# how do i drain my batteries (bottom balancing)



## brainzel (Jun 15, 2009)

You have to check the data sheet of your cell.
There is typically a discharge rate and a min. discharge voltage.

F.ex. Sky Energy LiFePo4 100Ah:
http://www.i4wifi.cz/img.asp?attid=255157
recommended discharge rate: 0.3C means 30A for a 100Ah cell (100*0.3=30)
discharge cutt of voltage: 2.5V

So you have to drain the battery until it hits 2.5V @ 30A discharge current.

You can take a 0.1 Ohm / 200W resistor or any other load that takes a constant current.

A meter with a relay to cut of the load at 2.5V would be helpful.
Such as a JLD404 or VAM4020P (max.20A).

Otherwise you could buy a Unit that does all this for you.
Junsi 3010B (max. 10A load) f.ex.


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## evnz (Jul 24, 2010)

can i do all 47 x 90ah as one 3.2v pack at 0.5 c


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## gravelydude (Sep 6, 2008)

In a word--- No. You have to discharge each cell individually to the bottom (i.e. 2.5 volts), and then wire up the pack and charge together. This minimizes the difference between cells at the bottom. It is a tedious process. 

JACK


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## Zak650 (Sep 20, 2008)

Hi,
Check into a Powerlab, either 6 or 8. They can automate the process and give you a computer graph of what's happening real time. I run an inverter to make use of the power I'm draining from the pack if possible.


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## swoozle (Nov 13, 2011)

You do not need a specific discharge rate for bottom balancing. The idea is to hit a target resting voltage on the lower knee; it doesn't matter how you get there.


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## Ziggythewiz (May 16, 2010)

I think it would be easiest to drive the car down to what you think is 20% (assuming shipped at 60%), then finish draining each cell individually using a control circuit like a powerlab or programmable volt meter.


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## onegreenev (May 18, 2012)

The initial discharge with the vehicle is fine if you have them installed already in the vehicle. If not you could connect them to a heater coil but you MUST watch them close because when you get to a low voltage and hit the knee they will drop fast. 

If you have time to watch them then yes you don't need to worry about the load too much. It will be harder to get a nice smooth stable 2.5 or 2.8 volt static doing it that way. 

I recommend the PowerLab 8. Us a source that will take your cell to like 2.8 volts under load then put them on the PowerLab 8 and do a final discharge using my PowerLab 8 setup that will take them to 2.6 volts static using a CC/CV discharge. Nails the final voltage pretty good every time. I use the Regen function of the PowerLab 8 to recover the drained power. Once all your cells are done then you install them or connect them all together as you would in your vehicle and charge them to your set voltage. 

I am assuming your not going to be using a BMS since your bottom balancing them. If that is the case then make your final end voltage somewhere like 3.5 or 3.55 volts per cell average. You will have some at lower and higher voltages at the top but they will all be at the same voltage just off the top when they settle after a charge. My cells all settle to 3.335 volts after a charge to 3.65 volts. I have a few that reach 3.76 volts but those all settle to 3.335 volts resting. That is what you want. 

Pete 

Many just use 3.5 volts and I can concur that is a great final voltage to stop at as the cells will be full at that point. Any extra you might decide to squeeze out will not be of much benefit and will more than likely be damaging if you take your cells to the max. 3.5 static is nearly the max anyway.


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## onegreenev (May 18, 2012)

I had a cell on a load using a motor as the load and when I was at 3 volts I went in to get quick cup of coffee and then back out and that was no more than like 3 minutes max and the cell was at a point well below 2 volts in that short time. Ouch. Watch out when your cells reach 3 volts under load. The drop from that point is FAST if your using a large load to discharge your cell from. That motors load was around 100 amps. So things happen fast at those currents. I did not make that mistake again.


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## brainzel (Jun 15, 2009)

If you are going to invest into a single charge/discharge unit, check out the new Junsi iCharger 4010 Duo

charge current 0.05A - 70A (2000W max.)
discharge current 0.05A - 70A

about $350 - $420 Dollar


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## kennybobby (Aug 10, 2012)

*Yes you can do a bulk discharge that way*



evnz said:


> can i do all 47 x 90ah as one 3.2v pack at 0.5 c


for about an hour and a half, or until the voltage hits about 3.0. Then you might reduce the discharge rate to C/3 until it hits 2.8. Then C/20 until it hits 2.7 or however low you want to take them down. The Bestgo 100Ah prismatic cells consist of 10 individual 10 Ah pouches connected in parallel, other prismatics may do the same or may have a huge number of parallel foils contained within the plastic housing of the cell, so it doesn't really matter that you have 47 cells in parallel since each cell internally is a parallel circuit. Good luck and let us know how you do.


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## evnz (Jul 24, 2010)

*Re: Yes you can do a bulk discharge that way*



kennybobby said:


> for about an hour and a half, or until the voltage hits about 3.0. Then you might reduce the discharge rate to C/3 until it hits 2.8. Then *C/20* until it hits 2.7 or however low you want to take them down. The prismatic cells consist of 9 or 10 individual 10 Ah pouches connected in parallel, so it doesn't really matter that you have 47 cells in parallel. Good luck and let us know how you do.


do you mean 2c ?
they are winstons and they say not to drain them before the first charge any idea why?
also what do i use to drain that much ?


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## kennybobby (Aug 10, 2012)

*Re: Yes you can do a bulk discharge that way*



evnz said:


> do you mean 2c ?
> they are winstons and they say not to drain them before the first charge any idea why?
> also what do i use to drain that much ?


No that is C/20, or about 5 Amps. 2C would be 180 Amps.

Near the end of discharging you want to slow down the rate so they don't fall too low too quickly for you to shut off--that is one way to damage cells, overdischarge.

Do you have any information on your cells? i was assuming they were of the LiFePO chemistry--if they are something else then ignore what i said and follow the manufacturers instructions.

To discharge you will need a large wattage load with a fairly low resistance, such as the electric burners (eyes) of a stove. You can configure them in series or parallel to adjust the resistance for the current you want. Plus you can heat up your lunch if you get hungry...


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## dougingraham (Jul 26, 2011)

The method I used is simple.

I charged each cell to full using the typical algorithm. Charge to 3.6V terminate charge at C/20, in my case with 100AH cells this is 5 amps. Discharge the cell at the maximum rate my charger can do which for the iCharger 3010b is 30A to a cutoff point of 2.5V. Because of voltage drop in the cables the actual cutoff is 2.7V. Enter the cell capacity into a spreadsheet along with the serial number. Repeat for every cell. Sort cells by capacity. Install in car and charge the pack. Knowing the capacity of your cells will catch one that is just bad before it goes into the car. Sorting them into order doesn't take too long and lets you watch for things based on position in the pack if you are so inclined.

The reasoning behind why this method works is simple. The thing you are trying to protect against by bottom balance is overdischarge induced cell reversal. This happens when you keep driving your car until it wont move anymore and your cells are not bottom balanced (or perfectly matched). The discharger is computer controlled and stops the discharge at exactly the same place on every cell to within measurement resolution of the device. (For the iCharger 3010b this appears to be about 15mah in a 100AH cell.) When you are driving the car you are pulling high currents similar to what the cells see with a CC type discharge. You can tune the bottom balance if you want to but there is very little if any benefit from doing so and you will spend quite a lot of time doing this. The resting voltage (overnight) after a full charge is surprisingly close at within a couple of thousandths of a volt. There is a lot of difference while under charge but I have selected a pack cutoff voltage such that the lowest capacity cell just reaches 3.6v. And because I tested and sorted the cells I know which one this is.

Best wishes on your builds everyone.


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## PStechPaul (May 1, 2012)

The individual cell BMS unit I am designing will be able to do bottom balancing using the charge shunt resistor. I have a 0.47 ohm 15 watt resistor so it will be able to discharge the battery at up to 5 amps, which is C/20 for a 100A cell. This can be done while the pack is still in the vehicle, although you need to provide a way to remove the heat. A 100 cell pack would dissipate 1500 watts, so this would essentially be like a typical space heater. It would take 20 hours to bottom balance a fully charged pack, but just 8 hours for a pack that was near empty. I think this can be built for about $3/cell and (if I sold it) would be about $10/cell. It would also need a master unit (about $50-$150) which would communicate with the individual BMS units and the charger and/or motor controller and safety interlocks. So for a 100 cell pack the cost would be about $1100, and since each cell has its own BMS unit, you can add or replace cells without making any changes to the master unit. 

I will be using this for my much smaller pack with 2-10 Ah cells, but it seems that the design as it is will work with 100Ah cells (or maybe a bit larger), and I could upsize or downsize some of the components to make it more economical or more capable. 

Please let me know if this is a worthwhile project to pursue more seriously. I can probably have a prototype working in a few month's time. I have already shown the schematic for my concept and I would offer this as open source, but also sell PCBs, kits, and complete units for a reasonable price and enough to recoup NRE and some profit to make it worthwhile.


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## nzev (Nov 20, 2008)

*Re: Yes you can do a bulk discharge that way*



kennybobby said:


> for about an hour and a half, or until the voltage hits about 3.0. Then you might reduce the discharge rate to C/3 until it hits 2.8. Then C/20 until it hits 2.7 or however low you want to take them down. The prismatic cells consist of 9 or 10 individual 10 Ah pouches connected in parallel, so it doesn't really matter that you have 47 cells in parallel. Good luck and let us know how you do.


Typing yourself smart?? Prismatic cells such as Thundersky, Winston Battery, Sinopoly, CALB are not made up of individual pouches, the only pouch formed is the outter PVC case.


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## Duncan (Dec 8, 2008)

*Re: Yes you can do a bulk discharge that way*

Hi 

I thought you were going to use a BMS?
With a BMS you top balance - not bottom

If not I have a 10 ohm 4Kw heating coil that we could make into a discharger for you


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## evnz (Jul 24, 2010)

sounds good but how do i get it to stop at a voltage with out watching every one


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## brainzel (Jun 15, 2009)

You can use a JLD404 or a cheaper VAM4020P.
These devices can switch a Relay at a previously defined voltage.

Additional to the Junsi 3010B:
this device offers a discharge circuit with an external resistor.
And you can improve the accuracy of voltage measurement extreme by using the "balance" port.
These two little extra wires would take the voltage directly from the battery and not through the current carrying wires.


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## Red Neck (Feb 1, 2013)

*THE FOLLOWING IS FOR NEW CELLS FROM MANUFACTURER ONLY*

By far the easiest way is to use (you can get a used one with bad batteries on the cheap) UPS or several of them. I use a 12v UPS for 4 cells or 36v (had three 12v lead batteries in series in it originally) per 12 cells.

Just plug a load in (600w heat gun for the 12 100Ah cells only takes a few hours) and
wait. UPS units switch off at 10.5 v for each 12v lead acid battery.

Every time the UPS shuts off early enough to prevent any single cell to go too
low and then I still manually discharge them a bit with 50W car light bulbs to get them lower, to about 2.6v on the lowest cell and then each individually
until they settle at about 2.5v. I connect them in parallel then and wait for a day or two. 

Don't worry about 2.5v being too low. The paranoid low v alarms for LifePo4 are mainly due to fear of any single cells hitting negative direction when under load than 2v or 2.5v etc being really as harmful in its own. Low discharges are not as bad as most think. It is overcharging that will kill a cell quickly.

By the way, Sinopoly batteries (the current ones) are not bottom balanced out of factory and diverge strongly at below 3,2v so bottom balancing really needs to take place. If you then stay between 20% above bottom and about 10% below full charge, you will get an extremely long lifetime out of them.

Note that when discharging them, under load you will need to go below 2.5v since they will bounce back strongly several times. 

If you feel uncomfortable going to 2.5v, you can discharge and balance them at 2.7v, etc. I do it at 2.5 because it allows me really close matching. Even I
might do it at 2.7v in the future.


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## onegreenev (May 18, 2012)

Take the pack down to 3 volts then one at a time. Doing in packs of 4 with the UPS is not safe. The reason is you DONT know which ones are bottom ragged and the purpose of the bottom balance it to remove that. 2.6 volts is too low as its well into the discharge curve and you could easily loose cells if any drop below that point long before the UPS knows. If they are all equal already why are you balancing them?

The safe stop voltage is 3 then single cell discharge to a specific level. 2.8 is fine. That is well into the discharge curve too.


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## Red Neck (Feb 1, 2013)

*I FORGOT TO MENTION THAT I RECOMMEND THIS FOR NEW CELLS FROM SAME PACK FROM FACTORY ONLY!*

If you used cells before or they are definitely at different states of charge, then you need to treat them individually from the start.

I did over 200 (NEW) Sinopoly cells by now with the UPS , which actually does not discharge them enough on its own. Even those which are completely out of whack from each other. So far, I have seen the lowest one at 2.5 under load when the UPS cut off. After the UPS switched off, it bounced to 3v, while others bounced to 3.1 and one to 3.3.

The UPS shuts off just right for the most out of whack cells or still a bit early and way to early for balanced cells since you still have to deplete them further manually at that time and in several go's, since the still bounce back strongly.

The UPS should not be feared with new cells but it is best to treat each cell individually from where it shuts off, so even if you forget a load on a cell (has happened to me a few times, forgetting a 50w light bulb on a cell) it will not get reversed but more depleted. I had my lowest cell at 1.2v resting and simply charged it quickly to 2.5 and it showed no reduced capacity or anything. Even that Lithium professor mentioned in the threads here mentions that the chemistry is not that sensitive even to complete discharge. You just can't do it in series as it reverses a weak cell which does kill it.

You need to understand that the manufacturers fear low voltages because of fear of ramming weak cells into negative territory under load when near depletion due to reversing polarity. Otherwise, the cell killer is overcharge.

Keep in mind that even if you have three cells at 3v under load (still quite full) and one dropping to 1.5, the UPS will shut off. And this is an unlikely scenario for new cells from same pack.

That is not to say one should discharge them below rated bottom voltage regularly but doing it once or twice a year is not an issue at all and even storing them at 2.5v is not an issue if they are disconnected (no parasitic loads)... 

To those with less experience and with NEW cells, straight out of factory, I would say, use UPS, let it finish, then bring down stronger cells individually to level of the rest and tie them in parallel, then load them some more and bring them to say 2.7v (resting). Then let them be for a day or two.. Then, connect them in series with voltage which corresponds to some 3.47v with a charger that shuts off when that voltage is reached. Then you're golden...


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## onegreenev (May 18, 2012)

Kinda helps clarify when you fill in the blanks. I can see that with new quality cells. Thanks for focusing the lens.

Pete


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## Red Neck (Feb 1, 2013)

I need to be more careful since someone can easily kill components if details are lacking. Your post made me see I missed a vital piece of info


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