In order to use the battery for as long as possible and not damage it, it is important to keep cell parameters within certain limits. A good BMS ensures that you don’t have to worry about this. The BMS keeps an eye on each cell and intervenes when necessary. For example, by switching off the dischargers when they continue to discharge while one or more cells are already empty.
This intervention ensures that the battery always remains in optimal condition.
Function of the BMS #
In general, a complete BMS does the following:
- Monitors each cell voltage and temperature. These are the most important features. When these values are within limits the cell will last longer.
- Protect when a problem threatens to occur. Before the critical value is reached, the BMS will switch off the charger and/or inverter, depending on the type.
- Balancing the cells. No two cells are alike. This causes one cell to empty faster than the other, for example due to self-discharge of the cell. Balancing ensures that the full capacity of a cell can be used, even after many times charging and discharging.
Selecting the type of BMS #
There are several brands and types of BMS on the market. From very simple ones that only switch off based on a fixed voltage, to advanced ones with, for example, Bluetooth connectivity.
Switching off the connected devices can also be done in several ways. In general, we can distinguish BMS based on the method of controlling the current flow:
FET-based BMS #
Many – especially cheaper – BMS on the market have integrated FETs. The advantage is that the installer does not need to connect power relays. This simplifies the installation. However, there are many disadvantages to this. Depending on the type and number of FETs, the charge and discharge current are limited to a fixed value. It is not possible to expand the installation for a higher charging or discharging current at a later stage. Also, the integrated FETs (usually MOSFETs) are specified for a voltage only a few to at most tens of volts higher than the maximum voltage of the BMS.
There are many stories on the Internet of chargers, including MPPTs, that fail and therefore give a higher DC charge voltage than they should. For example, directly the voltage of the connected solar panels. Since this voltage is higher than what the FETS are rated for, the FETs will break down. The danger is that FETs usually continue to conduct when they fail, which will overcharge the cells. The BMS will not be able to turn off the charger and the battery cells get damaged.
BMS with option for power relays #
Most professional BMS on the market are able to control power relays. The installer can connect a power relay of their choice to the BMS. When the BMS detects a problem it will disable the power relay.
The installation of this type of BMS takes a little more time, but is much more flexible and also safer.
The 123\SmartBMS includes the option to connect one or more power relays, for example, a power relay for the charger and a separate one for the inverter. Of course, a single relay is also possible. In this case, set the 123\SmartBMS to Critical Mode.
When selecting a power relay, go for an energy efficient one. This will make sure that the relay will not drain the battery too quickly, so the BMS and relay can operate for a long time, even when the battery is not being charged frequently.
Examples of energy efficient power relays are the 123\SmartRelay (12V/24V 120A/100A), 123\PowerSwitch (48V 100A and 200A peak for 1 minute) and Kilovac EV200 (12V/24V/48V 500A – this one consumes consumes more but is still more efficient than many other relays).
Selecting the BMS operating limits #
To protect a battery, it is best to set the BMS battery limits a little under the maximum limits. This gives some safety headroom. By setting the minimum voltage a little over the minimal acceptable value, some energy will still be left in the battery when the BMS turns off the loads. Thus, the BMS is still able to operate for a long time even when the BMS gives a “low voltage error” and turns off it’s discharge signal relay. The 123\SmartBMS consumes very little current (several mA’s), thus it can operate for a very long time on the small bit of energy left in the battery.