Li-Ion automated charge equaliser - Page 1
Forum section: electronics / general

author: Brian Drury
author_rank: Newbie
timestamp: January 05, 2018, 07:45:39 pm
content: 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. Each battery will have n flying cells where n = (number of static cells)-1I have chosen to refer to the device as an equaliser to distinguish it from the more common technique of cell ‘balancing’ which usually involves the application of dump loads across individual cells should the terminal voltage exceed some pre-defined limit.Testing my unit required the development of 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. The test unit is arduino based and works very well.The flying cell approach has several advantages:1. No wasted power2. No software3. Low cost4. Increases the battery capacity by (2 * cell count)-15. Maximises energy transfer by reducing outliersThe 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 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.

author: Inverted18650
author_rank: Frequent Contributor
timestamp: January 06, 2018, 11:20:45 pm
content: Very interesting and I am following. Did you record the cells internal resistance prior to the start of the experiment? May I ask what BMS unit you are using? I am working toward the same goal, eliminating wasteful dropper resistors. Cross-current equalizing to keep all the precious energy my solar panels worked so hard to create, in the loop.

author: Brian Drury
author_rank: Newbie
timestamp: January 07, 2018, 01:36:16 pm
content: I checked the cells for capacity and internal resistance using an Opus BT-C3100 prior to constructing the pack. The cell capacity varies a bit but they are all between 2Ah and 2.5Ah.The internal resistance measurements vary between 0R1 and 0R15 even for repeat measurements on the same cell. I think the contact resistance accounts for most of this. Anyhow, they were all below 0R2.I based the BMS design on an arduino nano. See the attached schematic and video clip here:https://1drv.ms/v/s!AtcxErupGaCfgasl1EFwQoj6JfbN_g

author: Rerouter
author_rank: Super Contributor
timestamp: January 13, 2018, 12:56:25 pm
content: I believe the flying battery idea was not well received is that the switching current spikes while equalizing weared the batteries, and slowly degraded them, especially if there was only 1 battery being used as a flying one.If you want to simplify your design, might i suggest this guy?http://www.ti.com/lit/ds/symlink/mux36d08.pdfHe is a 2 channel mux, that can withstand up to 40V, so your round robin would work, you would just need to increment the 4 address lines, then he would balance between your pack cells and your flying cell, using the 170 ohm channel resistance to prevent spikes while reducing system efficiency it would increase cell life.Equally if you did need to discharge a cell you could just switch on a load across the flying cell.

author: Brian Drury
author_rank: Newbie
timestamp: January 13, 2018, 03:01:12 pm
content: I have attached a simplified schematic of the equaliser to help explain how it works.S1, S2 & S3 are static cells wired in series.F1 & F2 are flying cells.Q1 – Q8 are MOSFET’s.The MOSFET’s are switched in pairs. Q1 & Q3, Q2 & Q4. (Q1 & Q5 are P type)The top 14047 controls Q1, Q2, Q3 & Q4.So, if Q1 & Q3 are ON then F1 is in parallel with S1. If S1 has more charge than F1 then current will flow from S1 into F1. Alternatively if F1 has more charge than S1 then current will flow from F1 into S1.When Q2 & Q4 are ON then F1 is in parallel with S2. If S2 has more charge than F1 then current will flow from S2 into F1. Alternatively if F1 has more charge than S2 then current will flow from F1 into S2.The bottom 14047 controls Q5, Q6, Q7 & Q8.The basic action is as above so charge is distributed S2 S3 & F2. Also, because the two 14047 are not synchronised there will be occasions when F2 is in parallel with S2 and F1.If we start with S1 charged and S2, S3, F1 & F2 discharged then eventually the charge from S1 will be evenly distributed between all 5 cells.The flying cells are switched at about 1Hz and the switch transition time is nanoseconds therefore the flying cells can be considered to be in time division parallel with the static cells. The ‘OFF’ time is a tiny fraction of the ‘ON’ time.Conceptually, we have 8 static cells in parallel with 7 flying cells. Taking this one step further an individual electron may consider the cells to be in series and in parallel at the same time. This takes a bit of thinking about.The flying cell equaliser works if the series resistance through the MOSFET’s and wiring is kept to a few milli Ohms. The suggestion by Rerouter to use a mux with series resistance of 125 Ohm is orders of magnitude too high. There will simply not be sufficient energy passed on each cycle to be useful.As for the cell degradation theory I have seen no published data to support this. Perhaps Rerouter can advise where his information comes from?

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 13, 2018, 09:49:30 pm
content: LT has a chip that does something similar.http://www.linear.com/product/LTC3305In practice, because cells used in battery packs are carefully matched to each other, it would be very unusual for them to go out of balance enough for an active solution to make sense. Even Tesla just uses shunt regulators.

author: Brian Drury
author_rank: Newbie
timestamp: January 27, 2018, 12:11:51 pm
content: This is true and the battery management electronics is similar although with a much larger number of cells. My point is that if you have the electronics and the cells why not arrange them differently to avoid the need for dissipative balancing with shunt resistors?The worst case situation for ‘top balancing’ is when one cell is low relative to all the rest and a significant amount of power must be wasted from all the other cells.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 27, 2018, 04:17:24 pm
content: Wasted work. (Trust me, I did this for quite some time.)Any miniscule energy recovered by "non-dissipative" (actually: less dissipative) balancing is offset by cost and environmental factor caused by the equaliser BOM alone, let alone any engineering costs, by several orders of magnitude.The issue is, literature gives you an impression that li-ion cells are much more leaky than they are.In reality, differences of self-discharges are almost nonexistent (since self-discharge itself is almost nonexistent), and coulombic efficiency is at around 99.95%, and differences between the cells are, again, order of magnitude less.Monitoring and replacing broken cells makes more sense (in a bigger system anyway).Preparing yourself to equalize "slightly broken" cells requires so much investment added to all cells, 99.99% of which are just fine, is ridiculous.