method for measure max battery load

[davidefromitaly]

i don't know if this is the right way to measure the max load that a battery can reach but i have try this with my multimeter:

set the multimeter (analog type) at 10amps scale

direct touch the - and + with the two cable

this is the results (all AA size):

ni-cd 750mah varta: 7,5amps
ni-mh 1600mah energizer: 5,5amps
alkaline philips powerlife: 3,5amps

i have try some cr123a but at 5amps scale they go out of scale (so much more than 5amps i think) and at 10amps scale they shut-off as there a internal switch in the battery

 

[evan9162]

All this is measuring is how much current that the battery can deliver in a near short circuit.

The current you measure in this situation depends on three things:

1) The internal resistance of the batteries. Lower internal resistance means a battery can deliver more current into a short. However, this factor becomes smaller depending on #2 and #3

2) The resistance of the current shunt (plus probes, wires, etc) in your DMM. If the resistance of the DMM is too high, then the effect of the internal resistance of the battery becomes lessened, and the short-circuit current that you measure starts to depend entirely on #3.

3) Battery open circuit voltage. Higher voltage into a specific resistive load = higher current.


Using Ohms law, the expected short circuit current for a battery is then Ishort=Vbat/(Rcell + Rdmm).

A NiMH AA cell should have around 20 milliohms (0.02 ohms) of internal resistance. Theoretical short circuit current is 60A. I've measured 13A on my DMM with an Energizer AA NiMH cell.
A NiCD AA cell should have around 14 milliohms (0.014 ohms) of internal resistance. Theoretical is about 85 amps.
A AA Alkaline cell should have around 120 milliohms of internal resistance (0.12 ohms). Theoretical is about 12 amps. I've measured a fresh Ray-O-Vac Maximum AA alkaline at 10.6A.
A 123A cell should have around 250 milliohms (0.25 ohms) of internal resistance. Theoretical is about 12 amps.



If you assume that the resistance of your meter is 0, then in reality, the AA NiCD should show the highest short circuit current, since it has the lowest internal resistance compared to cell voltage.

Some have used the momentary short circuit current of a 123A cell as an indication of cell life - the term "flash amps" is used to describe this number.

 

[davidefromitaly]

ok my method is wrong... /ubbthreads/images/graemlins/icon23.gif

how type of resistance can simulate a load of 6amps? because i need to know what kind of battery can drive a 6amps bulb.

thanks for the help evan9162

 

[evan9162]

Well,

If you're testing one cell at a time, it'll be quite difficult. Let's assume that at 6A, your cell will drop to 0.9V. So for I=6, V=0.9, then R=0.15 ohms. And in reality, that's pretty darn close to the resistance of the 5A scale of your DMM, since your DMM was showing 7.5A "shorting" the cell.

If you have multiple cells available, then you can put 3-4 in series. 4 in series (with the cell's voltage dropping due to a 6A load) would be about 3.6V. So if you got a 1/2 ohm, 30 Watt resistor, that would work to test out 4 cells in series at about 7A (6A would take some wierd in-between value).

One thing you could do would be to get a second DMM, then while measuring the current as you did before, measure the voltage across the cell with the second DMM, so you can see how much the voltage of the cells drop under that load. That'll also give you an indication of how the voltage drops under load.

 

[davidefromitaly]

i can use up to 20 batts in series... exist some method for distribute the amps load to each?

for example:
14,4v at 6amps from 12 batts is 6amps each
14,4v at 6amps from 20 batts is 3,6amps each

 

[evan9162]

If you put all the cells in series, then the same amount of current flows through each cell, so you can test a 20 cell pack in series (for 24V)

If you have two strings of 10 cells in series, and put those strings in parallel, then you'll have a 12V pack which half of the current flows through each string, so each cell will have 3A of current flowing through them.

So, for the 20 cell example, you'll have 24V. To test at 6A, you'd need a load that can dissipate over 100 watts!

However, if you test the series/parallel arrangement (12V, two strings), then you need a load that can dissipate about 70 watts. One thing to try would be a 12V, 75 watt MR16 bulb (you can pick up for about $5). That should simulate a 6A load very nicely.

 

[davidefromitaly]

i have try as you saw me: 2 multimeter togeter: one for amperes, one for volts:

the ni-cd 750mah under a load of 7,5Ah have 1V, wowww /ubbthreads/images/graemlins/buttrock.gif

the ni-mh 1600mah under a loaf of 5,5Ah have 0,7V... poooooor /ubbthreads/images/graemlins/thumbsdown.gif

now i have 2 possibilities:

1) 14-15 ni-cd in series but low autonomy

2) 20 ni-mh in series but without a soft-star the bulb probably blow...

what is the best configuration?

 

[evan9162]

You'll probably get the best performance from the NiCD batteries. NiCDs just perform better than NiMH at extremely high current levels.


The other thing this shows you is that the resistance the meter measuring current (leads and current shunt) is around 0.13 ohms (R = V / I). This is good to know, as you can factor that in when using it in other measurements, and you know how it will affect circuits you're testing.

 

[davidefromitaly]

ok, thanks to yours suggestion now i'm decided to use ni-cd batteries /ubbthreads/images/graemlins/thanks.gif

now how many batteries i can use for the max brightness whitout blow the bulb (the bulb is an alogen rated 4000hours at 12v)? how i can build a soft-start switch?