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Battery charging

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Jack.Straw

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A battery with a higher voltage does not increase the battery operating time.
The increased heat from a higher voltage is simply wasted. With fewer battery cells the amplifier will sound the same and the battery will last as long but the entire battery is smaller and lighter.
A 9V Ni-MH battery is too small to power a 1/2W amplifier for more than an hour. At continuous full power output a charge on the the little battery will last 15 minutes. Six AA Ni-MH cells make 9V and a charge will last for a week or two.
AG, i was shopping for the batteries and came back to re-read the thread because i could only find 1.2v AA's. The info above seems to be conflicting. Is a battery pack made from AA's going to provide longer play-time than a 9v? If so, should i do 8 of the 1.2v (9.6v) instead of 6?

Thanks again!
-Scott
 

audioguru

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AG, i was shopping for the batteries and came back to re-read the thread because i could only find 1.2v AA's. The info above seems to be conflicting. Is a battery pack made from AA's going to provide longer play-time than a 9v? If so, should i do 8 of the 1.2v (9.6v) instead of 6?
The cells inside a 9V battery are tiny AAAA ones that last for only a few minutes. AA cells are much larger so they last for hours.
If the supply voltage for an LM386 is higher than 9V then the extra voltage just makes waste heat instead of sound.
Use 6 cells or 8 cells.

6 Ni-MH cells are about 7.5V for most of a discharge but make 9V when they are fully charged and are still in the charger. They make 6V when almost dead.
 

Jack.Straw

Member
Ahh, now i understand. Thanks man! I'm still not sure if i should do 6 or 8 cells, but i think i will try both and see how it effects performance.
 

audioguru

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6 cells make an average of 7.5V and the output of the LM386 will be only 0.31W at clipping into an 8 ohm speaker.

8 cells make an average of 10V and the output of the LM386 will be only 0.48W at clipping into an 8 ohm speaker.

The difference in power is so low that you won't notice. It will be as loud as a cheap clock radio.
 

Jack.Straw

Member
Awesome... so if there isn't any noticeable difference i can go with 6 cells and save $$ on batteries.

I do notice the sound difference as the 9v (not rechargeable) degrades. The volume level at which the sound starts to distort becomes lower. Are you sure that level won't be significantly lower, or happen sooner, using the 6 cell pack instead of the 8?

Thanks again AG.. i really appreciate your time!
 

audioguru

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Double or half the power is only 3dB which is barely noticeable. When a battery goes dead then the amplifier it powers cannot supply enough peak power any more.
 

millwood

Banned
Awesome... so if there isn't any noticeable difference i can go with 6 cells and save $$ on batteries.

I do notice the sound difference as the 9v (not rechargeable) degrades. The volume level at which the sound starts to distort becomes lower.
because akailin batteries have much higher internal resistance as they get depleted. so their ability to supply current dynamic gets worse and worse.

rechargeables have much lower internal resistance and are better in that regard.

Are you sure that level won't be significantly lower, or happen sooner, using the 6 cell pack instead of the 8?

Thanks again AG.. i really appreciate your time!
that's 9v vs. 12v. on a 8ohm load, and assuming no output voltage loss on the amp, that's 4.5 Vp vs. 6v Vp, or (4.5^2/8/2)=1.25w vs. (6*6/8/2)=2.25w rms.

assuming 1v voltage loss, that's 0.75w rms vs. 1.5w rms.

so on and so forth.
 

audioguru

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The LM386 is a low power amplifier so of course it has voltage loss.
With a 12V supply (nearly 10 Ni-MH rechargeable cells) its max output is only 6.5V p-p into 8 ohms at clipping which is only 0.53W, not 1.5W.
 

Jack.Straw

Member
Maxim sent me some samples of the MAX713 so i've been reading the documentation and planning the circuit. We sell Rayovac batteries through my work... so i'll be using their "Hybrid", 2100mAhr Ni-MH cells. I contacted Rayovac for their recommendation on charge rate, max charge current, and termination method. They recommended C1 as the fastest safe charge rate, a max current of 2.1 amps. However, my wall-wart is only 600mA. Does this mean i need to setup the IC up to charge at a C/4 rate?
 

audioguru

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If you try to take more than its rated current then the wall-wart will get hot and maybe blow an internal fuse, melt or catch on fire.
 

Jack.Straw

Member
I guess i'm confused at the charge rate notation. Does C/4 mean it that, for 2100mA batteries, it should pull 525mA and take about 4 hours? I see notations like C/2 and then also 2C. What does a 2C rate mean? That you're supplying the batteries with twice their rate mA?
 
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audioguru

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Battery charging is not perfect. Some power is lost by heating the battery. So a 2100mAh battery charges at C/4 which is 525mA for longer than 4 hours, maybe 5.6 hours.
 

Jack.Straw

Member
Would you mind checking my math/logic on the components before i order them? The MAX713 documentation is attached.

Battery pack: 6x AA Ni-MH 2100mAhr (7.2v)
Wall-wart: 12v DC 600mA

____________________________________________________

It says to calculate the value of R1 with the following formula:
R1 = (minimum wall-cube voltage - 5v) / 5mA

I'm not sure where to get the minimum voltage, so i was working with 10.5v as a minimum for the 12v wall-wart. Does that sound about right?
R1 = (10.5v - 5v) / 5mA = 1.1k ohm?


____________________________________________________

It says that the charge current is controlled by voltage sensing resistor 'rSense'. I figure I will set the fast-charge current to C/4 since my wall wart can only do 600mA. The calculation for rSense is given as:

rSense = 0.25v/(iFast)

where

iFast = (capacity of battery in mAh / charge time in hours)

so..

rSense = 0.25v / 525mA = .4761 ohm?


____________________________________________________


PGM pins:
PMG0 - Open
PGM1 - Open
(6 cells)

PGM2 - BATT-
PGM3 - BATT-
(264 minute timeout, voltage slope enabled, iFast/8 tickle charge rate.)

VLIMIT = REF (2v)

Am i close?

____________________________________________________


Thanks so much for your time,
-Jack
 

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