Strange behavior from Battery Tender

Discussion in 'General Electronics Chat' started by earthtodan, Feb 4, 2012.

  1. earthtodan

    Thread Starter Member

    Dec 3, 2008
    24
    0
    Hi all,
    I use a 1.25A Battery Tender to warm my motorcycle battery (AGM), and occasionally my car battery (900AH sealed lead-acid). The Battery Tender has three charge indicators:

    Solid Red: Battery = less than 80%
    Blinking green: = 80% to 100%
    Solid green: 100% and tending

    If I unplug it from solid green and plug it right back in, it indicates solid red for a minute, and then starts blinking green. This also happens right after I park the vehicle and plug it in, although it stays red longer. In other words, I unplug it at 100% and it jumps right down to 80%. Why would it do this?

    My theory is that the tender only achieves a full surface charge, which disappears when I unplug it. Would unplugging it and plugging it back in actually make it charge the battery further?

    Dan
     
  2. #12

    Expert

    Nov 30, 2010
    16,278
    6,789
    It's not broken. Your theory is right.
    It will charge the battery further...by about .00001%
    Not worth the bother.
     
  3. EB255GTX

    Active Member

    Apr 30, 2011
    54
    2
    First off just a point of interest (sorry to be picky!) but your battery is very unlikely to be 900AH - that's huge! Typical is more like 40-60AH. It's probably 900CCA. Anyway, the other thing that caught my eye is that you are using the charger to "warm" the battery.... what do you mean? That battery will get warm as part of the charge process but that's not the ultimate aim, the aim is to charge/maintain it.

    Anyway, with regard to your question - when you unplug the charger it's perfectly normal for the battery terminal voltage to drop. In order to charger the battery we need to put current into it, and in order to get current to flow into a voltage source we need to apply a voltage higher than the battery terminal voltage. When you remove the charger the terminal voltage of the battery drops to around the nominal 12V, but the battery is now charged, so it can supply current at around 12V until it is flat again. A discharged battery can still have around 12V nominal aross the terminals, but as soon as you try and pull current from it, that will drop rapidly.

    So, unplugging and replugging the charger won't do anything for you, and might be bad for the battery. The battery tender is designed to be left connected to the battery, that way it won't go through a boost/float cycle once charged, it will just maintain the battery charge.
     
  4. earthtodan

    Thread Starter Member

    Dec 3, 2008
    24
    0
    Yes, you're right about the terminology. 900CCA for the car battery, and maintaining the motorcycle battery. I recharge the car battery after I do an hour stereo tuning session with the car off.

    So then, is it a good idea to always put the motorcycle battery on the tender when I park it, or should I only do it once in a while?
     
  5. EB255GTX

    Active Member

    Apr 30, 2011
    54
    2
    Assuming you have one of these: http://batterytender.com/motorcycle.html
    The designed use of the unit is to leave it connected permanently.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,002
    3,231
    If you use your motorcycle at least once a month or so you shouldn't have to charge the battery. An AGM battery self-discharge rate is 1-5% per month.
     
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