NiMh Batteries in Series (12v) Charger Question

Discussion in 'General Electronics Chat' started by Carlos Almeida, Jan 29, 2015.

  1. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    Hi Everyone,

    I read this old post:

    http://forum.allaboutcircuits.com/threads/charging-2x-10-eneloop-12v-4-000-mah.72246/#post-504454

    And although it answers most of my questions, I just want to make things "safer" and get an updated opinion on this subject.

    I am planning to wire 10x AA NiMh 3,200mAh (1.2v) in series to make a power supply of approximately 12v, I spent lots of hours reading about recharging different types of batteries (which drove me off my original "project" with li-po batteries...) so I am aware that there's no cheap/easy way of charging NiMh in series perfectly, so my objective is finding the safest and less battery-harmful solution.

    I was thinking that, if I have a 12v 300mAh DC charger, can I "safely" use it to charge the battery "pack" all-together for a controlled period of time (6 - 8h)? I know that it will reduce the battery life-time but that is not very important to me because I only plan to charge them about twice a month, just want to know if it's doable without a huge risk.
    By the way, the charger is from Sony (probably from an old gadget that died some years ago) so it's safe to say that I can connect the 12v charger directly to the battery "pack" terminals? Would you recommend a power supply with a voltage a bit higher (15v for example) or this 12v one would be fine?

    PS: It's important to say that I would test the batteries one by one before the first charge to make sure they were at the same voltage (+/- 0.1v).

    I'm kind of a Noob on this so don't go easy on me, I like to learn and that's why I'm posting this thread.

    Thanks.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    The safest, simplest charger for NiCad or NiMh batteries is a constant-current charger (no specific voltage limit), where the charging current is controlled at 320mA (the 0.1C rate for a C=3200mAh battery). You use time to cut-off the charger after 14hours (you have to put 140% of C back in to fully-charge them, 14h*0.1C = 1.4C if they were fully discharged to begin with). Most of these batteries will stand a modest overcharge if they are charged at the 0.1C rate, but dont leave then under charge for more than 14h.
     
    Carlos Almeida likes this.
  3. ian field

    Distinguished Member

    Oct 27, 2012
    4,412
    782
    The maximum safe charge current for long term charging, results in a long time to charge.

    If you need rapid charging - constant current alone won't do!

    Most (decent) commercial chargers sense the small voltage reduction as the cell starts self heating when it reaches fully charged - this fall in terminal voltage is pronounced with Ni-Cd and more subtle on Ni-Mh, a Ni-Cd charger might cook Ni-Mh cells, and a Ni-Mh charger will probably shut off before Ni-Cd cells are fully charged.

    You can get off the shelf charge management ICs - the quick and dirty DIY method is by temperature sensing.
     
    Carlos Almeida likes this.
  4. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    Hi Mike,

    Thank you very much for this explanation, this certainly cleared some doubts I had.
    So, picking up what you said and after doing some quick research on Google, this is what I'm planning to do:

    1 - Will use a Laptop Charger of 19V 4.7A.
    2- Will use a Constant Current and Voltage regulator to Step Down the Voltage to 13V and the current to 320mAh.*
    3 - Will connect the CCV regulator directly to the "in serie" batteries pack and let it charge for around 14h (IF they are all discharged).

    Does the above look good?

    Regarding the Constant Current and Voltage regulator, I've looked at a few on Ebay and THIS one looked nice, pretty straight forward for what it seems. Any opinions about it? It supposely has 3 Lights (Red, Blue, Green) . Red I suppose is to say that it's working, Blue to say that it's charging and Green to say that it's charged. The calculations they present seem "logical" but as I said, I'm a Noob, can you give me your insights on the reliability of their "Fully Charged" method (copied below - bad English but understandable I think) ?


    "Charge turn lamp current factory default is 0.1 times the charging current;(Battery during charging current is gradually reduced, if the charge current setting is 1A, then when the charge current is less than 0.1A, blue lights turned off, the green light is on, which means that the battery is fullycharged)."


    Once again, thank you very much.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
    4,412
    782
    Voltage regulation is irrelevant - as the cells heat up at end of charge, the terminal voltage drops - without current limit (i.e. fixed voltage charging) that fall in terminal voltage would cause the cell to draw increased current and self destruct
     
  6. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    Hi Ian,

    Thanks for that, I actually spent like 2hours reading about some DIY Kits to charge NiHm and Lithium batteries but none of them mentioned that they would work with batteries in series. Some showed the circuits charging out-of-the-box packed batteries but for what I understand, the "packs" that stores sell already come with some sort of a protective circuit to keep them balanced while charging, so I'm not sure that my "homemade" 12v pack would work with those type of IC's.

    And, also important, those Circuits seemed way to "advanced" for a beginner like me, don't want to risk screwing up a circuit that is potentially dangerous (batteries...). Couldn't find a cheap one already made so that was a no go for me unfortunatelly :/

    On the other hand, since I'm planning to charge only twice a month, overnight charging is not a real issue, Constant Current seems to be my best friend for the moment :)

    Thanks
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    The constant current charger circuit for NiMh can be as simple as a LM317 regulator IC mounted on a suitable heatsink and a R = E/I = 1.25/0.32 = 3.9Ω resistor. The resistor would dissipate P= E^2/R = 1.25*1.25/3.9 = 0.4W, so use a 1W resistor. Starting from 19V, the power dissipation in the LM317 would be P= IE= ~0.32*(19-1.25-12) = ~1.84W, so a modest heatsink on the LM317 is required.

    Typical circuit (right off the data sheet)
     
    Last edited: Jan 29, 2015
    Carlos Almeida likes this.
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,493
    2,363
    There is a series of microprocessor controlled programmable chargers on ebay cheap, e.g. Imax, ebay 400635730382
    Will charge different kinds of bat. technology.
    Max.
     
    Carlos Almeida likes this.
  9. ian field

    Distinguished Member

    Oct 27, 2012
    4,412
    782
    To date - I've never seen cell balancing precautions on nickel battery packs.

    Its absolutely critical to safety on lithium packs, if a cell is overcharged it can vent with flaming gas - if a cell in a pack goes down the applied charging voltage gets shared among the other cells, whichever one reaches overcharged first can provide a fair bit of entertainment.

    Nickel cells seem to be in the habit of developing internal shorts, its most common when in a low state of charge (especially self discharged) if they fail fully charged they vent, maybe even burst - but the event is nowhere near as exciting as with lithium cells.
     
    Carlos Almeida likes this.
  10. ian field

    Distinguished Member

    Oct 27, 2012
    4,412
    782
    Its not that easy - you either have to make the charge rate so low that it takes at least 14 hours to reach full charge - or you have to include a charge shut off timer to prevent cooking the cells.
     
    MikeML likes this.
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Ian, please read the previous postings before making suggestions that have already been covered...
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
    4,412
    782
    Someone already pointed out your suggestion was silly?!!!
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
  14. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    Hi Mike, Ian, Max

    So, lots of information to digest above, that should take me a while to go through all the topics to get a bit more info, I'll try to reply in the correct order and make some comments.

    Regarding the Voltage drop when the batteries get hotter, with a normal charger I think I understand the concept that the battery would suck more current to counter-balance the voltage drop, which would harm it real bad (and the charger as-well I guess), however, the little Module I posted that makes the Current and Voltage Constant, wouldn't that fix this given that it would only allow the battery to suck up an X max amount of current?

    Mike, the constant current charger circuit you indicated does seem pretty achievable even for a newbie if he (in this case me) takes the time to properly study it and confirm everything with the experts in the community (such as yourself), I will certainly be giving it a closer look, meanwhile, if I can get my hands on a Constant Current Charger (or in this case, a normal charger with the Constant Current/Voltage module), the result is basically the same right? I want to test it as quickly as possible and ordering some parts that are on that circuit would take at least 20 days to arrive at my place :(
    Referring to Ian's comment on this, charging time it's not an issue for me and I can keep the factor time under control so if that's the only disadvantage I truly don't mind :)

    Max, thanks for the indication of the Imax Charger, that was one of the first things I looked at when researching this topic and although it's perfect for what I was looking for, it's extremely expensive for my low budget and in my Country a second hand one is still way above 50€ (without the transformer). It's already on my "favorites" but only for future projects, in this case it's not worth the investment, I'm still a newbie and am starting small, for the moment will work with accessible (+/- cheap) material because in the event of "blowing up" something (which will probably happen...) it will be easily replaceable.

    Ian, I also spent a few minutes on Youtube checking the "hazardous" events with different types of batteries and couldn't agree more, lithium batteries do give a good show (reason why I stayed away from them for this project), Nickel Batteries seemed quite resistant and took some efforts to produce anything out of the ordinary, usually they just leaked so they seem quite robust (reason why I chose them lol).
    Regarding the protection circuit, my bad, it was for the 18650 battery which is lipo (but physically seems like a Nickel battery...).

    Summing things up, if possible, I want to stick to NiMh batteries packed in series with a charging method that it's easy and that doesn't present a big risk, Time factor is not an issue and can be controlled strictly by me or by one of those main plugs with a timer. With the Circuit Module that makes the current and voltage constant, with a charging current of 0.1C and a charging time of HxC=1.40C, will that achieve my goal?

    Q: On the other hand, I also have the possibility of getting a lead acid battery with 12V 7A and a proper Lead Acid Battery Charger, I never "played" with Lead Acid Batteries other than to Jump-Start my car so I thought they were all pretty big, this one is not and would also fit (tightly) into my project, what are your opinions comparing this option to the NiMh in series?

    Thanks a lot for the time you have been providing me.
     
  15. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,762
    920
    a simple 12 volt, 100 ma wall wart. get a programable lamp timer. the kind you use when you go on vacation to make the lights in the house turn off/on everyday. set the timer for 14 hours and plug it in. come back and collect the battery later.
    or you could build your own current limit supply, google LM317 constant current and check the image returns for circuit schematics. very simple. and dont sweat the overcharge stuff with NiMH AA cells. if your lucky you might catch them while they hiss and sputter, but if you miss that moment youll just find battery shaped slugs that feel warm, but do nothing electrical anymore.
     
    Carlos Almeida likes this.
  16. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Carlos Almeida likes this.
  17. ian field

    Distinguished Member

    Oct 27, 2012
    4,412
    782
  18. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    Hi,

    So, a simple charger will be able to maintain the current constant even when the battery voltage starts to drop ? I do have some unused chargers around here somwhere...Will look for them just in case this is a valid option :)

    Thanks for the feedback
     
  19. Carlos Almeida

    Thread Starter Member

    Jan 15, 2015
    35
    0
    That's a really good "article" indeed, just learned a new thing and please correct me if I'm wrong. When the NiMh batteries are not in use I should keep them "charging" at a current of around 0.04C to extend their life time and avoid memory issues? Had no idea, that explains a lot.
    So I guess I would be good to go with the setup I described earlier today?
    What about Sealed Lead Acid Batteries, any opinion about those? Are they more or less reliable than NiMh? I read so many reviews about this but no REAL opinion from people that actually used both and can compare.

    Thanks
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
    4,412
    782
    If you go the timer controlled charge route - don't forget whatever percentage of charge might be left in the cells from last time - a full timed charge for cells that are only half discharged could result in cooked cells.

    Some commercial "dumb" chargers that rely on a timer, have a discharge phase at the start - I don't know the exact procedure, but they probably just connect a resistor across the cells for another timed period.

    The one I bought in the Lidl store before they started offering the MCU controlled ones, only does discharge monitoring on the cell nearest the control panel, so it could be a bit more sophisticated - maybe a comparator monitoring the terminal voltage that starts the timed charging period.
     
    Carlos Almeida likes this.
Loading...