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Battery Discharger, What Load?

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kufman

New Member
This is a project that I am building for my radio control cars. It is a high current battery discharger with auto cutoff. The goal is to discharge the battery at 20 amps until the battery reaches 5.4 volts. Then the discharger needs to shut off and clamp off so that is doesn't begin the discharge again when the battery voltage rises back above 5.4V. I have some ideas about how to do this, but my main dilema is my load. I don't really want to buy power resistors, so I plan on using FET's as the load. It will require a large heatsink and some descent FET's. Any Ideas? :?:
 

Pilot

New Member
As a quickie have a look at........

http://users.chariot.net.au/~samstain/CCD.html

You will notice that you need 2 x 0R22 50 Watt resistors.

Add a low voltage fan blowing onto the Fet/Resistors/Heatsink and power it off the battery you are discharging.

Eveything runs cool...............

Have fun

:lol:
 

wovk

New Member
20 A discharger?!?!?! that's for a 12V battery??

you can just put bulbs in parallel for your load
and put a comparator with your reference voltage (5.4V) with a relay that remove your load when your voltage goes above V ref.

but maybe bulbs is a kind of power resistor... :)
 

kufman

New Member
I currently use light bulbs, except they don't do a constant current as the battery pack falls from 7.2 V down to 5.4 V. I could use extra bulbs and limit the current with a couple of FET's. I would have to use an inline inductor to limit the surge current from the switching of the FET, or else I could just use the FET as a linear device.
 

bogdanfirst

New Member
so you need a constant cureent for discharging?
that can be done with a circuit and some resitor to disipate the power.
you can't do this only with resistrs.
but why do you wanna discharge the batt?
 

Pilot

New Member
Bogdan

A lot of people use this method to find out the time it takes to discharge the battery. In this way the battery capacity can be checked/verified.

One problem with bulbs is the change in resistance when they heat up.

If one or more bulbs fail during the discharge then any measurements taken are meaningless.

There are many commercial units available to do this job. All the reliable ones use resistors.

Having said all that - you should experiment.

:lol:
 

kufman

New Member
I have found all the parts I need to do the project. I decided to use power resistors that are mounted on a heat sink. I choose a resistance that is lower than i need and I plan on limiting the current using a hexFET and a voltage feedback system. The voltage feedback will come from a 0.01 ohm resistor mounted in series to the power resistor. At 20 amps I will have a .2V drop across the .01 ohm resistor. I will then run that .2v into an opamp and a reference signal into the other input of the opamp. The open loop circuit gain of the opamp will drive the gate of the FET. Since I am not using a fixed gain, the opamp will drive the fet until both input terminals are the same votage. I am also going to incorporate a simple timing device using a lm555 and an 8 bit bianary counter. The 8 bit's output will be connected to 8 LED's that will display the elapsed time in bianary. I am still working on the auto cutoff so that the discharger stops when I reach a certain input voltage and latches off. I think it will use some basic digital logic chips and a small relay.
 
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