Battery discharge is occasionally desirable, though batteries are usually flown down.  Weather, change of plans, battery break in and other reasons make a discharger useful.  Most chargers offer meager discharge capability with the Powerlab 8 offering about 4 amp, 100 watt capability.  Still, not much fun to condition and break in 4 - 6 batteries at that rate, or to come home with 4 fully charged batteries from a rain out.  There is also "regenerative discharge" on IChargers and Powerlab, but that implies lead acid battery ready to accept charge.  Hopefully we can share a few simple discharge ideas.

I had a few parts setting around including a 12 volt computer fan and shield screen and a few 25 watt resistors and decided to try to make a discharger.

Parts:

12 volt 120mm computer fan with screen

25 watt 20 or 27 ohm resistors

Miscellaneous solder, wiring, and battery connectors.

The copper bars energizing the resistor wires are just 14 gauge solid household wiring stripped and soldered.  The resistors are adhesed with silicone rubber to the screen.

Using 6 cells and about 24* volts gives about 1.2 amps/20 ohm parallel resistor loop and then about 28.8 watts/loop.  If you use 27 ohm, that is 0.89 amps/loop and about 21 watts/loop.  I had 3 of the 27 ohm resistors and wen to to Fry's. They were out of 27's and had 20's so 3 of the 27's were used and 4 of the 20's.  Calculated about (4*1.2) + (3*0.89) = 7.47 amps/hr or about 125 ma/minute.  Without a fan, the approximate 310F temperature max of the resistors will be exceeded and with a fan, still close.

The fan is 12 volt and 0.35 amps, so that gives about 40 ohms of resistance. A 10 ohm and 33 ohm resistor all in series will drop the voltage to about 12 and keep the fan happy. Of course that adds a bit of amp draw.

I run about 130 ma/minute or about 7.8 amp/hour, not quite 3C for break in of batteries.  I run 9 minutes to get just lower than storage voltage of the PL8 charger.  I am using batteries charged to about 4.16 volts and normally discharge to about 25%.  Obviously fully charged batteries would take another minute or so.  If I had a second screen to put on the fan, I might load a couple more resistors to up the load, but don't want to compromise airflow.

Considered adding a timer cutoff for the unit, available 24 volt systems fairly cheaply, as are low voltage cutoffs.  I didn't have either and most are Ebay China origin, and it works fine without if you don't leave it running.  In practice I never use it without carrying my smart phone with a timer set.  It would be easy to over discharge a battery, but the timer works for me so far.

If 5 cells, the resistance to control the fan is adequate, and if 4 cells, the 10 ohm is left out of the circuit.  And, yes the wattage will drop with voltage drop, so I might use more resistors for less cells and larger amp cells.

An enclosure of Formica was made about 7" long and a slide fit.  That helped airflow and shielded the hot resistors from touch.

I will say simple as it took longer to think about and obtain the resistors than constructing.  Not counting letting the silicone dry overnight.

*The 24 volts is an average approximation as the voltage is falling along with wattage.

I agree that the low voltage cutoff would be nice.  I tried to find a simple wire in board.  

http://www.ebay.com/itm/Low-Voltage-Disconnect-Module-LVD-24V-10A-Protect-Prolong-Battery-Life-/131791231226?_trksid=p2141725.m3641.l6368

Selectable and close for 6 cells, but not exactly as needed.  

Several circuit diagrams were found by Google search and the RC groups as you noted.  None seem "simple."

It seems like the ideal would be to run the discharge through the charger with the resistance discharge board perhaps in series taking the load, and the charger then able to break the circuit.  I am not sure quite how to do that or if feasible.

Along this line, but I haven't thought it through, on page 18, expanding the discharge mode,

https://www.progressiverc.com/media/iCharger%20206B%20Manual.pdf

Bathroom fan timer and an automotive relay of sufficient current carrying capacity.  Turn on the relay with the timer and discharge the batteries using the relay.

Turn the dial to the desired time, discharger comes on.  When the timer goes off, the discharger goes off.

For a 24 volt system, figure out the coil current of the relay and use and appropriate dropping resistor, or find a 24-volt truck relay -- I'd use the dropping resistor.

And fix it if it shows any sign of wonkiness that might lead to it being stuck on -- that would kill a pack dead.

I just use the charger. Hmmm....

Discharge them in parallel?  It'll still take forever, but it could be fairly unattended.

Randy, just a little curious about "45 minutes to storage from full charge."  How many in 45 minutes?  I ask because the discharge capacity is listed at 80 watts, or for a 5 cell pack discharging with some load and sitting at about 19 volts average while discharging (3.8v/cell), the discharge capacity is then about 80/19 or 4.2 amps.  IIRC, the storage on iCharger is about 1100 - 1200 ma down from full charge on 5 cell 2700 Zippy packs, so 4 batteries on a parallel board could/should go over 1 hr as they would give up 4.4 - 4.8 amps.  Of course that assumes good capacity cells and that the discharge rate is as listed. Perhaps it does better?

The connector to add the resistance into existing wiring in series

Thanks Fred, Interesting, I need to step up my charging game, I did manage to discharge my packs tonight, the hard way,, 7 flights, 

Randy,
When I charge or discharge, I usually look at the amps/ma of input or output and % of charge at the beginning and end, if available.  The actual amps of input/output give a suggestion of battery capacity if you know beginning and ending % as well.  Without the amps in or out, the times don't make a lot of sense.  All of that said, your system seems to be working well.
Thanks,
Fred

Using the iCharger as a controller caused the iCharger to shut down as the number of resistors increased and approached a 12 amp load.  The i206b lists 20 amps, but does not seem to like the heat.  The Discharger unit works great as stand alone and uses about 190 - 200 ma/minute so about 6 minutes/6cell 2700mah pack from full to just below storage.  Also used with a parallel board and more packs.  The down side is that you have to set a timer and shut it off, but it is simple.  I may add the CellLog as below since I already use one in the field.

The CellLog8M has a switch feature that will activate at a set cell voltage and serve as a cutoff.  The cube relay, earlier suggested by Tim, controlled by the CellLog8 instead of a timer has been used by others with good results and is fairly easy to hook up.  If not a cube relay, then a DC-DC SSR can be used, also written up.  The last RCgroup thread below will lead there, thanks to William DeMauro for getting me started on the search.


The best way I found if starting from scratch and want more discharge than your charger supplies, or if you want to avoid the heat load to your charger is below.

A balancer/discharger for stand alone use is the RCMart 3-in1 balancer discharger.

http://www.myrcmart.com/rcx-3in1-battery-balancer-discharger-voltage-indicator-150w-discharging-p-4767.html

One unit gives about 4 amps discharge, and they can be paralleled giving 4, 8, or 12 amps of discharge as described below, "An effective variable load automatic LiPo discharger."

http://www.rcgroups.com/forums/showthread.php?t=2406395#post31524795

Lucky you.  I didn't recommend resistors like that because of price.  A bargain price $49.99 used

http://www.ebay.com/itm/like/221923395778?lpid=82&chn=ps&ul_noapp=true

If you found it surplus/cheap, a good way to go.

For 5 cell at approximate 3.6v under load, 5 cell is 18v divide by 2 ohm is 9 amp and 162 watt.  A bit higher at first as the initial voltage is higher. 

Hi,

for those who are interested:

I also was in search for a light and simple solution to discharge batteries down to storage voltage during the WC in Perth in case I didn't use them on my flying day. Normal discharge currents of chargers are 1A only and then discharging takes too long.

I use 6s batteries btw. I discharge them on two H1 automotive headlamps connected in series. The full battery (6s) has 25,2 Volts and the design voltage of any automotive lamp is 12V, thus it was logic to use two in series.

You can discharge a full 6s battery down to storage voltage in approx. 15 minutes. That is a discharge power of approx. 100 Watt or approx. 4 Amps. You can monitor the battery voltage connecting a digital battery tester to the balance connector of the battery.

Pro's: it is a very cheap solution, discharge currents are approx. 2C only, so saves the battery
Con's: you need to monitor the discharge process as there is no automatic cut off at storage voltage and you may need to check whether automotive lamps can also be used to discharge 4s or 5s batteries. Also, you may need to wear sunglasses while monitoring the discharge process.

Sorry I have no picture available but try it. H1 or any other automotive headlamps (H7 or H4) don't cost anything.

Christoph

And to all: I think you take it too complicated, battery is significantly aging at full charge, means 4.10 to 4.2V ... if you discharge only fraction of capacity, you will limit it,...

That was my reasoning on the thread about charging for out of town contests - 80 - 85% charge.

The reason for the discharger is the first place was for "battery break in" as suggested by ThunderPower, and that was at 3 - 4C.  Discharge to "storage" is for the convenience of parallel charging multiple packs, a common starting voltage.  The used packs are charged up to storage and the unused can be at near storage in about 6 minutes at about 12 amps (2700 mah packs), then brought to storage. I believe that you are describing "regenerative discharge" to the car, and most chargers don't do that.  PowerLab, iCharger, and Reaktor are the ones that I recall.  You posted about having Reaktor clone of iCharger 306b as I remember.

A balancer/discharger for stand alone use is the RCMart 3-in1 balancer discharger.

http://www.myrcmart.com/rcx-3in1-battery-balancer-discharger-voltage-indicator-150w-discharging-p-4767.html

I bought one of these and used it for the first time yesterday.  Works a treat and a whole lot faster than using my charger.  It's a little slow on the final finish as the shutoff point is approached so I then stop it and do the last bit on my charger which seems to accomplish that last little bit faster.  Perhaps the charger has more sophisticated hardware/software that allows a more rapid approach to the shutoff point without overshooting.

Fred, thanks for the link to the thread.

I haven't read all of the postings, but a bank of lightbulbs makes a great load. You would need to experiment with which ones to use but it's a low cost way to accurately discharge.

What's a nice boy like you doing in a place like this?

Thanks for the information, guys.  I got a couple of the dischargers Fred mentioned and mounted them in a cookie tin.  The light bulbs are under the screen.  The blob to the left is a low-voltage monitor, https://www.amazon.com/gp/product/B003Y6E6IE/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1 , $2.40 including shipping.  Based on the advice above, I set the dischargers to 4.0 volts and the monitor to 3.8 volts in case I forget to disconnect the batteries after they discharge.  If Igor is present, I'll discharge my batteries into his car battery. 

I like Christoph's headlamp idea, too.  I usually fly until dark, so that would enable me to see to pick up my stuff. 

Howard, discharge to 4 volts?  Just to take the top off until flying the next day, or are you storing at that voltage.  I like the set up, and it keeps the hot lamps away from skin.

 Just to take the top off until flying the next day.

I thought about this for a while and am still not sure exactly what was meant.  To expand on what I think is meant, or add a thought, one could use the battery that is charged to power the charger to bring used batteries back to storage.  I could have an adapter to use the 6 cell pack as the power source for the charger and then use the charger to bring two 6s 2700 flown packs to storage and end up with all 3 at about storage.  The charger must accept greater than 12v input.

An example with simple calculations to clarify - Say 1800 mah used in a flight and about 600 mah to get a single pack back to storage, then I could bring 2 flown packs back to storage and at the same time remove about 1200 mah from the full pack leaving it at about storage. 

Of course the voltage step up and inefficiencies could use a bit more than 1200 mah from the full power pack, but with a little experience the real numbers could be worked out.  As the usage approached 2100 mah, perhaps one power pack to one storage, but again, the numbers can be adjusted.

If a fully adjustable charger such as the FMA PowerLab 8, the storage endpoints could be adjusted for your actual battery use so that one full pack into 2 packs, parallel, to storage could make 3 packs with about matching voltage and say 40% instead of 50% storage charge.  Once the numbers are worked out, the system should run itself and shut off.  That might just be done while you pick up after yourself and load the car.  If on the same plane and electric system, battery usage and replacement should be fairly constant.  If the voltage numbers get fairly close, then the 3 packs are ready for parallel charge for the next outing, or parallel charging to the about 50% recommended value for long term storage.

Great idea!

What transfer rate?  Will the system tolerate say 10 amps and serve as a new battery conditioner as well.  Same cell counts only each side, probably not since a resistor works?

I think that the concept above, using the charged battery as the power source for the charger is similar.  I can set a shut off for low battery on the power side giving similar safety.

I thought about this for a while and am still not sure exactly what was meant.  To expand on what I think is meant, or add a thought, one could use the battery that is charged to power the charger to bring used batteries back to storage.  I could have an adapter to use the 6 cell pack as the power source for the charger and then use the charger to bring two 6s 2700 flown packs to storage and end up with all 3 at about storage.  The charger must accept greater than 12v input.

An example with simple calculations to clarify - Say 1800 mah used in a flight and about 600 mah to get a single pack back to storage, then I could bring 2 flown packs back to storage and at the same time remove about 1200 mah from the full pack leaving it at about storage.  

Of course the voltage step up and inefficiencies could use a bit more than 1200 mah from the full power pack, but with a little experience the real numbers could be worked out.  As the usage approached 2100 mah, perhaps one power pack to one storage, but again, the numbers can be adjusted.

If a fully adjustable charger such as the FMA PowerLab 8, the storage endpoints could be adjusted for your actual battery use so that one full pack into 2 packs, parallel, to storage could make 3 packs with about matching voltage and say 40% instead of 50% storage charge.  Once the numbers are worked out, the system should run itself and shut off.  That might just be done while you pick up after yourself and load the car.  If on the same plane and electric system, battery usage and replacement should be fairly constant.  If the voltage numbers get fairly close, then the 3 packs are ready for parallel charge for the next outing, or parallel charging to the about 50% recommended value for long term storage.

The PowerLab 6 and 8, and the iChargers, Reaktors and a few others have Regenerative discharge, meant to allow the lipoly to discharge back into the power lead acid battery.  If the lead acid battery is full, headlamps or other resistance may be used to allow for some regenerative discharge.

The battery on the power side does not need a charging algorithm, nor does it have to be monitored.  If a full lipoly of adequate capacity is used as the charging battery, it may be discharged into depleted lipoly battery(s).  So a fully charged, unflown, lipoly may be discharged into flown lipoly(s) and the receiving batteries monitored as usual to control the process.

Needed is a charger capable of using a 24+V charging source, and an adapter for the lipoly to charger connection.  That adapter was easily made in my case from a male deans and the remainders of the original Zippy connectors as female donors, as below.  Also shown the system connected up and running, one lipoly for power into two lipolys in need of storage charge.

The power lipoly is also monitored, though not necessary once the process and proper numbers are worked out.  Perhaps not necessary, but desirable.  Howard's inexpensive monitor above could also be used to make sure the power battery is not overly discharged.

I thought about this for a while and am still not sure exactly what was meant.  To expand on what I think is meant, or add a thought, one could use the battery that is charged to power the charger to bring used batteries back to storage.  I could have an adapter to use the 6 cell pack as the power source for the charger and then use the charger to bring two 6s 2700 flown packs to storage and end up with all 3 at about storage.  The charger must accept greater than 12v input.

Yes, that is what I do ... or even better - I have 4 packs of old batteries taped together and connected paralel which does not have enough capacity anymore, so I do not fly them and I use them to power the charger, they have still enough capacity for 3 flights. I use also iCharger, it is well configurable to watch voltage, so no big stress.

On the discharger the balancing plug in, on the side of unit works on Thunder power battery's with white plug, but some Thunder power battery's have Black balance plug , were do I get adapter?

battery  Thunder power 5c 2700 mhA

I updated my discharger box with some new technology.  The original discharger consisted of the blue control module and a switching module connected to 150W worth of light bulbs.  New ones, https://www.banggood.com/AOKoda-CellMeter-8-150W-Discharge-Module-Set-with-Lipo-Battery-Balancer-p-1196376.html?rmmds=search&cur_warehouse=CN ,  have what looks like the same blue control module, but the switching module now has an input and an output so you can connect a whole string of switching modules, which you can now buy separately: https://www.banggood.com/AOKoda-CellMeter-8-150W-Discharge-Module-Set-for-AOKoda-Discharger-p-1196377.html?rmmds=search&cur_warehouse=CN .  I got two of the latter, so now I have 300W of light bulbs for each control module.  If you have the old discharger, you can add switching modules and put the original switching module at the end of the string.

I put the whole assembly plus a parallel battery board in a bigger cookie box (technically a biscuit tin: British shortbread cookies).