building battery pack 12 volt 500 mA

I am trying to build a battery power supply for my old wireless (Shure) microphone reciever which takes 12 volts 15mA. I want to use either rechargable batteries and or easy to obtain standard sized batteries if possible. I also want to have the unit reasonable small so I can take it around with me easily.

Thank you for any help.

 

[William Sommerwerck]

Parts suppliers (such as MCM) often carry battery holders. You should have no
trouble finding one that holds eight AA cells.

 

belliveaumagic@gmail.com wrote:

I am trying to build a battery power supply for my old wireless
(Shure) microphone reciever which takes 12 volts 15mA.

Are you sure (no pun intended) of that rating? Your title mentions
500 mA. 15 mA seems kind of low to me, but I guess it's within the
realm of possibility.

I want to use either rechargable batteries and or easy to obtain
standard sized batteries if possible.

The quick version:

Get a battery holder for 8 AA cells, put alkaline cells in it, and
enjoy. These cells are 1.5 V nominal; 1.5 x 8 = 12 V. This battery
will last something north of 150 hours at 15 mA.

Or, get a battery holder for 10 AA cells, put NiMH cells in it, and
enjoy. These cells are 1.2 V nominal, which is why you need two more:
1.2 x 10 = 12 V. Depending on the cells you use, this battery will
last something around 120 hours at 15 mA.

If you don't use the microphone frequently, get Sanyo Eneloop (or equal)
low-self-discharge NiMH cells, so they don't lose as much charge when
sitting. To recharge, remove the cells from your battery holder and use
a standard NiMH charger. You might want to buy a charger that can do 4,
6, or 8 cells at once, so you don't have to wait so long to charge all
10 cells.

For either option, if you can't find an 8 or 10 AA holder, it is
perfectly fine to wire smaller holders in series to get what you want.
This may also help you fit the cells into a compact package.

The long version:

If you want to hedge your bets, get some AA holders and a single-pole
double-throw (SPDT) switch, and wire it like this:

- ---[8 cells]---*---[2 cells]---+
| |
To +-----o o-----+
receiver \
o SPDT switch
|
+ -----------------------+

If you have alkaline cells, put 8 of them in the holder labeled "8
cells" above and flip the switch to the left. If you have NiMH cells,
put ten of them in all the holders and flip the switch to the right.
This lets you use whichever type of cell is handy.

If you really want small size, see if the receiver will operate well on
an input of 6 to 9 V DC. If you don't like what this does to the sound
quality or range, then this option is out, but if it works OK, consider
using a 9 V alkaline or lithium battery. If the receiver really does
use only 15 mA, this will be good for roughly 30 (alkaline) to 70
(lithium) hours of use.

If you want really long runtime, and can tolerate a slightly larger
package, you might consider using a 12 V sealed lead-acid "gel cell"
battery. These are available in various sizes, from 1.2 amp-hours on
up. The 1.2 Ah size (about 4"x2"x2" and 1.3 lb) would run a 15 mA load
for about 80 hours. The 5 Ah size (about 4"x3"x4", 3 lb) would run for
about 330 hours. These batteries have flat tab terminals on top that
mate with 0.187" push-on female connectors from the hardware store. You
can get a "wall wart" charger to recharge this kind of battery;
disconnect the battery from the receiver before recharging.

Your local Radio Shack will have some parts for this. You might also
consider using an online parts supplier such as Digi-Key ( digikey.com )
or Mouser ( mouser.com ); you'll have to pay for shipping but the
selection is much wider and the prices are better.

Most of the battery holders you will find are black plastic. Mouser and
Digi-Key sell some aluminum ones from Keystone Electronics that are more
expensive but may last longer if this device gets beat around a lot.
You might also consider placing all of the battery holders into a small
plastic box; that way if you do drop it, the cells stay inside the box
rather than scattering.

Standard disclaimers apply: I don't get money or other consideration
from any companies mentioned.

Matt Roberds