A Passive Circuit of Battery Management Without Power Supply Designed for Commercial Space

LIAO Luwei,XU Qifeng,XIN Yubao,SONG Junchao,WANG Yongkang

Shanghai Institute of Space Power-sources,Shanghai 200245

Abstract:Recently for lithium-ion batteries in satellites and spaceships,the Battery Management System (BMS) has become essential to enhance life and guarantee safety.However,most of the balance management circuits need complicated cell sensing and duty control modules.Such circuits are too costly to commercialize.In order to reduce the cost,weight,volume and energy consumption of BMS,this paper proposes a new battery management circuit.The designed circuit is passive and small in size.The charge voltage is regulated by increasing the bypass current through an adjustable reference chip.Experimental results show that the bypass current increases linearly if the charging voltage is in the range of 4.05 V to 4.2 V.Also after several charge-discharge cycles,the differences between batteries visibly decrease.The proposed circuit is small in size,low in power consumption and economical,making it ideal for commercial space.

Key words:power system,lithium-ion batteries,BMS,commercial space

1 INTRODUCTION

1.1 Background

Aerospace technology has developed rapidly in recent years especially in the civilian and commercial fields.Nearly 120 venture capitalists around the world invested 13 billion USD in commercial aerospace companies in 2017[1],including business titans such as Masayoshi Son of Softbank,Mask of Space-X and Bezos of Amazon.According to the market forecasts,the global commercial space market will exceed 1.7 trillion RMB by 2020.The Chinese market alone,including carrier rockets,satellites applications and space broadband internet,will reach 800 billion RMB.Satellite communications account for 69% of the manufacturing and launch market.The rest is split between Earth observation,situation awareness and exploration[2].However the commercial space market is difficult to make profits because of the high costs at present.Therefore,reducing production cost is an important measure for the further development.

As a core component of space technologies,the weight and size of the power system directly affects the production and launch cost of space products.The batteries and BMS account for 50% of the power system’s weight and 70% of its volume.As a result,lithium-ion batteries have been utilized to replace nickel-cadmium batteries and nickel-hydride batteries because of their higher energy density and low cost[3].However,in the area of BMS technology,there has still been no major breakthrough.

1.2 BMS Technology

Lithium-ion batteries must work at their right voltage range of 3.8 V-4.2 V.If the charging voltage is higher than 4.25 V,irreversible damage will be done to the batteries and even cause safety issues.Experiment results show that the battery capacity decreases to 88% after 50 abnormal charge and discharge cycles.In addition,an unequal charging voltage between batteries doubles the probability of this happening.Therefore,lithium-ion batteries must be equipped with a balance management and over-charge protection circuit[4].

The BMS is divided into passive and active modes.The active mode refers to charging the high voltage level battery for the low voltage level battery.Passive mode means that excess power is consumed by resistors to restrict charge current and achieve balance.The passive mode is more suitable for aerospace applications because the circuit structure is simple which has an advantage in weight and volume.However,the commonly used BMS for lithium-ion batteries is uneconomical and takes up a lot of volume,weight and power consumption.

LU[5]and other authors introduce some key issues about BMS and give a simplified passive mode structure of BMS,as shown in Figure 1 here.The sensing module contains amplifiers and ADCs,which will be expensive and hard to program.Finally the sensing module establishes the average charge voltage value and outputs a duty cycle signal to drive the transistor in parallel with the batteries.The bypass current in the transistor restricts the charging current.This makes sure that the charging voltage is equalized and safe.However those circuits require discrete transistors,amplifiers and digital chips,consuming much energy and space.Also the common battery management needs extra power sources to drive these devices.

To reduce the cost,Moon-Young[6]used a regulated voltage source to replace the ADCs in the sensing module.Unfortunately such a circuit contains transformers and DC-DC converters which have no advantage in volume and weight.To reduce volume and weight of the sensing module,Thomas Blank[7]and Federico Baronti[8]used the battery monitoring chip LTC6803 and isolated the DC-DC converter.Those methods reduce volume and weight,but the cost is still high.Moreover,all the above systems require an extra power supply,which is not efficient.

Figure 1 Common structure of BMS

In order to reduce the weight,volume and energy consumption of the battery management system,we recommend a circuit working in passive mode without an extra power supply.The circuit contains 4 resistors and one adjustable reference regulator.The proposed circuit is efficient and small in size.It will be applied in a small civilian communications satellite with a lithium-ion battery series which is currently in its trial and analysis phase.

2 PROPOSED CIRCUIT OF BMS FOR COMMERCIAL SPACE

2.1 Operational Principles

Figure 3 gives the detailed structure of the designed circuit.The whole circuit is in parallel with the battery and can realize balance management and overcharge protection at the same time.Compared to the traditional passive circuit structure,it is more refined and contains only four resistors and an adjustable reference regulator.Crucially,it is passive,requiring no additional power supply.The internal circuit of the adjustable reference regulator is shown in Figure 2.This chip contains a low voltage amplifier,a BJT transistor and a 2.5 V reference voltage generator.The chip can be considered as part of a linear regulator.By using this chip,we do not need a specialized amplifier chip and standby power supply,thus reducing the cost and power consumption.Resistor R1and R2collect the charging voltage.The collected voltage will be compared with the reference voltage inside the chip.When the charge voltage increases to the threshold value,the voltage on R2will increase beyond the reference voltage 2.5 V.Then the amplifier outputs a high level voltage to turn on the BJT transistor and a bypass current appears in the transistor.The bypass current flows through R4and the voltage on R4is proportional to the bypass current.Therefore,negative feedback is established.For the deep negative feedback,the voltage at both ends of the amplifier is equal.So the change of the sample voltage is equal to the voltage across R4.This means the bypass current is proportional to the charge voltage.

Figure 2 Internal circuit of the chip

Figure 3 Detailed circuit

The bypass current can be expressed as follows：

In this paper,R1and R2is set as 30 kΩ and 51.5 kΩ.R4is set as 1.5 Ω.The change ratio of the bypass current is around 0.33 mA/mV.As the battery voltage increases,R3decides the highest bypass current and protects the chip.The highest bypass current can be expressed as：

In the equation,0.3 V is the saturation voltage of the BJT transistor.

The simulation result in Figure 4 shows that the threshold voltage is around 4.05 V.

The bypass current is 3 mA when the charging voltage is 4.06 V and increases to 50 mA if the charging voltage becomes 4.2 V.The bypass current is no longer linear as the battery voltage beyond 4.2 V.This means R3has restricted the bypass current and protects the chip.The maximum power consumption of one circuit is only 200 mW.

Figure 4 Simulation result

2.2 Limitations

The designed circuit operates under low voltage.However it can only be applied to a lithium-ion battery with capacity under 6 A?h,because the saturation current of the transistor integrated in the chip is less than 500 mA.This means the bypass current is limited.For further application,we need to use discrete devices to replace the integrated chip but this will still be economical.

2.3 Applications

Figure 5 shows engineering implementation of the designed circuit.B+is the anode of a lithium-ion battery and B-is the cathode.N24 is the adjustable reference regulator and is packaged as a DIP-8.Lithium-ion batteries can be soldered directly to the printed board via electrodes.This engineering structure is applied in a single lithium-ion battery scenario.

Figure 5 Single lithium-ion battery application scenario

Figure 6 gives the method of application for a lithium-ion battery series.Each lithium-ion battery is equipped with a proposed circuit.When the charging voltage of any battery increases abnormally,the bypass current will increase at the same time.After several charge and discharge cycles,the voltage difference decreases,ensuring that the battery does not overcharge.In this case,we recommend separating the battery pack from the control circuit to allow for better mass production.

Figure 6 Application in Li-ion battery series

3 EXPERIMENT RESULTS

Figure 7 shows the experimental result of overcharging protection.The common voltage of a single lithium-ion battery is 4.0 V.When the charging voltage exceeds the activation voltage (4.05 V),the transistor in the regulator opens and the bypass current builds.The bypass current increases linearly as the charging voltage increases.The regulation rate of the bypass current can be flexible by changing the impedance of R4.The regulation rate is 35.4 mA/ V if R4is 40 kΩ.The experimental result shows that the proposed circuit prevents an excessive charging current when the lithium-ion voltage increases to a high level.

Figure 7 Experimental result of overcharging protection

Figure 8 Process of balance management

Figure 8 shows the process of balance management.The battery series contains 12 single lithium-ion batteries and the common voltage range is 45.5 -50 V.During the charge-discharge cycles,the voltage differences in the batteries decrease.After 15 cycles (16 h),the charging voltage difference decreases from 13 mV to 5 mV.This means the proposed circuit is validated for balance management.

4 CONCLUSIONS

This paper introduces a passive battery management circuit for commercial space.The proposed circuit contains only one integrated circuit and four resistors which is efficient and small in size.Experimental result confirms that this recommended circuit can regulate the charging current based on the charging voltage to protect lithium-ion batteries and realize balance management at the same time.Compared to common battery management circuits,the circuit is very economical without extra power sources and expensive ADCs.We believe that the design of the circuit can be integrated for commercial space.However,this circuit is designed for lithium-ion batteries with a capacity of less than 6Ah as the transistors in the chip cannot operate with large currents.