Simulated resistive load for batt testing?

[dll932]

OK, I know just hooking up a VOM to a battery won't give good info since it won't place a load on the line. I also know I can buy battery testers that'll test "standard" cells (AA, C, 9V, etc.) with a load, but I'd like to test batteries with voltages like 3.6V, etc. Anyone know what resistors would make suitable loads to put in parellel with the VOM at various voltages?

[slipstick]

It depends entirely on what load (current) you want to put on the battery.

Ohms law V = IR (Voltage V = current I times resistance R)

So Resistance R = Voltage V divided by Current I

e.g. if you want to use a load of 200mA (0.2A) on your 3.6V battery

R = 3.6 / 0.2 = 18 ohms.

Steve

[dll932]

Thanks! Actually, this is more about cellphones, cameras, drills, etc. I can see I'm gonna have to get current draw figures.

[JohnMuchow]

Be sure that the resistors you buy can deal with the power levels they need to dissipate (as heat):

Power = Current times Current times Resistance
P = (I-squared)(R)

OR...

Power = Voltage times Current
P = VI

The second equation is easier when a single resistor is used. The first one is esier to use, IMHO, when you have multiple resistors in series or parallel.

To ensure that the resistors aren't blazingly hot, don't exceed 50% of their rating. That is, if you have 5 watts being dissipated, buy a 10 watt resistor or use multiple resistors in parallel or series to divide the power dissipation among them.

[dll932]

Thanks! About the beefiest battery I'd test would be 18v, 2a.

[vintage1]

Thanks! About the beefiest battery I'd test would be 18v, 2a.

see if you can find an old lab RHEOSTAT on ebay or elsewhere..

These are big variable resistors that can take quite a lot of current.

http://cgi.ebay.com/Milvay-Rheostat-size-6_W0QQitemZ6586453034QQcategoryZ361QQrdZ1QQcmdZVie wItem

[Chippie]

Here's a circuit to simulate a variable load....can take up to 40amps+

[JohnMuchow]

Here's a circuit to simulate a variable load....can take up to 40amps+Aren't transistors #3 to #6 always biased full on? With their gates connected to the +++ lead, the gate drive can't be adjusted.
Or, is it very late and I'm missing something. :)

[Dan Baldwin]

John, I think this is what he meant, but it's would be a very tricky current source to use. I believe that the turn on would be very sudden. Setting a particular current would take a very steady hand, and the current would drop off very quickly as the battery voltage lowers. Transistors hooked in parallel that way have a tendency to not play well together because of minor differences in turn on voltage. One transistor ends up taking most of the current. A small resistor in the emitter of each transistor would help to equalize the load, and give it a more gradual turn on.

Dan

[dll932]

What I was thinking of doing was to make a box with a rotary switch hooked to various resistors-not good enough?

[vintage1]

I am amazed. A rheostat that does all you want can be bought secondhand for far less than all these compnenets, and its simpler, and it is more reliable...

Why not use a bunch of car headlamp bulbs if all you want is to dump power..

[JohnMuchow]

Dan, that's more what I was thinking it should be. I agree about the emitter resistor.

Vintage1...well, we never said it was a better idea, just another one. :)
And it's a heck of a lot smaller and lighter than the rheostat.

[simingx]

I have a 400 watt electronic load that I posted here a while back... If there's any interest I'll dig up the thread. :)

[dll932]

Sure, if you can. Thanks!

[simingx]

Here you go...
http://www.rcgroups.com/forums/showthread.php?t=200602