Rechargeable batteries

Discussion in 'The Projects Forum' started by agroom, Nov 3, 2011.

  1. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
    1
    I'm working on a project that I want to permanently install 4 NiMH AAA rechargeable batteries (in series). The plan is to hook the batteries up to a charging plug and charge it with an external DC wall plug. I don't really know what the technical names are called, but here's a picture of what I have in mind.

    1. I basically don't know anything about recharging batteries, so I'm afraid of "over charging" them. I've read the battery charger wiki page and it specifically said intelligent chargers work well for NiHM batteries:

    For Ni-Cd and NiMH batteries, the voltage across the battery increases slowly during the charging process, until the battery is fully charged. After that, the voltage decreases, which indicates to an intelligent charger that the battery is fully charged. Such chargers are often labeled as a ΔV, "delta-V," or sometimes "delta peak", charger, indicating that they monitor the voltage change.

    These are the batteries I have. Many of the other rechargeable ones I've seen on DX say they are overcharge protected and I don't see that listed for these. Is that a problem or will it be okay? I'm assuming the more overcharging that happens, the short the battery will live. I could also find ones with OC protection. If I did that, would that mean I could potentially use any wall plug DC converter to charge them?

    2. Where do I find an intelligent charger & plug like the one in the picture? 4 AAA batteries would require a 5v charger because they're in series right? Or do I use 1.2v?

    If this is successful, there's other projects I'd like do the same for only with different sized batteries like 3v CR2/123 and 3.7v Trustfire 18650/14500s. Which I assume would use the same techniques, just scaled to the voltages of these and not the AAA I'm using for this project.

    There's probably some questions I didn't ask since I don't know what they are, so any extra advice or information is greatly appreciated!
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Easiest way to do it is to slow charge tham at a rate less than 10% of "c" rate which is 1100 mA/hr for the cells you chose. Just charge them at 100 mA or less and don't worry about overcharge. Use a "wall cube" with a series resistor to limit maximum current. Example: use a charger with abut 8V out at 100mA and put a 30 Ohm (1/2W) resistor in series with the cells.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Those are not rechargeable batteries.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    Are you sure that's OK for NiMH? I think that was OK for NiCad but will overcharge the MiMH batteries.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    It is a problem, for the reason you noted.

    I'm skeptical that there really is such a thing, but I have to admit I don't know one way or the other.

    You need ~5V if you're leaving the batteries in place and not moving them into the charger. There are plenty of good chargers but I'm not aware of one that does what you want, to plug into the 4 cell pack. You may need to hack one to add the power cord into your device. But again, I think most chargers are designed to work for 1 to individual 4 cells, and not 4 in series.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You must learn about charging batteries before causing an explosion and/or a fire. Go to www.batteryuniversity.com and read about how to charge batteries with a certain current, not voltage and how to detect when fully charged.

    5V is not enough voltage to fully charge a 4-cells Ni-Cad or Ni-MH battery. It will be near 6V when fully charged.
    An old Ni-Cad can be charged for a few years at 10% of its rated mAh current but a Ni-MH battery loses its life at such a high trickle charge current.
    The max voltage and max allowed charging current for a lithium rechargeable cell must never be exceeded.
     
  7. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    They sure are!
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    I don't think that's true of today's NI-MH cells, it's not what the battery suppliers call out. I have some I have used for many years that way. When NI-MH first came out, the max allowed trickle charge rate was about c/20 but they improved them since then.

    http://www.onlybatterypacks.com/up/pdf/batterycharginginstructions/NiMH INSTRUCTIONS.pdf




    http://www.powerstream.com/NiMH.htm

     
    Last edited: Nov 3, 2011
  9. Smoke_Maker

    Active Member

    Sep 24, 2007
    126
    15
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    My Energizer Ni-MH charger is stupid because it is simply a dumb timer. It charges for about 10 hours at about 0.14C then its timer stops the charging. Therefore it seriously over-charges cells that are not completely dsischarged and it over-charges cells that have lower capacity. If the power fails while charging and the power returns then the timer starts all over again to over-charge.

    A good charger measures the cell's voltage so it never over-charges.

    A "trickle-charge" is a continuous very low current charge. An "over-night" charge is at about 0.1C for 10 to 14 hours then it is turned off.
     
  11. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
    1
    Could you explain the math behind this please? I know the basic V=IR and have taken AC/DC 1&2 which went over most of this, but I'm not getting this specific application. Would the 30 Ohm resistor in series with the cells affect their output to the device? Or do you mean put it in between the charger and the cells, but not between the cells and the device?
     
  12. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
    1
    I saw someone else reply that they are, and typically you don't see them as rechargeable in stores, but they do make rechargeable. I these quite frequently.
     
  13. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
    1
    This is an example of what I'm referring to. I see in the description it specifically notes this:

    These fully protected cells simplify usage with their integrated PCB. Perfect for your single cell application these cells are not for pack building as doing so will defeat the PCB protection which is matched to the correct voltage on a single cell. If you are building a pack please choose one of our other cells and the appropriate PCB.

    So my interpretation is that it's possible if you charged just a single battery, but not multiple in series (i.e. building a pack). So this wouldn't work for my application.

    This is actually the project I'm trying; not verbatim, but essentially the same. He shows connecting the 4 batteries in series and charging with just a standard wall plug. I was very skeptical about this setup and wanted to ask about it.

    Since I posted this, I decided to change this up a bit if possible. I'd like to use strip LEDs instead of wiring individual lights. However, strip LEDs take 12v so I'd need to use 4 3v batteries (or similar) ILO 4 1.5v.

    I'm able to purchase non-rechargeable 12v batteries that would fit inside the hoop, but they're kind of expensive and I'd like to get away from replacing batteries. If there were a single 12v rechargeable battery that I could remove and plug into a charger that would work great too. However, I can't use 4 3v CR2s and do the same thing. Because hoop weight and balance is a major issue, 4 batteries in a single location would significantly off-balance it.
     
    Last edited: Nov 8, 2011
  14. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
    1
    Essentially isn't what I want something like this, only that I'm using fewer batteries? This is likely just 10 1.2v batteries connected in series. So wouldn't connecting 4 CR2 rechargeable batteries in series be the same thing?
     
  15. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    Well, it would probably work at least for a while. Hard to tell without charger specs. The likely problem is shortened battery life due to overcharging, but it could take a long time to notice this. [EDIT] Looking a bit more, it appears he used a genuine "charger", not just a wall-wart power supply. If that charger was designed to charge NiMH batteries, it should be just fine.
    Really tough, IMHO, given your space and weight constraints. You could consider 3 or 4 Li-ion batteries (which are 3.7V nominal) in series. I scavenge old laptop battery packs to get mine. But those are too fat for your application. Li-ion has the best power density though, so if you could find the right size cells, they could be ideal. Expensive.
    I think the problem with any 12v battery will be capacity in milliamp-hours, mAh. AA's are great for lots of capacity for the money. That's why they can power a lot of LEDs in parallel.
     
    Last edited: Nov 8, 2011
  16. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
    1
    Yep, definitely an issue. The LED strips I purchased use about 650mA/foot max, so at ~9ft, I'll be consuming just under 2A. 650mA though would be all the LEDs on bright, which probably won't happen, so I'm sure the current draw will be significantly less. I can get 3.7v TrustFire Lithiums that have about 2400mAh, so it'll run for probably a few hours. I'm completely fine with this, but would want to compliment this with some kind of 1hr quick charger.

    I've made several standard LED hoops and use 2 3v CR2 batteries. These last a good day or two of normal usage (5-6hrs). I could also reduce the # of LEDs on the strip by cutting it up and spacing them out more. But at present, the 1hr life is not an issue. Because I'm using 5050 RGB LEDs, this 'should' give me more flexibility on options for colors and potentially integrate a controller that can manipulate the LEDs rather than being stuck with a single design. I suppose that's the trade off: awesome lighting effects or long life.

    Astral Hoops already makes something exactly like this, but they charge a LOT! I know they don't use the 5050 RGBs since you can't individually program them, but I figured I'd start easy for now. I'm sure in a few years I'll figure this out, but at least it's not a pipe dream since something like this has already been done. It also says they use NiMH batteries.

    *sigh* Too bad my wife couldn't pick a different past time :p
     
    Last edited: Nov 8, 2011
  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Depends on the chemistry: if you stick with NiCD or NIMH you can just push current into the stack of cells forever as long as the current stays below about c/10. With Lithium cells, you can NOT and each cell has to be charged to a very precise final voltage (not the voltage of the string) which is why each individual cell must be measured and the charger must have a shunt to bypass the fully charged cell(s) as the lower cells are brought up. This is called "balancing" and the battery will have a short life if the charger can't do this. Designing Lithium chargers is not for the novice.
     
  18. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The No-Name 3V Chinese batteries do not haver a detailed datasheet. They might be Li-Fe type.
     
  19. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    You can also get the AA NI-MH batteries in 2000mA-hr (or higher) pretty cheap in four packs. If you are going to try to fast charge batteries, I would stay away from Lithiums.
     
    agroom likes this.
  20. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
    1
    Maybe someday in the future I'd look into designing my own charger, but for the time being I figure why try to reinvent the wheel. Here's what I've gathered from comments and material in links.

    NiCd/NiMH - I can trickle charged at c/10, so I'm sure I could find just a standard wall plug close to this. I have 4 AAA NiMH 1100mAh, so I should find a charger with ~110A? This seems fine, but not an ideal charging time. 1100 x 10% = 12 hours. For an 1100mah 2/3 A it says I can do a 500mA max peak charge rate? Is this safe if I know the batteries are fully discharged (or < 10%) and only charge for 1 hour?

    Lithium-ion - Need to be charged to a specific voltage, so creating a battery pack that aligns them in series is either not possible or requires some special tecquinque so that each battery is charged individually, I believe someone referred to it as "balancing."

    I started reading battery university last night and rather than jumping to what I wanted, I'm going through all the basics. Tonight I'll be starting on battery types, then moving to charging.

    I have long-term goals for learning all this information, but I was hoping for this project there would be some simple, ready-made solutions I could incorporate to have done for the end of the month. Like I said, I'm not looking to reinvent the wheel, I'd rather just have it work :) I suppose every application is different, and nothing is as simple as it sounds, but I was honestly hoping I'd find something like, "setup these 4 batteries and use this charger".

    Thanks for everyone's replies, I'll hopefully figure this out. It can't be too difficult...I see battery packs and chargers about everywhere I look!
     
Loading...