Battery management system

Contents

1. Abstract
2. LiFePO4 battery issues
3. Battery capacity improvement with battery cell equalisation
4. Passive cell balancers
5. Active cell balancers
6. Regenerative braking with BMS
7. Solar bike BMS
8. Notes
9. BMS links
10. References

1. Abstract

LiFePO4 batteries, as well as lithium-ion batteries, doesn't like to be overcharged or over-discharged. Therefore a fail-safe circuitry is mandatory. It shuts down the battery pack when the voltage of one of the battery cells becomes outside the safe range, for A123 LiFePO4 batteries this is about 2V to 4V. The fail-safe circuitry is part of the so called battery management system (BMS).

A BMS contains quite a lot of electronics to be able to measure all the individual cell voltages. Power MOSFETs are used as switches to shut down the battery. A BMS is often equipped with extra functions.

2. LiFePO4 battery issues

2.1. LiFePO4 battery energy storage efficiency versus voltage

When the battery efficiency is 100%, the stored energy is U * I * t. The battery efficiency however varies as function of the voltage. Between 3.0V and 3.4V the efficiency is high; here most of the energy is stored. At the end of the charging cycle, not only the voltage rises rapidly but the battery efficiency is very low too. In other words, there is less energy stored in the battery between 3.6V and 4V.

The evidence of the limited energy storage above 3.6V is that after charging, the voltage drops quickly without load. See the charge curve.

2.2. Charge curve

2.3. LiFePO4 batteries overcharge tolerance

Usually it is recommended that LiFePO4 batteries should be charged until the voltage reaches 3.6V. They can however be overcharged to about 4V without degradation or safety issues. To store more energy it doesn’t make sense to overcharge a LiFePO4 battery.

3. Battery capacity improvement with battery cell equalisation

Upon discharging, the entire battery pack is shut off when the weakest cell drops below the lower voltage limit. It is clear that the other cells are still not completely empty. Upon loading plays the same problem. To get the maximum capacity out of a battery pack, the cells must therefore be balanced. Cell balancers can be dissipative or nondissipative.

Here is a practical example from Charles Richter that shows the capacity improvement that a balancing BMS can bring about.

4. Passive cell balancers

Also called bleeding cell balancers or dissipative cell balancers. Resistors are used to bleed the energy from the good cells, in order to match the voltage to those of the bad cells. It is clear that this is wasting a lot of energy because the good cells are in the majority.

5. Active cell balancers

Also called nondissipative cell balancersors or distribution cell balancers. An distribution cell balancer moves energy from the good cells to the bad cells. This can be done capacitive or inductive.

5.1. Capacitive cell balancers

A common method is using a charge shuttling balancing circuit, containing a flying capacitors, for every two neighbouring battery cells. Each cell contains a balancing circuit that can move energy with a capacitor to cells above or below in the cell string. Over time, all cell voltages will be equal.

The number flying capacitors is the number of battery cells -1.

It is clear that when the good and bad cells are on the opposite ends of the cell string, the charge would have to travel through every cell. Because of the cell-to-cell transfer loss, this kind of capacitive balancing has a maximum efficiency of just 50% in practise. The maximum number of battery cells in series that can be used without killing the efficiency is about 12.

A serious disadvantage of the capacity cell balancer is that it requires a certain cell voltage difference to function, but on the flat section of the LiFePO4 discharge graph the voltage differences between cells are very small.

5.2. Inductive cell balancers - transformer based

Energy transfer in power supplies is commonly done inductively, with coils and transformers. We see only capacitor based power supplies at lower power levels. Inductive cell balancers are faster and have a higher efficiency than capacitive cell balancers. At a transformer based cell balancing BMS, the primary winding is used for the battery pack and the secondary windings are used for the individual cells.

Transformer based cell balancers can be divided between bottom cell balancers and top cell balancers.

Bottom cell balancing. A battery cell receives energy from the entire battery pack.
Top cell balancing. The entire battery pack receives energy from a battery cell.

Here is a bottom cell balancer that uses a flyback transformer with a winding for every cell.

At a transformer-based cell balancer, the transformer can be used at a convenient way to measure individual cell voltages. If a battery cell is connected to its secondary winding, on the primary winding arise a voltage pulse proportional to the cell voltage.

5.3. Inductive cell balancers - inductor based

An inductor based cell balancing BMS is the patented PowerPump cell balancing technology from Texas Instruments. It uses a buck-boost converter to transfer the energy from one cell to the other. It is used at the TI chipset BQ78PL114 / BQ76PL102. See PowerLAN Gateway Battery Management Controller with Power Pump Cell Balancing.

Unfortunately, there is no e-bike BMS on the market that uses the TI chipset. Currently Per Hassel Sørensen is developing an advanced BMS with this chipset.

6. Regenerative braking with BMS

Hub motors without freewheel can be used for regenerative braking. Instead of using the brake when driving downhill, the motor is used as generator and the energy is stored into the battery. Regenerative braking is no problem for the BMS; it is the same as if a battery charger is connected.

If the battery is full, the BMS switches the battery off:

While the motor/generator is abruptly switched off, braking is stopped, this can cause a dangerous condition.
The voltage of the motor/generator at high speed can exceed the BMS maximum voltage, this can damage the BMS.

7. Solar bike BMS

7.1. Required solar bike BMS functions

Over- and under-voltage protection for each cell
Battery cell balancing
Overcurrent protection
Logging of individual cell voltages to detect bad cells
Temperature measurement
Estimating the battery state of charge by Coulomb counting
A sophisticated cell balance algorithm

7.2. Cell balancing efficiency

When charging batteries from the mains, the loss of the cell balancer is of little importance. The solar bike battery however is charged by solar panels, which power is limited. Here a high efficiency of the battery cell balancer is crucial.

7.3. Required cell balancing power

The cell balancer has to fill the bad cells fast enough to keep up during discharging.

P = average load power * cell capacity tolerance

I don't know the A123 26650 battery capacity tolerance, let we assume it is 5%. The required balance power is 2.5W at an average load power of 50W.

7.4. Fast acting DC fuse

The BMS is equipped with a fast over current protection. But the use of an additional fuse in series with the battery is necessary too. Note that automotive (blade) fuses are too slow and are not suited for 36V.

We need a special high current fast acting DC fuse, such as the 30A 250VDC fuse 0324030.HXP from Littelfuse. The resistance is 1.82mΩ.

7.5. Fuse holder resistance loss

The resistance of fuse holders is often larger than the fuse itself. These values are measured at 8A:

Fuse 5 x 20mm F16A, R = 4mΩ

Chassis mount fuse holder. R ~ 1mΩ

In-line fuse holder. R > 10mΩ

Panel mount fuse holder. R > 20mΩ

8. Notes

It cost extremely time to balance a battery pack with full and empty cells. Replace only cells with approximately the same charge as the other cells.

9. BMS links

10. References

"A cost optimized battery management system with active cell balancing for lithium ion battery stacks" Infineon Technologies, Carl Bonfiglio, Werner Roessler.
"How to Efficiently and Safely Charge a Lithium Iron Phosphate (LiFePO4) Battery" Texas Instruments, Jinrong Qian.

avdweb