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Lifetime of rechargeable battery (sealed lead acid)

  1. Jun 20, 2011 #1

    Redbelly98

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    I have a cordless electric lawnmower that runs on sealed lead acid batteries. It is on it's second battery, which has noticeable degraded performance and will need replacing soon.

    My question is why the original battery, which was rated at 17 Amp-hours, lasted twice as long as this second battery that is rated at 22 Amp-hours. The two batteries are the same physical size, externally -- so it's not the case that the 22 A-hr battery is simply a "scaled up" larger version of the 17 A-hr design.

    Some other details...

    17 A-hr battery: lasted 3 years, at which time I thought it had died, which turned out to be wrong. One of those years, I was mowing a friend's lawn as well as my own, so it's pretty reasonable to say this battery lasted the equivalent of 4 years and was still in working order.

    22 A-hr battery: has lasted 2 years, and is noticeably degraded (as I mentioned earlier)

    Also, the two batteries are different brands, which might be an explanation for the different lifetimes. However I am wondering if there is some fundamental design difference that would allow a 22 A-hr battery to be the same size as a 17 A-hr one, but also would result in shortened lifetime. Mainly I am thinking about the size (area or thickness) of the electrodes, or the total number of electrodes per cell being different -- or something along those lines.

    Thanks in advance for any insights.
     
  2. jcsd
  3. Jun 20, 2011 #2

    SpectraCat

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    Well, first off, I assume you are accounting for external factors like increased mowing due to increased rain-fall (or whatever) and things like that? I figure you probably have thought of that, but it's worth asking. Also, changes in the storage conditions (humidity and/or temperature) could also be possible contributing external factors.

    Assuming all of that is about equal on a year-by-year basis, then the most likely explanation I can think of is that the 22 A-hr battery has signifcantly larger internal losses than the 17 A-hr battery. This could just be due to a manufacturing defect, or it could be a design difference as you are speculating. Have you looked online to see if there are similar reports of reduced lifetime for these batteries compared to other brands?

    As to how they can pack more amp-hours into the same size packge, your speculations are pretty much dead on. By decreasing electrode-thicknesses and increasing the convolution of the electrodes, they can increase the surface area of existing cells, and also make them smaller, so that they can fit more cells in a given battery. However, by doing this, they also tighten the manufacturing tolerances, so that a defect that would have been acceptable in another battery with thicker electrodes, leads to a large parasitic current due to short-circuiting or near short-circuiting in a thin-electrode battery. In a nutshell, batteries with thinner electrodes can deliver higher peak currents faster, while those with thicker electrodes are more suitable for deep-cycling, where the battery is discharged almost completely and then recharged, over many cycles. I am not sure what the most important figure of merit is for your lawnmower battery, but I would guess a deep-cycling battery is what is required.
     
  4. Jun 21, 2011 #3

    Redbelly98

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    Thanks for replying.

    Yes, I am pretty much assuming the amount of mowing is about the same on a year-to-year basis -- except, as I said, for the one year I was mowing a friend's lawn in addition to my own. In the winter, the mower and batteries were stored in my basement and kept on the charger, so the two batteries had the same treatment during winter storage as well.

    I haven't looked, that's a good idea.

    Now that I have read your post, I am inclined to think the 22 A-hr battery probably does have thinner electrodes -- in which case the electrolyte solution would occupy a larger volume. A larger quantity of electrolyte would explain the higher charge capacity, while the thinner electrodes explains the shorter lifetime -- either irreversible damage from sulfation happens in a shorter time, or perhaps a defect in the electrode has a greater effect as you suggested.

    I don't see how the 22 A-hr battery has more cells, as you suggested, since that would change the overall battery voltage. Both batteries are 24 V -- actually, each "battery" consists of two standard 12V batteries connected together, which the mower manufacturer sells as a 24V battery. 2V per cell is standard for lead acid batteries.

    My two figures of merit are battery lifetime, and how much of my lawn I can mow on a single recharge.

    With my original battery, I could mow about 3/4 of my lawn on a single charge. I was hoping to mow the entire lawn, which is why when I replaced it I went for the higher 22 A-hr battery. I was surprised that it had the same physical dimensions as the 17 A-hr one, but at the time this was a real selling point as it meant it would fit in the lawnmower without my having to modify the parts that hold the battery in place.

    At this point, it is more important to me to have the battery last a long time. It is my routine now to mow either the front or back lawn, and then recharge the battery before finishing it up another day. Being able to mow it all at once would have been nice, but I am used to this routine now that I have been doing it for 5 years.

    Thanks again!
     
  5. Jun 27, 2011 #4
    Pb-acid batteries should be kept charged regularly, and not allowed to be discharged and stored in the discharged state, since this allows sulfate to accumulate on the plates, which basically can wreck the battery's performance.
     
  6. Jun 27, 2011 #5

    mheslep

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    Battery life for a given battery design is a primarily a function of these usage parameters: 1) number of charge/discharge cycles, 2) depth of charge/discharge per cycle, and 3) temperature during charge/discharge operation.
    I suspect this might be the culprit. Could it be that, since you couldn't finish the lawn with the 17AH, you mowed (say) half the lawn or less at a time, and then stopped to recharge a less than fully depleted battery? Then w/ the 22AH unit you fully depleted it each time? Full depletion will age the battery faster per charge/discharge cycle. So does topping off the charge to 100% (vs ~90%), though that's usually a function of your battery charger. On the other hand this may be offset if you were subjecting the 17AH unit to twice as many charges in a season.
     
    Last edited: Jun 27, 2011
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