DIY battery testing problem

[navi]

I have only recently gotten into rechargeable batteries and am trying to educate myself about the technology involved.

I decided to try to measure the capacity of some NiMH D cells I have. The first cell is a 7dayshop branded 4000mAh rated D. I connected it fully charged in series to a 1Ω 5W power resistor. I expected, from Ohm's law, a current of 1.2A given a battery Voltage of 1.2V.

I then tried 0.33Ω and 0.22Ω resistors giving currents of 0.38A and 0.41A respectively.

I then tried a 8000mAh Tensai D cell that HKJ had tested as being capable of 8779mAh at 3A. With the 1Ω resistor again I measured a current of about 0.3A.

Obviously I am doing something wrong but I am not sure what. Why do I not get the results I was expecting from Ohm's law.

Thanks in advance.

 

[alpg88]

you missing time from your equation.(capacity maH) you supposed to let the cell discharge to 1v under that load than calculate capacity by multiplying current by time it took to discharge to 1v, for single cell.

but current in this set up will be changing depending on soc. the best way to find out true capacity is to run it thru hobby charger.

 

[navi]

I understand about time in relation to capacity. My question is more to do with why do I not observe the current predicted by Ohm's law?

Thanks.

 

[NeilP]

Hobby charger..like an iCharger106 or 310b..they can connect via US to the PC and you can plot a discharge graph.


But, what you are missing from your thoughts is also battery IR..the Internal resistance. or resistance of your connections as well as the time factor as already mentioned

Yes in theory a 1.2volt cell through a 1 Ω resistor should give 1.2 amps using ohms law. but there is voltage sag to consider and the continuous discharge rating of the cell.

An 18650 sized Nano Tech LiPo cell of 1.2Ah has a continuous discharge rating of 15C. that means 15 times the capacity, so you could draw 15 x 1.2 = 18 amps. That would flatten the battery in 1/15th of an hour so ..4 minutes.

But try and draw 18 amps out of a standard Lithium Ion 18650 cell..it anin't gonna happen. Not looked up the spec..but what does a standard Li Ion 18650 cell give? 2C maybe ?

D cells I don't know., they don't actually publish C ratings from what I can see.

Google is your friend, I won't do your work for you . Look up C rates for NiMh D cells and see how much current you can expect to draw.

 

[NeilP]

I got curous

http://www.7dayshop.com/products/7d...chargeable-batteries-4000mah-2-pack-7DAYDGTG2
are these the ones?

UMMMM


energiser graphs here


http://www.7dayshop.com/products/7d...chargeable-batteries-4000mah-2-pack-7DAYDGTG2


2500mah cells.


I'd not give those 7 day shop cells much credit. i Know I have their AA cells . not the best..

Get your self an iCharger ( and a power supply to run it) and try discharges at various currents

 

[navi]

I already posted a reply to alpg88. I'm not sure how long it takes to get moderated as I'm new here.

NeilP, those are the 7dayshop ones. I bought them to power a baby swing but have been disappointed with their performance.

However the Tensai 8000mAh that HKJ tested here:

http://lygte-info.dk/review/batteries2012/Tensai D TR-C8000 8000mAh (White) UK.html

also did not give the expected current. Would it be because the Tensai cell is brand new?

I most likely will get some form of battery analyser, as I said I am new to this and am just trying to understand all the concepts. Thanks.

 

[wrf]

[..]I connected it fully charged in series to a 1Ω 5W power resistor. I expected, from Ohm's law, a current of 1.2A given a battery Voltage of 1.2V.

I then tried 0.33Ω and 0.22Ω resistors giving currents of 0.38A and 0.41A respectively.

I then tried a 8000mAh Tensai D cell that HKJ had tested as being capable of 8779mAh at 3A. With the 1Ω resistor again I measured a current of about 0.3A.
[..]

If your resistors are not getting warm when you perform this test, then you may have unexpected resistance in your circuit (including your meter), or your batteries are not performing as expected.

Suggest you use your meter to accurately measure the resistance of your 1 ohm resistor. When it is connected to the battery, measure the voltage across the resistor. That takes the resistance of your meter out of the circuit as opposed to directly measuring amperage. If you don't see close to 1 volt across the resistor, troubleshoot the rest of your circuit (or the battery).

 

[navi]

You were correct wrf. It was unexpected resistance. I was using some cheap ebay leads with alligator clips. When I removed thrm from the circuit the current rose for all resistors. With the 1, 0.33 and 0.22 Ohm resistors and the Tensai D cell I measure 0.92, 2.6 and 3.5A respectively. I think I can further improve the connections so I'll work on that. Thanks for all the help.