Connect with us

Battery connection JST

Discussion in 'Hobby Electronics' started by Jason S, May 21, 2005.

Scroll to continue with content
  1. Jason S

    Jason S Guest

    This might seem like a really dumb question, but can you connect two (2)
    9-volt batteries (rechargeable or non-rechargeable)....
    (a) in parallel (for longer operation of a device I suppose), or
    (b) in series (to get double the voltage)???
    I'm guessing the answer is yes, but I just need to be sure!

    I have a small plastic case with 12V circuitry inside it, and there isn't
    enough room for a 12v SLA battery. But enough room for two 9-volt batteries
    to be inserted. One 9-volt isn't enough power for the circuit, but thought
    if I can connect two of the 9-volt batteries in series, I might be able to
    double the voltage and get up to 17 or 18V .... which could then be
    rectified down to the required voltage.

    No smart-ass replies thanks...
    Thanks in advance.

  2. Ken Taylor

    Ken Taylor Guest

    Connecting in parallel without extra tricks is generally a no-no as they
    won't share current evenly and one will drag down the other. Connecting in
    series (assuming they are the same sort of battery, such as your 9V
    instance) is okay. You will want a regulator to do your 12V conversion for
    you - how much current does you circuit draw? I'm guessing a 78L12 would be
    the job if it's low-current, otherwise a 7812. Either IC is available at
    Dick Smith and they probably even have a data sheet for them (or use Google
    for more info).


  3. You could use a 9V and two AA or AAA size batteries in series to get 12 V.
    The 9V would discharge quicker tho. No messy regulators. Whats the
  4. Jason S

    Jason S Guest

    Thanks for your in-depth reply, Ken.
    Yes, they will be the same sort of battery. Current probably won't be any
    more than 0.5A, so I will probably use a 1A regulator.

  5. Jason S

    Jason S Guest

    Nice thought, but I don't like the idea of mixing different voltages,
    battery shapes, and current. I would rather connect 2x 9-volts in series
    and regulate the voltage I think. I'm not too concerned about the "messy"
    circuitry as you would describe it, as its only 4 or 5 components to make up
    a basic regulator, and I have the necessary gear to produce a nice little

  6. Pete

    Pete Guest

    Current probably won't be any more than 0.5A

    A 9v battery isn't going to provide anywhere near that amount of
    current, I'm afraid.

    A typical nicad 9v battery (which are usually 8.4v or sometimes even
    7.2v) is around 200 to 250mAh, and they only get that rating for loads
    of around 10% of that. You might get a bit better results from a NiMh
    battery, but it won't be a lot better.

    A typical alkaline 9v battery is good for around 600mAh, but again,
    they're only designed to provide about 25mA.

    A lithium 9v battery might be provide 1Ah or a little more, but they're
    only designed for *momentary* loads of up to maybe 20% of that, while
    for continuous loads, it's more like just 10-20mA.

    So, regardless of which chemistry you choose, with a 9v battery, you're
    only going to be able to supply perhaps 5% of the 500mA you're looking for.

    As for using a 7812 type regulator, the quiescent current alone could be
    as high as 10mA, which wastes half of the current the battery will let
    you have, all on its own.

    And dropping 18v down to 12v at 500mA is wasting an awful lot of power
    in heat.

    I don't suppose there's any chance you could fit nine or ten AA cells in
    the space you have available? Jaycar has 2300mAh NiMh AA for $3.75 each
    at the moment, and brand name AA alkaline would be maybe 2800-3000mAh,
    but pulling 500mA continuously from either of those would still be a
    stretch, for the same reasons stated above.

    Regards, Peter
  7. Jason S

    Jason S Guest

    Thanks Peter, ill keep all that in mind. But I'm sure I won't be using
    anywhere near 0.5A anyway.... it's only a 12V microwave sensor module and a
    wireless transmitter module. It was like worst-case scenario.
    Maybe you can tell me the average power consumption of the average
    single-phase microwave sensor/detector? Maybe then I can decide the battery
    usage. 9 or 10 AA's not very practical.... but if I have to, I have to.

  8. Pete

    Pete Guest

    Maybe you can tell me the average power consumption of the average
    I found a microwave motion detector module via google (at ), and it says that it
    draws 50-60mA. Whether that bears any resemblance to what you have, I
    don't know.

    Are you sure what you have is microwave? PIR units are much more
    common. A Crow 304Mhz wireless PIR ( ) draws about 15mA when idle,
    and about 50mA when operating.
    Nine AA batteries wouldn't take up all that much more volume than two 9V
    batteries, but obviously only you know what space you have available.

  9. Jason S

    Jason S Guest

    Peter, I bought a microwave detector today, and works great! Works very
    similar to a PIR, only doesn't require line-of-sight, and can be hidden
    easily... that's what I originally wanted. Yes, I found that it draws very
    little current, close to what you specified.... which is great!

    Peter, do you know much about RF remote control? I'm asking you because you
    seem to know a lot about electronics.
    Is there such thing as a small RF transmitter (including an encoder) thant
    sends a signal to a receiver (including a decoder!!!) up to 30m away or
    more, whenever power is applied to it for a short time? I know it sounds
    crazy, but there won't be anybody to press the button on the transmitter (to
    the circuit I want to hook it up to). The decoder is throwing me off also,
    because some RX/TX modules I've seen in the market require a "decoder".
    Sounds complicated.
  10. quietguy

    quietguy Guest

    You could do what you want with an older (ie cheap) used RC set as used for
    model planes - tho it depends on what you want the receiver to do when it gets
    the signal as to whether or not this would suit you

  11. Pete

    Pete Guest

    If you think about it, that's basically what a garage door opener does.
    Pressing the button on the keyfob turns the transmitter on, and the
    receiver pulls in a relay and opens the door.

    Take a look at places like Oatley Electronics in Sydney - - there might be something
    there that will do what you want. Perhaps their TX7 and RX7 units -
    even though they're four channel, you only need to use one channel.
    You'd need to be sure that you could turn the transmitter on with
    whatever you're driving it from, though.

    30 metres might be a bit of a stretch, depending on what's between the
    transmitter and receiver.

    Regards, Peter
  12. Ken Taylor

    Ken Taylor Guest

    If you can get hold of the National (or other manufacturer) switching
    drop-in replacement for the more common linear regulator you'll have a much
    more efficient end-result. Can't recall the numbers off-hand, but they're on
    the National site.

  13. Jason S

    Jason S Guest

    ok Ken, thanks for your input. Ill check it out.

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day