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I am new to this forum. While I have been experimenting with electronics for years, I guess I am still a novice. Please help me understand why my battery pack doesn't run a 12 V LED lamp very long.

I have 8 AA rechargeable, fully charged batteries connected in series. They say they can deliver 1.2 V and 2000 mAh. I know that voltage drops as the batteries are discharged, so decided I needed a boost converter module to deliver smooth 12 V output.

The input meter on the booster shows 9.89 V which is about what I would expect with no load. As soon as I turn on the bulb the voltage drops to 6.43 V. The output meter shows I am getting 12 V out and I am drawing 0.85 A. I believe that sounds about right because it is a 10 W, 12 V LED bulb.

I calculated that the light should burn for about 1.9 hours (I originally, incorrectly stated 19 hours). I know there is resistance that I don't how to measure, so I was ready to accept 20-25% reduction.

But the lamp burns for only about 5 minutes.

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  • \$\begingroup\$ What happens to the battery voltage during these 5 minutes? Does it drop further? Do the batteries get warm? And what brand of batteries are these? \$\endgroup\$
    – Jonathan S.
    Commented Jan 20, 2022 at 20:08
  • \$\begingroup\$ Datasheet for the boost converter, batteries, and LED bulb? Also, you are "drawing 0.85 Ah" -- shouldn't that be amps, not amp-hours? \$\endgroup\$
    – Null ♦
    Commented Jan 20, 2022 at 20:10
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    \$\begingroup\$ Measure the voltage across each cell with the lamp on, and throw away any cell producing < 1V under load. Also, 1.2V * 2Ah = 2.4Wh, *8 = 19.2Wh, so you can't expect to get 190Wh out of them. \$\endgroup\$
    – user16324
    Commented Jan 20, 2022 at 20:17
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    \$\begingroup\$ How did you end up with 19 hours, with a battery of 2 Ah and lamp needing almost 1A? \$\endgroup\$
    – Justme
    Commented Jan 20, 2022 at 20:19
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    \$\begingroup\$ Do your calculations again, 1 AA contains 1.2V * 2000mAh = 2.4Wh, 8 AA contains 2.4Wh * 8 = 19.2 Wh, You are using a 10W led light, which consumes 10Wh every hour, so in theory, they only last 2 hours, not 19 hours \$\endgroup\$
    – Ferrybig
    Commented Jan 20, 2022 at 20:33

3 Answers 3

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As soon as I turn on the bulb the voltage drops to 6.43 V

Something is terribly wrong. That is only 0.8 Volts per cell. Much too low. Try to measure the input current. Maybe the boost is not operating properly for some reason and is consuming vast amounts of input current (although it would be getting hot if that were the case).

Also, consider using 10 AA batteries in series and powering the lamp directly. You will probably have much better results.

Each cell has something like 2 Watt-hours of energy in it. A little more actually. So that means 8 cells have 16-20 Watt-hours. So your calculation of expected battery life appears to be a good ballpark number. I would guesstimate about 1.5 hours to allow for various normal expected losses.

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  • \$\begingroup\$ Thank you. Yes, I made a 10x error in my calculation. I will try your suggestion of using 10 in series without the booster. \$\endgroup\$
    – Kevin Reagan
    Commented Jan 20, 2022 at 23:47
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    \$\begingroup\$ Just saw your comment about the battery terminals getting hot. That is definitely a problem you should fix. Maybe that is the biggest problem. \$\endgroup\$
    – user57037
    Commented Jan 21, 2022 at 1:30
  • \$\begingroup\$ Thank you, I will get a better quality battery pack and see if that solves it. \$\endgroup\$
    – Kevin Reagan
    Commented Jan 21, 2022 at 2:54
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Your AA rechargeable cells must be very old, not fully charged or very cheap to have their voltage per cell drop to only 6.43V/8= 0.8V.

A Name-brand AA Ni-MH rechargeable cell can supply 1A for almost 1 hour when its 1.4V has dropped to 1.1V. A name-brand AA alkaline battery can supply 1A for half an hour when its 1.5V has dropped to 1.25V. A very cheap no-name-brand AA "Super" heavy duty battery from China has trouble producing more than 50mA.

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  • \$\begingroup\$ They were brand new fully charged. On my first test I used Amazon Basics Recharge NiMH, 1.2V, 2000mAh. On my second test I used brand new fully charged Energizer Recharge Plus, NiMh, 1.2V, 2300mAh. \$\endgroup\$
    – Kevin Reagan
    Commented Jan 20, 2022 at 23:44
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You measured .85A at 12V. With an input of around 10 volts, the current draw would be right around 1A. You say the cells are 2 Amp-Hour. With those numbers I'd say the most run time you would get is 2 hours.

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  • \$\begingroup\$ Yes, thank you. I made a 10x error in my calculation. \$\endgroup\$
    – Kevin Reagan
    Commented Jan 20, 2022 at 23:45

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