Tech Info: EV Power's Lithium Battery Management System

By Rod Dilkes, EV Works Pty Ltd

Simple, Effective Battery Management

The simplest, most reliable and cost-effective large format lithium ion (LFP, LiFePO4) battery management system. Lithium Ion batteries require cell balancing during charging and protection from over-charge and over-discharge. This seemingly simple function is of utmost importance in mission critical environments. It is also one of the most difficult functions to achieve with absolute safety and reliability. The EV Power Battery Balancing System has been developed specifically for large format Lithium Iron Phosphate batteries such as Thunder Sky and Sky Energy LFP cells. It can also work with other Lithium Iron Phosphate battery brands with minimal modification. This system has been in continuous development for several years and has evolved to the point where we can claim it to be one of the most reliable and effective battery management systems on the market.

Cell Module

The EV Power Cell module is a unique patent pending design. It has had several years of field testing and proven itself to be safe and reliable in the most demanding applications. The smallest unit of the BMS is the cell module suitable for Thunder Sky or equivalent LiFePO4 cells. It can operate as a standalone cell balancer or in combination with other modules to monitor and balance a LFP pack. A red LED operates when the voltage exceeds 3.6V as the regulator dissipates excess charge from the cell. A green LED is normally on when when the cell voltage is between 2.5V and 4.2V. Signal wires connect in series with other cell modules via the one wire link the circuit is normally closed if all cells are OK (2.5V - 4.2V), open otherwise. Connected in series with other cell modules and a master unit provides complete battery protection from overcharge and over-discharge. Epoxy Encapsulated to prevent moisture and dust interference. Gold plated cell terminal connectors. One wire link between cell modules Soldered signal connections for reliability. Signal connections isolated from cell terminals. Non-polarized (bidirectional) signal connections for easy installation. Low profile, does not protrude above terminal bolts. 800mA shunt current capacity. Reverse polarity protected. Internal fuse to prevent cell damage from incorrect installation. Simple to install and easy to understand. Suitable only for LiFePO4 cell chemistry

Master Control Unit

We have a number of master unit models in development and production. The EV Power system consists of Cell Modules, one for each cell, and a master control unit. The cell modules perform the balancing function and also monitor the voltage of each cell in the battery. This system is designed specifically for road going electric vehicles with LFP battery packs with a nominal voltage of 12 - 300V. The TS90-EV2 Master Control Unit oversees the battery pack. It interfaces with the cell modules via a proprietary one wire interface. The master unit monitors a heartbeat signal through the cell modules. If there is a problem with a particular cell this heartbeat will be broken and the master unit will take some action to prevent further charging or discharging of the battery. There are outputs to activate or deactivate external devices as required to protect the battery.

• Supply Voltage 36-350VDC (HV model), 12-36VDC (LV model)
• Supply Power 1W max. @ 144VDC
• 1.5kg
• 200 x 170 x 55mm
• One signal input channels for monitoring banks of cell modules and/or temperature probes.
• Charger Relay (Relay is closed when system is OK and charger is plugged in) Contacts: NO/COM , Max Voltage: 250VAC /
• 24VDC, Max Current: 20A Suitable for single phase or three phase chargers.
• Alarm Light Relay (Relay is active/closed when the system is not OK) Contacts: NO/COM, Max Voltage: 30VDC, Max
• Current: 2A
• Throttle control PWM output reduces motor throttle in the event of a system error.
• Main contactor control relay will drop the main contactor in the event of a system error and no response from reducing the throttle.
• The system is designed to work in unison with the TBS E-Xpert Pro battery fuel gauge. This provides comprehensive battery state-of-charge information.

EV Power BMS Features:

• Modular - one small balancing module bolts directly to each cell.
• Simple - A one wire interface is all that is required. No wiring birds nest.
• Scalable - Battery balancing from one cell up to any number.
• Fail Safe - a unique patent pending safety interlock system prevents any cell from going over/under voltage.
• Hermetically sealed - cell modules resist dust and moisture
• Flexible - the master control unit can work with almost any charger.
• Reliable - analog electronic cell modules, no computers in high EMI environment.
• Safe Installation - No high voltage signal wire connections.
• Easy Cell Status indication - An indicator on each cell.
• Cost Effective - low cost per cell for large format Lithium Ion batteries.
• Designed and manufactured in Australia

About Battery Management

All lithium Ion batteries require a battery management system (BMS). There is no way around this. There are numerous examples of catastrophic failures where no BMS has been used.

At a minimum a BMS must have the following basic functions:

1. Balance the cells during charge to level the state of charge of all cells in the pack.
2. Protect each cell in the battery from going outside its safe voltage range.

There is a lot of hype about battery management systems for Lithium Ion batteries. Should a BMS have cell by cell voltage monitoring and reporting to a central display unit? Should cell temperatures be recorded and reported? Should the BMS be able to bypass large charge currents? What about active balancing and a CAN bus interface?

The answer is, NO. These functions are not essential. Sophisticated BMS’s may provide an impressive list of functions but are prone by their very complexity to problems both in installation and usage.

• SIMPLICITY = RELIABILITY.
• SIMPLICITY = LOW COST
• SIMPLICITY = EASY TO UNDERSTAND

The EV Power BMS is the simplest system in the world that will actually provide balancing and fail safe voltage protection of large format LiFePO4 batteries. Extra monitoring functions can be easily layered on top of a simple BMS. A simple BMS should form the core of any large format LFP battery.

Modern LiFePO4 cells are very safe and manufactured to close tolerances. With a BMS they will not explode or catch fire. They should be highly reliable and operate with the minimum of intervention from the BMS. It is there as the last line of defence.

The EV Power system designed for LiFePO4 cells of 40Ah and larger performs the basic functions simply and well. It does not rely on digital microcontrollers on each cell but to date has proven itself to be fail-safe and highly reliable. This has been proven in many road going electric vehicles which are demanding applications for a BMS. Digital microcontrollers on each cell will be more likely to fail. This coupled with potential communications interference in noisy BMS applications could result in unreliable systems which may not be fail-safe.

Our cell modules are epoxy encapsulated against moisture and dust. This IS important because it is moisture and dust that will cause electrical problems in other systems down the track. Also important is that it does not rely on any connectors which can wriggle loose and fail. The EV Power BMS is a genuine one wire system where all the connections are soldered. It can monitor any number of cells in up to four banks and has provision for temperature monitoring also.

As well as high quality with gold plated terminal connectors it is reasonably priced. This is all you need in a BMS.

Frequently Asked Questions

Q: Why do I need a Battery Management system?
A: Lithium Ion batteries are made up of two or more cells connected in series. Each cell has a slightly different capacity due to manufacturing tolerances and environmental conditions. After several charge/discharge cycles the cells begin to go out of balance and so a balancing system is required to keep the cells all at the same state of charge. The BMS also prevents overcharge and overdischarge of any cell in the battery which is essential when using Lithium Ion batteries.

Q: What digital outputs does your BMS provide?
A: It simply balances the battery pack and provides connections to the charger and motor controller to prevent over charge and overdischarge. Digital information can be obtained from the TBS Battery Monitor or EVMS also stocked by EV Works.

Q. What charger should I use?
A: Almost any switchmode SLA charger with the correct peak voltage is suitable. However there may occasionally be problems with cell balancing due to the incorrect charge curve. In EV Works we now have a range of Zivan NG3 chargers with a new charge curve ideally suited to charging LFP batteries.

Q: Will the BMS work if the batteries are in different locations around the car?
A: Yes... However, it is advisable to keep all the cells in one physical location if at all possible. Cable resistances, temperature and moisture differences in different locations of the car can result in unforseen problems and shortened battery life. This has nothing to do with the BMS, it is about design and common sense. Also placing batteries in car crumple zones SIGNIFICANTLY COMPROMISES SAFETY in the event of an accident. Remember that batteries are the fuel tanks of an EV and due consideration should be given to locating them, even at the expense of load space. It is also important to maintain weight distribution between the front and rear axles of a car.