Please help me create simple mAh counter.

Discussion in 'The Projects Forum' started by HugoW, Oct 8, 2008.

  1. HugoW

    Thread Starter New Member

    Oct 8, 2008
    2
    0
    Hi,

    for my RC model elecric planes, I would like a very simple, lightweight gauge that tells me the amount of mAh that left the battery. It should hook up between the battery and the rest of the electronics, and only have a 4 digit display. A reset would be nice, and the max current would be say 80 amps, take 100 to be sure. Voltage is 22.2 V from a 6S LiPo, but it it is possible to use anything from 10 to 40V, that would be nice. I don't want datalogging, I only want to measure what came out of the battery pack

    I know there are many dataloggers commercially available, but I like diy and I don't need the logging abilities.

    Any help / input / idea is appriciated,
    Cheers,

    Hugo
     
  2. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Fuel gauging is a complex problem, unfortunately. I am actually designing 6 fuel gauges right now for a small power system on an autonomous vehicle, it has been very difficult.

    I ended up going for a TI part "bq20z95", since it is a single chip solution and doesn't require a lot of external components. The kicker is, it has about 300 registers to program. Fortunately, TI has software to work with an interface board to program the part. It also has charge balancing, which is a necessary feature if you value your battery life and longevity. It only works up to 4 cells, so it cannot work in your app.

    I took a look around a few sites and cannot find a 6 cell gauge, I'm sure they are out there though. It really seems that there isn't a simple solution though. Perhaps if you are fine with a ~15-20% inaccuracy, you can rig something up based upon the voltage of the cell, rather than coulomb counting.

    Steve
     
  3. neon9

    Member

    Oct 8, 2008
    15
    0
    First of all you can check the battery voltage on the battery that will tell you absolutely nothing. to check a battery a typical load must be applied and even then because of loading and the amount of testing time will vary the results. solution put a meter across the battery then apply the load and see if it maintains the voltage to be usefull. the electrolite ph factor check is the only way to find health on a battery. there is not and never will be a meter that will tell you the battery live . variation in temp., cycles time cycles of type of operations, all come in to to be considered. good luck
     
  4. HugoW

    Thread Starter New Member

    Oct 8, 2008
    2
    0
    Hi Steve,

    thanks for the reply. The voltage is indeed a problem, but if I drop voltage I need to increase current (power to prop remains), which requires bigger (heavier) controllers for the motor. Which I seriously want to avoid. I'll look into the bq20z95 direction, and see if I can find something similar or a way to fool it.

    @neon9; I'm not looking for a life-gauge. I just want to know how much I discharged from the pack, compare to how much I charged into the pack, and so determine whether there's enough juice left in the pack for another flight before charging again.

    Cheers,

    Hugo
     
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