X2 BMS – new technologies

X2 BMS introduces five new BMS technologies adopted for systems in yachts

1. Introduces new X2-technology

..where the “X2” stands for management of two different batteries by combining the start lead battery (or aux) and the lithium battery into one system.

  • No insulator device (Isolator) needed between batteries. (Lead battery combiner / voltage relay does not work with lithium and isolators have drawbacks).
  • Power saving due to BMS will disconnect Charge relay when not charging (i.e. no hold power to Charge relay which makes a non-latching relay power efficient as a bi-stable relay).
  • No pre-charge effect when neither Charge relay nor Load relay connects (Load relay will be pre-charged when start up).
  • No DC/DC device needed to control different charging and/or insulate alternator charging from starter.
  • All chargers are able to charge all batteries included in the system.
  • Wind and hydro generators operate safely according to the manufacturer’s recommendation, which does not allow the battery voltage to be disconnected when the generator is spinning. (Starter / Aux is always connected in the X2-system.

2. Introduces new Charge Management System (CMS)

  • Manage all chargers and disconnect charging including solar and alternator when battery is fully charged.
  • Benefit of CMS is to avoid float charging which otherwise shortens the life of the lithium battery. Charging is reconnected automatically, only if certain discharge has taken place.
  • The built-in Charge Management System (CMS) interrupts charging to the lithium battery when it is full and then continues to charge the lead battery (starter / Aux). CMS works fine with X2 BMS’ dynamic balancing which do not require the absorption charge.

3. Introduces new Intelligent Dynamic Algorithm (IDA) to perform balancing

  • Will improve balancing during limited time periods when balancing can take place.
  • IDA will boost balancing up to 10 times in comparison with other systems, which are unable to balance when chargers switch to float or are turned off.  

When a marine charger switch from BULK to FLOAT and voltage drop from 14.2 V (28.4) to 13.5 V (27.0), the difference in cell voltage will disappear and no matter if cells are balanced or not. However,  X2 BMS with Intelligent Dynamic Algorithm has the ability to continue balancing despite no cell delta.


X2 balancing will not occur when the cells are in balance, regardless of cell voltage. Other poor systems with balancing boards on the cells will always start shunting as soon as the cell voltage exceeds a certain level. For balancing-board systems, balancing/shunting will take place for all cells even the lowest, because balancing boards lack intelligence from the central processor. If all cells are balanced / shunted, no balancing takes place. Balancing boards also have continued standby current which can drain and damage the battery in event of LVP. 

4. Introduces the new Low Voltage Protection as the safest of all existing battery systems

Low Voltage Protection (LVP) will cut off all discharging if the lowest cell is below threshold and after a time delay with several alerts and Pre-Alarm includes 2x buzzers.

  • LVP at two different levels with different time delay.
  • BMS will not trigger LVP-1 shut off due to voltage drop when battery is low and power consumption is very high. In such event BMS Control will alert the Pre-Alarm with a delay of 3 minutes.

The warning system makes it safe to utilize 93 ­– 95% of the true capacity. When about 80% of lowest cell has been utilized, “Low Battery LED” starts flashing. When 93 – 95% of lowest cell has been utilized, “Pre-Alarm LED” starts flashing. The duration of Pre-Alarm / Low Battery is 27 minutes, where the final 2 minutes includes a Buzzer alarm.

The battery monitor (external) measures the current and makes an estimate of the state of charge (SOC) based on the capacity set in the battery monitor. Most often, the SOC calculation is not entirely true, because the battery monitor has not been synchronized for a while, but also because the capacity set is not the same as the true capacity of the lowest cell.

Please note, difference between 80% of nominal capacity vs. 95% of true capacity (when Pre-Alarm on the BMS Control) can be as much as 50% of capacity utilization. (True capacity can be 127% of the nominal). A great warning system based on cell status will increase capacity utilization by up to 50% when required, which should be taken into account when comparing systems ($ / usable Ah).

5. Introduces inbuilt pre-charge for Charge and Load relay

Inrush currents into capacitive components are a key concern in switches and relays. When initially connecting a battery to a capacitive load, there is an inrush current as the input capacitance is charged up to the battery voltage. With large batteries and powerful loads (charger, inverter, alternator), the inrush current can peak 1000 A or more for just a millisecond. 

In a X2 BMS controlled system, all chargers are always connected to starter or aux battery and will keep all capacitors* (chargers) pre-charged all the time. The X2-system eliminate need of pre-charge for the Charge Relay.

When pressing START button, BMS will be powered up during a limited period and Load relay will close with a delay of 5 seconds.  During these 5 seconds, the Inverter, Multi (charger/inverter) and other capacitive loads at load side will be pre-charged. The current will be by-passed thru the power resistor before Load Relay closes and connects the lithium battery.

The pre-charge power resistor is mounted next to the X2 BMS. The resistor is different for 12 and 24 V systems and has a low resistance to be efficient for 10 kW loads.* (Resistance is included in BMS Kit-2 and as a standalone price 10 €).

*) Capacitive loads are all components that includes a coil, such as: Inverter, Multi/combo, alternator, motors as electric wind lass, winch and bow thruster. In most installations, capacitive loads will be chargers and loads, i.e. both load and charge relay must be protected by pre-charge.

The relay is bridged  by a wire between two pins. This picture illustrates an arc created by a 3000 W inverter. If the system isn’t pre-charged, an arc will affect the relay on the inside in the same way.

The pre-charge power resistor (green) limits the inrush current to 2.5 A. The current is sufficient for inverter and other loads up to 10 kW.