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I am doing a simple experiment to analyse the discharge characteristics of a Lithium Ion battery to see how long it takes to discharge.

I have a 12V 4000mAh battery. Once charged, using a voltmeter I find that it has about 12V across it. I then put it in a simple circuit with a 6 ohm resistor. I estimated that this is the resistance that would cause it to discharge in 2 hours. After 10 minutes the voltage has dropped to 5V. This seems wrong and I may be doing this incorrectly.

Can anyone suggest a better way to measure the discharge characteristics of this battery.

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    \$\begingroup\$ Discharging a Li-Ion battery like that can damage it, a 12 V battery probably has 3 sets of cells in series. You should not discharge Li-Ion cells below 3 V per cell so the discharging should stop at 9 V. Properly measuring the capacity of such cells requires special equipment. \$\endgroup\$ – Bimpelrekkie Feb 25 '16 at 11:33
  • \$\begingroup\$ I would also like to add that the voltage should probably be closer to 13V (maybe 4.2V per cell, 3 cells, makes 12.6V). How are you charging the battery? Maybe it isn't fully charged like you think it is? \$\endgroup\$ – Josh Jobin Feb 25 '16 at 11:38
  • \$\begingroup\$ Thanks. I had suspected exactly that. What sort of "special equipment" would be required. Would it be within the budget of an amateur ? \$\endgroup\$ – Dom Feb 25 '16 at 11:39
  • \$\begingroup\$ @joshJobin I don't think it is fully charged. Even after 48 hours the light on the charging plug is still red. I think the battery is damaged. I may buy another but I would like to avoid damaging the battery next time. \$\endgroup\$ – Dom Feb 25 '16 at 11:41
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    \$\begingroup\$ @FakeMoustache The seller lists it as "Li-Ion" and the pack itself states "super polymer lithium-ion battery" so it's understandable that OP got confused. \$\endgroup\$ – jms Feb 25 '16 at 12:29
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2A isn't a trivial draw rate!

It's very possible that your battery pack's internal resistance is many times that. If your battery pack was getting noticably hot, then you probably spent more power heating up your batts than heating up the external "test" resistor.

Try with a 24ohm resistor & a voltage meter. When the meter registers 9v, turn it off & call that "discharged" if it lasts longer than an hour, then you were definitely wasting most of your power against the batteries' internal resistances.

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    \$\begingroup\$ I voted down your answer because it is most unlikely that lithium battery would have even 1 Ohm of resistance, let alone more than 6 Ohms. Also, you compared draw rate directly to internal resistance, which is sloppy thinking or writing. Finally, if the battery really is 4Ah, as stated, 2A is a high but not unreasonable draw rate. \$\endgroup\$ – mkeith Feb 25 '16 at 21:10
  • \$\begingroup\$ @mkeith your opinion is your own, but you may want to check up on the various different lithium battery chemistries. considering the price of that battery pack and the mfgr specified 4800mAh rating, it's far more likely to be one of the higher-storage, but terrible discharge-rate chemistries with very high internal resistance. \$\endgroup\$ – Robherc KV5ROB Feb 25 '16 at 22:01

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