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Forum Index : Electronics : a new way to balance lithium ion battery
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Tinker Guru  Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Running for a set period at the moment. The cells seem to be balanced after one day. Presently testing how long to leave them before re balancing is worthwhile (when cell difference >40mV, after one day its around 15mV).l The lipo battery bank is brand new. It made a decent size hole into my savings. I want the maximum service life out of these lipo cells. Klaus |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Maybe you understand how this balancer works differently than I do  .
The supercaps absorb charge (if it was higher than the cell just sampled) from a cell and dump it at the next lower cell in the sequence. If there is no difference the supercap's potential remains the same, no charge transfer is taking place. There is no "pulling down" of any cell the way I see it. Anyway the lipo bank is brand new and I should hope that "sick" cells are a long way down the track. The whole idea of my battery management should ensure that. Still not convinced about your 'random' sequence advantage. I'm sampling the 8 cells for a fixed time (one day). Sequential sampling ensures each cell gets sampled an equal number of times during this period. Random sampling might have one cell sampled 10 times more than another. It may even out if one samples continuously for a very long time period but that is not my intention. So, sorry, I'll stick with the less complicated electronics of a johnson counter.  Klaus |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
If one battery cell has a lower voltage than average, the previously charged ultracap will discharge into it to raise the voltage slightly. That is agreed. But the Ultracap will now have given up some charge in doing that, and be at a slightly lower voltage. When it is switched to the next cell in the switching sequence, the ultracap will recharge from that cell, and always that same cell, thus pulling down that cell voltage slightly. The next pass around the switching sequence will repeat exactly, where each cell impresses it's terminal voltage first onto the ultracap, then onto the next cell in the sequence. The whole battery will definitely equalize over time, there is no argument about that. This is a truly great idea !! With a fairly small number of cells there should not be any problem at all. What I am suggesting is that with a much larger higher voltage battery, with a large number of cells, the equalisation process will be faster and more even if the switching sequence were totally random. The reason for this is that a "problem" cell will not always load down the same next cell in the switching sequence, but every pass through the "problem" cell will subsequently jump from, and to, a totally different place in the battery, not the same next cell every time. One very easy way to generate a random sequence from the circuit you already have would be to continuously drive your Johnston counter with a fast oscillator, maybe at several Khz, or tens of Khz. Then place edge triggerd latches on each output from the Johnston counter triggered by your slow relay sequencing 555. When the 555 changes state to select the next relay, whatever count happens to be present in the very fastly changing Johnston counter at that instant in time is frozen and latched. Something like a 74C574 has eight flip flops with a common positive going edge triggered clock, and it also has very user friendly pin outs. http://www.msarnoff.org/chipdb/74574 Cheers, Tony. |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
OK, I have finally worked out what has been nagging at me. The thundersky battery pack has a very flat voltage vs state of charge curve while the Lead acid batteries do not. Will the Lead Acid batteries contribute anything to the load while both banks are connected together? I have this feeling in my bones that the the LiFePO4 battery bank will contribute most of the cycling and the lead acid bank almost nothing. Not sure I can back this up with a logical argument, short of visiting Klaus with an arm-full of clamp meters 
Any thoughts? Yahoo I'm confused, no wait... maybe I'm not... |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Yahoo2, you are 100% correct, although this can work to ones advantage too. The Lithium will discharge to around 70% before the lead acid starts to contrubite, but where the advantage is Lithiums dont handle going below 70% very well and doing so shortens the cycle life from around 5000 cycles to 3000 cycles, so to have the lead acid pickup the bottom end and reduce the drop/demand on the Lithiums going greatly below 70% extends the life of the Lithium cells. In general daily use the lead acids will not be used but in the case of a few dull days and extra daily consumption where the Lithium might be drawn below 70% then having the lead acid pick up the slack is a great advantage. If you have a lead acid bank thats getting near replacement and to use lithium for the replacement, then there is big advantages of not throwing the lead acid out and using it for assisting the lithiums and protecting them. Think of it as a reserve fuel tank. Pete. Sometimes it just works |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Thanks Pete, I was hoping that would happen and you confirming it makes my decision to use the LA batteries in tandem worthwhile. Do you happen to know the charge state to bank voltage relationship of these lithium batteries? At the moment they are at 26.8V if fully charged(level is MPPT controller chosen) after sunset and my Wattsup meter says the lowest voltage was about 25.6V over night. Klaus |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
OK Tony, I get the point. Having only 8 cells to play with and no plans to convert my car to electric drive the "large number of cell" situation never entered my mind .
But, if I should happen to go ahead and build that electric dream boat with a 48V propulsion motor, would 16 cells come into your 'random sequence' advantages or is the sequential 16 step method just as effective? Keeping in mind all the while that a 200Ah cell is a decent size capacity and 50F of supercap packs quite a punch of charge to make the equalization process speedy unless one or more cells are really bad. Klaus |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
[quote]Do you happen to know the charge state to bank voltage relationship of these lithium batteries? At the moment they are at 26.8V if fully charged(level is MPPT controller chosen) after sunset and my Wattsup meter says the lowest voltage was about 25.6V over night. [/quote] Im not sure i follow your question? Lithium batteries should remain at full bank voltage being the same as the charge voltage, lithium dont drop back in voltage like a LA will when the charge power is removed (hence a little baffled by your question) Also a charge voltage of 26.8v sounds too low to me for lithium cells, and would expect more like 28-28.5v to be required, then im no expert on lithium so would pay to double check (ask Trev as he will know off hand) Pete. Sometimes it just works |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
All gets a bit tricky Klaus, I was given to understand that the typical voltage slope for a LiFePO4 was between 3 to 5 mV per 1% SOC (24 mV to 40mV for the 8 battery pack)so if we start at 90% the voltage should drop between 1.9 and 3.2 volts by 10% SOC. I would look at your 1.2 volt drop as showing a decrease in the SOC of between 29.5% and 48% overnight. roughly 1.5 to 2.5 Kw would that be in the ballpark? BUT given how much that temperature, load, chemistry, etc, affects voltage, getting a true resting voltage reading may not be easy.(even more so with some lead acid batteries in the mix as well) 3.35 volts seems like a correct RESTING volts for a thundersky that is charged over 60% SOC. The only way I can think of to pinpoint the starting SOC accurately, is to charge until the voltage starts to rise at a higher rate as you get close to 100% charge. I agree with Pete, 26.8V seems way too low for a charging or float voltage?? I'm guessing that's not what you meant. I have to admit I am a little gun-shy of parallel banks, took me 10 call-outs over 7 months to find two faulty cells in a system with 5 banks. On the plus side, it really sharpened up my diagnostic skills 
edited twice for spelling mistakes  I'm confused, no wait... maybe I'm not... |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Sorry Pete if I sound confusing, the addition of the lithium battery bank is a new learning curve for me. Anyway, what happens is the MPPT controller is set to charge vented flooded LA batteries. It does that by initially charging until the voltage goes to 28.9V and after a while at that level it drops back to the 26.8 I mentioned above and stays there for the rest of the day. I will just have to observe it carefully and shed load on cloudy days since only 410W of solar panel (on a tracker) are charging at present. Today (sunny w occ. cloud)that amounted to around 70Ah of charge. Solar input will soon rise to 1.2KW so even on cloudy days the combined battery bank should get fully charged. I set the low voltage disconnect to what Trev suggests as the minimum lithium bank voltage (24V) and he also said the ~29V max is OK. Its just the bit in between that's obviously quite different to the LA battery bank. I wish I had a simple meter that tells me the % of charge in the battery bank but with two different battery technologies that might be not be doable. Klaus |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hi Klaus, I have had a chat with a couple of people that have some experience with winston LYP battery packs. They both commented that with low charge and discharge rates it is possible to hit 100% state of charge and get within a few % of total discharge with conservative voltage settings. I asked " How do I tell when the battery bank is full?" Answer I Got, "Any voltage above 3.5 volts per cell at C/100 Amps charge current is fully charged." (that is 1% of bank capacity in amps) Given that your upper charging limit is 7% capacity it is possible that you could get very close to 100% by the time your charger switches to float at 3.6125 Volts per cell. They thought that getting this close to 100% full would not reduce battery cycles because it is a very slow charge, provided the pack is balanced and not held for long periods over 3.5 volts by the controller.( I guess that is determined by what amps the float stage is triggered on). They both thought your float at 3.35volts a cell was spot on the money. The area that they raised some concern was at the bottom end. Their view was that for intermittent and low discharge rates 3.0 volts per cell in some circumstances could be within 2 or 3 % of flat. That is not much of a safety margin if the Low Voltage Disconnect is measuring the pack total voltage and not individual cells. It is possible for the batteries to get to 5% SOC and still be at 3.15 Volts at discharge rates under 0.1C. I have to admit, this has got me scratching my head a bit. I'm trying to design something for myself with Winston Lithium Yttrium Phosphate's that will work with very little supervision, looks like I need to do a bit more research. I'm confused, no wait... maybe I'm not... |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Thanks Yahoo, I appreciate all that information. So it looks as if I'm doing the charging bit right though I have not yet figured out just what exactly triggers the change from full to float charging on the MPPT controller. According to the makers it should go through the full cycle every day but it does not do so by observation. The charging current drops off gradually first and the unit actually disconnects the battery briefly several times, presumably to sense its voltage drop with the charge removed. The reason why it does not every day reach the 29V level (does not go higher than 26.8V) is perhaps due to the solar charging not being consistent every day nor is the load on the batteries. Hence my question how do I know when the batteries are full and you answered the LYP side of that. Regarding low voltage disconnect, the 24V is a compromise as I'm using a battery combiner kit board from Oatley for that job. This has a latching relay with 80A contacts. For 24V operation it unfortunately has a larger hysteresis than for the designed 12V. I set it for disconnect at 24V but it won't reconnect until 25V have been exceeded. If I set the disconnect voltage higher it may take very long before the LYP bank gets charged again since the LA batteries stay connected to the inverter and need to be charged first to that reconnect potential. I will look into sensing a single LYP cell with a different comparator and route its output to operate the latching relay. Thanks for that idea, why did I not think of it myself .
Klaus |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
An ideal solution would be to stop the draw on the LYP's sooner so that they stay inside the 70% DOD target for maximum cycles. That would eliminate the risk of one cell with a lower voltage at the bottom end going into reverse charge. I don't know how to do that without a full blown battery monitoring system. Most charge controllers that I see are not that sophisticated, the constant voltage/absorption stage finishes at a set amps then goes to float. The amps setting to go from float back to absorption is usually 10% to 20% higher, it is pretty unusual to see them cycle with lead batteries but I have seen a 700mA charger on a truck battery do it. It might be due to the low resistance of the LYP's. I would not put these guys advice in the "proven fact" category until you get a handle on how deep your own batteries are cycling, I am hoping they will send me a couple of test graphs so I can see what they are describing. I'm confused, no wait... maybe I'm not... |
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norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
My experience as a rcer and LiPo batteries(not LiFePo4 but they are now available at reasonable prices for rcers), agree with the info of Yahoo2s. Whilst I have only approx 100+ cycles, have taken them down to below 5% with no failures as yet. In my rc planes (thermal gliders mainly)the battery pack runs the rc receiver and the electric motor. The Electronic speed control(ESC) feeds power to both (5v to receiver and battery voltage to motor) When the battery voltage drops down below 3.2v(can be adjusted on some ESCs) the power is cut to the motor but left to the receiver. This safeguard maintains power to the receivers for approx hours of extra flight time.(receivers use bugger all power) I use mainly 2.2 AH batteries, the intelligent balancer chargers we use display the AHrs charge, have many times put 2.1+ AHrs on charge back into the batteries. Have learnt to trust them, but no doubt will lose a plane at some stage. Chargers also handle discharge/charge cycles, but this feature is used on occasions just to test the batteries. Lithium batteries have been a godsend to rcers and on a larger scale will probably do the same to our RE systems. The technology and availability of these batteries is constantly changing. The ability of lithium batteries to give high discharge rates down to a low DOD, has seen them used reliably in ultralights, motorbike etc for starting purposes, is not to far away that we will see them used in our road vehicles for starting. Demand will drive the prices down where Li will hopefully replace LA. We come from the land downunder. Vic |
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Don B Senior Member  Joined: 27/09/2008 Location: AustraliaPosts: 190 |
In all this discussion focusing on methods of balancing the charge variations in LiPo batteries, sight should not be lost of the importance of the related comment regarding paralleling LiPo and LA battery banks to give what is perhaps the best currently available energy storage system (if properly managed). The principle of using the LiPo battery to handle the shallow cycling, backed up by the paralleled LA battery to handle the occasional deeper cycling would appear to give the longest substantial capacity battery cycling life currently available. I think that Gordon and Klaus should be well pleased about how this technique is evolving, thanks to their inputs. Regards Don B |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
I took your advice and use now the voltage of a single LYP cell to control the low voltage cutoff setting. For now I set it at 3.2V but that's adjustable. The neat thing doing it this way is the Oatley kit I use (which is easily modified for this purpose) has now only 0.1V hysteresis. It disconnects the bank when the sampled cell's potential falls below 3.2V and it reconnects at 3.3V. There's a catch however, since I sample a cell of the LYP battery bank it wont reconnect to charge up automatically since the cell voltage can't rise while disconnected from the MPPT controller & LA battery bank. Maybe a large diode bypassing the disconnect relay in the charging direction would work to overcome this? Another thing I learned from my relay/ capacitor equaliser: The capacitors I used do not hold their charge very long and it drops considerably overnight. Consequently when the equaliser gets reconnected in the morning there is quite a difference in the capacitor charge voltage and the LYP cell voltage. This is not good for the relay contacts which experience a much higher than normal inrush current on their first cycling. I am now re doing the PCB to include a series resistor from each capacitor terminal to one LYP cell so the caps remain trickle charged to that cell potential even if the sequencer is switched off. I choose resistors that limit the current to about 1 mA which should be enough to overcome self discharge but not enough to effect sequencer operation. I should know by tomorrow if this mod works as expected. I will also use resistor wire (instead of copper) links in series to the relay connections to further prolong their life. Klaus |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
That is not how I understood the advice I was given. I am sure that they were monitoring each individual cell and the LVD was based on the voltage of the LOWEST cell. I assume that the voltage difference between cells would be tiny but start to spread as they get close to full charge or empty. The idea is to not have one cell getting dangerously low. I was thinking of using something like a cellog-8m that can do a output for a relay on low volt, over volt and volt difference for each cell. Been pondering Norcold's comments about how his LiPo's are running pretty sweet, I think a big part of that is just the number of cells. Statistically the chances of an out of balance cell in an 8 cell pack is 840 times higher than a 3 cell pack. I dug this graph up from somewhere a while ago, its from a 90Ah LYP 4 cell bank. Hope you can read it, it is pretty grainy, the low discharge rate and temperature curves are interesting. 
I'm confused, no wait... maybe I'm not... |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Each and every time the Lipo`s are charged they are also balanced, the chargers (that I have) will not charge or even detect a Li battery if the balance leads are not connected. These charges charge NiCd NiMH Pb as well as the Li families. They auto detect cell count etc. That LiPo go so low with their charge and still prove to be reliable shows how important balancing is also it seems to be a feature of the Li family of batteries. We use 2,3,4,6 cell batteries normally with some rcers using 12 or more in their large scale models. Li batteries have advanced the use of electric motors in RC, battery and motor combos are now well below the weight of comparable glow motors and fuel. Impatiently waiting for Li family to do this for our RE system, thus my interest and bit off subject input to this thread. We come from the land downunder. Vic |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Its been good, I have had a look at your suggestion of using a hobby charger to monitor and balance a pack and realised that I should buy one anyway. very handy piece of Kit
youtube, balancing with the Icharger 3010B With everyone's input and seeing Klaus post his experiences, I am a lot closer to buying a set of batteries myself. I'm confused, no wait... maybe I'm not... |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
The Icharger, I believe is supported by Logview(freeware software)if so you will find this will give you very fine and usefull recorded data and graphs. I use Logview with Powerlog 6S, a piece of RC hardware that has applications also in RE especially logging wind turbines. Allows logging of rpm,temp,current,voltage and more. Klaus`s and other posts have me thinking also its time for some action, although I feel Li batteries of large capacity will drop in price fairly fast, as they did for rcers. Just being patient is the problem. We come from the land downunder. Vic |
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