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Why Does Battery Drain so Quickly?

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Thazatoon

New Member
I have a portable LCD screen rated at 12VDC 8W. I measured the current with an amp meter and it came in at 400mA. I assumed I would be able to power the screen with 10 rechargeable AA 2400mAh batteries for about 5 hours.

Wrong! The batteries last no more than 40 minutes before the appliance switches off and the power supply is exhausted.

I then tried 12 AA batteries and measured the voltage at 16.4 volts. The appliance only lasted 50 minutes before switching off. Measuring the batteries afterwards I was still getting 14 volts but there still isn't enough power to switch the appliance back on.

Can anyone explain what is going on?
 

crutschow

Well-Known Member
Most Helpful Member
Did you measure the current with the batteries connected that you are using?
 

Sceadwian

Banned
Are you sure you're measuring your current draw right?
 

Thazatoon

New Member
the screen is rated at 12V. I don't know if it will operate at a lower voltage than that.

Also, I've tried a 12V racing pack made up of 10 X C Cells. That's rated at 4700mAH and so far has powered the screen with no interruption for 2 hours. Could it have something to do with the internal resistance of the AA cells? Or could it be a phoney capacity stated on the battery? They are Vapextech cells.
 
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colin55

Well-Known Member
One solution is to add a voltage regulator and supply it with a higher voltage. This way you will be able to run the batteries down to their last drop.
 

Thazatoon

New Member
Hayato, thanks, if I bought some non generic AA rechargeable batteries such as Uniross or Ever-ready, could I expect to see longer power times? Or should I try Li-Ion instead? I have 2 X 7.2V Li-Ion batteries ready for testing.

colin55, thanks, I hoped to avoid the voltage regulator route as I wanted to avoid any extraneous electronics.
 
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colin55

Well-Known Member
Drawing anything more than the "10-hour rate" (or 14-hour rate) is going to severly limit the energy you are going to get from the cells. By adding extra cells and a switch-mode regulator is going to reduce the current and increase the delivery time.
 
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Hayato

Member
Usually Li-Ion batteries charges last less then NiMH ones.

You may experience longer times with Eveready, Uniross, Energyzer... But I don't think that will be much.

The batteries you've tested were new ones? Or used ones?
 

Hayato

Member
Colin, I think that adding a switch mode regulator to a battery bank won't help at all. In the fact, the batteries will deplete faster.
 

Sceadwian

Banned
One obvious question to get out of the way.
Are these all brand new never before used batteries?
 

Sceadwian

Banned
colin55, Nimh's are rated to give 1C output without significantly effecting their rated capacity. With the 14 hour rate thing you might be talking about Lead Acid batteries, but not NIMH's.
 
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Thazatoon

New Member
They are all brand new batteries never used. They were bought as a 12Volt pack already tagged and heat wrapped. Charged fully and then tested and timed. After discharge, I recharged them again and re-tested. Same discharge time each time.

The charger is fine as I am also using it to charge the 4700mAh C Cells and that's been powering the monitor for 3 hours now.

Googling and searching info on AA cells, someone has claimed that high capacity rechargeables have high internal resistance. Then you get contrary information that says AA Ni-Mh cells are good for high current applications.

Surely 400mAh isn't too much current for a brand new 2400mAh AA battery? But it's the 40 minutes discharge I find shocking.

Something must be wrong surely?
 
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Hayato

Member
Try to measure up the charge other way.

Try to load the batteries with 2 paralleled 39 ohms x 5W resistors. And then monitor the current and voltage.

One other thing, when the LCD turns of due the depletion, have you tried to measure the current?
 

Sceadwian

Banned
I don't know how accurate the NIMH model I have is but when I simulated 12 cells in series in LTSpice and the run time with a 35ohm load which is about ~400ma continuous the battery doesn't drop bellow 12 volts for almost 5 hours. The current has to be going somewhere. Or they're not being charged fully, if anything else is going on I can't explain it. Are you absolutly sure the charger you're using is actually fully charging them? How are you measuring their charge state? No-load voltage is just about useless for NIMH capacity checks.
 
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audioguru

Well-Known Member
Most Helpful Member
Simply look at the curves on the datasheet of the battery from a battery manufacturer.
Energizer's new AA NI-MH cell can supply 490mA for over 4.5 hours before its voltage drops below 1.2V. It can supply over 2A for almost 1 hour. It is rated at 2450mAh.
The "new" ones can hold a charge for months.
 

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colin55

Well-Known Member
When you measure the current with an ammeter in series with a lead it introduces resistance and also changes the impedance of the supply and the current taken can decrease considerably. The circuit may be taking more current than you think.
The second reason why the cells do not last longer than 40 minutes can be due to inferior cells. They may be "rubbish."
They may not be able to provide the high discharge current.
Try discharging the cells over a 10 hour period with a resistor. Measure the voltage each hour to work out the current and see if the cells have the capacity stated. This will decide if the cells have the rated capacity and if they are fully charged.
Another solution is to put two packs in parallel.
Finally you can buy a reputable brand and see if they perform any better.
You can also get battery packs from lap-top recyclers and put two in parallel.
There is more than one way of killing a cat than choking it with butter.
 
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