Li-ion Battery Management Systems (BMS)

Lithium Ion Battery Management System Technology

The BMS or Battery management System is one of the unique differences that separates older Lead Acid products with the newer emerging Lithium technology.
Read below to see what a BMS does and why it is important for the safety and reliability of Lithium Cells.

Cell Balancing

Cell balancing is very important with Lithium Cells and batteries as it has both a safety aspect and ensures longevity of the product.
When more then 1 cell is connected together it is important that the terminal voltage of the cells relative to one another are kept within a few millivolts.
Unlike Lead Acid where if one of the internal cells reaches its maximum voltage then it begins to offgas until the other cells reach their maximum voltage and even out.
Lithium cells cannot be charged in this way and can be dangerous if allowed to do so.

If one of the cells is allowed to exceed its maximum voltage rating then it can start to heat up and reduce its service life. If the maximum terminal voltage is exceeded for a long enough period or by too much voltage then the cell can heat up to a point where it will continue increasing in temperature even if the charge voltage is removed, this is called thermal runaway and certain chemistries are more prone to this phenomena than others.
If this is allowed to occur the cell can expand, offgas violently, and even explode or burst into flames so it is vitally important that a cell balancing strategy is used.
Altertek BMS's use passive cell balancing which "bleeds" energy off cells that are out of balance in relation to other connected cells allowing the terminal voltages to equalise during charging.

Temperature Monitoring

As described earlier, certain chemistries of Lithium Cells can be prone to a condition called Thermal Runaway if overcharged, too much current is drawn or an external heat source is applied which could be a hot operating environment next to a piece of equipment or a hot country.
In any of these cases or a combination of cases it is vitally important that the cell temperatures are constantly measured to ensure that they do not overheat.
In the event that the temperature build up is caused by the electrical loading of the cells then the BMS can disconnect the battery, or if the BMS is capable of it engage any cooling equipment first before disconnecting the battery.
If the temperature build up is caused by an external heat source then a good quality BMS can alert you to a pending fault condition through a communications interface or illumination of an Indicator.
Altertek BMS's all have multiple temperature sensors for the connected cells and an onboard PCB temperature sensor as an extra layer of protection as well as onboard fault LEDs and the ability to connect an off board indicator.

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Under & Over Voltage Protection

As described in cell balancing section above, a cell or cells exceeding their rated terminal voltage can have a detrimental effect on their service life but more importantly can cause a cell to rupture violently in flames and smoke.
On the other end of the scale running a cell below its rated minimum voltage can be just as dangerous and detrimental to service life.
Cell balancing will keep cells in check but what happens if the charger fails or is incorrect for the cells chosen and provides a voltage that exceeds the cells rated voltage.
Or conversely a cell or cells are allowed to discharge past their minimum voltage.
The under and over voltage protection circuits constantly monitor the cells and will disconnect the charger and / or load ensuring the pack remains within safe limits.
All Altertek BMS's come with over and under voltage protection circuits as standard, and some models include both a primary software and secondary hardware circuit for added levels of protection.

Short Circuit Protection

Short Circuit Protection is a must have safety function when using Lithium cells and batteries for 2 primary reasons.

The first is that Lithium cells can output in a short circuit scenario many times more power that a lead acid and we could be looking at current in the thousands of Amps depending on the scenario which can cause damage to wiring, instantaneous heating of connected metallic items to red or white hot, Arc flash which can be damaging to eyes and the actual damage to the cells themselves.

The second reason is that for a Lithium battery to be able to be sent by normal transport methods the battery has to be tested to UN38.3 test standards which part of this testing is a prolonged short circuit test. Any lithium battery without short circuit protection will not fair well in this test and in some cases there will be no battery left to examine.

So Short circuit protection is critical to protect the battery, the installation and to be safe to transport from one place to another.
Altertek BMS's all come with Short Circuit Protection and we always recommend using MOSFET switching so the system is fast enough to disconnect in a fault condition without damage occurring.
The switching can either be done on the BMS or through a separate solid state switch adequately rated for the load but ultimately controlled by the BMS.

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