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Forum Index : Electronics : lipo battery bank equalizing update
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Tinker Guru  Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
The idea of using a super capacitor to equalise the cell potentials of a LIPO battery bank has now been trialled for around 6 months since I first posted it here. The idea is sound and it works far better than I had hoped. I regularly measure the individual cells and they are within less than 10 milli Volts of each other during _any_ stage of the charging process. It was not straight forward to get it to work so well, some bugs always seem to attach themselves to my new ideas  .
Basically, this type equalising is a slow process so, if the individual cell's potentials are very different, its better to charge each individually first to get them off at an equal handicap. One annoying bug was some of the in lead fuses would blow occasionally. This took a while before I discovered a pattern, it always happened after a prolonged overcast when the bank was not charged up to equalising potential. This is determined by the MPPT charge controller which decides when there was sufficient solar charge to initiate its equalising cycle. Anyway, what happened is this: I assumed LIPO equalising is no different from other battery types and starts once the batteries are fully charged. To that effect I was sensing the solar panel output voltage which, during bulk charging, sits around 34V. When the bulk charging was over this increases to around 40V and this voltage increase was used to switch on my super capacitor LIPO cell equaliser. Now, if this did not happen for some days the charge stored at the super capacitor leaked away to some potential low enough to cause a considerable inrush current once my LIPO cell equaliser was turned on again, sufficient to blow one or more of the 20A automotive blade fuses I had fitted to the equalising leads. So a different initialising method was required and what I came up with is far better BTW. I now use current sensing, as soon as the battery bank charging current passes a preset level the LIPO equaliser turns on and runs as long as a charging current above this level flows into the bank. A picture of this gadget is here: 
The little hall effect current sensor (blue box) runs of a single 12V supply and has a nominal 20A rating with a + - 60A capability. The output sits at 2.5V with no current flowing and goes above or below this depending in which direction the current flows. So its easy to combine with a 2.5V reference and a comparator do do the job. LIPO cell equalising now starts as soon my preset charging current is exceeded and continues throughout the charging time. This can only be good for the cells, no more late equalising to even out the charge and no cell needing to bleed off excess charge as is used on the in between terminal post equaliser strips. Another change I implemented was to make the sequencing interval depended on charge transfer. Now the super capacitor remains connected to each cell for a minimum of 5 seconds but should it not have reached an equal potential to that on the cell by then it remains connected until that happens, before switching to the next in line. This guarantees the maximum efficiency of charge transfer. Klaus |
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fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Nice work Tinker , can you give a bit better detail on the schematic? . So am I correct in thinking what you are saying, is you have a 12V powersupply to balance while the battery is in its charge cycle and a super cap for active balancing when discharging. PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Not quite Phil, let me explain. What you see on the picture is just a current activated *switch*. This feeds 12V power to run the sequencing electronics on my LIPO equaliser. It senses current in the negative lead from the MPPT controller to the battery bank so this switch only turns on while the batteries accept charge above a preset level. I described the sequential equalising idea in more detail here about 6 months ago. Since I obviously don't want to run this all the time, especially since relays are involved which have a limited mechanical life, I let this current sensing circuit select when its worthwhile to balance my LIPO battery bank. The supercap (2 x 25F in series) connects sequentially in turn across each LIPO cell, thus transferring charge from the high cell to the low cell. This happens during the entire charging time, it stops when the bank no longer accepts charge (its full) and it stops over night when no solar charging happens. This is active balancing, as you call it, during the charging cycle. Nothing is done during overnight discharging, I found the battery cells do not drift far from being equal in that time and whatever difference they may have by morning immediately gets actively equalised as soon as the solar charging is on and supplying charge current. Klaus |
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| fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
It sounds like the system has changed a bit from your original Thread , are you going to post a full diagram or schematic on how to build the balance system . Heres the LINK to the origional thread and there is no diagrams there. PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
No plans to post full diagrams of my LIPO balancer here fillm. Reason being I spent a heck of a lot of time to get all the bugs out of it and would hate somebody overseas just pinches it and put the info onto their website, claiming it was their idea - this has happened to Oztules, if you remember him posting here. However, for those genuinely interested for their own battery bank I could make an exception, via PM only though. Actually, with all the info I have already supplied it would not be too hard to come up with something similar if someone has a moderate understanding of electronics.
But you were right, the unit I now run has definitely metamorphosed into something quite different than what you see in those first post pictures  but the idea has not changed. Klaus |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
If they want to do that they will anyway with what you have posted thus far. In actual fact i find it works to opersite and to post all the information gives you a far greater right to claim as it will be date stamped here. I could draw a schematic from your information and claim it as mine right now as you have not provided that so my post date would give me the first claim. I always find i get far more back by giving the circuit away in full, then trying to keep it close to your chest. I do think there is much better and efficent ways to do equalizing then the method you choose, as its a bit like using a fire hydrant to fill a thimble and a bulldozer to move the thimble full, but it works for you and that is all that is needed. Pete. Sometimes it just works |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
HAHA, Pete, I think you are right with your analogy .
OK, I take up your suggestion about posting full details, so, what is the "better and more efficient" method you are thinking about? I know that anybody could reproduce my idea with what details I posted so far, I just want them to do their own leg work . I have no plans whatsoever in a commercial reward from that idea, its just a hobby for me anyway. Besides, taking up this idea and building it their way by anybody reading this forum is far better in an educational sense than following plans IMO.
I should warn them though, the power of a 200Ah LIPO cell is awesome. It instantly vaporizes PCB tracks if something is done wrongly - lessons learned the hard way here. BTW, the LM723 based power supply you posted here some time ago, which I had built with some small modifications, proved invaluable to test low voltage / high current circuits as in my project above. Klaus |
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