Why Does Battery Drain so Quickly?

[Thazatoon]

Thanks for all your responses.

The charger I'm using is a generic 7.2V - 12V charger with a 1A or 2A charging selector. I use it for charging 12V C cell racing packs and had no problem with it so far. After charging I just put voltmeter probes across the terminals, so I assume this isn't a good way of determining their worth.

I'm going to grab 10 brand new energizer cells today and charge them all with a plug in charger, then test them on the appliance. Will let you know the results later on tonight.

 

[ecerfoglio]

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?

If you are getting 14 volts from the batteries under load then it may be that the batteries have a high internal resistance and can't provide the device with enough voltage when it demands current spikes.

Try adding a big capacitor in parallel with the batteries (as close to the device as you can).

If you are getting 14 volts from the batteries unloadded , what is the voltage under load?

 

[Hayato]

If you are getting 14 volts from the batteries under load then it may be that the batteries have a high internal resistance and can't provide the device with enough voltage when it demands current spikes.

Try adding a big capacitor in parallel with the batteries (as close to the device as you can).

If you are getting 14 volts from the batteries unloadded , what is the voltage under load?

A BIG capacitor, would be just a "charge thief".

Thanks for all your responses.

The charger I'm using is a generic 7.2V - 12V charger with a 1A or 2A charging selector. I use it for charging 12V C cell racing packs and had no problem with it so far. After charging I just put voltmeter probes across the terminals, so I assume this isn't a good way of determining their worth.

I'm going to grab 10 brand new energizer cells today and charge them all with a plug in charger, then test them on the appliance. Will let you know the results later on tonight.

The recharger is very important. If you do not charge your batteries properly, then you'll have poor performance.

 

[Sceadwian]

Thazatoon, have you determined what the voltage dropout of your LCD is yet? That's going to be as critical at this point as the charging scheme is, because according to my simulations a 10 cell NiMH pack will drop bellow 12 volts after only 30 minutes and then slower taper off to 11 volts over the next 4 or 5 hours. If the dropout voltage is fairly close to 12 volts 10 cells are not going to cut it. And from what you've stated so far you can't charge more than 10 cells full. A fully charged 12 cell pack requires around 17 volts to fully charge.

 

[Hero999]

I think it's more to do with the minimum voltage that the display will work down to rather than the capacity of the batteries. If the display shuts down at just over 11V then it's no surprise that it doesn't last very long.

 

[Sceadwian]

Simulated mind you a 10 cell pack with a 400ma load is supposed to have 11 volts for at least 5 hours.

 

[unclejed613]

LCD displays have a lot of internal power supplies. you have the inverter for the backlight, the inverter for the LCD voltage, regulators for the cpu and screen controller. if the monitor has an audio amp, it will probably have a regulator for that too.... with a lot of regulators, inverters and such, the chances that just one of those devices has a high dropout voltage is very good. if the CPU detects a monitored voltage drop out, it will shut everything down. also if one of those batteries has a slightly high internal resistance, chances are that one of the inverters is going to run marginally (maybe even with a "jumpy" output), and the display cpu will trigger a shutdown for stuff like that as well...

 

[Sceadwian]

I'm gonna guess a 12 cell pack will work IF you can full charge it. The charger you listed so far can't change a 12 cell pack.