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Discussion Starter #1
I am wondering if this situation occurs?......... For example if you are charging 2 cells in series, say [email protected] and apply a set 7.0v, (3.5v ea). If in this situation (one cell starting Voltage at 2.3v and the other at 3.1v), could it be possible for one cell to take 4.5v and the other 2.5v and the ruin the higher voltage one? Assuming no BMS is used?

thanks
francis
 

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I am wondering if this situation occurs?......... For example if you are charging 2 cells in series, say [email protected] and apply a set 7.0v, (3.5v ea). If in this situation (one cell starting Voltage at 2.3v and the other at 3.1v), could it be possible for one cell to take 4.5v and the other 2.5v and the ruin the higher voltage one? Assuming no BMS is used?

thanks
francis
Damage probably wouldn't happen right away, since the curve is so flat even at different SOC. What type of power supply/charger is it?

However as the higher cell reaches the knee the voltage would increase pretty quick and could easily overcharge. (It probably wouldn't be enough voltage to make it blow up, but it could damage the cell, make it swell or even vent)

If your only choice is a 7v charger you need to bring down (discharge) the 3.1v cell to match the other cell before putting them in series.
 

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Discussion Starter #3
Using 7v and 2 cells are only an example to simplify things. I will be series charging 20x200ah cells at a time. My charger is capable of charging 12v-80v at 1/10th volt increments and amperage limiting to 5-60amps and the varying adjustments above can be set for 1 min to several hours and also can be set on a percentage basis and also can be varied according to temp variations. The above can also be adjusted in both Absorb and Float settings. The charger will also throttle down to 1/10th amp when preset voltage is reached no matter what voltage the batteries start at. Anyway it is a solar MPPT charger/controller running off my solar panels and digital/computerized.

But my question is if I am charging x number of cells at the proper voltage (3.5-3.7v), can a bad cell "run-away"? In Lead, the bad cell/battery will boil or get hot ------- so what will a Lithium do? thanks
francis
 

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Using 7v and 2 cells are only an example to simplify things. I will be series charging 20x200ah cells at a time. My charger is capable of charging 12v-80v at 1/10th volt increments and amperage limiting to 5-60amps and the varying adjustments above can be set for 1 min to several hours and also can be set on a percentage basis and also can be varied according to temp variations. The above can also be adjusted in both Absorb and Float settings. The charger will also throttle down to 1/10th amp when preset voltage is reached no matter what voltage the batteries start at. Anyway it is a solar MPPT charger/controller running off my solar panels and digital/computerized.

But my question is if I am charging x number of cells at the proper voltage (3.5-3.7v), can a bad cell "run-away"? In Lead, the bad cell/battery will boil or get hot ------- so what will a Lithium do? thanks
francis
This would be the route of the whole BMS debate. The ideal thing to do is make sure the cells are at the same SOC, now you can choose to bottom balance, top balance, or if the cells haven't been cycled yet trust good old Jack R. and just wire them in series and charge them (I don't quite trust that one) So I would top/bottom balance them depending on your goals and use of the pack, if you will have a BMS, will it shunt, monitor etc?

With lithium there is potential to do exactly as you mentioned, a cell that hits the top first could "run-away". I used a few small packs of 100AH TS at work, (we replaced LA with Lithium in a battery backup setup charged from an alternator) We top balanced the cells because they would commonly be in a float charge off the alternator, and the system it was powering could shut down at any low voltage level we programed. I was impressed though that after top balancing there was virtually no drift after many test cycles. At the bottom, a couple cells would drift as they hit the knee, but we tweaked the software to shut down as the first cell hit the knee. But every single charge cycle the low cell would come up and end at the same voltage as the other cells every single time.

So there are benefits to top and bottom balancing but it depends on your application.
 

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Bottom ballance all of the cells by putting them in parallel and discharging them to 2.3V. Put them in series and start charging them. Track the voltage of each battery. The weakest cell will increase in voltage the fastest. Measure the pack's voltage when the first cell reaches 3.5v and set your charger to charge to that voltage. If the final voltage is too low due to one cell being really weak, swap it out with a new one, bottom balance and find the weakest cell by the same method.
 

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Discussion Starter #6
Thanks guys. Just like some of you about to take the Lithium plunge, I am trying to "figure what is best for me". I ordered a couple of Celllog-8's and got advice from someone NOT using a BMS that he balanced and top charged and ran his pack for a couple of cycles then found four of the lowest cells and four of the highest and hooked up the Celllog-8 to get a reading of where to stop discharge/charge. Every few months he repeated his setup and has been fine ever since. (but I am still not against BMS :)


Francis
 

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Thanks guys. Just like some of you about to take the Lithium plunge, I am trying to "figure what is best for me". I ordered a couple of Celllog-8's and got advice from someone NOT using a BMS that he balanced and top charged and ran his pack for a couple of cycles then found four of the lowest cells and four of the highest and hooked up the Celllog-8 to get a reading of where to stop discharge/charge. Every few months he repeated his setup and has been fine ever since. (but I am still not against BMS :)


Francis
This method has been recommended against by numerous people because the Celllog-8 will put a small drain on only a few cells of your pack and will cause long term inbalance. You will have to check your pack every month or few months. There are better ways where you wouldn't have to micro-manage your pack at all.
 
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