hi ,
I am makeing a wierd tester thingy, one if the uses that I would like to include would be discharging a battery to a set voltage and measuring the capacity of it,
I have a hall sensor on order to measure amperage with , and I have a Load of old 2n3055 transistors,
Is it possible to use them and a big heatsink to effectivly short the battery and thus discharge it safely ?
How would one go about connecting it if possible,
thanks,
The old 2N3055 can be used, but it needs a large base current.
At least use a resistor to limit the maximum current. For example a 5W or 10W, 2.2 or 4.7 ohm for a 1.2V battery.
Connect the (-) side of battery to GND.
A single 2N3055 to discharge the battery, emitter to GND, collector via 2.2 or 4.7 ohm resistor to battery (+).
The Hfe for the 2N3055 would around 50 to 100. Say 1A / 50 = 20mA base current.
At 1A, the 20mA is enough to drive the 2N3055 into saturation, that is good.
Arduino output pin via 150 ohm or 220 ohm resistor to base of 2N3055.
What kind of batteries ?
Normal NiMH 1.2V AA and AAA ? There are good chargers that also discharge and calculate the capacity.
This website is over-the-top for testing batteries and chargers: http://lygte-info.dk/
I bought recently this one: Review of Charger Powerex MH-C9000
It is something that I should have bought many years ago.
I scrounged a 5w 0.5ohm resistor from the same place I got the transistor, not any good for this?
I have a decent enough hitec x2 400w charger, I frequently use it for what I am describing, but I want to include it in my super wierd tester thingy if I can, just for the fun of learning I guess.
the batteries will be LIPOs. I know i know dont over charge or under charge them or they puff and vent and smoke and then burn, I have worked with them for over 4 years now and know not to abuse them. So obviously I would also need to monitor the voltage fo the cell and stop the discharge at a safe level.
Single cell Lipo batteries ? What mAh ? Do they have a protection circuit board included ?
You better not discharge above 1C, and I would use 1/3 C. So a full discharge would be 3 hours.
Suppose 4000mAh at 1/3 C = 1.3A discharge current.
Voltage 4.2 to 3.0, let's use 4.0V.
4.0 / 1.3A = a safety resistor of 3 ohm.
You could use PWM and a RC filter, so the 2N3055 (1 or more) are used analog.
The safety resistor to discharge is then only needed to prevent that the battery would explode, and can be lower.
With that example of 4000mAh, you could use 2 resistors in series for 1 ohm total.
Perhaps you can use the resistor to measure the current.
they will be single cells for now,
up to 6.5ah cells,
They can be safely discharged at 40c, so I dont think that I will have a problem with that.
I tried using the resitor to sense current, but apparently as the current increases it will waste more voltage and thus throw my voltage reading off, that of course is in another application mind you. I dont know what would happen if I use it to discharge.
A hall current sensor can be indeed easier, with a resistor the ground currents could be a problem.
The 40C is for new batteries. Old and bad batteries will not be happy with that.
Do you have an idea about the schematic ?
The 2N3055 can be used up to 3A, above that, the gain is dropping.
On the basis that battery capacity is normally specified at 20, 10 or 5 hour rating you need to design your system based on how you wish to specify your battery rating. I presume you wish to evaluate the Ah capacity of a battery
On the basis that it is 5 hours, you should base your maximum current on Ah/5, so for a C5 cell rated at 6.5Ah, the discharge current should be limited to 6.5/5 = 1.25A
Ideally you should employ a constant current discharge regime, whilst keeping a close eye on voltage. This then makes calculation and determination of battery capacity relatively simple (only one variable - time; since Ah capacity is Amps x Duration. (voltage doesn't come into the equation) You discharge the battery until the voltage has fallen to the minimum value specified by the manufacturer (see their discharge data curve)
In the past I've used such techniques to evaluate the Ah capacity of large wet acid batteries. Monitor voltage whilst discharging at constant current. Achieving constant current is relatively easy using something like the 2N3055 you suggest by fitting the load resistor as an emitter follower whereby the gain is near enough unity and by feeding the base with a constant voltage the voltage across the load resistor is kept constant, hence the load current is constant. The variable "resistor" is the transistor itself and it of course disposes of the voltage as heat. By correct selection of the emitter load (say 80% of the theoretical load value), the transistor does 20% of the work.