Cheap load for mAh measurement?

[kalel]

I would like to get one of these to get some approximate values. Seems to be capable of measuring V/A like the USB power meters too, could be useful.



The unit is not expensive, but the load could be. They seem to give some 7.5 ohm resistors. Product rating comments mention that they can get very hot, but this surely depends on the power going through.

E.g. at the low end: 1.5V / 7.5Ohm = 0.2A * 1.5V = 0.3W

When testing batteries, the voltage will drop eventually. That means the current drawn by the resistor will decrease. Resistor temperature will also affect resistance and current slightly. Do I need a constant current load to measure the Ah value of a battery with this unit, and if so, is there a cheap (being the most important part) way to obtain or improvise one? Slow discharge is fine if needs be. It does not need to be adjustable.

[jeroen79]

Ebay/aliexpress/etc offer a 10A adjustable constant current load for €15-ish.

Or you can put something together with a transistor.
Feed the transistor's base through a resistor from the 5V supply of the discharge tester.

How crucial will the current be anyway?
Fluctuations in current wouldn't matter, the thing constantly calculates how much charge went through.
And it displays the current so you can make adjustments

[kalel]

Or you can put something together with a transistor.
Feed the transistor's base through a resistor from the 5V supply of the discharge tester.

That's a nice idea. Relatively cheap (for low currents) and adjustable too (although probably more stable to use a resistor than a potentiometer).

How crucial will the current be anyway?
Fluctuations in current wouldn't matter, the thing constantly calculates how much charge went through.
And it displays the current so you can make adjustments

Yes, you're probably right there and it makes things easier to just use a resistor if that's good enough for a rough estimate of the capacity. I'm guessing it would matter if the current changes so rapidly that the unit isn't fast enough to measure and factor in the changes. It probably won't change rapidly at all with a battery, so that should be fine.

[anishkgt]

Or you could try this also
eBay auction: #http://www.ebay.com/itm/18650-li-ion-lithium-Battery-Mobile-Power-Capacity-Tester-Meter-discharge-load-/292125965820?hash=item4404107dfc:g:IrYAAOSw4shYBclk

or maybe wait for Lexcelon's battery analyzer