two 12V SLA batteries in series, two 12V chargers

Discussion in 'General Electronics Chat' started by toledobythesea, Jun 6, 2012.

  1. toledobythesea

    Thread Starter New Member

    Aug 19, 2008
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    An 80-year-old friend helps control his diabetes by riding his bike about 2km to a coffee shop a few times a week. He depends mostly on one leg, the knee of the other was ruined by many years as an amateur baseball catcher. Return trip is slightly uphill, I'd say about a 10-15 degree grade, so for parts of that he needs the assist of the Currie 12V bike motor, otherwise he has to get off and walk.

    The original two 12V SLA batteries (12 AH as I recall) were fine for about a year. Then we got some 8AH ones from another friend who works for an alarm company, by regulation they have to replace alarm batteries after a year or two even if they've never been used. In the process of replacing the batteries we fried the Currie 24V charger (we assumed white wire was negative and red wire was positive when for some reason, red was negative and white was positive!). So we found a 28V wart adapter and for the next year or so he'd plug that in for a couple of hours after each ride.

    What I've noticed is that with both setups, one battery went south long before the other, based simply on checking them with a little PulseTech conductance tester that estimates CCA's. For example one of the original 12AH batteries measured 170 CCA or so (still does, I put it on a maintainer for a day or so every month or two). Whereas the other measured less than 50 CCA. Similar difference between the two 8AH batteries, the good still measures a bit over 100 CCA.

    I'd say my friend does this routine about 150-200 times a year, so he and I aren't unhappy about the longevity of these batteries and we're basically getting them for free. However, I have a couple of small maintainers I got on sale and since I know very little about circuit analysis beyond the most simple kind such as a single voltage source and load, I'm wondering if it's possible to charge the two remaining batteries that look good to me while they remain connected in series, each connected to one of the 12V maintainers, without having to install a switch to open the series connection.

    Grateful for any pointers or reasons why this might be a stupid idea!
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Charging the batteries individually is better that charging them together in series.

    If you are using two chargers, make sure that the supplies are floating, i.e. both +ve and -ve terminals are not connected to GND nor CHASSIS.
     
  3. toledobythesea

    Thread Starter New Member

    Aug 19, 2008
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    Thanks. I'm pretty sure that when charging they are what you call 'floating'. My reason is that in order to charge we unplug the batteries' connector to the controller and use that same connector to connect to the charger (in my first post I didn't mention that the reason we fried the controller was that we had to re-wire after the original charge plug came apart after a year's use, so we thought it would be a good safety feature to use a single replacement plug, using it for running and charging at different times, that way there's never be any chance of turning the motor on while charging).
     
  4. K7GUH

    Member

    Jan 28, 2011
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    23
    I'm guessing you don't know if the motor circuitry contains protection against over-voltage, reversed flow, and other hazards. I have a similar situation with an electric handicap scooter which uses two 12 volt batteries in series. From time to time (e.g. once a month) I disconnect the +24 and -24 connections and charge the batteries separately. This tops the batteries up so far as possible, given their age and unknown history. The internal 24 volt charger applies a float charge from day to day.

    FWIW, if you shop around, you can find 7.5 amp-hour 12 volt batteries for as little as $20 (brand new).
     
  5. toledobythesea

    Thread Starter New Member

    Aug 19, 2008
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    Thanks. That's right, I don't know for sure what hazards the controller and throttle are susceptible to, but I have a guess. I suspect the controller tolerates reverse voltage because one time my friend had the battery compartment apart and back together again and when I visited him to test it, the rear wheel turned backwards (nobody got hurt because the bike has a rear derailleur) but when I corrected the polarity and the motor needed to overcome the chain friction et cetera, the motor wouldn't turn. Then I temporarily put in brand new ones from my own electric bike and the motor turned in the right direction but within a few seconds the throttle seemed to fail completely on (so much so that the bike took off and hit his wife's car!). New throttles for this bike are cheap, less than $20 and I have instructions for testing to confirm that throttle is bad. If necessary he'll buy another controller as well, about $60 from what I've seen on the internet.

    We're in BC so we get gouged a bit for batteries, but the local Walmart does sell little booster packs that have a 7AH SLA battery inside for $29 plus taxes, so for about CAD 70 we could get two more. Friend likes the light battery and I figure his very short trips don't deplete them enough to affect the number of possible recharges (my estimate is that he uses battery power for less than 10 minutes between re-charges). However, I'm confident that one of the 12AH's and one of the 8AH's each have some life left. I don't know much but I do know that such mis-matched sizes in series with a single charger will charge even more poorly than two matched ones, which was part of my motive for asking the question about using two 12V chargers instead. Plus the wall-wart is not what you'd call a smart charger, whereas the two maintainers I have (they go up to about 2 A and have three states and automatic float mode) mean my friend doesn't even have to remember to unplug them after a few hours.
     
  6. toledobythesea

    Thread Starter New Member

    Aug 19, 2008
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    Just noticed I originally wrote: "so for parts of that he needs the assist of the Currie 12V bike motor". I mean to write "24V motor".

    thanks to MrChips and K7GUH for the replies.
     
  7. bill heil

    New Member

    Jun 6, 2012
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    could i hook up a 6 volt battery to a 12 volt bilge water pump? what would happen?
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Likely nothing will happen - the pump will not run.
    BTW - you should start a new thread for this.
     
  9. dumb as a stump

    New Member

    Jul 4, 2012
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    As an ex fire alarm and sec tech simplest and cheapest fix is to find a 24 volt emergency light unit (that is for gell cells) and remount the charger board into a smaller case. find a company willing to sell you a unit they take out and charge them in series. A gell battery requires a low charge rate and that is what these units do.
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    To check this, use an ohmmeter to measure between both of the output charging leads to the safety ground (long terminal) on the AC cord (when it's unplugged, of course) and to chassis. There should be no continuity if the outputs are floating. If there is continuity, then you can't use two of them to charge both batteries at the same time.
     
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