New Beetle BMS Alternatives Analysis

January 2004

Battery management systems are divided into Monitors and Balancers. Monitors monitor each battery voltage, temp and charge/discharge, then display this information to the driver. Balancers ensure each battery within the string are maintained at a constant similar voltage. Balancers are required, as all batteries are subtly different, and a weak battery can slowly destroy the whole pack.

The intention for the New Beetle project will have a custom developed Battery Management System (BMS) which both monitors and balances the pack.

Monitor Requirements

• Monitors battery voltage
• Monitors each pack temperature (could monitor battery temperature)
• Monitors charge/discharge

Monitor General Designs

In General

There a few general designs for battery monitoring systems:

• Local or remote (i.e. is the monitor on the battery or on the dash)
• Digital, analog or hybrid

Constraints

• Complex circuitry
• If programmable, must use PIC Microcontrollers and supported by PicBasic

Alternatives

Local Analog Voltage Monitor

2 colour LEDs with Zener diodes on each battery (analog)

Alternative	Description	PROS	CONS
Local Analog Voltage Monitor	2 colour LEDs with Zener diodes on each battery (analog)	easy to build no isolation issues	only monitors voltage have to stop, open the battery boxes to see voltage nothing about state of charge
Central Digital Voltage Monitor with Remote Analog Sensors	Using PS8602 optical isolator, feed to single central computer		does not require a network accuracy of optical isolator? (what does the LHB use?) can add temp sensors for each battery box can be isolated can implement algorithms for state of charge	lots of wires going to central computer subject to line noise and errors
Central Digital Voltage Monitor multiplexed to Remote Analog Sensors	A voltage to frequency chip converts the battery voltage to pulses. This is fed to an analog isolator. This goes to a multiplexor for the battery box. The central computer selects a battery, then reads the frequency. The frequency is then a function of the voltage.	only a couple of wires to the central computer possibly better accuracy than straight analog can add temp sensors for each battery box can be isolated can implement algorithms for state of charge simpler network protocol	requires wires for multiplexor addressing complex to make
Remote Digital Voltage/Temp Monitor multiplexed to Digital Sensors	Use small uProc plus circuits to measure each battery voltage and temp. A multiplexor selects the various monitors, via isolation. (i.e. hardware addressing)	better accuracy? can add temp sensors for each battery box can be isolated can implement algorithms for state of charge simpler network protocol less subject to noise	requires wires for multiplexor addressing complex to make
Remote Digital Voltage/Temp Monitor on a Bus with Digital Sensors	Use small uProc plus circuits to measure each battery voltage and temp. Info is put onto an isolated bus (EvilBus, RS485, CAN, etc.) to the central computer (i.e. software addressing) - master/slave protocol	only a couple of wires to the central computer bettery accuracy can be isolated can implement algorithms for state of charge less subject to noise Flexible: bus can be used for all other information (pack current, charger status, etc.) Can be integrated into a battery balancing system	complex software to handle protocol complex to make have to develop sensor addressing scheme

Battery Balancer

There are several balancer designs (shunt, pair balancer, central charger). Lee Hart's Battery balancer is a well designed system with integral safety built in. It is a central charger system which routes power to the weakest battery using relay control boards. This system is design for 16 batteries in two packs, as well as only two temperature monitors. The design will have to be modified for the requirements of the New Beetle EV project.

Selection for the New Beetle Project

After many prototypes and analysis, the Lee Hart Balancer will be modified for monitoring and balancing for the New Beetle project. For the following reasons:

• The LH Balancer has integral, buit-in safety to ensure no software bugs or hardware noise can charge no more than one battery at a time.
• It is a proven design, in use by a number of EV hobbyists who are electrical engineers.

There will have to be a number of modifications to the LH Balancer for the New Beetle EV project:

• Add control to be able to simultaeousnly quickly monitor individual battery voltages while charging any given battery.
• Add up to four battery pack temperature sensors
• Control up to 4 packs with up to 8 batteries each

BMS Parts