Rechargeable Batteries

[BeginnerMatt]

Hello everyone, this is my first post so be patient with me haha. So I want to build an amplifier to power a 8ohm 8 watt speaker. I have the design of the circuit all figured out except for power. The amplifier chip (LM386) will run on 200mA of current using power supplies from 6-12V. I was originally going to use a 9V non rechargeable battery but don't want to waste money by continuously buying 9V batteries. I'm also planning on making a few and giving them to my friends and would like to use rechargeable batteries, something cheap maybe NIMH or NICD? I don't mind buying the batteries individually and constructing a battery pack but the problem is how to charge it. I can't afford to buy my each of my friends a LI-PO charger. So I'm curious if there's an easier method to charge batteries. I would also like the batteries enclosed in the speaker enclosure to make it tidy. So the batteries will be unable to be taken out to charge. They will need to be connected to the enclosure for charging. Thanks in advance!!

[IanB]

This is not super easy. There is no convenient rechargeable battery configuration that will provide 9-12 V (or even 6 V).

To keep things really simple and reliable, I would suggest putting 8 NiMH cells in a battery holder to give you about 12 V. You would want to use the modern "pre-charged" variety as these are much more reliable. You can charge them if you can rig up a current limited supply of about 200 mA. Use a charging jack that disconnects the amplifier while the batteries are being charged. Charging the 8 cells in series at 200 mA will provide a smooth and safe charge without any great danger of overcharging. Just disconnect the charger after 12 hours or so and the batteries will be nicely charged.

You could in theory use lithium ion batteries, but you would need two cells in series to give a 7.2 V pack, and the charging solution for this would be much more complicated.

[BeginnerMatt]

Thank you! If I were to wire 7 NiMH batterys 1200mAh @ 1.2V, and were to wire them in series, I would have 8.4 volts and 1200 mAh correct? I could then buy this cheap charger to charge them.

http://www.all-battery.com/TenergySmartACChargerfor8.4V-9.6VBatterieswiStandardFemaleTamiya-01026.aspx

but would the current output by these batteries destroy the amplifier?

[IanB]

Most standard AA size NiMH batteries are about 2000 mAh, which would be fine with that charger. You could put 8 in series and that would give you 9.6 V nominal at 2000 mAh.

The amplifier will be quite happy as long as the supply voltage is within range.

[Zero999]

Thank you! If I were to wire 7 NiMH batterys 1200mAh @ 1.2V, and were to wire them in series, I would have 8.4 volts and 1200 mAh correct? I could then buy this cheap charger to charge them.

http://www.all-battery.com/TenergySmartACChargerfor8.4V-9.6VBatterieswiStandardFemaleTamiya-01026.aspx

That's fine but be aware the battery voltage could be as high as 1.6V per cell when hot off the charger. This shouldn't be a problem but it's something you need to be aware of in future.

but would the current output by these batteries destroy the amplifier?

No, why would it? The voltage is correct so it'll be fine. Lots of beginners ask this question and I've wondered why. Learning Ohm's law should provide reassurance this won't happen.

Having said that, you should beware that protection from over current is often necessary when the power supply has the capability to deliver large amounts of energy. For example, if you were designing an audio amplifier to run off a car battery, you would need to include a fuse to prevent a fire if something goes wrong and causes a short circuit.

[mariush]

My advice would be to forget about LM386 and try to use a more modern audio amplifier chip.

Why?  Well, it's an easy to use IC but it's after all a class AB amplifier which means it's kind of inefficient ( with 10v in and 0.2A you basicaly give it 2 watts of power so the output is probably going to be less than 1.2-1.5 watts in a 8w speaker, not very loud) and a lot of power will be wasted.

Also, there seems to be two "versions" of lm386, one that works at 4-12v and one that works at 5-18v - if you use the first one, the people you give this to may use 8 x 1.5v alkaline batteries (which have about 1.65v when fresh) and kill your device.

I assume you chose LM386 because it's available in DIP package (the one with long pins, easy to solder on board) and probably because there's a schematic available already on lots of sites.  If you want DIP package, I'd suggest looking at a chip like TPA3122d2n : http://uk.farnell.com/texas-instruments/tpa3122d2n/ic-amp-audio-pwr-15w-d-ster-20dip/dp/1755372
It's a class D stereo chip (but can also be configured as mono) and there's schematics inside the datasheet for both modes of operation.
It may look scary at first but if you read the datasheet, it explains there how to choose every component and what's important.
There's also a tutorial made by Afrotech on Youtube where it uses this exact chip : http://hackaday.com/2014/06/09/afrotechs-guide-to-class-d-amplifiers/#more-124134   (the video is in the page)

This IC needs at least 10v to run (and it would prefer more).  As for how to power it, I would suggest using a lipo battery from sites like Hobbyking : http://hobbyking.com/hobbyking/store/__86__85__Batteries_Accessories-Li_Poly_All_brands_.html 
They may seem more expensive compared to 8 rechargeable batteries but they hold more charge and you could also use a very basic lipo charging chip to provide charging capability to your project. There's not many in DIP packages, but there's a lot of that that can still be easily soldered onto a pcb and don't need lots of parts, for example something like this MCP73812 : http://uk.farnell.com/microchip/mcp73812t-420i-ot/ic-li-ion-li-poly-charger-sot23/dp/1627187
Basically, you plug a usb cable at the chip's input (it can handle up to 6v) and the chip charges the lipo battery at up to 450mA and stops when the battery is charged.

Of course, the battery would only give you about 4.2-4.7v  so you'd need something to boost this to above 10v.  There's plenty of switching regulators capable of working from 4v to boost to about 12v which decent efficiency. 

[BradC]

Nobody has mentioned small 12V SLA batteries. Cheap, ubiquitous, tolerant of abuse and easy to charge.

[BeginnerMatt]

My advice would be to forget about LM386 and try to use a more modern audio amplifier chip.

Why?  Well, it's an easy to use IC but it's after all a class AB amplifier which means it's kind of inefficient ( with 10v in and 0.2A you basicaly give it 2 watts of power so the output is probably going to be less than 1.2-1.5 watts in a 8w speaker, not very loud) and a lot of power will be wasted.

Also, there seems to be two "versions" of lm386, one that works at 4-12v and one that works at 5-18v - if you use the first one, the people you give this to may use 8 x 1.5v alkaline batteries (which have about 1.65v when fresh) and kill your device.

I assume you chose LM386 because it's available in DIP package (the one with long pins, easy to solder on board) and probably because there's a schematic available already on lots of sites.  If you want DIP package, I'd suggest looking at a chip like TPA3122d2n : http://uk.farnell.com/texas-instruments/tpa3122d2n/ic-amp-audio-pwr-15w-d-ster-20dip/dp/1755372
It's a class D stereo chip (but can also be configured as mono) and there's schematics inside the datasheet for both modes of operation.
It may look scary at first but if you read the datasheet, it explains there how to choose every component and what's important.
There's also a tutorial made by Afrotech on Youtube where it uses this exact chip : http://hackaday.com/2014/06/09/afrotechs-guide-to-class-d-amplifiers/#more-124134   (the video is in the page)

This IC needs at least 10v to run (and it would prefer more).  As for how to power it, I would suggest using a lipo battery from sites like Hobbyking : http://hobbyking.com/hobbyking/store/__86__85__Batteries_Accessories-Li_Poly_All_brands_.html 
They may seem more expensive compared to 8 rechargeable batteries but they hold more charge and you could also use a very basic lipo charging chip to provide charging capability to your project. There's not many in DIP packages, but there's a lot of that that can still be easily soldered onto a pcb and don't need lots of parts, for example something like this MCP73812 : http://uk.farnell.com/microchip/mcp73812t-420i-ot/ic-li-ion-li-poly-charger-sot23/dp/1627187
Basically, you plug a usb cable at the chip's input (it can handle up to 6v) and the chip charges the lipo battery at up to 450mA and stops when the battery is charged.

Of course, the battery would only give you about 4.2-4.7v  so you'd need something to boost this to above 10v.  There's plenty of switching regulators capable of working from 4v to boost to about 12v which decent efficiency. 

I already ordered the parts for the LM386 using IanB's advice. After I make this small circuit I will definitely try making an amp like the one you mentioned. Being able to charge with a USB cable would make it a hell of a lot easier to charge, good information thank you!

[BeginnerMatt]

In addition, as good as this sounds to have a micro controller charging a lipo battery it seems kinda dangerous for a beginner to be attempting. What would you suggest me do to better understand charging using this method?

[mariush]

The MCP73812 is not a microcontroller, it's a dedicated charging IC.  It constantly monitors the battery and stops charging it automatically when it's full. there's nothing to program.

[BeginnerMatt]

Is there any IC's that can control multiple cells? because it seems like ill be needing more power than the standard 3.7V of a single LI Ion/Poly cell to power the amp.