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Old Sep 03, 2009, 09:53 PM
Registered User
United States, AL
Joined Apr 2007
822 Posts
Mini-HowTo
How to build a constant current battery discharger

By discharging a battery at a constant current and graphing the voltage as a function of time you can accurately gage the performance of a battery. Products such as the CBA allow you to do this but they are expensive and require a computer. Here we will learn how to build a no-frills constant current discharger.

The basic device we are going to build can be seen at this link: http://www.webx.dk/oz2cpu/radios/dc-load.htm We will make a few small changes to make it fit our use better. I've uploaded the schematic from that page here to save it for posterity. Our goal will be to discharge a 2200mAh 3s LiPo pack at 50a for under $50. I'm no electronics wiz, but I know what the basic circuit symbols are and understand v=ir and p=iv. If you learn those simple things you can understand how this thing works and make small changes to fit your needs.

First, a look at the schematic and our minor changes:
The device labelled "7809" is a 9v voltage regulator which provides a stable reference voltage to the circuit. It is critical and can't be changed. The two capacitors on it are per the spec sheet from any manufacturer of the voltage regulator and ensure smooth operation.
The big triangle is an op-amp. It measures the battery discharge current and controls the device to keep the current at a constant level. It is a tiny 8 pin chip with two op-amps on it, we only use one of them.
The "sense resistor" is a 0.001 ohm shunt that provides a small voltage drop that the op-amp reads as battery current.
The transistor(s) are mosfets that regulate the battery current. A mosfet is sort of like pinching a garden hose to control how much water can flow through it. The mosfet's resistance is controlled by the op-amp to keep the current flow constant. The 100 and 0.1 ohm resistors are if you use more than one mosfet - the 0.1 ohm resistance helps balance current flow through the mosfets and the 100 ohm resistors dampen changes in mosfet resistance to prevent oscillation due to manufacturing tolerances in the mosfets.
The potentiometer varies the reference voltage to the op-amp, ie. it sets how much current flow the op-amp will keep across the sense resistor.
The left switch is simple an on-off switch. The right switch switches between constant current and constant resistance mode. We will delete this switch and hard-wire the circuit for constant current.
The other small components are as called for on the mosfet/op-amp spec sheets and simply help the circuit perform ideally.

How many mosfets one needs depends on how much power you want to dissipate. A 3s LiPo at 50a is around 500w of power. You can look up the specs for different mosfets, but what it comes down to is how well you can cool the mosfets. 500w is a lot of heat and it's hard to get so much out of a tiny electrical part the size of a BB. Devices such as the cc400 can dissipate 400w of power but cost around $400 - a bargain as other devices cost much more.

So our plan of attack is this: Build the control circuit and use it to test single mosfets to see how much power we can reliably put into them. Our goal is 75w per mosfet(7a * 11v = 77w) and seven total mosfets to get 50a. For testing purposes we will replace the 180k ohm resistor with an 820k ohm resistor. As pictured the circuit will provide 50a of current adjustment. With the resistor change the adjustment range will be reduced to 11a and provide finer control for testing.

At mouser.com you can buy the parts for the control circuit and 3 mosfets for about $14(plus shipping). If you omit the switch it's only $12! Three mosfets because we will surely burn some up testing our cooling ideas. We got our parts today and will build the thing this weekend. The big hurdle will be figuring out a way to cool the mosfets. Commercial devices use fans and large(ie. expensive) heat sinks to dissipate the heat, we'll try out some no-frills cooling options which is how we will make this thing so inexpensive. Next we'll also review changes to make if you want to use this for a significantly different current/voltage range(ie. 6s packs, more/less current, etc).
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Old Sep 03, 2009, 10:00 PM
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Miami Mike's Avatar
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Did you know that RCGroups has a DIY Electronics forum?
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Old Sep 03, 2009, 10:07 PM
Registered User
United States, AL
Joined Apr 2007
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Doh! There's so many categories here you can't see them all!
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Old Sep 04, 2009, 02:41 AM
CamLight Systems
New York City, USA
Joined Oct 2003
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Hopefully a moderator can move it over, it's perfect for the DIY forum.

Biskit, I'm glad that you're moving forward with this! I think you'll be able to get 75W safely from a CBA-sized heat sink (and fan) if you're using a large case MOSFET (TO-247, TO-264) with a low thermal resistance. Good luck this weekend!
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Old Sep 04, 2009, 07:04 AM
Space Coast USA
hoppy's Avatar
Space Coast
Joined Oct 2000
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I would never have seen this in the DIY forum so it's not all bad. I vote to leave it here FWTW.
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Old Sep 04, 2009, 07:48 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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"..At mouser.com you can buy the parts for the control circuit and 3 mosfets for about $14(plus shipping). If you omit the switch it's only $12!..."

biskit,

It looks like you forgot to attach and upload the ASCII text with the parts list and the Digi-Key part numbers.

I can understand your having missed the DIY Electronics subforum on the first try. But if your interest is in RC and electronics, you are really missing a lot of good stuff over there.

Look at it this way, if you want to search for something about electronics, would you rather search one subforum and go through a few hits or search the entire forum and go through thousands of hits?

Hoppy,

I scan the DIY electronics forum once a week or so and just read the thread titles. Then I mark it read and go away. As a result of that forum I have built things like Dan Baldwin's great DIY Terminator for A123 charging and a wonderful variable low voltage cutoff I can use for when I am discharging batteries in the course of getting them seasoned.

Just a suggestion, please take this as well meant and for the betterment of the forums for all.

Jack
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Last edited by jackerbes; Sep 04, 2009 at 07:54 AM.
Old Sep 04, 2009, 10:20 AM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,987 Posts
Quote:
Originally Posted by Miami Mike
Did you know that RCGroups has a DIY Electronics forum?
I can personally atest posting here is two orders of magnitude more fun than the DIY area.

Two orders of magnitude more hits here too.
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Old Sep 04, 2009, 11:20 AM
ProgressiveRC
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Seattle, WA
Joined Dec 2006
1,996 Posts
Right on. Agreed, never would have found it otherwise. I'm looking forward to the build.

- David
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Old Sep 04, 2009, 01:02 PM
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Miami Mike's Avatar
Miami Lakes, Florida, USA
Joined Mar 2003
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Quote:
Originally Posted by rich smith
I can personally atest posting here is two orders of magnitude more fun than the DIY area.
I can personally attest that the DIY Electronics forum was created for a reason.
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Old Sep 04, 2009, 01:06 PM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
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Quote:
Originally Posted by Miami Mike
I can personally attest that the DIY Electronics forum was created for a reason.
Yeah, to weed out forum nazis, trolls, and troublemakers in general. That's why it's not so much fun.
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Old Sep 04, 2009, 01:59 PM
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You can post the construction article in the DIY electronics forum, since that's what it's there for, then post a link to it in this forum.

Quote:
Originally Posted by rich smith
I can personally atest posting here is two orders of magnitude more fun than the DIY area.
Two orders of magnitude more fun is good, but I'd rather have two orders of chilli cheese fries.

Dan
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Old Sep 04, 2009, 02:17 PM
Registered User
USA
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Quote:
Originally Posted by Dan Baldwin
Two orders of magnitude more fun is good, but I'd rather have two orders of chilli cheese fries.

Dan
Here you go!



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Old Sep 04, 2009, 02:23 PM
Pompano Hill Flyers
Miami Mike's Avatar
Miami Lakes, Florida, USA
Joined Mar 2003
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Quote:
Originally Posted by rich smith
I can personally atest posting here is two orders of magnitude more fun than the DIY area.

Two orders of magnitude more hits here too.
It's kind of like driving on the wrong side of the road then, isn't it?
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Old Sep 04, 2009, 02:33 PM
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United States, CA, Norwalk
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Quote:
Originally Posted by PeteB
Here you go!
Thanks Pete. You'll never know how much I appreciate that.

Dan
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Old Sep 04, 2009, 05:56 PM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,987 Posts
Quote:
Originally Posted by Miami Mike
It's kind of like driving on the wrong side of the road then, isn't it?
That is a perfect description of my experiences in the Battery/Charger area.

I LOVE this place.
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