect output of the smallBMS NG can be used for this purpose. The Load disconnect output is normally high (equal to battery voltage) and switches to a free-floating (open-circuit) state in case of imminent cell undervoltage. DC loads with a remote on/off terminal that activates when pulled high (to battery plus) and deactivates when left free- floating can be controlled directly via the Load disconnect output. See [→Appendix A] for a list of Victron productswith this behavior.
For DC loads with a remote on/off terminal that switches the load on when the terminal is pulled low (to battery minus) and switches it off when the terminal is left free-floating, a Inverting remote on-off cable can be used. See Appendix A [23] . 3.2.2. Controlling DC loads with a BatteryProtect The smallBMS NG can control the remote on/off terminal of a BatteryProtect to manage load disconnection. A BatteryProtect will disconnect the load when: The input voltage (battery voltage) falls below a preset threshold (adjustable in BatteryProtect), or when The remote on/off terminal is pulled low. Page 5 Installation