# Battery Discharging



## Guest (Dec 10, 2010)

I want to discharge a small group of batteries in parallel to 2.8 volts each then charge them up to like 3.1 resting volts then put them in series to match my other cells. I have a few that I want to marry into the pack. The main pack is sitting at 3.1 resting volts right now. How would be the best way to discharge the batteries if I don't have a tool to do the job? I have a charger that I can manually keep the amperage up at about 15 amps. I must keep an eye on the charging but the charger works well. I'd like to use something that will discharge reasonable fast but the pack is only at 3 volts. What would work good? 

Pete 

PS. These are HiPower Cells.


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## Guest (Dec 10, 2010)

What is the basic resting voltage of the lithium batteries after they are fully charged and been sitting for a day?


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## ZEVUtah (Apr 10, 2008)

gottdi said:


> I want to discharge a small group of batteries in parallel to 2.8 volts each then charge them up to like 3.1 resting volts then put them in series to match my other cells. I have a few that I want to marry into the pack. The main pack is sitting at 3.1 resting volts right now. How would be the best way to discharge the batteries if I don't have a tool to do the job? .


You can buy the tool to do the job for about 30 bucks on Amazon. Why not do it ?

http://www.amazon.com/gp/product/B000AMBOI0/ref=pd_lpo_k2_dp_sr_2


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## major (Apr 4, 2008)

gottdi said:


> I want to discharge a small group of batteries in parallel to 2.8 volts each then charge them up to like 3.1 resting volts then put them in series to match my other cells. I have a few that I want to marry into the pack. The main pack is sitting at 3.1 resting volts right now. How would be the best way to discharge the batteries if I don't have a tool to do the job? I have a charger that I can manually keep the amperage up at about 15 amps. I must keep an eye on the charging but the charger works well. I'd like to use something that will discharge reasonable fast but the pack is only at 3 volts. What would work good?


Hi Pete,

Put your cells in parallel. Discharge into a resistor. Maybe several of those series motor field sets in series, they should do 100A. May need to use a fan on them. Use an ammeter and voltmeter. They should discharge at a fairly constant voltage until they get to the knee of the curve. Then they dive in voltage fast, so watch it closely and stop when that happens. Then let them rest until voltage stabilizes. Then repeat with a higher resistance load so you just get a few amps and take it down to your desired voltage. Maybe like a car headlamp for lower current load. Leave them set parallel connected without any load for a long time to balance and stabilize voltage. Recheck and adjust with increasing higher resistance, lower current loads.

I'd do something like that.

major


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## Guest (Dec 12, 2010)

Would this work to discharge to the knee? 

http://www.harborfreight.com/500-amp-carbon-pile-load-tester-91129.html


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## rwaudio (May 22, 2008)

Those will work, but you can't put a load on them for too long. They are ment for testing batteries for about 10 seconds then have a few minutes of off time or they overheat. If you were using the 500A version on a 3.2v cell and keep the current down you might be ok.


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## dtbaker (Jan 5, 2008)

the little 1ohm-25watt resistors I got are pretty cheap ($1.50 ea). I'd say even somebody like me could put together a tray of them to have a fairly high load. They get really hot though, like 200 degF, so you have to be careful.


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## piotrsko (Dec 9, 2007)

actually light bulbs are the traditional way to build a load.


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## Guest (Dec 13, 2010)

We no longer use conventional light bulbs. I'd like something that actually shows voltage and amps drawn. Being able to watch voltage and have a known amperage draw to me would be critical and make things much easier. So with that, what are my options? 

Pete


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## jockepocke (Nov 24, 2009)

You will just have to measure voltage drop across resistor to know current (if you first have measured resistance, of course). 

You might have a lot of crap to scavenge resistors from... what about an old Sauna heater? Or a stove? Maybe you could waste the energy by electrolysis of a water tank? That will be very easy, and safe is you use good ventilation (preferably outdoors). Problem is you might need a series configuration for that, I am unsure of voltage needed...


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## jockepocke (Nov 24, 2009)

Thinking more about it, what about "too thin" wire submerged in water (for cooling)? If current is limited to protect the batteries, worst thing that should happen is the wire burns off, and then just replace with a longer one! Be sure to fill upp with water as it boils off! 

Yeah, and you could scavenge old water boilers?


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## Guest (Dec 13, 2010)

No old heaters or heating elements. How would I know the amp draw if using an old heating element? I can hook up my multimeter and watch the voltage drop but what about amperage draw. I'd want something in the range of 50 amps to 100 amps so I don't have to wait all day for the darn thing to discharge. I also don't want to discharge it at a faster rate than 100 amps either. How do I determine those values with only a simple multimeter in hand?


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## jockepocke (Nov 24, 2009)

gottdi said:


> No old heaters or heating elements. How would I know the amp draw if using an old heating element? I can hook up my multimeter and watch the voltage drop but what about amperage draw. I'd want something in the range of 50 amps to 100 amps so I don't have to wait all day for the darn thing to discharge. I also don't want to discharge it at a faster rate than 100 amps either. How do I determine those values with only a simple multimeter in hand?


Measuring the resistance will guide you, although it rises with temperature, so you will have less current than calculated as the thing warms more! 

Measure the resistance across the load, and calculate the expected current U=R*I and U=3,2 v and I=100 A gives us R=3,2/100=0,032 Ohm. Hope you have a good meter!  

OR you can hook up the load to the battery, and let current flow, and measure the voltage drop to determine how many A you use. (U=R*I again)

EDIT: Wait, dont forget internal resistance of the batteries! Although I understand it is quite low for Li?


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## dtbaker (Jan 5, 2008)

if you have your electric heater for the car handy you could run some leads to that.... probably pulls 1000-1500 watts with the fan on, right?


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## Guest (Dec 13, 2010)

No heater. It's an old air cooled VW. I removed the stock heater.


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## EVfun (Mar 14, 2010)

gottdi said:


> No heater. It's an old air cooled VW.


So basically, its just like stock 
(I've owned a few)


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## Guest (Dec 13, 2010)

Yeah! Bout as much heat as your buggy.


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## Guest (Dec 13, 2010)

I will integrate a heater into the stock heating system. A big hair drier (sort of) connected to each heating tube with insulation.


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## Lordwacky (Jan 28, 2009)

ok well, need a load.

if you have an electric hot water heater? or can salvage one, or buy a element. then you shoudl also have a shunt for measureing the current from your car. You can use wire the shunt in series with the heating element then use your amp meter in the car to read the current. then measure the voltage accross the batterys... obviously you'll need to let the batteries rest before you can read the actual voltage.

how did you bottom balance your cells to begin with? you should be able to use the same setup.


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## Guest (Dec 15, 2010)

I like that idea. I have an amp gauge and shunt out of the car I can use. I also have my multimeter. I could scrounge up a heating element. Then check to see what readings I get and monitor the batteries and amp draw and log it. A fully manual discharger that I can monitor voltage and amperage. 

I have not bottom balanced any cells. I purchased them and now need to balance out a small pack for use in one of my projects. The four main packs are all the same at this time so no changes will be made to those. 

Pete


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## Lordwacky (Jan 28, 2009)

hmmm ok lets think about this a bit more what is the average hot water heating element rated at ..... 1500 watt , maybe 3000 watt. 

so P=VI so these thing run at 110AC RMS, or 220AC RMS so for example

1500 watts/ 110Vac rms = 13.6 amps

and R = V/I so 110Vac RMS/ 13.6 = 8 ohms maybe 6 or so when it is cold.

but you are discharging at ~3.2v so the current you would get out is I = V/R = 3.2/6 = 0.53 amps.....best case 

Unless you want to be at this for a month or two you need something with a lower resistance.

your target resistance should be 3.2v/50 amps = 0.06 ohms..... so back to the drawing board... you might be in coil of wire mode.

also check your shunt/ amp meter it may not be able to read current as low as 50 amps if it was designed to high current.

tough problem to do on the cheap. I think Dimitri modified some of his mini BMS units to do discharge to a set voltage... I don't think he was charging too much for them.... may want to do that route... if he still has some.

you haven't bottom balanced? I thought you were a member of the Rickard clan?


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## Guest (Dec 15, 2010)

I am sure we can figure something out that will resist enough to pull some serious amps. Mine is a 500 amp shunt with matching amp gauge. Basically it is just shorting out the battery on a known wire thickness and or resistance to create heat. Like a big fat toaster. There should be something real cheap and easy for doing this that is accurate and fast. 

Pete


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## Roy Von Rogers (Mar 21, 2009)

Go to a local appliance parts place, get the cheapest dryer rewire kit you can get, spread the coil out a bit, dont cut, use your ohm meter to get close to the ohms you need via formula.

Hook up with heavy aligator clips (jumper cables?) and watch amperage, adjust amps by moving aligator clip, till you see amperage wanted.

May want to put some fan air across so you dont burn the element out.

Or get some nichrome wire and make your own heater/load.

Roy


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## Guest (Dec 15, 2010)

Mmmmm. Big fat toaster. The clothes drier. Nice. I can do that. I will keep you all posted on the results.


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## jockepocke (Nov 24, 2009)

gottdi said:


> Mmmmm. Big fat toaster. The clothes drier. Nice. I can do that. I will keep you all posted on the results.


It is quite easy to burn off resistive wires with a lot of current... maybe submerge in water?


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