Flash Amp Question

[DavidD]

I am not familiar with this function of the DMM, but recently noticed it being cited in another thread. I tested 3 Duracell 2650s resulting in a 9, 7.5 & 5. I also tested 2 Eneloops; both tested over 10.

What do these numbers tell me? Do they tell me the overall health of the cell? Do they indicated Tom/SilverFox's 'Vibrancy' quality? The other thread indicated that 10 (for AA's?) is good. Is there a different 'good' number for AAA's?

DavidD

 

[Mr Happy]

The so-called "flash amp" test is a somewhat crude test, and as you saw the results can be rather variable.

In general, higher numbers are good, and a NiMH AA cell can typically produce 8-10 amps when in good condition. Smaller values indicate increased internal resistance in the cell, which may indicate cell aging or poor quality to start with.

I'm not sure what values to expect from AAA cells as I have never tested them this way, but I'd guess somewhere in the 6-8 amp range.

It's not a good idea to do the test too often as it is hard on the cells. No device likes a dead short across its output. Never attempt a flash amp test on a large battery as the current will damage the meter.

 

[eebowler]

http://candlepowerforums.com/vb/showthread.php?t=183084

 

[Brooke]

Hi:

Another of my interests is in very early DC motors. One of the called the G.W. Moore Build it Yourself!, a real DC Motor, see:
http://www.prc68.com/I/MotorKit.shtml
is an educational kit based on patent
1857209 Toy Electric Motor, G.A. Moore, May 10, 1932, 310/1 ; 310/40MM; 310/46; 434/380

Since it's a kit I didn't want to build I just measured the wire and found it calculated at 0.07 Ohms and measured 0.084 Ohms. The thing is that the motor is specified to turn 6,000 RPM when run from a single No. 6 Dry Cell, i.e. from 1.5 Volts. That means the Dry Cell is supplying about 17 Amps!!!

It turnes out that in the late 1800s and through the mid 1900s "Pocket Amp meters" were comonly used to measure the short current capability of No. 6 Dry Cells both in the filed and in the store prior to purchase. After measuring some of these meters it's my belief the specification of 10 milli Ohms comes from the newest model Eveready Pocket Amp meter, see: http://www.prc68.com/I/No6.shtml#ERPAM

The 0.2 seconds may be the setteling time for the needle?

These meters came with one test lead, the other terminal was the case. The test lead was less than a foot long.

For some Measured Flash Amps see:
http://www.prc68.com/I/No6.shtml#MFA

I have a good design for a single "D" cell No. 6 battery adapter and am working on a parallel "F" cell adapter.

It's far from trivial to get the test circuit resistance low enough so that the current is being set mostly by the battery under test. No solid state components can switch 40 Amps at 1.5 VDC so automotive relays seem to be the best choice. Crimped connections also offer lower resistance and better strain relief than soldered connections, see:
http://www.prc68.com/I/PowerPole.shtml#Crmp

Have Fun,

Brooke

 

[Mr Happy]

Since it's a kit I didn't want to build I just measured the wire and found it calculated at 0.07 Ohms and measured 0.084 Ohms. The thing is that the motor is specified to turn 6,000 RPM when run from a single No. 6 Dry Cell, i.e. from 1.5 Volts. That means the Dry Cell is supplying about 17 Amps!!!

This is not true, actually. The battery will be supplying much less (very much less) than 17 amps.

The calculation of I = V/R only applies to resistive circuits. When an electric motor is operating it is an electro-magnetic circuit and different, more complex, calculations apply. For instance, due to the power in equals power out principle, the current drawn by the motor will increase when you put a mechanical load on the output shaft. When the motor is unloaded, the current will be quite low, depending on the efficiency of the motor. A "toy" motor is unlikely to be that efficient, but it will still restrict the current draw to a fairly low value.

 

[Brooke]

Hi:

Yes when the motor is running. But when it's stalled the full current flows and it's easy to stall a small motor.

The sound the motor makes also is much different when being driven from a high Flash Amp capable source or something that's weaker, like a lab bench DC power supply that can supply the average current but not the peak current. Most of my DC motor web pages have links to videos where you can hear then running.

I think the motor example is parallel with Switching Mode Power Supplies that use an inductor. That's to say a battery with strong Flash Amp capability will do a better job than a lesser battery.

Have Fun,

Brooke

 

[Mr Happy]

The sound the motor makes also is much different when being driven from a high Flash Amp capable source or something that's weaker, like a lab bench DC power supply that can supply the average current but not the peak current.

Ah now. I think that is a measure of the inefficiency of those small educational motors. That observation is educational in itself, in fact

As you observe, the motor does not have much power to show for all that current it is sucking out of the battery. In effect it is wasting a lot of energy and draining the battery faster.

An efficiently designed motor for real applications will not draw anything like the same kind of large current spikes, and will run just fine on a power supply that doesn't have a high flash amp capability.

Motor stall is a different question of course, and even efficient motors will draw large currents in that case. If left for too long the windings might overheat and burn out. Very large industrial motors will have a high current trip to protect them against damage if they are overloaded like that.