Case/Golf carts smart lithium battery

fristCase brief (Customer request)

Golf carts smart lithium battery is used in electronic golf trolley. The nominal voltage is 25.6V, nominal capacity is 6500mAh. The PCM should have balance function, overcharge protection, over-discharge protection, overcurrent protection, short circuit protection and high temperature protection, which also with bq34z100-g1--I2C. Besides, the battery pack should have 4P XLR connector and be put into a black housing (size is 178*100*60mm). Most importantly, the cycle life of battery can’t less than 1500times.

The continuous working current of golf trolley is 2-3A, peak current is 10A which can last 1S. The working voltage range is 22-29 Volt.

Solve the case (case analysis)

1.Difficulties:

①The structural design of golf carts smart lithium battery must be accurate because of the limitation of size. If the permutation and combination of battery cell and placement position of PCB can’t design well, and then we can’t meet the size requirement.

②The PCM need to be developed. According to the requirement of customer, we need to choose communication protocol“ bq34z100-g1--I2C” to collect the battery information. However, it increases the development difficult because of the limited of battery size.

③When I2C communication protocol connects with XLR connector, we can’t mix up the pin. If it is misconnection, the golf trolley can’t receive the right information.

2.Key points:

① Optimizing the structure of PCM and battery pack.

② Collecting the requirements information.

③ The size of black housing should be precise, the outlet hole is consistent with the size of electronic wire.

Set up the project (project design)

1.Ideas for desing:

① Selects high quality battery cell which can meet the requirement of cycle life.

② According to the size requirement, the lifepo4 battery is designed to be two layers. The base layer is 12 cells standing on end and top layer is 4 cells which are horizontal. Due to the effect of housing’s screw hole, the 4 cells need to stagger this position. This design is both beautiful and can meet the size requirement, but it also increases the difficult of insulation and welding.

③ Developing a suitable PCM which can collect battery information, such as voltage, capacity, temperature, remaining capacity, charge and discharge current and cycle index. What’s more, we will add a MOS tube on I2C which can protect the IC.

④ Using 65℃ temperature controlled switch.

⑤ Designing the housing and handing it to the mould factory to make it.

⑥ Confirming the design and content of label, the size of label is consistent with the label size on housing lid.

2.Concrete plans:

① The number of this case: 60008-4.

② Cells: 26650 3.2V 3300mAh. After combination, the cycle life more than 1500 times.

③ Battery pack structure: 8S2P.

④ PCM: Single cell overcharge protection voltage: 3.77±0.06V. Single cell over-discharge protection voltage: 2.25±0.1V. With communication protocol“ bq34z100-g1--I2C”, overcharge protection, over-discharge protection, overcurrent protection, short circuit protection and high temperature protection, charge recovering.

⑤ Electric wires: 18#, 220mm(including XLR connector)⑥ The housing is black which has waterproof function, size is 178*100*60mm.

Implementation (process introduction)