Why less than ten Eneloops in series?

Discussion in 'General Electronics Chat' started by RichardO, Jun 21, 2014.

  1. RichardO

    Thread Starter Well-Known Member

    May 4, 2013
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    I was reading the card that my Eneloop batteries came on and it says:

    • Do not connect 10 batteries or more in series.

    Why? I am guessing that is is either be unsafe or will reduce battery life. Any thoughts?

    I have, ahem, 16 batteries in series in an existing battery pack so this is a potential problem. Considering, I am already "riding the batteries hard", I don't really have much choice of going to fewer batteries.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    http://e-baterie.com/uploads/Katalog_Sanyo.pdf

    I find no mention anywhere in a search, of why that is a limit, or what danger is posed by it.

    The common problem with batteries in series is a weak battery in the string can become reversed during discharge of the battery pack. Also voltages higher than 12 volts(10 in series) present greater danger to the user and for ANTI-litigation reasons this is chosen as a limit. (a guess, but it is a good bet that this is the real reason)
     
  3. RichardO

    Thread Starter Well-Known Member

    May 4, 2013
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    Kermit2:
    Your observations closely match mine. I just wish I could find some confirmation.

    I just wish the manufacturer would say why there is a limit and what can be done instead... Oh, well with lawyers (potentially) involved I don't expect much that is useful.
     
  4. Markd77

    Senior Member

    Sep 7, 2009
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    I suppose you could monitor the voltage of each half of the pack and stop if either reaches your chosen cutoff voltage. Presumably you are charging the batteries individually (not 16 in series).
     
  5. RichardO

    Thread Starter Well-Known Member

    May 4, 2013
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    I am not sure what is to be gained by monitoring the center of the battery string. Could you give more details?

    I have been using a commercial charger but taking 16 batteries in and out of the battery holders is a nuisance. My intent is to make a dedicated charger to do all of them (in series) while still in the case. I know that this is not ideal but I am willing to charge at a real low rate to limit battery heating and overcharge.
     
  6. Markd77

    Senior Member

    Sep 7, 2009
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    The most likely problem is what Kermit2 said. If you have 16 cells and discharge them to 1V per cell (16V total) there is a good chance one of the cells has dropped way below 1V and is being damaged. If you discharge until either half of the cells drops to 8V there is a better chance of stopping before this happens. There is a similar problem charging a lot of cells in series, the pack can only be charged to the capacity of the weakest cell (or the cell that was discharged the least) . Back when people used to use 6 or 8 NiMH batteries in a pack for RC cars, etc. the manufacturer would try and match the cells in each pack. You probably don't have that luxury.
     
  7. RichardO

    Thread Starter Well-Known Member

    May 4, 2013
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    Thanks Markd77. I will have to think about this some more. I really don't want to put any stuff in the battery pack that draws power since it defeats the purpose of using the Eneloop batteries.
     
  8. adamclark

    Member

    Oct 4, 2013
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    2 banks of 6 series parallel together?
     
  9. RichardO

    Thread Starter Well-Known Member

    May 4, 2013
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    I need all the voltage I can get to keep efficiency high. The batteries power a bicycle headlight that draws as much as 15 watts at full brightness.
     
  10. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    I tend to believe the cell reverse voltage idea as well as the below 12v or 15v idea for keeping the recommended number of cells in series to 10 or under.

    Using more than 10 cells in series results in a voltage above 15v, and once you get higher the danger from a shock gets higher as well as the danger from an accidental short circuit. Many people (including myself sometimes) dont realize the problems that can come up if you short out 10 cells that are in series. The current can be very high and cause fast heating leading to fire or explosion.

    The danger of a reverse voltage cell is mostly about it charging in reverse with HIGH current. This can also cause big problems, and a reversed cell is very likely in high current application where several are in series because there is always one that is weaker than the other and so looses its charge faster. What a pain.
    Reverse diodes? Reverse diodes might help, but the cell could still see 0.5v (Schottky of course) of reverse voltage so that might not help either. Never tried it though so if you have some cells you dont care about, do a little careful experimentation and let us know.

    BTW monitoring the voltage of every cell in a pack of cells does not require much power usage. You could use very low current comparators that would monitor each cell and report one that is too low with some indicator, or shut the circuit down. In fact, using a microcontroller you might be able to use a multichannel ADC to monitor the cells, checking once per second and sleep otherwise, which would use very very very little power (on the order of 2 years of continuous operation with two or three AA cells).

    I once thought about building a 100 cell pack of AA cells, for use with a soldering iron. Found out the iron requires AC only so i bought a 12v iron instead.
     
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