Rechargeable Mini amp?

[Solidhex]

  Couldn't find a thread specific to this. I'm going to build a LM386 based mini amp and was looking for a convenient way to use a rechargeable battery in it. Of course I could just use a regular rechargeable 9v and use an external battery charger but I think it would be WAY more convenient to keep the battery inside the amp at all times and use an external power supply to charge the battery via a regular barrel 9v jack. This thread was very useful http://www.diystompboxes.com/smfforum/index.php?topic=34343.0   Especially the link provided by Apehouse http://minidisc.org/headbanger.html  . It has a stereo LM386 based headphone amp schem that shows how to hook implement a battery that can be recharged by an external power supply. Here's the schematic: http://minidisc.org/schem.gif
  I have a question. Is the 12v charger just a garden variety 12v DC power supply or does it have to be some sort of specific battery charger version? All the chargers I see seem to be hardwired to charger kits.

--Brad

[Top Top]

I have thought about this using a battery for a moped/scooter. I have this extra one that I got from a friend who salvaged it from a building he was working on.

My dad uses a similar battery that they throw out at his work periodically (backup battery to some industrial machinery computers) when he goes hunting to charge his cellphone throughout the week and he told me he charges it before he leaves from a regular 12v wallwart. I had considered using something like that in a portable amp.

[amptramp]

Check the actual voltage on the rechargeable 9 volt batteries.  Some are 7.2 volt (six cells), some are 8.4 volt (seven cells), but I have yet to see a NI-CD or Ni-MH battery in a 9 volt format that actually puts out 9 volts.  You can get 12 volt lead-acid gel cells here in larger case sizes, but they would be heavy.

[Derringer]

I just ordered and received these and fresh from the charger, they measure at about 9.8 volts. I can't comment on how long they'll stay 'punchy' yet, but I'd say they're off to a good start.
http://www.all-battery.com/4piecesof9v250mahnimhhighcapacityrechargeablebattery.aspx

Also, another option if you have the space, get one of the 9.6V rechargeable battery packs + charger used for RC cars. My son has a RC car that he barely uses, so I've been using the packs for breadboard work and have even powered multiple pedals off of one when I'm testing things out.

[Brymus]

I have some of those 9V rechargables and they are AWESOME,
I had mine in use for at least two months before they went belowe 9V,they actually hold a charge longer than a fresh alkaline does.
Most rechargables wont even stay charged for more than a month even when not in use...

Edit  Mine are only on their second charge so still could have some problems in the future but so far I am really impressed with them.

[Solidhex]

Ah

  Thanks! Great info. Those RC car battery packs seem like a great idea. I was planning on using a 1790 enclosure with a speaker inside so I'd definitely have room for one of those. This sort of charger could easily be adapted to connect to a regular barrel dc jack.... http://www.batteryjunction.com/ch-un180.html
  Mini amps eat through batteries like crazy. Powering one via a wall wart defeats the purpose of the convenience and self contained aspect of them. Unscrewing the back and popping rechargeable constantly is a pain so that really sounds like the ticket...

--Brad

[PRR]

> Is the 12v charger just a garden variety 12v DC power supply

Yes.

Assuming 12V supply and 9V battery, 12V-9V= 3V difference, the 330 ohm resistor passes 3V/330= 9mA current into the system. When the switch is off, the entire 9mA flows into the battery.

This is a VERY crude and weak, but safe, charging plan. Worst-case, 12V through 330 into a 6V battery is 18mA * 6V = 1/10th Watt into the battery. If the battery does not take it as electro-chemical charge, it heats the battery. 1/10th Watt in any respectable battery is hardly any heat, certainly not enough to be dangerous. Hardly warm.

> I just ordered and received these
"9V 250mAh NiMH high capacity rechargeable Battery"

A 250mA battery charged at 9mA will take 27-30 hours to re-charge. That's probably fine for light use. Small amp or pedal a few hours per day. It would be very lame in other work: a cordless drill should re-charge over lunch-break, a cellphone should top-off while you shower. QUICK chargers are MUCH more sophisticated, dumping huge current with many safety systems.

[Solidhex]

Saw this at Radio Shack the other day....   http://www.radioshack.com/product/index.jsp?productId=3999878 only $20
You could cut the cables attached to the battery, attach an external barrel jack to it. Then wire a dc power plug like this: Http://www.smallbearelec.com/Detail.bok?no=136  to the remaining wire so it can be connected to the recharge station. The battery pack is a little big but I'd put up with it for the convenience of the whole recharge thing.