7.2ah and 7ah in series? how bad is this?

Discussion in 'General Electronics Chat' started by Thuktun, Apr 7, 2015.

  1. Thuktun

    Thread Starter New Member

    Apr 2, 2015
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    Okay, I know this is bad, but how bad? I have 4 6v 7.2ah SLA batteries and I need 36v, maybe 48v. A buck convertor is not an option, it has to be more batteries. Locally I can buy 12v 7ah batteries for $34 each and 7.2ah for $60 each from a different retailer. So obviously I would prefer the 7ah battery/ies. I'm probably better off buying all new batteries but $ is tight right now. The 6vs are of varying ages but have been kept float charged and are in good condition. I wouldn't dare put 7.2ah in series with 4ah but how much difference can 200mah make? Any thoughts?
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If they are good or slightly used batteries it's doubtful that a few hundred mah difference is going to affect much.

    Now that said the battery nazis should be along shortly to say otherwise based on a theoretically ideal world.
     
  3. WBahn

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    Mar 31, 2012
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    Why wouldn't you dare put a 7.2Ah battery in series with a 4Ah battery? The effective capacity of your effective battery will simply be 4Ah.
     
  4. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    What's the purpose of the build ??
    What about using 24v batteries instead ??
     
  5. Thuktun

    Thread Starter New Member

    Apr 2, 2015
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    If I put a 7.2 and a 4 together wouldn't the 4 drain faster and then start charging from the 7.2?

    It's an electric bike.
     
  6. WBahn

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    Mar 31, 2012
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    How would one battery charge from the other if they are in series.

    Once you try to exceed the 4Ah total draw that the lowest rated battery is capable of your terminal voltage will drop because you, in essence, have a dead battery.

    Now, you may have issues associated with charging them in this configuration because one battery is getting cycled much more deeply than the other.
     
  7. nsaspook

    AAC Fanatic!

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  8. WBahn

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    Mar 31, 2012
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    That makes no sense. How does being in series with a 7.2Ah battery cause it to be damaged when, presumable, it wouldn't be damaged if operated at the same charge/discharge rates by itself?

    If you are operating it at a charge/discharge rate that is high enough to damage the 4Ah battery, they you are operating out of spec and would damage the battery whether it was in series with another battery or not.

    Any time you construct a system the limitations are set by the weakest link. As long as you abide by those limitations you are fine.
     
  9. Dr.killjoy

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    Apr 28, 2013
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    Last edited: Apr 8, 2015
  10. Thuktun

    Thread Starter New Member

    Apr 2, 2015
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    Not in Canada once you include shipping.
     
  11. Dr.killjoy

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    Apr 28, 2013
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    Yeah but 12v 10ah vs 12v 7ah is a pretty good difference and have you tried searching ebay ??
     
  12. nsaspook

    AAC Fanatic!

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    It's mainly due the cells SOC discharge rate affecting the Ah capacity of individual cells in each battery in a string of much different capacity cells. The lower capacity cells would drain to a much lower SOC (flat) needing a complete bulk/absorption/float charge cycle to maintain full capacity while the higher capacity cells would require less before they were full (lowering charging current in the string with the nominal charging voltage for pack) when charged in series. Over time this causes the Ah imbalance to increase rapidly as weak cells get weaker from chronic undercharging/deeper cycling and the higher capacity cells are over-charged in the need to pushing current for the weaker ones to generate the needed voltage from the string.

    A BMS system would check the individual cell (or groups of cells) and rebalanced the distribution in a system of same capacity cells, or run an equalization cycle to slightly over-charge each cell to the same capacity. With very different capacity cells (batteries) the best solution is to individuality charge them after every discharge cycle so the affects of string charging are eliminated.
     
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