I have built a simple clockwork logic charge equaliser for a series connected battery of 8 Li-Ion cells. The design is based upon the familiar flying capacitor charge equaliser but this uses flying cells rather than flying capacitors.
Testing the unit required an automated battery management device to periodically measure the terminal voltage of each cell whilst alternately applying a load and then charging the battery to see how well the equaliser works.
I have chosen to refer to the device as an equaliser to distinguish it from the more common technique of ‘balancing’ which usually involves the application of dump loads across individual cells should the terminal voltage exceed some pre-defined limit.
This approach has several advantages:
1. No wasted power
2. No software
3. Low cost
4. Increases the battery capacity by (2 * cell count)-1
5. Maximises energy transfer by reducing outliers
The reason for this post is to see what other engineers think about the basic idea. As far as I can tell this is not something that is commonly done, in fact I have not seen anything published regarding this approach.
No doubt most people will realise that 2018 will see the start of an explosion in the use of battery technology and currently Li-Ion is one of the leading chemistry types so this is a very topical subject with huge application potential.
As this is my first post on the subject I shall not overload it with schematics and circuit descriptions but simply provide some empirical data obtained by operating the device.
In the following graphs the cells are reclaimed 18650 cells taken from old laptop batteries. The charge current is 405mA and the discharge rate is 1.3A. The horizontal axis is time with a tick interval of 10 Seconds. The vertical axis is in volts. The top two traces indicate the application of charge current or load.
The BMS is set to apply a 1.3A load until the lowest cell voltage reaches 3.5V. It then turns off the load and switches on the charger. The charger stays on until one of the cells reaches 4.0V when the charge is switched off and the load is re-applied.
The first graph is without the equaliser. You can see that cell 7 has more charge than the others and cell 1 is a bit low. As expected, the performance remains the same for multiple cycles because there is no cell balancing.
The second graph simply carries on where the first graph stops but this time the equaliser is switched on. The initial discharge looks similar to the first but as charging takes place the charge on cell 7 is being equalised with cell 6 which flattens the rate of rise on cell 7.
You will also notice that without the equaliser charging terminated after 65 minutes but with the equaliser charging took 188 minutes on the second cycle.
Static cells 1 and 8 have only one flying cell to share power with therefore I would expect them to take longer to stabilise than the other static cells.
The power required to charge to 3.8V without the equaliser fitted is 13.5Wh. The power drawn is 13.4Wh to reach 3.5V
The power required to charge to 3.8V with the equaliser fitted is 29.5Wh. The power drawn is 29.3Wh to reach 3.5V
So, the equaliser provides an additional 15.9Wh or + 118.7% for + 87.5% extra cells.
My conclusion so far is that the equaliser is highly beneficial. Not only are the cells now working in harmony the energy available from the pack is increased by 118.7% with no wasted power.
I intend to carry out more tests and will be interested to hear what others think. Also, it would be great if anyone has data they can share using alternative methods.
Testing the unit required an automated battery management device to periodically measure the terminal voltage of each cell whilst alternately applying a load and then charging the battery to see how well the equaliser works.
I have chosen to refer to the device as an equaliser to distinguish it from the more common technique of ‘balancing’ which usually involves the application of dump loads across individual cells should the terminal voltage exceed some pre-defined limit.
This approach has several advantages:
1. No wasted power
2. No software
3. Low cost
4. Increases the battery capacity by (2 * cell count)-1
5. Maximises energy transfer by reducing outliers
The reason for this post is to see what other engineers think about the basic idea. As far as I can tell this is not something that is commonly done, in fact I have not seen anything published regarding this approach.
No doubt most people will realise that 2018 will see the start of an explosion in the use of battery technology and currently Li-Ion is one of the leading chemistry types so this is a very topical subject with huge application potential.
As this is my first post on the subject I shall not overload it with schematics and circuit descriptions but simply provide some empirical data obtained by operating the device.
In the following graphs the cells are reclaimed 18650 cells taken from old laptop batteries. The charge current is 405mA and the discharge rate is 1.3A. The horizontal axis is time with a tick interval of 10 Seconds. The vertical axis is in volts. The top two traces indicate the application of charge current or load.
The BMS is set to apply a 1.3A load until the lowest cell voltage reaches 3.5V. It then turns off the load and switches on the charger. The charger stays on until one of the cells reaches 4.0V when the charge is switched off and the load is re-applied.
The first graph is without the equaliser. You can see that cell 7 has more charge than the others and cell 1 is a bit low. As expected, the performance remains the same for multiple cycles because there is no cell balancing.
The second graph simply carries on where the first graph stops but this time the equaliser is switched on. The initial discharge looks similar to the first but as charging takes place the charge on cell 7 is being equalised with cell 6 which flattens the rate of rise on cell 7.
You will also notice that without the equaliser charging terminated after 65 minutes but with the equaliser charging took 188 minutes on the second cycle.
Static cells 1 and 8 have only one flying cell to share power with therefore I would expect them to take longer to stabilise than the other static cells.
The power required to charge to 3.8V without the equaliser fitted is 13.5Wh. The power drawn is 13.4Wh to reach 3.5V
The power required to charge to 3.8V with the equaliser fitted is 29.5Wh. The power drawn is 29.3Wh to reach 3.5V
So, the equaliser provides an additional 15.9Wh or + 118.7% for + 87.5% extra cells.
My conclusion so far is that the equaliser is highly beneficial. Not only are the cells now working in harmony the energy available from the pack is increased by 118.7% with no wasted power.
I intend to carry out more tests and will be interested to hear what others think. Also, it would be great if anyone has data they can share using alternative methods.
