Sunday, 26 October 2014
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Start-up develops shift-based battery management system

A Scottish start-up has developed a battery management system that extends lifecycle by working individual cells in ‘shifts’ with allotted rest periods.

The software and hardware solution can work on any battery type, according to CHEEVC of Aberdour, which will nevertheless initially focus on small lithium-ion batteries for e-bikes and scooters.

The rate at which a battery or cell can be charged or discharged is limited by the rate at which the active chemicals in the cells can be transformed. Forcing high currents through the battery results in incomplete transformation of the active chemicals, reducing the battery’s effective charge capacity. It also causes unwanted, irreversible chemical reactions to occur because the transformations cannot keep up with the current demands.

The unwanted chemical transformations consume some of the active chemicals, causing the battery to lose capacity and thus age prematurely.

The introduction of ‘rest periods’ during charging and discharging allows more time for the chemical transformations to take place and for the reaction to stabilise.

‘It has been known for some time that you could rest the whole battery, but you get intermittent power,’ Barrie Lawson, chairman of CHEEVC, told The Engineer. ‘What nobody had thought of doing before was having access [to] individual cells in the battery — that’s what enabled us to do that [rest the battery] without losing any power.’

CHEEVC’s system, termed cyclic redundancy, provides regular, repetitive rest periods for each cell during charging and discharging.

Each cell is bypassed sequentially under software control, using two or more field effect transistors (FETs) in a switching matrix.

Crucially, the cells all age at approximately the same rate, which is important because presently battery age is a function of the lowest-performing cell. Lawson claims that, overall, battery lifecycle can be extended by around 30 per cent (depending on cell chemistry and construction) while capacity, and therefore range, can be extended by around four per cent (with a redundancy ratio of 1:7).

CHEEVC has recently been awarded a SMART Feasibility Award from Scottish Enterprise, worth £90,000, to develop a commercial prototype for e-bikes.

‘The design is scalable, but for a start-up company such as us the hurdles to overcome to get into an a big auto manufacturer are quite big, so we thought it better to walk before we can run,’ said Lawson.

According to Lawson, another advantage of the cyclic redundancy is improved reliability. In the event of a cell failure, the faulty cell will be isolated from the chain and replaced by the resting cell, allowing the battery to continue operating without affecting output performance. This is particularly important for mission-critical and airborne applications.

‘The failsafe thing is very important in an aircraft drone; you don’t want it falling out of the sky if something goes wrong with one of the cells,’ Lawson said.


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