Batteries for speaker system

[jsd]

I have an small amplified speaker system that I am making into a portable system. The adapter that came with the speakers is 12V 2A but I figured out that unless the Vin drops below 6V, the system works fine; the system automatically turns off at 6V. I don't notice any difference in sound quality at lower Vin. At 9V the current fluctuated between 300mA and 500mA according to the bench DC power supply.

That said, i bought an 8xAA battery holder and attached a barrel plug to it. This plug came with 18AWG wires already molded on which I cut to 3ft. I also put a rocker switch on the positive lead in case I want to disable the battery pack for whatever reason.

The batteries I'm using are 8xAA Eneloops, but I'm not sure of their age. My Maha charger indicated between 1800 and 1870 for all 8 using the refresh analyze function and default 1000mA charge, 500mA discharge settings.

According to my digital multimeter, the current is fluctuates between 420-450mA using my battery setup. I ran the system continuously for 4.5 hours at a party a couple days ago and haven't touched it since then. Today, I've had the speakers running for about 30 minutes and under load the battery reads 10.0V and open voltage is 10.1V. I'm thinking this isn't right because the batteries should be depleted by now according to my calculations. How is this possible? My multimeter is a Radio Shack model from the mid-1990s if that matters.

Is it 9.6V under load or open voltage that I should consider the batteries fully depleted? I'm trying to figure out my run time right now -- I know that I should get at least 4 hours but not sure what is the maximum run time because I do not want to damage the batteries by over discharging them. Is this a legitimate concern or is it after some point the batteries the voltage under load would drop too much to power my speakers -- keep in mind automatic shut down doesn't happen until 6V which is 0.75V per cell.

I would like to add something to turn off the battery power below a certain voltage, but I feel like this would be overly complex for this application.

 

[Knight_Light]

You have to measure nimh batteries underload to get any kind of capacity readings. You can't just put a voltmeter to it. Having said that you can discharge Eneloops down to 1 volt without any issues (you can even go to .9V). When I calculate theoretical safe runtimes for my gear on eneloops I measure how much current the gear uses and divide that into 1700 (this should leave a partial charge in the battery when done). If you want you can get a fairly accurate battery tester for around $80 (ZTS Multi Battery Tester - ZTS MBT-1)

 

[Lynx_Arc]

I think you need to instead of measuring voltage where the system cuts off you need to measure current where you can tell it is starting to cut out because when nimh are nearly depleted I think they soon have trouble maintaining current output. It is possible the system also starts to fail at 300ma or so regardless of voltage. It is also possible the system starts to shut off when a certain power level is gone below or voltage times current.

 

[jsd]

The 10V measurement is under load and 10.1V is open voltage so that is 1.25V per cell under load and 1.26 open.

As far as power level, when I had the speakers on the bench DC power supply, I was expecting current to increase as I decreased the voltage, but I saw no such change which was not expected.

After my first post above, I ran the speakers another 2 hours, which means I've run the speakers for 7 hours on battery so far and still have 10V under load which doesn't make sense to me if the current is 420-450mA according to my DMM because the batteries are only charged to 1800mAh according to my Maha charger.

This is already longer than I had ever intended to be able to run the speakers. My intent was to have 4 hours minimum, and 6 hours was better; anything beyond that not really necessary.

Although now I do wish I have more than one DMM so I can measure voltage and current at the same time.

 

[jsd]

Ok, the speaker system fooled me. It turns out there is a rechargeable battery inside, it appears to be 4xAAA of some sort wired together. Once I disconnected this battery, the current went down to about 100mA, occasionally spiking to 450mA. After I plugged it in, the current went back to the 420mA figure I had before.

So it seems that the system tries to charge this battery for a period of time and then stops after some amount of time. Also one of the LEDs turns off with the internal battery disconnected. Also the LED went from blinking to solid -- I guess blinking is charging. I did not previously notice this behavior. I think this internal battery is dead because it won't run off the internal battery, but if I use 3 Eneloops with some jumper leads, it does run, but at significantly reduced volume level. The speaker output with 3.6V supplied to the connector for internal battery is very low so I won't replace that.

 

[Lynx_Arc]

It now makes sense that is will run down to 6v as that is the voltage of the internal battery. Without defeating the internal battery or bypassing it completely there is no sense in making any measurements using a secondary power source. Depending on how powerful the speaker setup is and the type of music played through it even with 4AAA you could see well over 4 hours of runtime. I have a 9vdc input on my old stereo boom box and it will run off 6v at 150ma at low levels and I once used a 4.5Ahr lead acid battery at night for 4 days in a power outage to power it.

 

[jsd]

The internal batteries are NiMH or NiCad. They're shrink wrapped have just have a 3-wire lead coming out. I have no idea what's inside.

From what I can tell, the speaker system has two operating modes. One is low power using internal battery that works with 3 or 4 rechargeables, 3.6 to 4.8V. This is adequate for personal use in a quiet area or small room, but insufficient for a small outdoor event. The other mode is external power which works from 6V up to 12V and is significantly louder which is why I sought a portable external power option. At 10V, it seems to be working fine though; I couldn't find any 10AA holders which I why I went with 8AA. My guess is that the amplifier needs the higher voltage to produce the higher sound levels but the internal battery cannot provide it. I don't want to go SLA at this time because I have plenty of Eneloops around and SLA are heavier than I want to carry on my back.

As far as music, it's mostly going to be rock and maybe some electronic music, maybe some metal like Godsmack but not stuff like death metal.

 

[Lynx_Arc]

A 3 wire lead may mean it has a thermistor to detect overheating. You can buy a 6 cell and 4 cell battery holders and wire them in series to get 10 cells but be sure your device can take close to 15v input as 10 nimh hot off the charger will be over 14v till they rest awhile and drop down to the 13v+ range.