Problem with SLA 36 volt battery charger

Discussion in 'General Electronics Chat' started by logicrules, Dec 5, 2009.

  1. logicrules

    Thread Starter Member

    May 3, 2009
    16
    0
    Hi,
    I am having trouble with an SLA 36 volt battery charger.
    The charger works, as far as charging the batteries, but the ‘cutoff to float’ does not kick in; the LED indicating the charge has been completed remains red. The LED does not turn green, indicating the charger is in the float mode.
    When I charge the batteries now, I wait for the normal time that it previously took to charge the batteries per trip, (guess work) and then disconnect it from the batteries. Upon measuring the voltage on the batteries; it is at or near 41 volts. If I plug it back in the charger goes into the float mode; the LED turns green and it maintains the proper voltage.
    My procedure for charging is to connect the batteries first, and then to plug the charger into 110 volts A/C, and the same for disconnecting.
    I fear that one day I will forget to disconnect the charger, and overcharge my batteries, and ruin them.
    Is there a way I can find out which part of the circuit is failing, and replace it?
    Thank you for any assistance you may provide.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hello Logicrules,
    Welcome to AAC.

    To make your posts easy to read and respond to, please use the default font in the text box. Using the custom fonts and sizes interfere with the ability of users to re-size the text to their preferred size in their browsers. It also makes a real mess if people try to reply with quoting.

    I suspect that your charger is OK, but your battery or batteries may have one or more dead or shorted cells.

    Charge your battery/batteries until it/they reach 42v, and then let the battery stand with no load for at least 8 hours. Measure the output voltage again.

    Depending on the battery chemistry, you should read between 38.1v and 38.7v when the internal temperature is 25°C (77°F). This is approximately 2.117v to 2.15 volts per cell; you have 18 cells in series to get the 38.1v to 38.7v output.

    If you have a dead/shorted cell or two in there somewhere, your charger will still try to charge the pack up to 42v anyway. The one shorted cell will cause the other 17 cells to be significantly overcharged.

    You could try testing the charger using a large-value aluminum electrolytic capacitor rated for at least 80v, with a 1.3k Ohm 2 Watt resistor connected across the leads. The charger should charge the cap within a few seconds, and go into float charge mode.
     
  3. logicrules

    Thread Starter Member

    May 3, 2009
    16
    0
    Hi,
    Thanx for your help. Sorry about the font; I just did a copy and paste from MSWord. I will keep this in mind.
    The largest capacitors I have, on hand, are two, 50V-4400uF and two, 100V-100uF. I am not quite sufficient in cap specs but I believe I can get 100V-200uF. Would that be enough to make the test?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Sure, use the two 4400uF caps in series so that you have a comfortable margin on the voltage rating.

    Easier to just check the battery/batteries after letting it/them sit for several hours. If they're much below 38v, you know you have a battery problem.
     
  5. logicrules

    Thread Starter Member

    May 3, 2009
    16
    0
    Hi, I borrowed a charger and it worked fine. The voltage was 41.9 after charging. After a short trip I again, used my charger, and the same condition exists; the float mode does not start. At this point I am positive the problem lies with the charger.
    What controls the float mode start, and can I change the components to get it back in proper working condition?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    OK, fair enough.

    Post a schematic of your battery charger.
     
  7. logicrules

    Thread Starter Member

    May 3, 2009
    16
    0
    I do not have a schematic and I have no idea where I could get one. It is the 'standard' charger that comes with a bicycle conversion kit. One would assume that these chargers are basically the same. The only things I am sure of are the voltage 36v, and the output amps 1.6.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    We don't know where you are.
    We don't know what is "standard" where you are.
    What manufacturer did you buy this conversion kit from?
    Does it have a part number?

    Why don't you ask the manufacturer for a schematic?

    If they won't give one to you, will they repair it?

    We are a pretty good group of people, and lots of us have a lot of talent. However, we need more information than what has been provided to us in order to give you the help that you seek.
     
  9. logicrules

    Thread Starter Member

    May 3, 2009
    16
    0
    I am in Texas. The charger was made in China.
    I have made several attempts to reach who I thought might be the manufacturer, all to no avail.
    The information printed on the back of the charger is in Chinese except for the following:
    36V, 80W, 1.7A DC, a lot of Chinese, and on the bottom 0519-05110760.
    Possibly someone has done some work on a charger from China, and can help me.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Can you open it up without destroying it?
    Do you have a digital camera?
    Could you take some pictures of the circuit board and post them?
     
  11. logicrules

    Thread Starter Member

    May 3, 2009
    16
    0
    Hi, I opened it, and found a 'normal' looking cicuit. I inspected it for any abnormalities, and found none but circuits do not always give a visual clue to their malfunctions.
    I used a webcam to take a few photos but they were not of good quality but here they are:
    I hope this helps; otherwise I will have to purchase a new charger.:mad:
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, those photos are so small and fuzzy that they are basically worthless. I do appreciate the effort. It's tough to get good photos of such circuits.

    You need a higher-resolution camera, and you need better lighting.

    The best lighting you'll ever find for taking pics of electronics is outside, in the middle of a very overcast day. The lighting is remarkably even, and there are no harsh shadows.

    An experienced photographer can simulate the "overcast day" by using a "bounce flash" technique to get plenty of indirect lighting, which softens the shadows.
     
  13. logicrules

    Thread Starter Member

    May 3, 2009
    16
    0
    Thank you for your efforts.
    My question remains the same: What controls the float mode start, and can I change the components to get it back in proper working condition?
    Possibly, I can find a circuit/schematic of a 36V charger somewhere on the "net that might provide some information. Preious searches did not help.
     
  14. VoodooMojo

    Active Member

    Nov 28, 2009
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  15. logicrules

    Thread Starter Member

    May 3, 2009
    16
    0
    Thanks VoodooMojo,
    I sent them an email, as they do not have search capabilities in English. I'll see how that flys.
     
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