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I'm trying to understand the notion of bottom balancing a series battery of lithium cells and I've reached a sticking point in my understanding. Help would be greatly appreciated.

Once I've individually "bottomed" all of the cells and assembled the battery, I should charge it. I have no balance leads that would permit me to monitor individual cell voltages, and so I don't know if the states of charge for each of the cells are maintaining balance over the charging cycle. If I terminate charging when the voltage of the pack is a simple multiple of the overcharge threshold of a single cell, then am I not at risk of overcharging some individual cell? And if I terminate charging prematurely, then doesn't this defeat the object of balancing the cells to get the maximum capacity out of the battery? In short, how should I determine when to terminate the charging cycle?

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    \$\begingroup\$ Basically to balance the cells you need a balance wire or something connected to each battery tab (cell to cell balance circuit), don't think the question is answerable based in the no balance leads available \$\endgroup\$ – user2813274 Nov 9 '16 at 13:29
  • \$\begingroup\$ The people who promote bottom balancing say that the only wires on the pack must be the end wires... any current flow at all through intermediate wires will only contribute to balance drift and defeat the object of bottom balancing. This is why I'm perplexed. \$\endgroup\$ – Andy Kay Nov 9 '16 at 13:45
  • \$\begingroup\$ It can be packaged neatly with a small balance circuit across the tabs of each battery, but for charging those would need a bus or something to communicate if any cell is under/over voltage, or possibly just trickle charge such that the per cell boards can dissappate faster than the charger current if needed, and then the charger knows its done charging when the current draw matches all cells dissipating - still not quite what your describing \$\endgroup\$ – user2813274 Nov 9 '16 at 13:48
  • \$\begingroup\$ The strategy you propose is flawed, although possibly better than other flawed strategies. \$\endgroup\$ – Sean Houlihane Nov 9 '16 at 13:55
  • \$\begingroup\$ Flawed in what way? \$\endgroup\$ – Andy Kay Nov 9 '16 at 19:01
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I can help with a different POV as I have Bottom Balanced for several years.

Let's clear up one thing quick. You do not give up or do away the Balance Leads. They are still used to monitor things time from time. Jus tnot used for charging.

OK start with a different POV with a Stupid Question. What is the capacity of a battery when at 100% SOC? You cannot answer the question. Next Question. What is the capacity of a battery at 0% SOC. Easy answer, 0 AH for any battery or type.

LiFeP04 charge and discharge are very flat, and you cannot determine the SOC from voltage. Only place where SOC is accurate is at the 0% SOC or Bottom (2.5 volts), and 3.65 volt at 100% SOC. Capacity at 25 vpx is 0 AH, and at 3.65 is unknown.

So what is the capacity of a Bottom Balanced pack charged with 90 AH. It has 90 AH each and every cell. All cells will reach 2.5 volts at the same time, thus eliminating over discharge. No cells in a series string will have any energy left to drive weaker adjacent cells to reverse polarity thus destroying it.

When you buy say 100 AH Prismatic Cells, the capacity of each cells varies -5 to + 10%. That means the weakest cell can be 95 AH, and the strongest 110 AH, a 15 AH distance. So say you make a 8S pack, and Top Balance. What is the capacity? 95 AH right? Sure you have cells with more than that, but to use it means you would have to drive the weaker cells into reverse polarity and destroy them. When you charge the cells to 100% SOC just means they are 100% SOC and not squat about capacity. Capacity is determined by the weakest cell in the chain.

Nothing destroys a lithium battery faster than over discharge. Not much a problem with other chemistries, huge problem for Lithium. Right now your mind is stuck in a box; Thou Shall Fully Charge My Battery.

That is a sure fire way to destroy Lithium Batteries. Change the charging strategy and logic. Start from a know point of knowing both Capacity and SOC which is at 2.5 volts = 0 AH capacity. Now wire the batteries in series and charge until the first cell reach roughly 3.7 volts and terminate charge. Note voltage, AH input, and which cell shot up to 3.7 volts very quickly at termination.

Example 95 AH went in, on a 8S LFP pack the voltage was say 27.1 volts, cell 3 went high. You know now you have a 95 AH pack and need to set charge voltage to just slightly less than 27.1 volts.

When Bottom Balanced if you discharge too deeply, no cell is destroyed, they all hit 0% SOC at the same time and the voltage collapses from 11 volts to useless in a few seconds. In a Top Balanced system one cell will be fully discharge before any others, and can be destroyed byy adjacent cells if not caught very quickly.

There is one more huge benefit from Bottom Balance. Double to Triple cycle life. By limiting SOC to less than 100% adds cycle life. Limit to 95% SOC on the charge cycles, and limit discharge to 90% DOD, and you get Double to Triple cycle life. All you gotta do is get out of that Thou Shall Fully Charge My Battery box you are trapped inside of.

Why is Top Balanced used commercially? Two reasons. Consumers are idiots and cannot be trusted. Sure enough some idiot will burn something up and look for deep pockets. 2nd reason is it is very easy to design and build Top Balance Chargers. It generate up sales, and shortens battery life which is good for the manufacture. Stupid Consumers will never know.

Bottom Balance takes skill and understanding of what is going on. Use the Balance Leads to check voltages from time to time. Do a full discharge test every 100 cycles and check Balance.

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Bottom-balancing will not allow you to extract the full energy capacity of every cell. The only way to do that is by active rebalancing or putting the cells in parallel instead of series.

That said, bottom-balancing is a perfectly reasonable way to avoid the need to rebalance with every cycle. Having bottomed all the cells and put them in series, the next step is to charge the whole stack while monitoring every cell voltage (this can be manually with a multimeter). Once any cell reaches 4.2V or your chosen threshold, consider the whole stack to be fully charged. You may want to let it rest for a while and re-measure the open-circuit voltage of that limiting cell and of the pack.

Note that pack OCV and use that as your target for future charge cycles. Every so often, perhaps every 20 charge cycles (adjust depending on how well-matched the cells are) you will need to re-check the cell voltage balancing at top and bottom of charge, and rebalance as necessary. If that sounds like too much work, build a BMS :)

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  • \$\begingroup\$ Yes, it's the "while monitoring every cell voltage" that I'm enquiring about. That would not be possible in the absence of balance leads, and I don't think bottom-balance advocates like those things very much. If balance leads can't be dispensed with (as you're suggesting here) then one might as well used a balanced charger and leave it to top balance the cells on each cycle. I think you're confirming my misgivings. \$\endgroup\$ – Andy Kay Nov 9 '16 at 13:51

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