# What is top balancing? What is bottom balancing?



## stealthE (Jan 31, 2016)

What is the difference?


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## major (Apr 4, 2008)

Top balance = all cells are same Voltage at 100% SOC (State of Charge). 
Bottom balance = all cells are same Voltage at 0% SOC. 
SOC refers to the pack in this case.



stealthE said:


> What is the difference?


Different pack management strategy. You can read volumes on the subject.


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## miscrms (Sep 25, 2013)

Basically, every cell is a little different. Which is why you generally need a Battery Management System to monitor, balance and protect the battery. But full BMS systems are generally expensive, so folks have come up with various strategies to try and do without one, or with only a partial one. 

When the cells are top balanced, ideally you can charge to a known pack voltage without risking overcharging the weaker cells. But you have to have a way to stop discharging before the weakest cells are over discharged and damaged. This can be done with a relatively cheap set of cell voltage monitors, and/or by establishing a max Ah discharged and/or pack voltage limit that leaves plenty of margin at the bottom of the pack.

When the cells are bottom balanced, ideally you can discharge to a known pack voltage without risking over discharging the weaker cells. Now the challenge is to avoid over charging the weakest cells, again by monitoring cell voltages and/or establishing a safe max pack charge voltage that leaves plenty of margin in the top of the pack.

In both cases you trade some of your potential range for reduced hardware cost. There are folks who will argue that either approach can work well, but without (and sometimes even with) regular maintenance you can get into trouble and prematurely ruin cells. The more aggressively you push the limits to try and get range back, the more likely such events probably are. With cell voltage monitoring at least you have a good idea of how hard you are pushing the cells on each run.


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## dougingraham (Jul 26, 2011)

And if all cells have exactly the same capacity top and bottom balance are the same. Unfortunately the cells are not the same. Of the 60 cells I bought they varied in capacity from 98 to 105 AH for 100 AH cells. If I top balanced my pack then if I manage to over discharge the one cell that has a capacity of 98 AH will be empty first and ruined if I continue to drive. If I bottom balance and then run the car completely down all the cells go flat at the same time. This is what I did and I found out that it works really well to keep you from destroying cells. When the pack dumped it was like turning off a switch. I had no choice but to pull over and park. Since I was just a couple of blocks from home I waited 5 minutes to let the cells recover. I managed to go an additional 15 feet. With bottom balance, dead is dead. Of course when you charge the weakest cell is the one that reaches full charge first. When that cell gets full you have to stop or you will ruin that cell first and bad things can happen if you don't stop.


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## Sunking (Aug 10, 2009)

dougingraham said:


> And if all cells have exactly the same capacity top and bottom balance are the same.


That is not correct. 

Top Balance only SOC voltage is equal at 100%, AH Capacity is not equal, just at maximum capacity which is not defined. Example 100 AH cells are not exactly 100 AH. With Chi-Com cells capacity tolerance is -3 to +10%, or 97 to 110 AH for a 100 AH cell. The weakest cell determines the pack capacity.

Bottom Balance has both equal voltage and capacity at 0% SOC. AH Capacity of every cell will be equal in all cells at all SOC voltages up to the maximum capacity of the weakest cell at 100% SOC.


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## dougingraham (Jul 26, 2011)

Sunking said:


> That is not correct.
> 
> Top Balance only SOC voltage is equal at 100%, AH Capacity is not equal, just at maximum capacity which is not defined. Example 100 AH cells are not exactly 100 AH. With Chi-Com cells capacity tolerance is -3 to +10%, or 97 to 110 AH for a 100 AH cell. The weakest cell determines the pack capacity.
> 
> Bottom Balance has both equal voltage and capacity at 0% SOC. AH Capacity of every cell will be equal in all cells at all SOC voltages up to the maximum capacity of the weakest cell at 100% SOC.


I said "And if all cells have exactly the same capacity top and bottom balance are the same. Unfortunately the cells are not the same. Of the 60 cells I bought they varied in capacity from 98 to 105 AH for 100 AH cells." And how is that not correct?

If all the cells have EXACTLY the same capacity then top and bottom balance are the same. And you could make such a pack if you picked through a few thousand cells in order to get 100 to match for capacity. Unfortunately this probably would not last more than a few years. Around 25 years ago you could buy matched for capacity NiCd packs. Because of the method used to charge NiCd's they would all be top balanced at the end of the charge. When you ran them down a matched pack would just stop working. The cliff would occur on all cells at the same time. Weird experience as you think something broke.


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