Technology

Why does the Cell Pack as one unit provide a better battery?

Essential for professionals when designing a lithium battery bank, is to have one battery instead of several 12 V blocks connected in parallel. Having one battery, as a homogeneous Cell Pack, will offer the best capability and performance of following reasons:

  • A battery bank as one Cell Pack can be better monitored and controlled by a central BMS as the Master. Balancing is performed by all the cells together and more capacity can be utilised safely.
  • A central BMS as Master configuration also has the advantage that the balancing is centralised in the BMS unit and therefore does not have electronics inside the battery (12 V block) consuming standby power. It is essential to not have any standby power when safeguarding the battery in the event of low voltage protection. This also means that the battery bank can be stored for a long period during the winter without requiring a recharge due to standby power from the cell boards / electronics inside the 12 V batteries.
  • An additional benefit is that a cell pack can be customized to best fit the dimensions of the battery compartment and maximise the capacity of that specific space.
  • Last but not least, the system (BMS) will be much better without any mosfets acting as battery switch carrying all the system current – as for drop-in batteries.

X2 BMS Pro is developed for lithium Cell Pack that consists of 3.3 V LiFePO4 cells. Makes it possible to build a custom-made battery as one Cell Pack.

X2 BMS Pro is developed for High Performance Cell Pack with large cells (several paralleled)

A Cell Pack is connected in an order of only 4 individual cells for 12 V and only 8 individual cells for 24 V. At image 3P4S, where each individual cell is 3 cells in parallel – creates a “big” cell where all cells will have the same voltage (status) as it was one homogeneous cell. These four big cells (3.3 V) are connected in series to get the battery voltage (about 13.3 V).

One battery will offer the best performance by monitoring each individual cell and balancing all cells by a centralized BMS. However, a cell pack as one battery places higher demands on the BMS to manage large cells. X2 BMS Pro has a unique capability to balance large cells by the Intelligent Dynamic Balancing (IDA) after the charging has been disconnected.

Maximum compatibility

X2 BMS Pro has eleven dedicated outputs to manage; different load relays, different charge remotes / relay, as well as Alternator control and cooling.

The system is designed to have an integrated backup solution.

Any charger including remote controlled will be able to supply power to the backup – at any time – even when BMS is forced OFF (no power) or system failure.

X2 BMS Pro is compatible to connect Victron Multi / Quattro or Mastervolt Combimaster. 

X2 BMS is also compatible with all alternator controllers by the unique field pre-disconnect – AFP output (will pre-disconnect for all the five conditions which are able to disconnect the charge relay).

X2 BMS Pro has two RJ45 terminals as CAN.  Comparison show that there are benefits to connect Victron Multi and/or Wakespeed regulator via digital output controlling a NC- relay versus via CAN. (Described in the User Manual).

Manages alternator cooling by controlling external in-line blower

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  • NEW Feature introduced 2024

    X2 BMS Pro has output to control external in-line blower based on temperature input from the alternator.

    • Will monitor the temperature for one or two alternators.
    • Will control in-line blowers, to cool the alternators.
    • If a fault occurs so that the temperature rises and the alternator is at risk of overheating, the BMS Control alerts with flashing LED as first level and the second level high-yielding buzzers.
    • Settings of different temperature by DIP switches.

Cell Balancing performed by IDA

The IDATM feature explains why X2 BMS Pro will outperform other BMS with active balancing at 2 – 4 amps.

X2 BMS Pro has a unique capability to perform the initial balancing when the cells are new – and can do it without absorb voltage.

  • Will improve balancing during limited time periods when balancing can take place.
  • IDA will boost balancing up to 5 times in comparison with other systems, which are unable to balance when chargers switch to float or are turned off.  

To obtain the best battery life and capacity utilization, it is important that the cells are in balance and never exceed their voltage limits.

IDA works by keeping different states of charge of unbalanced cells in memory. After the charger has been switched off or changed to Float, the voltage will be lowered and delta will disappear regardless of whether cells are still unbalanced. However, the unique IDA-balancing will continue balancing for several hours if a significant unbalance has been detected. Setting can be optimized with DIP switches regarding cell size.

In comparison, IDA is a unique feature and no other BMS can perform accurate balancing when battery voltage is below 13.8 V. This is because it is not possible to read out any accurate difference between the cells when the voltage drops and it does not matter if the BMS is allowed to balance.

Some argue that active balancing is more efficient than passive. However, the facts about the difference can be illustrated by following example:

  • Assume that one cell will be balanced (1.5 A passive) during five minutes, which can be the repeated balancing when battery is fully charged (and the initial balancing is done).
  • If the charger is charging at 50 A /12V, it takes two seconds to recharge the energy lost for the passive balancing done during five minutes (1.5 A / 3.5V).

Cell Balancing LED; is on when balancing is ongoing. The LED is Long-flashing when perfect balanced (require battery fully charged and voltage >13.8 V).

The balancing is performed with a fine-tuning algorithm as well as with an Intelligent and Dynamic Algorithm (IDA). Balancing LED indicates when balancing is in progress. X2 BMS Pro uses passive balancing with attached heatsink and integrated dual temperature sensing. Max balancing current is 1.5 A and will be boosted by IDA when large cell differences are detected.

Our testing has shown “IDA passive balancing” to be faster and more accurate compared to active balancing due to algorithms (IDA) and settings being more sophisticated for passive balancing versus active. This is also due to cell voltage being affected and not showing true value when active balancing charge one cell and discharge another cell. For passive balancing, it is only discharge i.e. the same impact downwards (voltage drop) which affects cell reading on cells that are being balanced.

IDA-balancing was introduced 2021 to improve balancing due to the limited time period when measurements of accurate cell differences can be performed.

 

Low Voltage Protection

Low Voltage Protection (LVP) will cut off all discharging if the lowest cell is below threshold and after several alerts and Pre-Alarms. The final Pre-Alarm includes 2x high-yielding buzzers with opening in the front (2x 92 dB).

  • LVP at two different levels – unique design.
  • LVP-1 will protect when the lowest cell is at about 6 – 8% SoC.
  • The LVP-algorithm will not trigger a shut down due to voltage drop when battery voltage is low and power consumption is very high. In such event the BMS Control will alert a Pre-Alarm with a delay of 3 minutes.

The pre-warning system makes it safe to utilise 92% of the true capacity. When about 80% of lowest cell has been utilised, “Low Battery LED” starts flashing. When 92 % of lowest cell has been utilised, “Pre-Alarm LED” starts flashing during 27 minutes and the final 2 minutes includes a Buzzer alert.

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 estimate 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. The Amp-hours settings can be adjusted according to BMS Control true values.

Please note, difference between 80% of nominal capacity vs. 92% of true capacity can be as much as 50% of the capacity utilisation. True capacity can be 127% of the nominal. A great pre-alarm system based on cell status will increase capacity utilisation by up to 50% when required, which should be taken into account when comparing systems ($ / usable Ah).

X2 Tech combines lithium and lead-acid batteries

..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.

Benefits of an integrated lead-acid battery – X2 TechTM

  • Backup battery and ability to supply power together with all chargers (when BMS is off / on)
  • Pre-charge to avoid damage the charge relay. The lead-acid battery will keep chargers (=capacitive loads) pre-charged (at 12.7 V) which means no inrush current when lithium connects. 
  • Protects from load dump, from chargers connected to charge bus (i.e. avoid transient voltage spike in the event of interrupted high current).
  • Supply controllers with input power; to manage wind and hydro generators and avoid damage when BMS disconnect the lithium battery.
  • Allow chargers to show voltage when they are ON and thereby allowing X2 BMS sensing charge.
  • All different marine alternators can easily be connected, including Yanmar and others, which require voltage at output.
  • Charging with high current without energy loss (current will not pass any isolator, nor DC/DC device).
  • Charge the starter battery via solar and other chargers when BMS is off (during winter storage).
  • Long Term Shore Power Mode (invented by Batterybalance) When using shore power for extended periods, there is possibility to avoid the float charge as well as to avoid cycling the lithium battery. Instead of float charging / cycling lithium, the starter or aux battery can be float charged by shore power and solar/wind, which also supply the DC power to the Load bus. In the Long Term Shore Power Mode, the X2 BMS is forced off and the lithium battery stored at 50 – 70% SOC.
  • All chargers will be able to charge all batteries included in the system. The priority will be lithium first and secondly the non-cycled lead-acid battery (-ies). This priority is the same as for an isolator 1:2. The difference is when lithium is fully charged, charge relay will open and the lead-acid battery (-ies) will be absorb charged a short period before charger change voltage to float charge.
  • No isolator device needed between the 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 (=no coil power @1.7 W).

Charge Management System

  • 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’s IDA balancing, without absorption charging. 

Built-in 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. 

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.

*) 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 the pins. This picture illustrates an arc created by a 3000 W inverter. If the system isn’t pre-charged, an arc will affect inside the relay. An arc inside the relay will damage the relay.

The pre-charge power resistor limits the inrush current to 3.3 A. The current is sufficient for inverter and other loads up to 10 kW. Will manage a certain level of parasitic current from other consumers.

Design details

TERMINALS

The terminals for connections are Phoenix, Würth and Molex (highest quality, some of them made in Germany). The C-terminal is a Molex that clips in and is secured. This is the connection of tap wires to the cells. Tinned wire size between C-terminal and fuse box is AWG 18 (0.75 mm2) and from fuse box AWG 16 (1.5 mm2).

The terminals for connections are Phoenix, Würth and Molex (made in Germany). The C-terminal is the connection of tap wires to the cells. Tinned wire size between C-terminal and fuse box is AWG 18 (0.75 mm2) and from fuse box AWG 16 (1.5 mm2).

FUSES and CONNECTIONS

Inputs are fused with ATO (automotive fuse) in an external fuse box. X2 BMS does not use resettable fuses (PPTC) for several reasons,
– higher resistance when compared to traditional fuses.
– change resistance at different ambient temperature
– slower response time compared to traditional fuses,
– can not be replaced by the user when they fail (=safety issue)

The advantage of PPTC is that they are cheap and can be mounted on the circuit board.
The advantage and disadvantage of ATO are the reverse as above.

At the same time, the external fuse box has the advantage that it becomes a natural connection point where tap wires can be extended to the desired length of good dimensions.
Another advantage is that if several contactors (relays) are connected in parallel, the fuse can be changed/adapted to what is required. Even the pre-charge is fused in the external fuse box, both incoming and outgoing.

 

Extensive experience with electronics in harsh and humid conditions set design requirements to protect in two areas:

  • Conformal coating of the circuit board that protects in humid environments and condensation
  • Protect terminals from moisture and water penetration

The BMS has several terminals and all of them are protected and placed inside the enclosure. 

The circuit board is protected by conformal coat of a superior quality and is heat-cured. Meets the highest defence standards in both Europe and the US.