Active cell balancing for electric cars is 92% efficient, and compact
Linear Technology has revealed its solution to the thorny problem of active cell balancing in electric and hybrid vehicle batteries.
Implemented in the newly-released LTC3300, the topology uses one fly-back transformer per cell and allows energy to be passed anywhere up and down stacks of up to 96 cells, with excellent efficiency.
“We think this is the first time this topology has been employed. Any number of cells can be charged and discharged at the same time obliviously to one another,” Sam Nork, director of Linear Tech’s Boston design centre, told Electronics Weekly. “The nice thing about this approach is that it is independent of cell voltage. It can take charge from a selected cell and re-distribute to 12 or more neighbouring cells, or take charge from neighbouring cells and directly charge a single cell.”
Conceptually, the firm divides the stack into blocks of six cells (see diagram), with each block allocated a single 3300 that controls switches in the primary and secondary of each cell’s transformer.
By appropriately timing the two switches, the transformer can fly-back energy in either direction.
Cell 1 is being charged from the cell 1-12 stack, while excess charge in cell 6 is being delivered to the same stack.
What makes universal energy transfer possible, is that the secondaries (in parallel) are not just wired across the block concerned, but are wired across the series combination of the block and the block above (see diagram). This means blocks are interleaved. For example, the LTC3300 charging or discharging individual cells 7-12, takes or delivers energy to cells 7-18 in series.
Interleaving allows energy from any cell to be delivered to any other.
“You can figure out where all charge is going to providing you know cell voltages, balance currents, transfer efficiency, and how the cells are connected,” said Nork. “You have to figure out the state of charge and capacity of each cell, after that, it is all maths.”
Two serial busses, one to the 3300 above and one to the 3300 below, allow data to be transferred along the whole battery.
Voltage monitoring is via separate chips, which are available from several manufacturers.
Why not add voltage monitoring to the 3300? “We chose not to have multi-amp switching on the same silicon as precision ADCs,” said Nork.
The firm has built a demonstrator that can handle 5A of balancing current.
“Efficiency quite good, 92% in both directions,” said Nork.
The transformers are around 15mm across and straightforward. “This topology does not require a high turns ratio. Basically you are building a 48V:4V transformer. We have used 2:1 turns ratio, with around 10µH of winding inductance.”
Effective operating frequency is 200kHz,” This is a sweet-spot,” said Nork. “You can probably double it, but there is an efficiency trade-off.”
Why balance cells
The batteries of electric and hybrid cars have long strings of cells – up to a hundred lithium ion cells, for example – which could be charged and discharged from the two end terminals if only identical cells were available.
However, normal manufacturing tolerances are enough to prevent this strategy working. Li-ion cells are the extreme case as, due to their 1% charge voltage regulation requirement and unpleasant over-charge characteristic, not even two cells can be charged in series without some additional control.
During charging, lower capacity cells become full first, and are damaged by over-charging should the remaining cells be charged.
Accurate per-cell voltage monitoring allows a switch, usually a mosfet, to bypass current around full cells through a resistor.
This passive balancing allows the whole battery to be 100% charged without damage, at the expense of heat generation as cells are by-passed.
During discharge, lower capacity cells are depleted first, and permanently damaged if they are Li-ion cells dragged below 2.5V.
Passive balancing cannot help here, so dc-dc converters – either inductive or switched-capacitor – can be employed to pass energy to the depleted cell from healthier parts of the battery. This is active balancing and allows 100% of battery capacity to be used, as well as cutting heat dissipation when used in reverse during charging.