Can you use cordless tool batteries to run a router?

Discussion in 'Modding Forum' started by Hogwild, Sep 23, 2011.

  1. Hogwild

    Hogwild LI Guru Member

    Hi everyone:

    I have a spare lithium ion battery here from a B+D cordless drill. It says "26wh" on the battery. I assume this means 26 watt hours? Does that mean it can (theoretically) deliver 1 watt for 26 hours?

    Is it safe to use cordless tool batteries to power a router? I have a Buffalo WHR-HP-G54 that I'd love to power by battery. If so, can anyone send me to a link or some reference? I couldn't find anything through Google. I can solder, but otherwise have minimal electronics knowledge. Unlike many, however, I can follow instructions.


  2. HennieM

    HennieM Network Guru Member

    If the battery delivers around 12V, it can certainly run your router. A battery delivers the most stable direct current (DC) available, so battery power is better than the DC you get from a wall adapter. There are a few things to keep in mind when you run electronics, in particular a computer such as your WHR, off a battery though, which include:

    1) If your battery goes flat, and the delivered voltage drops to a certain level (usually around 7.5V in 12V devices which run at 5V internally), you can fry some things in the router. You therefore have to ensure your battery, or rather, the voltage that the router gets, always delivers somewhere between 8 and about 18V (assuming a 12V/5V device).

    2) 26Wh does mean 1W for 26 hours. It also means 26W for 1 hour or 13W for 2 hours. I have no idea how much power a WHR draws. If it's similar to a WRT54GL, it would be around 3 to 5W. However, as Li batteries get older, the Wh drops significantly, so you might get much less from your battery.
  3. Hogwild

    Hogwild LI Guru Member


    Thanks for that info.Very easy to understand.

    How would I go about reducing the battery voltage down to something the
    router can use. Or is there some way of knowing if the router has a voltage regulator in it?

    Can you point me to a relevant resource? I looked through some theory, but couldn't find
    what was relevant, and I couldn't find any applied, hands-on HOWTOs or anything similar.

  4. HennieM

    HennieM Network Guru Member

    As mentioned, I don't know the Buffalo routers, so I don't know what voltage it needs, nor if it has a voltage regulator built-in. However, in the above post I assumed the WHR is similar to the WRT54s, in that it has a 5V regulator built-in. This 5V regulator can normally handle voltages between 7 and 18V or so, but you could look up the specs of the particular VR in your WHR (if it has a VR).

    You further need to find info on your battery's voltage, and the router's required voltage and current (Amps) drawn. I don't have a reference for you. You might find references on this site, such as, and other - search for "hardware mods" and the likes - such posts might have voltage, current, VR, etc. notes.
  5. Hogwild

    Hogwild LI Guru Member


    Thanks, I guess I didn't read carefully enough. Your last post certainly made it clear indeed.
    My Black and Decker drill battery is rated at 18V (26wh). When I measured it with a DMM, it was 18.1

    The Buffalo AC adapter is rated at: 5vdc 2.5A

    The WHr-HP-G54 supposedly contains some sort of internal voltage regulator:
    "If your power supply states an output voltage higher than 3.3 volt (WHR-HP-G54, +5v, others usually 5 or 12 volts) then the PCB contains a power regulator to lower the input voltage to 3.3 volt DC."

    ...and according to same page in the section about building a POE inejctor for the router:
    "Also easily powered with a simple 7805 (+5VDC) voltage regulator ($1.59 at Radio Shack) and your standard homemade PoE injector feed and tap (using pins 4+5 and 7+8, the blue and brown pairs). Heatsinking of the regulator is suggested due to ~700mA @ 5V max current draw. "

    Another web source tells of someone who used a TISR78105 (5v version) switching voltage regulator, so that would validate the first posting(s).

    On the latter webpage, he also lists using two capacitors in the circuit, but doesn't elaborate. My experience is limited to soldering and basic testing with a DMM, so I wouldn't know what to do next.

    Can you help me from here? If not, is there a web resource where Icould look next for help?


  6. Hogwild

    Hogwild LI Guru Member

    Okay, so I managed to pick up a couple of voltage regulators at a local surplus electronics store,
    as well as some heastsinks, heatsink compound and mica pads to dissipate excess heat.

    I got a 5v voltage regulator for the Buffalo, and a 12V regulator for my newer Asus RT-N12 router. I want to power it
    via battery too.

    I know I need some capacitors. If anyone can help, I'd be happy to try to draw up a circuit diagram
    or do any other simple task that might help someone else to have a battery-powered router.

  7. HennieM

    HennieM Network Guru Member

    Hog, you don't really need capacitors if you power off a battery. As mentioned above, the smoothest power available comes from a battery, and capacitors are used to smooth the power. It further seems that the WHR-HP-G54 runs on 3.3V internally, and it has such a 3.3V Voltage Regulator built-in to bring the normal 5V supply down to 3.3V. You can therefore supply, in my estimate, your WHR-HP-G54 with any voltage between 4.5V and 8V, as most 3.3V VRs can handle that. Some more info for you here

    A VR can only bring voltage down, and a VR typically needs a supply of least 1.5 - 2V above its output voltage; i.e. to get 5V out of a 7805 VR, you need to supply the 7805 with at least 6.5 to 7V. A VR also has a maximum voltage it can handle. So, if you have an 18-24V battery, you should probably go witha 7812 (12V VR) in between:

    18V-BATT === 12V-VR === 5V-VR === WHR

    or, with a battery that supplies between 12V and 17V, you can leave the 12V VR out:

    12V-BATT === 5v-VR === WHR

    If you have a SOMETHING that charges the battery at the same time that the battery is connected to the router, then you'll probably need a rectifying circuit and capacitors on the SOMETHING side:

    SOMETHING ≈≈≈ Rectifier+Capacitors === 12V-BATT === 5v-VR === WHR

    If the something is a wall charger that supplies 12V DC or so, you would not need any rectification or caps as the wall supply already has that built-in.

    If you have SOMETHING that continuously charges the battery, you might need something more complicated than just a rectifier and some caps, as you need to trickle charge the battery when the battery is full, and charge more aggressively when the battery is flattish, or the battery's life will be shortened significantly.

    As mentioned above, the critical point for the WHR would be when your battery becomes partly flat, and the supplied voltage to the WHR, or more specifically, to the 3.3V VR inside the WHR, approaches 4.5V or lower - it is then when the current drawn can get very high (relatively speaking) and the VR and other parts can fry out.
  8. Hogwild

    Hogwild LI Guru Member


    Wow, thanks. That is a whole lot of info you gave me. And very easy to understand.

    I think for now, I'll stick with powering the Asus RT-N12, and worry about the Buffalo
    another time.

    Let's see, the RT-N12 takes 12v@max 1A. So at full load, that would be 12 watts.
    The cordless drill battery is rated at 26wh.
    That would only give me about 2 hours at maximum draw on that cordless drill battery, right?
    Of course, I'm mostly going to be using this RT-N12 as a wireless client or client-bridge, feeding it to one wired client, so I'd assume I won't get near max load, right?

    I don't think I will want to be charging the battery. I want to keep this simple-I'm challenged enough when it comes to this stuff. :confused:

    Let me check..that voltage regulator I have is a National Semiconductor LM340T12.
    If I'm reading the spec sheet correctly, under "Absolute Maximum Ratings", they list the LM340 series DC Input voltage at 35V. So this VR should do the trick, yes?

    For other people who might want to build their own voltage regulators, apparently, you can,
    with extra components, could build a variable output VR circuit. This would allow you to
    use the same voltage regulator, regardless of the router (more or less), right?

  9. HennieM

    HennieM Network Guru Member

    Yes, it seems the LM340 can handle up to 35V input, so that is of no concern with your 18V battery. However, the output max current is 1A (1000mA). If your router indeed draws close to 1A, you'll be running close to the max of the VR, and would generate a lot of heat in it - a heatsink is recommended if so. What you might find though, is that the RT-N12 draws much less current. On WRT54GLs I found the normal curent to be around 200 - 300mA, but best to test your particular router with all the connections it would have if in full use. Remember that you must measure the current to the router and the voltage being applied, and then recalculate the OUTPUT current you would get when the output voltage is exactly 12V (P = Iin x Vin. P remains constant, so calc: Iout = P/12.)

    In terms of building a variable output VR circuit: That is easily done by placing a variable resister between the neg./GND leg of the VR and GND. This will allow you to have an output of 5V to perhaps 12V from a 5V reglator. However, the max ratings (V, A) still apply to the VR.
    Alternatively, you can build a circuit that does not even use a VR, but manipulates the output voltage via traditional components (capacitors, resisters, etc.). However, we are lazy, we want cheap, and we are pressed for space nowadays, so traditional circuits are not so much in use any more - it's all ICs lately, so VRs are commonly used.
  10. Hogwild

    Hogwild LI Guru Member


    Thanks again. Sorry I didn't respond back til now. Duty called.

    So, I did some reading, (oh, who am I kidding-I watched some Youtube videos) about the subject.
    In them, I learned that the higher the input voltage, the more power you lose to heat output from the voltage regulator.

    Would I be better off using a lower voltage battery or will this be okay?

    Also, you say I don't really need capacitors, as my input source is a battery. But what about when the battery
    starts to deplete? Does that affect the need for capacitors. Again, newbie questions, but

    a) I'm a newbie and
    b) I want other newbies to be able to read this and understand it easily.

    Thanks again,

  11. HennieM

    HennieM Network Guru Member

    It might very well be true that you generate more heat the higher the voltage of the source. However, the most heat in the VR and subsequent parts, is generated by output (from the VR) current.

    Capacitors can not make up the lost power when the battery starts going flat. Caps store power temporarily, and then discharges it as the load asks for it, but this "temporary", in a power supply, is usually fractions of a second or perhaps seconds at most. Caps in a power circuit is just there to smooth power.
    You might add some small caps after the VR, as the output from a VR usually have a little bit of a kHz ripple in it, but we're talking 0.01uF caps or so. For powering a router, which has it's own VR and other smoothing components built-in, the caps are unnecessary.
  12. Hogwild

    Hogwild LI Guru Member

    Okay, then. Thanks very much Henni. I'll try to hook up the circuit when I can
    and get back to you all about whether it's working or not.

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