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Discharge down a LiPo pack

Old 09-01-2013, 01:15 PM
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solentlife
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Default Discharge down a LiPo pack

OK ... so we have fancy chargers of all descriptions .. some high powered, others budget ...

You have various liPo's ranging from 800mAh 3S up to 3000mAh 5S ... in fact 2 boxes of them ! just in 3S 2200 there are 8 LiPo's ...

You want to store them - but to discharge to storage level on budget charger such as a 6 ... will take forever at the restricted 1A max ... and in fact as I'm looking at a 4S 1600 nano now - at 16.49V ... the charger has dropped the discharge rate to 0.3A .. so I'm looking at even longer to get her down even though setting was 1A ...

My post is investigate other ways instead of buying a higher rated charger ...

a) Run the LiPo's in a model - I did that with my 2200's in the glider other evening - then next day weather was better than planned and I charged em back up again !! But in reality - fine at the field where you can run a prop or EDF and not worry too much about the "airflow" ... but at home ?

b) Light bulb - car light bulbs set up so you can discharge various size Lipo's by having multiple bulbs and connections ... OK - anyone have a design / apparatus doing this ? Lets say caters for 2S up to 6S ?

c) Resistor bank - again set up for various LiPos ... Anyone have a design / apparatus doing this ... 2S up to 6S ?

d) Motor set-up on a bed with a load which is NOT a propeller .. ie a flywheel ? Anyone have a desigbn / apparatus doing this ... 2S up to 6S ?

OK maybe the 6S pushes the limits of d) a bit far as many budget motors stop at 4S ...

I'd like to see some recc'ds of bits and pierces to put together to do the job ... commonly available at local Electronics supplier - Tandy / Radio Shack etc. and how to hook it all up together ready to connect that LiPo ...
With the motor - we need a good load item for it ... ie a flywheel or similar that can be balanced and fitted to make the motor load up sufficiently.... without burning it out.

Oh - if anyone thinking of starting a Safety Issue over this .. please take note that I am asking for SAFE and reliable ways to do it ... if it has heat involved - needs heat shield etc. Motors safely covered etc.

Nigel
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Old 09-01-2013, 02:01 PM
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hello Nigel,quick question...how long will a auto light take to discharge a 3cell 2200mah 25c battery safely to discharge storage mode.would you go past the storage level and then balance all cellsback to storage level?

i really enjoy my powerlab6 but must admit it is a little annoying to my wife who sits by my side on the couch working on her computer and watching tv while i charge or discharge. she never complains but i often come home with packs to be discharged from full charge.

what would be a formula for calculating the 12volt bulbs rate of discharge for different batteries and higher mahs. my highest mah rating is 4000/40c/4cell zippys.

i'm sure this threads been covered before but i'm hoping to see new ideas of ways to help folks who have lots of batteries to discharge at one time. great idea for a topic Nigel. i'll take a picture of a motor setup on a stand to run while screwed down on a work bench away from any areas wind could damage anything...but i never use it any more.

lastly,i could see the bulb idea mounted on a board in a row[or rows] with deans plugs,but what would make a super cheap auto cut off so no accidents of over discharging can be put in the design. i once forgot the bulb connected and ruined a battery that way. also bulbs for auto get preaty hot so say 10 on at once may be great during winter,but not so much in your hot summer days..lol.
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Old 09-01-2013, 02:59 PM
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I have found the best way to discharge is to fly. Sorry - could not resist.

Bigger chargers usually have about 50w discharge. Bulb's work but no warning if they are too low. Just have to plug a meter in the balance port and watch.
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Old 09-01-2013, 03:38 PM
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Heads Up RC has this item for about $20.00. According to the write up you can preset discharge voltage limits, which is the feature that interests me most. I've been considering ways to discharge batteries too; sometimes I charge a few up and then the weather gets bad before I get a chance to fly. I feel that by the time I've constructed something with bulbs or resistors, I'd have spent more than the price of the unit, and certainly invested more time. Think I'll include one in my next order.

http://www.headsuphobby.com/3-in-1-B...ncer-G-155.htm

Tom
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Old 09-01-2013, 03:57 PM
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That's going to be discharge limited also.

Easiest I've found, various combos of tail light bulbs and this:
Battery Checker and Low Voltage Alarm

Note-unless you are OCD sitting and watching a wattmeter or some other NON-alarm measuring device is a sure way to over discharge the battery. I've done that with them sitting right next to me. It's like watching paint dry.
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Old 09-01-2013, 04:03 PM
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Originally Posted by tr4252 View Post
Heads Up RC has this item for about $20.00. According to the write up you can preset discharge voltage limits, which is the feature that interests me most. I've been considering ways to discharge batteries too; sometimes I charge a few up and then the weather gets bad before I get a chance to fly. I feel that by the time I've constructed something with bulbs or resistors, I'd have spent more than the price of the unit, and certainly invested more time. Think I'll include one in my next order.

http://www.headsuphobby.com/3-in-1-B...ncer-G-155.htm

Tom
Looks good Tom with one missing factor.

Conspicuously absent - the discharge rate. On a 2oz item likely in the mA range. It would take nearly forever (days) to discharge a larger 6s pack.

Mike
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Old 09-01-2013, 04:57 PM
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The heads up balancer would take almost forever to discharge a battery. It's not really intended for that purpose.

One way I've found that works well and quickly is to discharge the full battery into a used battery. You do this simply by connecting the two together with a resistor to control discharge rate. This way you end up with both batteries half charged and ideal for storage. You can do it through a wattmeter if you want to check the rate of discharge. This obviously only works if you have some charged and some empty batteries of the same cell count and similar size. It is quite an elegant and efficient solution because the power discharged from the battery is not wasted. You can do several at once, you just need to make up suitable leads.

On up to 3s batteries with a high charge rate (5c or 10c) you can get away with hooking them together without a resistor, but it's kinder to the battery to put a resistor in there. Something like a 1 or 2 Ohm wire wound high power resistor will do the job. The higher the resistance the lower the amps.

Or you could buy an iCharger 4010 duo that has two channels at 130W discharge power per channel... But they aren't cheap!
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Old 09-01-2013, 07:30 PM
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Originally Posted by JetPlaneFlyer View Post
The heads up balancer would take almost forever to discharge a battery. It's not really intended for that purpose.

One way I've found that works well and quickly is to discharge the full battery into a used battery. You do this simply by connecting the two together with a resistor to control discharge rate. This way you end up with both batteries half charged and ideal for storage. You can do it through a wattmeter if you want to check the rate of discharge. This obviously only works if you have some charged and some empty batteries of the same cell count and similar size. It is quite an elegant and efficient solution because the power discharged from the battery is not wasted. You can do several at once, you just need to make up suitable leads.

On up to 3s batteries with a high charge rate (5c or 10c) you can get away with hooking them together without a resistor, but it's kinder to the battery to put a resistor in there. Something like a 1 or 2 Ohm wire wound high power resistor will do the job. The higher the resistance the lower the amps.

Or you could buy an iCharger 4010 duo that has two channels at 130W discharge power per channel... But they aren't cheap!
One of those forehead slapping moments: "why didn't I think of that?"

I'm very interested in this idea, but ignorant. Do you hook up the batteries in parallel or series? What would be the best wattage resistor for 2 cell batteries in the 300 - 800 mAh range? Where do you place the resistor in the circuit?

Hope you don't mind all the questions.

Tom
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Old 09-01-2013, 08:03 PM
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You hook them up in parallel, 'positive to positive' and 'negative to negative'. In fact if you have a parallel battery connector you can use that.
The resistor goes anywhere in the circuit, just cut a wire and insert the resistor. A 0.5 Ohm should do fine for a 2s battery, that should limit the initial current to about 2A or so (current drops quickly). But get the high power wire wound ceramic type because the resistor absorbs quite a bit of power and will get hot.

Higher resistance is ok, it will just take longer.
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Old 09-02-2013, 02:33 AM
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Default 1S to 7S Discharger, at a fixed 2.2 Amps per cell

Originally Posted by solentlife View Post
c) Resistor bank - again set up for various LiPos ... Anyone have a design / apparatus doing this ... 2S up to 6S ?

Nigel
Yup
I've done something very similar to your requirements for a "Voltage Limiter" on my boat load full of A123 cells. As presently set up, it kicks in at 3.7 Volts per cell.

This unit would simply be plugged into your batteries balance connector, and will automatically discharge each cell to the required 3.7 Volts DC. As presently set up, the discharge rate is one ampere. But simply changing that power resistor to 1.5 Ohms at 10 Watts would increase the discharge rate to about 2.2 Amps. As built, when your LiPo is plugged in, a corresponding number of LED's will light up for each cell in your LiPo. And, when all of the cells are dragged down to 3.7 Volts DC, the LED's will go out, one by one.

Problem is, this circuitry is anything but simple, and discharging through the balance cable would be limited to two or three Amps.

You could discharge at a much higher current value by connecting a 50 or 100 watt power resistor across the battery power leads, but then there is no easy way to check individual cell voltages, or allow for battery packs from say one cell to six cells. Can be done, but that requires computer monitoring.

Take a look at this thread that shows the finished product. I've got this on a schematic, and a circuit board layout. As indicated, it's not a simple project. The schematic shows "one bank" out of the seven banks needed to handle up to a seven cell LiPo battery. FYI, the circuit diagram shows a common LM741 Op Amp. The circuit actually used a Microchip OpAmp, that is more accurate, and something I had a lot of on hand.

I've built of three of these units, and all three worked very well. They were used on a pair of 10 Amp, 7S A123 chargers that were scratch built. They are not being used now, because I've gone to the high powered Cellpro Powerlab 8 chargers that has built in cell balancing.

http://www.wattflyer.com/forums/showthread.php?t=65253
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Old 09-02-2013, 03:35 AM
  #11  
solentlife
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Cool

Originally Posted by rcers View Post
I have found the best way to discharge is to fly. Sorry - could not resist.

Bigger chargers usually have about 50w discharge. Bulb's work but no warning if they are too low. Just have to plug a meter in the balance port and watch.
Both - I already covered.... I fly to discharged other day. Second looking for a dedicated way not involving purchase of higher priced charger.

Nigel
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Old 09-02-2013, 04:09 AM
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Originally Posted by flydiver View Post
That's going to be discharge limited also.

Easiest I've found, various combos of tail light bulbs and this:
Battery Checker and Low Voltage Alarm

Note-unless you are OCD sitting and watching a wattmeter or some other NON-alarm measuring device is a sure way to over discharge the battery. I've done that with them sitting right next to me. It's like watching paint dry.
I took a second look at this product after your suggestion, and saw some possibilities I hadn't thought of. Having an alarm connected to whatever discharge system I end up with is a comforting thought. Also, though I didn't notice the weight listed, maybe I could put it in a plane to time the optimal duration of flight; something I only guess at now. The label close up mentions an LED, so maybe I could buy an extra one, remove the buzzers and rely on the light, if it's visible enough from the ground.

Thanks,
Tom
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Old 09-02-2013, 07:09 AM
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The alarm on those things is very loud, you should hear it in flight, no problem. A guy I fly with sometimes uses it in exactly this way and it works fine.
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Old 09-02-2013, 07:15 AM
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They are cheap, they are loud. I haven't used it in a plane so I can't vouch for it as a flight voltage indicator. That would depend on the plane noise, the distance, and your ears.
The LED is red and seems OK but I kind of doubt it would do well in daylight. Haven't used it that way either so can't say.
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Old 09-02-2013, 07:34 AM
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Originally Posted by JetPlaneFlyer View Post
The alarm on those things is very loud, you should hear it in flight, no problem. A guy I fly with sometimes uses it in exactly this way and it works fine.
Pal of mine uses them on his 450 helis and a) led is visible, b) alarm is loud.

I had not thought of using one - but I have a couple in my box....

New project I see now!

Nigel
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Old 09-02-2013, 05:15 PM
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Here is how this electrical engineer does it. I use a 2 Ohm 100 watt resistor on 3S packs. With a 3S pack the discharge current starts at 6.5 amps @ 78 watts and will trickle down to 5.7 amps when the battery voltage hits 11.5 volts. Only takes a few minutes on a 3.2 AH battery.

To make it work on a 6S pack will require 2 of the resistors connected in series.

Now with that said I use my I-Charger to do this as it is built to do just that. The I-charger is a constant power load of 20 watts and the resistor is in line, and I set the discharge current at 5 amps. The charger automatically terminates the discharge when the voltage reaches 11.5 volts where I set it.
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Old 09-02-2013, 05:29 PM
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Derek.... that's interesting.... now I have resistor figures to work on.

Yes the charger way is better as it terminates at desired voltage - but the B6 range only max at 1A.... but you cannot actually have it stay at that... it reduces to its programmed firmware levels often less than 0.5A.

That is why I am looking for alternative DIY way to do it ....

Nigel
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Old 09-02-2013, 05:37 PM
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Originally Posted by solentlife View Post
Derek.... that's interesting.... now I have resistor figures to work on.

Yes the charger way is better as it terminates at desired voltage - but the B6 range only max at 1A.... but you cannot actually have it stay at that... it reduces to its programmed firmware levels often less than 0.5A.

That is why I am looking for alternative DIY way to do it ....

Nigel
Nigel you can use resistors or any other load device manually. The problem doing it that way is you have to be right there monitoring the voltage to terminate the discharge. Otherwise you will go to far and risk damaging the battery.

Fortunately my Charger acts as a 20 watt constant power sink and can easily handle up to 10 amps when using an external power resistor to take the bulk of the heat. At 10 amps, only 2 volts on the charger. The resistor takes the rest of the load.
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Old 09-02-2013, 07:52 PM
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Have a look at these threads:

http://www.rcgroups.com/forums/showthread.php?t=822655
http://www.rcgroups.com/forums/showthread.php?t=1962098
http://www.rcgroups.com/forums/showthread.php?t=1901849
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Old 09-02-2013, 08:26 PM
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Originally Posted by solentlife View Post
but the B6 range only max at 1A.... but you cannot actually have it stay at that... it reduces to its programmed firmware levels often less than 0.5A.
The problem isnt the firware, it's a hardware limitation. The B6 is limited to 5 Watts of discharge power, so if for instance if you are discharging a 4s battery starting at 16.8V then you would be limited to only 0.3A.
The stated one amp discharge capability is very misleading. You could only ever achieve 1A if discharging a 1s LiPo... Always pays to read the fine print on chargers

Discharge power rating is often hidden away in the fine print, or not advertised at all. As a rule of thumb guideline expect discharge power to be around one tenth of the charger power.
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Old 09-02-2013, 09:13 PM
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Default Electronic Discharger

There are electronic dischargers that can handle anything we use. Post this question in the Electronic DIY section. I built them over 40 years ago.

A chunk of heatsink Aluminum. Some BIG devices hand wired Done.
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Old 09-02-2013, 09:34 PM
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Originally Posted by cyclops2 View Post
There are electronic dischargers that can handle anything we use. Post this question in the Electronic DIY section. I built them over 40 years ago.

A chunk of heatsink Aluminum. Some BIG devices hand wired Done.
Yeah, coming up with a device that will discharge the battery is relatively simple, a bunch of lamps, a few high wattage resistor and a heatsink, they will all do the job.

What's much harder is making the device stop discharging the LiPo at optimum storage voltage rather than continuing to discharge until the LiPo is dead flat and ruined.
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Old 09-03-2013, 12:45 AM
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Originally Posted by JetPlaneFlyer View Post
Yeah, coming up with a device that will discharge the battery is relatively simple, a bunch of lamps, a few high wattage resistor and a heatsink, they will all do the job.

What's much harder is making the device stop discharging the LiPo at optimum storage voltage rather than continuing to discharge until the LiPo is dead flat and ruined.
Yup
This is the territory where those microcontrollers really work out well. Take a look at the attached schematic. This could be built on one of those Radio Shack perf boards, and cost would be fairly reasonable.

But the microcontroller needs to be programmed with software for the discharge function. If enough people are interested in this project, I could do the programming, and send out only the microcontroller at cost, plus a few bucks for shipping charges. The remaining parts would be available from electronics supply houses such as www.digikey.com.

Or, MicroChip has their PicChip3 programmer for about $40 or so that allows anyone to program these units. I've done this hundreds of times over the years.

The schematic shows a push button switch. This switch would be pushed repeatedly until the proper number of LiPo cells shows up. The LED would flash once for one cell, twice for two cells, and so on.

Note that I'd use a PicChip 18F13K50 for the microcontroller, rather than the unit shown in the schematic. I've got a bunch of them on hand. This microcontroller also has a different pinout than that shown in the schematic!!!
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Old 09-03-2013, 03:15 PM
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solentlife
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I'm happy that the subject hasn't been dismissed as a crazy englishmans 'brainiac'.....

The AUTO stop of discharge is of course a much wanted item - BUT given that a low Voltage alarm can be hooked up to balance lead while main power lead is used to discharge ... via resistor etc. - this may be the way forward for me ...

One of the factors to consider is that discharge should be relatively quick BUT not so hard that low voltage alarm kicks in because of excessive voltage drop ....

So let's ask a question :

What would be a good amp rate to discharge packs ?

Using following as pack sizes :

2S low C up to 20C, up to 1000mAh
2S over 20C, up to 2500mAh
2S larger

3S up to 20C up to 1300mAh
3S up to 20C, over 1300mAh
3S 20C to 35C, up to 1800mAh
3S 20C to 35C over 1800mAh
3S higher

4S in similar steps ..

5S similar steps

??

Any suggestions ?

Or is it better to calculate discharge against TIME rather than amps ? Say a 30min cycle ?

Nigel
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Old 09-03-2013, 03:35 PM
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OK ... next bit :

variable resistors / rheostat ?

Is it feasible to create a discharger using a resistor as quoted earlier ... and also a variable pot resistor to adjust discharge rate ?

Suggestions of specs needed ...

Nigel
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