Anyone know of a GOOD battery tester AND/OR circuit anywhere on the web (or elsewhere)? I used to have a circuit which would test the battery under a load given some semblance (IIRC)of a "fresh" battery's internal resistance. (That was on a PC which we'll call a Pentium ¼ and has since given up the ghost; I think Reagan was still president.) I could be mistaken on the int ? thang but there was some reference to it.
I'm REALLY looking for something that REALLY tests under load, not some dime store POS. Cost me $500 couple weekends ago, so cost is little or no object.
Follow up: I believe that testing with a "modern" multimeter should do more than suffice, since the input resistance on a DMM is often in the megohm range. I did a bit of Ohm's law, calculating the parallel resistance of a battery's internal resistance (should be low, single digits) and DMM's input resistance (* megohms). This is what APPEARS to be the circuit made when testing a battery this way. E.G.:
1/Rtot = 1/R1+ 1/R2
1/Rtot = 1/2+ 1/10,000,000
1/Rtot = .5 + 0.0000001
1/Rtot = .5000001 Rtot = 1.9999996000000799999840000032, or damn near the battery's internal resistance (of 2 ohms).
So, the high input resistance of the DMM neither puts a big load on the battery nor gives some false representation if measured with a low resistance input tester (deflection type).
I transcribed bigger numbers, i.e. 10 ohms and 250Kohms, respectively, to see if the calcs would fall far away but they only seem to scatter a tad (probably WAY below the calibration of the DMM anyhow).
Anyone see any problems in this assumption? TIAA...
Aahhh, no. The meter impedance will not be in parallel with the batteries internal resistance.
What you want is to load the battery to an approximation of the current it will deliver in actual operation and confirm that the terminal voltage stays above some voltage for whatever time is required.
Without knowing the specific battery type/voltage and application, it is hard to recommend a specific load to test under.
So, then, Dave, would a DMM be a decent approximation of a battery's voltage? There is some allowable swing, no doubt (as per most altimeters' manuals).
The DMM resistance can be ignored completely, unless you consider a microamp important.
The internal (series) resistance of the battery can be used to determine whether the battery is any good or not, is that what you're looking for?
To determine the internal resistance you must get current flowing through the battery.
As Dave says, you will need to put a load on the battery... as close to what you will actually be using the battery with... or about 50% of capacity if actual load is unknown.
Here're the steps;
Check Battery Voltage unloaded (BVU).
Put a load (RL) on the battery.
Recheck the battery voltage loaded (BVL).
The difference between the voltage reading in step 1 and the reading in step 3 is due to the voltage drop across the internal resistance of the battery.
BVU - BVL = Eri
Calculate the Current using the voltage in step 3 and the value of RL;
i = BVL \ RL
Since we now have the voltage drop across the internal resister, and we know the current, we can calculate for the value of it's resistance (Ri) using;
Ri = Eri \ i
For example; a good AA battery;
RL = 15 ohms BVU = 1.53 V BVL = 1.49
So the voltage drop across the internal resister is BVU - BVL (1.53 - 1.49) or .04 volts (Eri).
The current is BVL \ RL (1.49 \ 15) or .099 amps.
The internal resistance is Eri \ i .04 volts \ .099 = .4 ohms.
The term impedance usually refers to the total opposition to an alternating current flow, and usually includes resistance and reactance, and is represented by Z.
A comment on testing batteries. All of the advice you have recieved is technically good and sound. But, consider this. Testing the battery under load for an extended time is more likely to cause the battery to fail during use than a fresh, un-tested battery is likely to fail. The heisenberg uncertainty principal applies: you cannot find the internal resistance of a battery without seriously reducing its capacity.
The best approach to getting good batteries is to use a NiCad rechargeable battery. It can deliver more current than alkaline, AND it's rechargeable. Charge the battery, measure the voltage under load as was suggested, and learn its voltage discharge characteristics when working properly. Before you use it each time, check the voltage to make sure it matches what you expect for a fully charged battery. Once every few launches, test the discharge curve again to make sure it has not lost a cell. Also, watch for a white crystalline substance around the terminals. If you see it, quit using the battery immediately.
I dunno... one need only take a reading for a second or two.
rechargeable.
Good advice... NiCad work at very wide temperature ranges, and deliver a good deal of current, too. (Needed if you're firing an igniter).
The only thing I don't like about them are their toxicity and that 'memory' problem.
Kokam Li-Poly High-Discharge Batteries sound good... they're 'Earth Friendly', don't have a memory problem, and are very light weight.
But they're expensive, require a different charger, and I haven't played with them much.
Example: Two 340 mAh Super High Discharge Cells connected in series gives one 7.4 volts, can be discharged at 6 amps, and weighs less than 3/4 of an ounce!!! Sounds very appealing for igniter ignition.
David, sound solution/suggestion, no doubt, but for one thing:
I'm the Rechargeable Undertaker. I'll read, re-read, re-re-reread and re-re-re-read manuals till I'm blue in the iris and STILL, I get three charges out of a .
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