12V Backup Battery Charging and Discharging at Any Time Design Scheme of 14.8V 2.6Ah Desk Type POS Machine

Foreword: compared with normal commercial motherboard, industrial motherboard is major in mains supply. As demand developing, many portable products have more and more functions and meet the requirements of industrial motherboard. There is no mains supply outdoor. Neither do backup battery charging circuit, relevant transfer and commutation circuit for motherboard. Therefore, the portable device cannot be used directly outdoor without matching lithium ion battery. Under this circumstance, the small portable product of industrial motherboard with capability of charging and discharging at any time has become a mainstream model. 12V backup battery charging and discharging at any time of POS machine sets up specific project on industrial motherboard with 12V mains supply to use portable device outdoor. This scheme realizes function of charging and discharging at any time on the same port by integrating relevant charging circuit, buck-boost circuit, input and output switching circuit. There is no need to separate the application of charging and discharging. The maximum power can satisfy the power demand of industrial motherboard within 100W.

Design requirements

I. 12V backup battery charging and discharging at any time design requirements of desk type POS machine:

12V backup battery charging and discharging at any time of POS machine sets up specific project on industrial motherboard with 12V mains supply. It can meet requirements of using portable device outdoor by integrating relevant charging circuit, buck-boost circuit, input and output switching circuit to realize the function of charging and discharging at any time on the same port. There is no need to separate the application of charging and discharging.

1）Design requirement of the type of lithium ion battery: 18650-4S/2.6Ah/14.8V. it can support the industrial motherboard with maximum of 25W to work at least one hour.

2）Input and output characteristics: input and output on the same port, and apply input/output automatic switching function.

1、Input characteristics: motherboard and lithium ion battery pack can charge together when there is mains supply (DC：12V/5A).

2、Output characteristics: the battery will output 12V/3A current automatically to satisfy the power supply of motherboard when there is no mains supply.

3、Status characteristics: change input/output status automatically by switching circuit. The machine won’t shutdown or restart led by retardation.

3）Charging and discharging indicator: based on the quantity of adjusted indicator light required by customer, the machine supplies alarm indication of sustaining charging during charging and discharging process, fully charge and low voltage.

4）Fundamental characteristics of guard plate

1、Over-charged protection voltage of single battery: 4.35±0.25V

2、Over-charged recovery voltage of single battery: 4.10±0.10V

3、Over-discharged protection voltage of single battery: 2.40±0.08V

4、Over-discharged recovery voltage of single battery: 3.00±0.10V

5、Over-discharge cut-off current of battery pack (10ms): 8~12A

6、Over temperature protection value of battery pack (restorable): 70±5℃

7、The finished product has protective function of short circuit.

5）Design requirement of cycle life: 300~500 times (national standard of charging and discharging)

6）Design requirement of housing dimension: refer to the actual dimension of battery housing.

II. 12V backup battery design scheme of desk type POS machine:

1） Intelligent buck-boost modular circuit: input 12V/5A to adapter for transition from DC/DC to CC/CV, which is the charging mode that is suitable for two lithium ion batteries in series, and then decrease the voltage of two batteries in series into 12V/2.5A constant voltage output status. At that moment, open automatic shift mode of input and output status.

2）Guard plate (PCM): it is protective circuit of rechargeable lithium ion battery pack. Because of the chemical characteristics of lithium ion battery, it needs protective function of intelligent capacity calculation, overcharging, over-discharging, short circuit, overcurrent, etc. so as to avoid combustion, explosion, etc.

3）Protection IC: it is the chip has main protective function in the design scheme. The chip monitors the cell on the functions of overcharging, over-discharging, overcurrent, short circuit, etc. at any time, which helps the battery works under safe, stable and efficient circumstance.

4）Temperature switch: it is designed for temperature protection. When the battery is heated up to 70±5℃ due to any issues, the temperature switch begins to protect the battery.

5）18650 lithium ion battery/18650/2600mAh/3.7V Li-ion cell（SANYO）

6）MOSFET: it works as the switch in protective circuit to make sure that the voltages of two ends won’t be up and down, so that the voltage will be stable.

7）Connector: it works as medical infusion pump to input and output current by lithium ion battery.

8）Battery housing: it reaches the fire rating of medical product and makes up the module of the whole battery.

Schematic diagram of design solution

Product pictures