Auto Shutoff Battery Discharger

[pctomlin]

Here is my latest DIY project that I built to compliment my charger box. Just got finished with the initial test run of an automatic shutoff battery discharger. I am going to use it to discharge unused packs back to storage voltage if I don't fly them. It was built to have flexibility discharging different S size packs. The voltage regulators used to supply the load are good from 1.5V to 37V so I should be able to use this on 2S all the way up to 8S batteries. Specifically I will be using it on 3S 2200mah and 6S 5000mah batteries. I never was too crazy about using storage mode on the charger, I don't like the heat that it generates in the charger. It is based off of some automotive/electronics relays and a CellLog 8M as the control device.

This is the stack of parts I used to make the discharger, the Deans connectors and additional wiring used to make it is not shown. Everything is housed in a Radio Shack project box. The four postion barrier strip was replaced with another eight postion barrier strip.


Here is the wiring diagram for the cut off circuit.

The LM317T regulators with the voltage adjustment legs shorted together for maximum voltage output (37V). Power in leads soldered on.


The regulators mounted to the underside of the heat sink. The battery ground wire is also connected to the heat sink.


Underside of the project box top showing the wiring from the switches and the heat sink.


Here is the finished wiring. Barrier strip on the left is the incoming 12V and Lipo battery connection to the switches and relays. The barrier strip on the right distributes the Lipo battery power to the regulators on the heat sink.


Here is how the whole unit looks discharging the very first battery - 3S 2200 mah.


Cell voltages before the start of the first test run.


Pack voltage before the start of the first test run.


First test underway. Pulling 18 amps, too much.


Disconnected four of the eight regulators to get the discharge load down to 9 amps. Much better. The regulators are pulling 2.25 amps each, not 1.5 amps each that was in the spec sheet.


First test run a success. Cells at 3.80V each at trip off.


Here is the pack voltage on the Watt Meter at trip. Whole thing probably took less than 10 minutes from start to finish.



The ultimate test comes next. Can I take this exact same rig with no adjustments and do the same thing with a 6S 5000mah battery, go from fully charged to storage voltage. Update to come.

[pctomlin]

I just finished the 6S battery discharge test using a Turnigy 6S 5000mah 40C pack. Hooked it up to the discharger with the same set up as the 3S 2200mah battery. The pack took 25 minutes to go from a full charge to 3.82V per cell. Monitored the relays inside the project case this time for heat and had no issues. The main power transfer relay got warm, maybe 100 deg/F toward the end of the cycle. This relay is 12V/30 amp rated for car headlights and such. I was a little worried about the 6S pack voltage through the relay. Here are the results.

Cell voltages of the pack prior to the start of the 6S discharge cycle.


Total pack voltage prior to the start of the 6S discharge cycle test.


Discharger engaged, pulling almost 5 amps from the 6S battery this time. About half the amps as the 3S battery.


Cell voltages at the end of the 6S discharge cycle test.


Pack voltage at the end of the 6S discharge cycle test. Removed a total of 2650mah from the battery in 25 minutes time.


Right now I am going to say I have been successful with making a Lipo battery discharger that is capable of doing 2S to 8S batteries with no adjustments. Both 3S and 6S batteries discharged at about 125 watts each (W/V=amps) thus the difference in discharge amps. I am thinking about a mod to the box to maybe add another switched bank of four regulators (the ones I disconnected) for the 6S discharge mode to bring this up to about 10 amps during the discharge. Should be relatively easy to do by splitting the 8 position barrier strip into two banks of four regulators with a switch to power up the additional four regulators for a 6S battery. Something to think about for a later date.

[ShaneJourdan]

Holy smokes, what a great project; and so well executed.

Now for the stupid question. What is it for? I guess I asking why?

My charges have whats called a "storage charge" system. So it will discharge or charge if needed.

[pctomlin]

Hey DJ, how's it going for you. You should know by now that I love to tinker and mess with stuff. I built it so I would not have to use my charger to bring my packs back to storage voltage if I did not fly them. It is fine to put them in storage charging them back up, I just did not like all the heat generated by my ICharger bringing a 6S 5000 mah pack back to storage from charged. Sooner of later I believe it will kill the charger. Plus, it is even more compact than my charger box set up.

I bought the switch today to segregate the 3S and 6S discharge circuits, I will post some pictures of what I did when complete. I am still deciding on 2 or 3 regulators for the 3S battery. Also debating on what to do on the 6S side, put 6 or all 8 of the regulators on the 6S side. Oh well have to do a little experimenting with different combinations of regulators hooked up to see what works best.

[ShaneJourdan]

Originally Posted by pctomlin

Hey DJ, how's it going for you. You should know by now that I love to tinker and mess with stuff. I built it so I would not have to use my charger to bring my packs back to storage voltage if I did not fly them. It is fine to put them in storage charging them back up, I just did not like all the heat generated by my ICharger bringing a 6S 5000 mah pack back to storage from charged. Sooner of later I believe it will kill the charger. Plus, it is even more compact than my charger box set up.

I bought the switch today to segregate the 3S and 6S discharge circuits, I will post some pictures of what I did when complete. I am still deciding on 2 or 3 regulators for the 3S battery. Also debating on what to do on the 6S side, put 6 or all 8 of the regulators on the 6S side. Oh well have to do a little experimenting with different combinations of regulators hooked up to see what works best.

Doing good my brother,

Absolutely awesome, I understand know and agree. I have had charged packs and sometimes not been able to burn them down and had to take my 6S 5000 packs back to storage i get it. This stuff is fun! Good on you.

[anthonynoahan]

I had gone through the post. The Auto Shutoff Battery Discharger may occur some times. This may due to the "Load" on the battery or something is running. This may also occur causing it to drain. It need for voltage draw in the vehicle while shut off. Please produce some more attachments for the more detailed view.





secondary current injector
relay tester

[pctomlin]

Hey Anthony,
I don't quite understand the question. The discharger has two separate circuits inside, a 12V control circuit and the battery discharge circuit. The control circuit can be powered by a 3S lipo or by the AC/12V transformer as show in the picture above. The control circuit is armed by turning on the toggle switch and then pushing the button on the momentary switch. This latches in the relays to start the battery discharging. The CellLog 8M is what controls the discharge cycle. When the pre-determined individual cell voltage is reached, the CellLog will trip open the relays and stop the discharge at that point. The heat sink cooling fan is powered directly off of the 12V control circuit.

[pctomlin]

The 3S/6S modification is complete. I added a switch to be able to turn on more regulators when I hook up a 6S battery. It is set up now and will discharge a 3S 2200 mah and a 6S 5000 mah battery in 20 minutes with a consistent storage voltage cutoff. The 3S set up discharges with a single regulator at 2.2 amps. The 6S battery discharges with an additional 4 regulators for a total of five when the switch is turned on at 7.5 amps. The discharge is about 750 mah removed from the 3S battery and about 2200 mah removed from the 6S battery.

Modification to the barrier strip jumpers to separate the regulators for 3S and 6S battery discharging. Power wiring for new switch installed.


All wiring re-installed, 6S discharge switch added and ready to put the case back together.


Discharger case with the 6S selector switch added.


Link to all the build pictures
https://www.helifreak.com/album.php?albumid=8896

[Just Another Flyer]

Nice, how much would it cost. The most I'll be discharging is 3s packs for now.

[pctomlin]

I have not put a pencil to it as of yet. I will see about doing that and post up a parts list.

[jdfelosi]

I know this is an old thread, but i'm wanting to build one, did you draw up a print for it?

[pctomlin]

The circuit for building the discharger is in the first post. The barrier strip on the left is for all the switch/relay and incoming power distribution. The barrier strip on the right is used to distribute the power to the LM317T regulators.

Here is the project box with the circuit board removed. The Dean's on the left are for the battery positive and negative and the other one is for the 12V trip circuit power. Both Deans connect to the barrier strip points associated with what they are to the specific points in the wiring diagram in the first post. The connection out of the top of the box is to the PC cooling fan.


Here is a picture of the bottom of the circuit board showing all the connections/jumpers to the different relay legs. These just loop over the end of the board and connect to the appropriate barrier strip point on the top side.


Parts List (RS is Radio Shack):
Coolmaster CPU cooling fan DI4-7H53D-Y5-GP
CellLog 8M
LM317T Voltage Regulator
9/16 OD X 1/4 ID rubber grommets (two)
8 Position Barrier Strip (RS 274-670 times two)
IN4001 Diode (RS 275-1101)
DPDT Miniature Relay (RS 275-0249)
Project Box (RS 270-1807)
Perf Board (RS 276-1394)
SPST Toggle Switch (RS 275-634)
SPST Momentary Switch (RS 275-618)
SPST 20A Toggle Switch (RS 275-019) for 6S load
Cole Hersee Automotive Relay RC-700112-DN (or equivalent)
Four 1" long standoffs
18awg silicone insulated wire
16awg silicone insulated wire
I used Dean's, can use other power connectors of choice
Terminal ends
Misc screws
Tap and drill bit set of appropriate size (6/32?)
CPU cooling fan connector (Fry's Electronics)

[jdfelosi]

Can we get this sticky'd???

It has my vote!! +1

[Spog]

For those of you interested in building a similar discharger, there are many more discharger examples and pics in the original design threads that this discharger is based on.

http://www.rcgroups.com/forums/showthread.php?t=1352331
http://www.rcgroups.com/forums/showthread.php?t=1901849

[jdfelosi]

I prefer the solid state route with the lm317's vs the resistor bank version, I set mine up pretty much the same way with the toggle for 1 vr for 3s, and 5 for 6s, only difference is instead of using the cellog, I use my iCharger 206b in DCHG+ mode. Been thinking of a control circuit with a rheostat or even a 4-6 position rotorary knob and different resistor values to be able to somewhat "tune" the regulator bank for certain voltages. I think there is a way to let the iCharger control the current, but I think it can only control +/- 2amps of the value of the load bank.

Thanks for this awesome guide Tons of great nuggets like this on the good ol' helifreak!!