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battery powered device, reviving some old UPS batteries...

Discussion in 'Power Electronics' started by BGB, Jan 26, 2015.

  1. BGB

    BGB

    154
    11
    Nov 30, 2014
    well, I am building a battery powered device (a small/modest-sized rolling robot, around 15x17 inches, or 30x34cm, with mostly cardboard construction), seemingly relatively low power (around 10A max), and had a few old/used UPS batteries (lead-acid, 7.0Ah, AGM/VRLA style).

    don't know full stats, but info from batteries:
    BATTSCO UN7-12 7.0Ah
    Valve Regulated Rechargable
    Constant Voltage Charge
    * Cycle Use: 14.5-14.9v
    * Standby Use: 13.6-13.8v
    * Initial Current: Less than 1.75A
    (some warnings and stuff)

    (elsewhere, found that they are AGM, couldn't find rated current, but "similar" batteries tend to have a max current of around 100A).

    both were reading 13.2v, so initially I figured they were probably still "good enough".
    trying to run the thing on them was pretty unimpressive.


    one failed pretty hard, voltage dropping to almost nothing as soon as I tried to run the motors (but the voltage would quickly jump back up as soon as the load was disabled).
    another was able to run one of the motors for a little while, dropping to around 4 volts, but fairly quickly faded out.

    note: the robot runs ok off a lab power supply and there is no evidence of excessive power use.
    side info: wheel gear reduction is approx 50:1, with 3D printed gears (and power-tool motors), and the frame construction is mostly cardboard. motor driver electronics is a dual H-bridge built using Darlington transistors.


    drained them a bit, got them both down to around 12.8v, and then started charging them (some yesterday and some today), first at 14.4v, but then looked at the side of the battery and noted that the rated cycle-charge voltage is 14.6-14.9v, so had been charging them a bit at 14.8v (them pulling currently 0.15A each).

    initial results looked promising, as they are able to spin a motor ok (which normally draws around 2A), and the battery "only" dropped to around 7v in a recent test while running the motor (and it seemed to hold up a little better, jumping back to 13.4v when the load stopped).

    ADD: after a few more hours charging, both batteries hold around 10-11v when running a power-tool motor. current battery voltages are 13.6 and 13.7v.


    don't necessarily need the full current, but a good solid 6A-10A would be nice.

    so, yeah, still pretty weak battery output...

    not really sure if anyone has had many relevant experiences with this type of battery.

    don't have any other UPS batteries available ATM, don't want to use a starter battery as this is a bit large/heavy/overkill for this...


    AFAIK:
    charge cycling may help (where the battery is run-down and recharged a few times);
    going above the rated voltage may result in "gassing" and a loss of electrolyte (very bad), whereas staying within the rated voltages should keep the gas from escaping too much (before it can recombine back into water in the battery);
    charging more at this voltage may help, but I don't really know how long is "safe?".

    thoughts?...
     
    Last edited: Jan 26, 2015
  2. BGB

    BGB

    154
    11
    Nov 30, 2014
    status:
    several days of intermittent charging and some cycling.

    can get around 2A for several minutes from the battery...
    box-robot needs around 5A to move both motors...

    yeah, these batteries, not so good...
    I estimate they are operating at around 1% to 2% capacity.

    may try to get some other (hopefully better) UPS batteries (or buy new ones, but similar-stats new batteries are around $17 to $20 each...).
     
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