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Help Building a battery pack

  1. Dec 6, 2005 #1
    I built a 7.2 V battery pack using two rechargeable 3.6V dc batteries. They eventually died and I purchased some different new 3.6 V replacements. Unfortunately, when these are charged they pump out a total of 8+ volts and shut down my device.
    I'm an electronics dummy other than the basics. I thought I was going to be able to buy a +1.2 to 37VDC Adjustable-Voltage Regulator LM317T from the Shack and just turn the switch (or adjust it) on it to 7.2 Volts, but upon seeing one in the store, there is no switch/adjuster (that I can see by shaking it around in the container), just a block and 3 metal pins.

    Basically, I want something that I can just attach to the battery pack output wire that will ensure that 1/ no more than 7.2 V DC gets output, 2/ it allows voltage less than 7.2 Volts (it doesn't stop the circuit if the battery is getting low), 3/ Is safe - I can just attach it into the wire and no real electrics shock protection (it's not going to build up a huge charge or anything), "just cover with electrical tape" type of project.

    I suppose whatever is suggested may cause heat, need heat to be released. Let me know/advice if that might be an issue.

    It's just for a small video camera - so doesn't use much juice.

    Thanks for doing the forum. Hope I get some good help, and can hopefully use the forum for some other projects.
  2. jcsd
  3. Dec 6, 2005 #2
    have a look at the 317 data sheet you will see that you need to use two resistors, they make a voltage divider and form the referance voltage for the 317. the data sheet shows how to calculate the values. you could also use a 7805 with a few diodes to lift the ground pin, that would also work. one thing about these regulators though is that they need some headroom to work so if your only looking to drop a volt use a low dropout regulator. the cheap and easy thing i would do: put two (rated current) diodes in series. done :smile:

    edit: pick up a pack of 1n4001's at radio shack
    Last edited by a moderator: Dec 6, 2005
  4. Dec 6, 2005 #3


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    If you want to try the LM317T you had in mind, just use this http://cache.national.com/ds/LM/LM117.pdf [Broken] the T designates a TO220 style package which can handle current up to 1.5Amps

    I suggest using the variable potentiometer as they indicate. You can approximate the values of resistors to use, but using their formula. Once you are getting the output voltage you want (7.2v), and taking care that you use the proper polarity, try it out on your load (video camera) under normal operation. Continue to measure the output voltage of your regulator while your camera is running. If it has dropped, you can adjust the pot back to 7.2v You now have your regulator adjusted under load conditions.

    After shutting down everything, you may replace that pot with a fixed resistor. Just disconnect one end of the pot and measure its resistance. You can now build your little regulator circuit with 2 resistors, 2 caps and the LM317.. If the IC does warm up under the load test, I would consider adding a small heatsink (using thermally conductive compound) between the sink and the TO220 tab. It would be nice to mount your circuit in a protected container (example a plastic fuse case). Be creative, as long as you remember to leave enough space for radiational cooling, you will be fine. If a jack was used to plug into the original power pack, you could even mount a similar chassis mount mate for it, in your regulator box.
    Last edited by a moderator: May 2, 2017
  5. Dec 6, 2005 #4
    if i'm reading the data sheet right the 317 needs at least 1.25v of overhead so it won't work well when his battery is 8 volts or less, plus i douht he'll be carrying a DMM around every time he has to adjust the voltage. he could use a proggramable shunt regulator but it would waste a lot of power. i will stick by my idea, he should use two or three diodes to drop the voltage.

    lets see what other people have to say.
  6. Dec 6, 2005 #5


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    Yeah, a standard dropout regulator is a bad idea in this application. The input-to-output drop required is too much to use in battery applications usually. For battery applications, you need to either use a low-dropout regulator (LDO), or a DC-DC regulator (often buck-boost topology).

    Check out Maxim's website or some other big player in LDOs to look for an adjustable positive regulator that fits the bill. Yes, you will need to dissipate some of the extra voltage drop as heat across the regulator when the batteries are at full charge. Also check to see how well-behaved the LDO is going to be as the input voltage drops below the minimum input voltage to support the output voltage set by the resistor divider. Does the LDO just keep dropping a couple tenths of a volt and not present any extra impedance? That's what you would like for the rest of the battery droop cycle.

    BTW, mugsby's idea is not bad, and it's definitely simpler than the LDO. The DC-DC solution would give you extra battery life, but would take a lot more effort on your part to design and build it. Also, instead of 3 diodes, you could use a transistor and two resistors to set the constant voltage drop -- an exercise left for the reader.
    Last edited: Dec 6, 2005
  7. Dec 6, 2005 #6
    Thanks for the discussions.
    I had 2 Black and Decker VersaPak NiCD 3.6 volt batteries that worked fine. Replaced them with Black and Decker VersaPak NIMH's that when charged seem to charge to much more than 7.2 volts, more like 8 or even a little over for some reason. When they are used for a while the voltage drops.

    Also, could there be something wrong with the VersaPak NiMH's when they are charging to much higher than the 3.6V? - or are these OK tollerances in the electronics world?

    I'm not sure how your drop volt ideas work when the voltage is going to drop, and I don't have much overhead to run something.
    Also - I don't have the "Adjustable-Voltage Regulator LM317T" yet, that was just my Electronics dummy thought on what might be the solution.
    I just want to build something very simple/inexpensive/safe that will keep the voltage at 7.2V when the batteries are outputting > 7.2, and something that won't stop the batteries output when it's running low and is 7.2 or below.
    Here's an example diagram that this E dummy can understand (I couldn't write it left to right as the editor strips the spaces) and you could always copy and paste.
    If you have the Radio Shack codes for any items you suggest, i'd appreciate (I don't think I know of any other electronics stores here).

    item? ---> heatsink

    Last edited: Dec 6, 2005
  8. Dec 6, 2005 #7


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    I've also replaced NiCds with NiMH cells and if the device allows a wide enough window for input voltage, it works well. In my application, this was fine..

    Now NiMH do self-discharge to a nominal 1.2V in about 2 hrs
    See this reference.. The graph illustrates self discharge of NiMH cells and shows 4 replications of experimental data. They will stay at 1.2V after that for much of the rest of their charge cycle..

    In your case, each of your batteries has 3 of these 1.2v cells probably stacked in series to give you the nominal 3.6V .. So the data here is still valid. In your new pack the 8.0V should drop to 7.2V within 2 hrs without any camera attached.
    Last edited: Dec 7, 2005
  9. Dec 6, 2005 #8

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  10. Dec 7, 2005 #9


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    Using mugsby's idea, one diode would be sufficient to drop the voltage within range of 7.2V Remember a 1N4001 turns on between 0.65 to 0.7V. So a single diode would take the 8.0V down to 7.3-7.35V
    But two diodes would take it down to 6.6-6.7V. (too low, target is 7.2V).

    As the pack self-discharges within first couple of hours to a nominal 7.2V, the drop across even one diode becomes too low 6.5-6.55V. So I wouldn't recommend the diode approach.
    Last edited: Dec 7, 2005
  11. Dec 7, 2005 #10
    Optimizer, the new batteries don't happen to be rechargeable lithiums, do they? You CANNOT charge those without a circuit that limits the battery voltage, otherwise they WILL START A FIRE. Please pardon the caps if it irritates anyone, but this is a serious safety issue. Some of these surplus lithium ion batteries are sold without the safety circuitry in place. Considering the safety hazard, I won't even use them. You can't get away with things like they do with other types of batteries. I even have a battery charger that lights up green when the batteries are charged but keeps pumping the full current into those batteries.

    Lithiums are nominally 3.6 volts each but run about 4.10 volts when fully charged.

    I wouldn't use them at all. Instead get a set of heavy duty AA NiMH batteries. Look for the 2000 to 2200 mAH rating. They weigh about the same, they take up less space than the lithiums, they have a slightly higher energy capacity in that setup (I think your lithium battery is 1800 mAH), and they take the abuse a lot better. Discount stores sell them for less than $10 a four-pack, and they will have exactly the voltage that you are looking for. You need six.
  12. Dec 7, 2005 #11
    seeing as most diodes test at .6v (never encountered .7 in the real world) and that he's get 8+ (lets assume 8.2 for lithiums as metakron states) then you have 7v (two diodes) and thats as good as it's gonna get unless he uses a (complicated, expensive) dc-dc convertor.

    edit: i don't think he wants to build this.
    Last edited by a moderator: Dec 7, 2005
  13. Dec 7, 2005 #12
    Not Lion

    Not using Lithium, I'm using what I mentioned earlier - Black and Decker Versapak 3.6V NiMH's. The Lithium and it's Lithium replacement for the Video camera died very quickly. I studied batteries a little, and the more I read about Lithium the more I would not even have them in a house (reminder... I must get those old ones recycled) - they seem a little too risky.
    When I started this reducing voltage venture, I looked in my Shack Beginners Electronics on sale book (that I've had for years but has collected a lot of dust), and first noticed Diodes, but it just described a small selection of them, Zenier? diodes being the closest, but not the solution (as I think it stopped the power either over or under whatever amount you wanted)

    Being a little curious what exactly does this diode do (shack doesn't provide descriptions etc on it's website). Can you touch them in operation without a shock. Will they get hot. Could anything make them blow or melt.

    I'm thinking about the voltage drop issue.
    I had a thought last night that I could always make the diode pack removable or build a switch, so they'll run fresh recharged batteries, but when the juice appears to run out I can switch it to batteries only - fine for el' cheapo video productions.

    I'll also test the "let the batteries stand for a while" issue, and see if they drop down to 3.6V in a hurry (I may have always taken them straight from the charger?).
    Black and Decker Versapak Gold NIMH 3.6 volt batteries
    In fact, I'll edit this with the stats as the occur:
    3-25pm Batteries out of B&D charger - Around 4.2 volts
    5-30pm Batteries out of B&D charger - Around 4.08 volts

    (By B&D charger was for the old Silver NiCD batteries - could it be of no use for the Gold NiMH batteries? The instructions for the Gold batteries say they can be charged in any B&D versapak charger from what I remember)
    Last edited: Dec 7, 2005
  14. Dec 7, 2005 #13
    What you are talking about is just a little strange, Optimizer. I do wonder how the battery pack before could have worked and the new one doesn't. Maybe the Gold pack has a slight difference in its chemistry? Battery voltage varies with chemistry and they keep trying new things to try to make batteries that customers like better.

    A diode performs two basic useful functions. It's primary function is to rectify alternating current, to act as a switch that passes current only one way. The other is that it drops a fairly constant voltage in a series circuit. The second function wastes power in a rectifier, but it is often useful for various circuits.

    One diode, maybe two, can be used to drop the voltage below 8 volts. There isn't any simple way to put together an automatic switch that cuts out the diode from the scheme. The best thing you can do is put in a manual switch that you could mark as "reserve." Place it across the diode so that it shorts out the diode when voltages get low.
  15. Dec 7, 2005 #14
    silver bats

    The Silver batteries I used before had been used for quite a while for Black and Decker tool purposes before I ever came up with an idea for using them to power a video camera instead of those Lithium Ions that seem such a waste/expense/don't run/last long. The silver batteries by that time may no longer have been able to charge to 8 volts (but I didn't need to test them when they were new, so I never knew what they would charge to).
    Well, we're getting quite a snow, so no diodes today, Santa will have to haul them in his sleigh.
    Yes, manual switch was what I was thinking, as it's just amateur video, and I'm used to those Lithiums running out of juice a lot - My NiMH's will be luxury when I get them working.
    Yes - automatic switch would be nice if there is such a thing.
  16. Dec 8, 2005 #15
    I have to wonder what the camera was designed to use in the first place. Six Nicad or NiMH cells makes 7.2 volts nominal, more when fully charged. Five of them make a good five, oops six volts nominal. I didn't even know that any of that type of battery could charge to quite that high a voltage.
  17. Dec 8, 2005 #16


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    Video camera batteries don't last as long as you would think (until you get a megabuck prosumer model like a Sony PD150 with $300 batteries) and some of the NiMH batteries on ebay are quite inexpensive and work well (at least the two I've tried on my Canon camera).

    A diode is a one-way valve of sorts. When used 'forward' it will cause the voltage to drop .6V or so (different materials may drop less) and it will not flow until the voltage is above this level. In 'reverse' it may be 100V or more before it starts to flow and it will likely be damaged since its not designed to handle much current in reverse.

    Like any electrial part, if a diode is dropping voltage it is turning electricity into heat and if there is a lot of power then yes it can get warm or hot or burn out. In this case that is unlikely, but for future reference keep it in mind. No difference in shock compared to the voltage the source has, stay below 40V and you are pretty safe unless you short circuit a battery (that can heat it up and blow it up).

    A zener diode is different from a regular diode, you buy it in voltage levels because its 'reverse' setting is ideal for determining voltage by whether or not it is flowing since you can buy it in different levels.

    Lets say you have a LED (light emitting diode) that has a forward voltage of 2.2V where you can see it lit up.

    If you have a 5V zener diode wired in reverse that is in series with the LED wired in forward, the LED will light up when the voltage is 7.2V or higher (5V + 2.2V = minimum voltage they turn on). Once the voltage falls below the point where the diodes will conduct, no current will flow. This way you could visually see when the battery is above/below a certain voltage by whether or not a LED is lit up.

    (NOTE: the forward voltage on the zener is likely 1V or so, so if you wired it up incorrectly in forward then it would light up as soon as there was 3.2V available, and it would also likely burn out since there would be too much current.)

    You also want like a 100 ohm resistor in series too for protecting the LED from too much current.

    So it goes: Battery+ --> resistor --> LED --> zener --> Battery-

    With some experimenting, you could come up with LEDs that will light up at different voltages (too high or just enough) and have a very low cost voltmeter to take with you to see whether or not you should flip the switch to wire in the other diodes.

    And you would need to experiment to make sure you understand what zener diode to use with the LEDs you get, each will have a different voltage where they turn on so unless you have good datasheets you'll need to test.

    mouser.com is going to have a much larger selection with much lower prices than RS and with no order minimum it works pretty nice even after the small shipping charge.
  18. Dec 9, 2005 #17

    That sounds real good too. Thanks. Hadn't thought of the indicator idea. Sounds good.

    Here's the latest (as the "edit" option isn't avail now)
    After 35 hours - Around 3.9 volts - and it's stopped snowing.
    So the voltage is slowly dropping.
    A few months ago when I first noticed the problem I tried running a B&D power screwdriver device for a short while to try drain the batteries a bit, but they seemed to retain their high voltage. Probably didn't run it enough?

    No, I've no idea what cells/structure make up a single B&D Versapak Gold NiMH 3.6 volt battery. I just know I'm trying to use 2 of them, which would be the same as a RC car pack (which maybe I should have got - but I got such a good deal on a 6 count of the B&D Golds).

    I wonder what the least expensive chargers high quality chargers are for NiMH's etc that monitor/look after all the aspects of charging properly. (might be worth it in the long run if it extends various batteries lives by a lot).
  19. Dec 9, 2005 #18


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    optimizer - a battery has a "nominal" rating which means approximate. A 12V car battery for example, is normally 12.8V when new and fully charged, once near 11.5V its fully drained.

    A battery will have a high voltage when fully charged and this will go down as it discharges. It will also have a lower voltage as you extract current from it since it has an internal ESR (equivalent series resistance) that will cause a small voltage drop.

    Some batteries, depending on chemistry, will actually increase voltage as they get 'activated' by being used and as their internal temperature increases.

    If you can run a single diode in series from the battery to the camera and get the required voltage drop to make this work, that's a 10 cent solution and about as simple as it gets.
  20. Dec 10, 2005 #19
    Optimizer: Black and Decker chargers that I have used have an irritating habit. Even after the batteries have charged, they keep pumping the full current through the batteries so they get hot. This leads to overcharge conditions very quickly. I haven't measured them before but this is probably your problem. I didn't think of that earlier. You need to take them off the charger when they get hot.

    Radio Shack has a selection of rectifier diodes. I don't know how much current your camera uses, but to be on the safe side you might want to go with a 2.5 or 3 amp diode instead of a 1 amp. If you know that it uses less than 1 amp, you can go with the 1N4001 to 1N4007 types. Any cheap silicon rectifier rated at almost any voltage will work if it can handle enough current.

    What I wondered about is what voltage the camera's own brand of battery put out. There can be such a thing as a 6 volt NiCad or NiMH battery pack. It goes right along with six volts as a standard for a lantern battery. It is also the voltage of a three cell sealed lead acid battery, and those have been used in camcorders. At full charge the lead acid battery can run as high as 7 volts or better for a short time.
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