Lithium Battery Smart Manual

Lithium batteries are easier to charge than lead-acid batteries. The charge voltage may vary anywhere from 14V to 15V for a 12.8V lithium battery and 28V to 30V for a 25.6V lithium battery, as long as no cell is subjected to more than 4.2V. Lithium batteries will become permanent damaged if they are over-charged.

Should a cell reach 4.2V, all charge into that cell will be dissipated as heat. However, this is impossible on a properly installed system.

We recommend 14.2V (28.4V) as the absorption voltage, but if you want to change it, we advise keeping it between 14.0V (28V) and 14.4V 28.8V). Float voltage should always be 13.5V (27V).

Because of the flexibility in charge voltages, up to 20 batteries can be connected in parallel without much problems. No damage will occur if there are small differences in individual battery voltages because of varying cable resistances or internal battery resistances.

Once the absorption stage has been finalised, the battery charger goes into float.

The storage stage is not per se needed for a lithium battery, but if the charger has a storage mode, set the storage voltage at the same value as the float voltage.

We recommend a charge current of 0.5C. This means that if the battery is completely empty, it will take 2 hours to charge the battery. A charge rate of 0.5C for a 100Ah battery is 50A charge current. The maximum charge current is 2C, for a 100Ah battery this is 200A. This will charge the battery in half an hour. But be aware that the batteries will produce more heat when high charge currents are used. More ventilation space is needed around the batteries and depending on the installation, hot air extraction or forced air cooling might be needed.

Lithium battery charge graph

The BMS will turn off all charge sources as soon as a battery cell voltage reaches 3.75V or if the battery temperature drops below 5°C or increases above 75°C. This means that all charge sources that are connected to the lithium battery must be controlled by the BMS.

The battery consists of lithium cells that are connected in series. The 12.8V battery has 4 and the 25.6V battery has 8 cells in series.

Why is cell balancing needed

Though carefully selected during the production process, the cells in the battery are not 100% identical. Therefore, when cycled, some cells will be charged or discharged earlier than the other cells. If the cells are not regularly balanced these differences will increase over time.

The same happens in a lead-acid battery, but there the cells self-correct without the need for cell balancing electronics because a small current will continue to flow even after one or more cells are fully charged. This current helps to fully charge the other cells that are lagging behind, thus equalising the charge state of all cells. The current through a lithium cell however, when fully charged, is almost zero. Lagging cells will not be charged further unless they receive "help" with this from cell balancing electronics.

Cells do not get damaged if they have different balance levels, but the imbalance will manifest itself in a (temporary) battery capacity reduction.

How does cell balancing work

The battery has built-in "active" and "passive" cell balancing. This ensures that all cells will be balanced. Each cell voltage is monitored and if required, energy will be moved from the cell(s) with the highest voltage to the cells with a lower voltage. This process will continue until all cell voltages are within 0.01V of each other.

When does cell balancing take place

The cell balancing process starts when the first cell has reached 3.3V. This depends on the level of imbalance. In case of a severely imbalanced battery, balancing can start at a lower voltage.

The cell balancing process generally takes place when the cell voltages are 3.50V. This can happen only during the absorption charge stage, as during this stage the charge voltage (14.2V or 28.4V) is high enough to allow the cells to reach a high enough voltage so the smaller cell differences can be corrected.

The cell balancing process is nearing completion when all cells have reached a voltage of 3.55V and the charge current has dropped below 1.5A. Balancing is complete when the charge voltage has dropped even further.

How to ensure that the battery remains balanced

A 2-hour fixed absorption period is recommended for lithium batteries, so that there is enough time for cell balancing to take place. It is important to regularly fully charge the battery. This so that the battery spends enough time in the absorption stage. A full charge once a month should be sufficient. However, there are some applications where the battery cells will become quicker unbalanced than usual. This is the case when the system is intensively used or if the battery bank consists of multiple batteries in series. To ensure a well balanced battery, a weekly full charge is required for:

It is not possible to speed up the cell balancing process

Please note that a higher charge voltage will not speed up the cell balancing process. The cells are charged by current and not by voltage. Feeding current into a cell will cause the voltage to increase over time, but this is a fixed process. Applying more voltage will not speed this process up. In addition to this, the balancing speed is determined by the maximum current rating (1.8A) of the active and the passive balancing circuits.

How to monitor cell balancing status

Use the VictronConnect app to monitor the balance status of the battery. The app will indicate 4 balancing stages, being:

For more information on these 4 stages, click the information text, located below the cell status listing, and a pop-up window will open up with an explanation of each stage.

Cell balancing information. From left to right: unknown, balancing, balanced and imbalance.

The app also indicates the number of days since the last full battery charge. If the full charge was more than 30 days ago, it will indicate "unknown". This means that the battery has not received its recommended monthly charge.

Nearly the whole available battery capacity can be used, with exception of the approximate last 3% of remaining capacity. Lithium batteries will become permanently damaged if they are discharged too deeply.

Lithium batteries can be discharged with high currents. The maximum discharge rate of the lithium battery is 2C. For a 100Ah battery this means a 200A discharge current. This current will discharge the battery in half an hour. However, we recommend not to discharge above a 1C rate. A 1C rate means that the battery is discharged in 1 hour.  For a 100Ah battery this is a discharge current of 100A.

When using a higher discharge rate, the battery will produce more heat than when a low discharge rate is used. More ventilation space is needed around the batteries and depending on the installation, hot air extraction or forced air cooling might be needed. Also, some cells might reach the low voltage threshold quicker than the other cells. This can be because of a combination of heat and ageing.

To be able to tell if a battery is too deeply discharged you will need to look at the individual cell voltages. As the battery is being discharged, the cell voltage drops. This is indicated in below discharge graph. When the battery is almost empty, the voltage will drop faster. This is the sign that the battery is almost empty. This happens at around a cell voltage of 2.80V to 2.60V. Further discharge needs to be prevented, otherwise the battery will get damaged. So as soon as one of the cells has reached this voltage the BMS will disable all DC loads.

The under voltage shutdown threshold is configurable, if it is set to a higher voltage the reserve capacity is greater than if it is set at a lower voltage. It is set by default at 2.8V and the range is 2.6V to 2.8V.

Discharge graph showing cell voltage at various depths of discharge for different discharge rates

The BMS will turn off all loads as soon as a battery cell voltage drops below the low voltage threshold.

Although a BMS is used, there are still a few possible scenarios where the battery can be damaged due to over discharge. This can occur if small loads, such as: alarm systems, relays, standby current of certain loads, back current drain of battery chargers or charge regulators, slowly discharge the battery when the system is not in use. In addition to this, the battery itself also has a small amount of self-discharge.

In case of any doubt about possible residual current draw, isolate the battery when the system is not in use. Do this by opening the battery switch, by pulling the battery fuse(s) or by disconnecting the battery positive cable.

A residual discharge current is especially dangerous if the system has been discharged completely and a low cell voltage shutdown has occurred. At 2.8V cell voltage there is approximately 3% remaining capacity and at 2.6V there is about 1% remaining capacity.

After shutdown due to low cell voltage, a capacity reserve of 1% corresponds with 1Ah left in a 100Ah capacity battery. The battery will be damaged if the remaining capacity reserve is drawn from the battery. A residual current of 10mA for example may damage a 100Ah battery if the system is left in discharged state during more than 4 days (100 hours).

If all cells are 2.8V, this means that the battery terminal voltage is 11.2V (22.4V) and if all cells are 2.6V the battery terminal voltage is 10.4V (20.8V). Be aware that the BMS will turn the loads off as soon as one cell drops below the low voltage threshold. This might not necessarily correspond with the battery terminal voltage. So, if investigating low voltage scenarios, always use the VictronConnect app to look at the actual cell voltages and do not just rely on the battery terminal voltage.

The battery sends a signal to the BMS in case of imminent cell under voltage. This is used by the BMS to  generate a  pre-alarm signal. This signal will give an advanced warning that the BMS is about to generate a “load disconnect” signal and that the loads are going to be turned off. This happens at a default cell voltage of 3.10V and the range is 2.80V to 3.15V.

Please note that older batteries might not support pre-alarm.