Thread Tools
Dec 10, 2001, 08:40 PM
Build'em and Crash'em
Ken Lapointe's Avatar
Thread OP

Help needed on controlable battery dumper

I bought a data acquisition card kit for my PC and set it up to read my power pack battery during discharge. The board also has 4 digital outputs that are programmable. What I want to do is program the discharger to stop discharging at whatever voltage I select. I can control a 5 volt digital output based on the voltage reading of the pack being discharged.

Does anyone know a good way to switch the discharge connection off and on based on a digital TTL line of 5 volts with a max current of 6 milliamps? I was going to use a relay but the ones I have would pull too much current through the coils and I'm not all that sure how to select the right one for this application.

Packs to be discharged will be up to 35 Volts and max current is 2 amps discharge.

Any ideas would be greatly appreciated


Sign up now
to remove ads between posts
Dec 11, 2001, 12:00 PM
Registered User
Mr.RC-CAM's Avatar
To use the relay, you need three more parts. A transistor, diode, and a resistor.

Take a 2.2K ohm resistor and connect it to the Digital Output Control Line. Connect the other side to the "Base" of a PN2222 transistor (or any other general purpose NPN). Connect the "Emitter" of the transistor to ground. Connect the "Collector" to the relay coil. Connect the other coil lead to your power source (+5V or +12V). Place a 1N4004 diode across the coil, with its Cathode on the power source side.

The relay should be a low current type. I like to use the 12V @ 20mA variety (720 ohm coil). But you can use whatever is appropriate for your needs.

There are other ways to do this, including using optocouplers, solid state relays, or FETs, but this example is cheap and works well.

Be careful -- a goofup will damage your DAQ/DIO card.
Dec 11, 2001, 12:22 PM
Build'em and Crash'em
Ken Lapointe's Avatar
Thread OP
Thanks for the help. I'll give it a try.

Dec 11, 2001, 08:07 PM


I would like to make a discharger for 7,8 and 10 cell packs that would cutoff @1.2 volts per cell
and have a discharge current of 30 amps. Does
anyone know how to make one?
Thanks in advance,
Dec 12, 2001, 07:11 AM
Crash Master
Gene Bond's Avatar
Yes, but it is more than you can describe here. Try RC Gizmos There's many other sites that have other circuits from the links here.
Dec 12, 2001, 11:40 AM


Thanks for the link, however I have seen these
kind of circuits before. I am looking to discharge flight packs at up to 30 amps. I am
wanting a fully variable current draw that I
can set with a dial like a rheostat from 1 amp
to 30 amps, depending on the size of the packs
being discharged. I have yet to see anything
like this being proposed.
Dec 12, 2001, 12:37 PM
Registered User
Mr.RC-CAM's Avatar
Thirty amp discharge is a bit steep. That is a lot of heat to dissipate! Should keep you warm and toasty this winter. Sorry, but I cannot offer any real help on such an endeavor.

However, I was at Borders Books last night. The magazine rack had a U.K. based electronics magazine that might help you. In it was part 1 of a 2 part construction article for a PC interfaced battery discharger. The parameters are set via a LCD based menu. I do not recall the name of the magazine, but if you hurry over to a Borders you should be able to stumble across it.
Dec 12, 2001, 03:06 PM
ClearView Rocks!
Quacker's Avatar

I could supply a schematic of a 30 amp discharger, but that would be only the begining of the story. Building a 30 amp controlled current sink requires substantial building skills to both properly monitor and cool the load. As an alternative, you might consider using your motor and propeller as the load itself. A current shunt and a cheap meter could be used to monitor the current. The current could be adjusted by an ESC as in normal operation. It then only need have a low voltage alarm to sense the lowest desired voltage. If this sounds good to you, then I can help you further.

Dec 12, 2001, 04:24 PM
I would be interested in the schematic for arguments sake, I have a new clubmember that is
a retired electrical engineer and I would show
it to him to see if his knowledge is all that
he says it is. I thought of the motor and esc
approach but I couldn't deal with the thrust
and noise. I currently have a setup that uses
a jeti discharger that I can vary the cell count
with through a pot and it has a built in cutoff
@.95 volts which according to what I have been
reading in the past year is too low. I have this
hooked up to a 12v auto lamp array that will not
draw more than 10 amps because of the limitations
of the jeti unit... it has a 10 amp fuse in the
top of it. I thought that I could use more lamps
and use two more jeti units in parallel?,... that
would take care of the 30 amp draw problem but
then I would have to know how many lamps would have to be lit to provide 30 amps all the way down
to 1 amp or whatever steps in between I deemed needed. And how would I switch these lamps on as to what current draw I would want, say 2 amps for a small parkflyer pack, 5 amps for a larger one,
10 amps for a speed 400 pack on and on to my full
sub c size packs that I would want to be able to
discharge at 25 amps. Actually I think that if I'm propping for 30 amps static then I'm actually
flying on 20 if I consider I'm not flying at full
throttle all the time. I guess I only need to
discharge at 20 amps. It seems that if I can
discharge closer to what I actually fly at then
the packs will be exercised better when I cycle
them. Let's say 20 amps now, o.k.
Dec 12, 2001, 06:49 PM
ClearView Rocks!
Quacker's Avatar
For starters, .95 volts per cell is just fine, especially at high rates when the cell's own internal resistance plays a larger role in determining the cell's terminal voltage. Sorry, ESC's in parallel will not work out electrically, so another means must be used. Light bulbs are a good load for high wattages. The more small ones you use, the closer you can come to the current you want by switching them out. My best suggestion is that you use a number of lamps that draw 1 and 2 amps each at 12 volts. Turn signals and side markers and brake lights are about the correct size. I'll look into model numbers that you can use. As for the low voltage alarm, do you actually want an alarm, or a relay that drops out?
Dec 12, 2001, 07:47 PM
Crash Master
Gene Bond's Avatar
I built one a few years back that discharged at 2-20A into a piece of niachrome wire... It glowed when dumping. I made a pwm current regulator from a chips worth of op amps and a FET with a good heatsink. I used it to match cells. I probably have some notes, but it's not a major design problem if you know the basics. If you don't know the basics, it's probably more than you should attempt, in case of a problem, you won't know what to do.

I'll never forget a college student who worked for me years ago. He called me at about 6am saying "I really need some help, I have to turn in this power supply project at 9:00 this morning, and every time I turn it on it starts smoking..."

We got together about an hour later, and I said "Turn it on". He did, it started smoking and he turned it off. I said "What are you doing? Turn it back on so I can see where the smoke is coming from!" Needless to say, he used too small of a heatsink, and didn't take long to get him on his way...
Dec 12, 2001, 08:24 PM
Registered User

Electronic loads

I suggest checking Ebay and other used / surplus equipment sights for "Electronic Loads". Saw a real nice on on Ebay for $20.00. With all of the companies folding, there are a lot of them on the market right now. They are several k$ new.

Dec 13, 2001, 02:08 PM

Jeti discharger project

I am enclosing a link to the jeti discharger I am describing, it is not an esc it is basically a cutoff device that you set the number of cells you are using by turning a pot with a screwdriver. It has the built in .95volts per cell circuit so you connect the battery pack to one side and the load to the other. It has a 10 amp max circuit so I thought that perhaps 2 of these units together could handle the 20 amps I need. If that's the
case, than all I would need is a bulb array that would provide the load ... however the way to wire it up to provide whatever current draw I want is what I need someone to figure out. Dave, I looked
on ebay and the only item I saw was #1674558358 it is very industrial sized and priced... neat though
Dec 13, 2001, 04:04 PM
ClearView Rocks!
Quacker's Avatar
I looked at the discharger on the web. I suppose that two of these in parallel would work, but not having intimate knowledge, I can't guarantee it. As for the resistance of the load, I suggest that you use a current sensing resistor along with a cheap digital voltmeter to measure the current drawn by the lamps. For 20 amps you'll need 0.01 ohms at 5 watts. The meter across the resistors will read 200millivolts at 20 amps. I checked with Digikey. They are out of .01 ohm resistors, but have .02 at 3 watts. Put two in parallel. They will get very hot--that's normal. The part number is: 13FR020. I dug around for auto lamp data. The specifications are hard to come by. Chances are that you will just have to buy some and play with it a bit. By the way, if you're doing all this to avoid "memory" problems with your batteries, you might want to save your money. If you take a modern NiCAD and repeatedly discharge it to exactly the same partial-discharge point, eventually (many, many cycles) you might find some sign of the much-misunderstood phenomena. When the discharge endpoint is varied or deep, as is natural with flight packs, memory (or more accurately “voltage depression”) does not occur. Moreover, the voltage depression is not severe and the remaining energy is still available if at a slightly lower voltage. In short, memory is a seldom seen malady that has been used to sell snake oil for many years. The information offered above comes as a result of detailed discussions between myself and Sanyo’s applications engineering, factory team.


Quick Reply

Thread Tools