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Thread: On the subject on POWER!

  1. #11
    willcaddy's Avatar
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    Quote Originally Posted by shakesc View Post
    Worth reading through the FRA rules for guidance on fuse rating
    The fuse rating is set by the discharge current of the batteries. You need to understand that and rate the fuse under it

    e.g. (from wiki but google does the job)

    For a 2000 mAh battery with a 15C rating, the continuous current that may be drawn out of the battery is 2000 mAh x 15 = 30000 mA, or 30 Amps (A) (divide by 1000).
    Quote Originally Posted by RogueTwoRobots View Post
    Also worth mentioning that the fuse should be rated below the burst current, not continuous. So if the above 15C pack had a burst rating of 45C, then the fuse would have to be rated below 90A (45C x 2.0Ah)
    Thank you both! This is really helpful

    Thank-You all for taking the time to help!

    Will

  2. #12
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    Hello all again,

    Just about to buy a battery but I was just wondering what sort of mAh and C rating would be a good bet to go with while using the BotBitz 85A ESCs?

    I'm not sure it it makes to much of a difference but just want to make sure before i buy one!

    Thanks again xD

    Will

  3. #13
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    The current delivery of a Lipo is found by multiplying the capacity in AH by the C rating IE a 2AH pack with a 40C rating can delivery 80 amps continuously. On a good quality pack, the maximum burst C rating is usually double the continuous rating. Its a good indication of an inferior pack if it's burst rating is much less than double the continuous rating.

    Working out the AH needed for a bot is difficult; most builders look at a similar design to see what has worked in the past and you eventually get a feel for it. I guess that you probably need at least a 2.2AH pack and a 3.3AH would give you more of a safety margin and running time - maybe somebody with an axe bot can give you a better idea. As long as you can spare the weight, its always handy to have extra capacity in case you have back to back matches with not enough time for a full recharge.

    Having a pack with a higher C rating and current capacity is a good idea; The pack will stay cooler which means you can start recharging it sooner (its not a good idea to charge a hot pack!) and the higher C packs can generally be recharged at a higher current for a faster charge time.

  4. #14
    Maxamuslead's Avatar
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    Well looking at Hatchet 2's build log Sam used a 2.2 mAh at 6s( not sure if this has changed) so that i am using a 5s, but depending of on how long it last and if i can find one, grab a lower mAh 6s one.
    Last edited by Maxamuslead; 3rd June 2017 at 17:51.

  5. #15
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    Quote Originally Posted by overkill View Post
    The current delivery of a Lipo is found by multiplying the capacity in AH by the C rating IE a 2AH pack with a 40C rating can delivery 80 amps continuously. On a good quality pack, the maximum burst C rating is usually double the continuous rating. Its a good indication of an inferior pack if it's burst rating is much less than double the continuous rating.

    Working out the AH needed for a bot is difficult; most builders look at a similar design to see what has worked in the past and you eventually get a feel for it. I guess that you probably need at least a 2.2AH pack and a 3.3AH would give you more of a safety margin and running time - maybe somebody with an axe bot can give you a better idea. As long as you can spare the weight, its always handy to have extra capacity in case you have back to back matches with not enough time for a full recharge.

    Having a pack with a higher C rating and current capacity is a good idea; The pack will stay cooler which means you can start recharging it sooner (its not a good idea to charge a hot pack!) and the higher C packs can generally be recharged at a higher current for a faster charge time.
    Thanks for the reply!

    If I got a 3300 mAh battery with a 30C rating then that would be 99 amps right? But my speed controllers are only up to 80 amps (with some higher burst ratings). Would this be to much for them and burn them out or is it a case of the speed controllers and motors only draw what they need?

    This is the thing I'm most confused about and I'm probably being over cautious because I don't want to burn anything out!

    Thanks again

    Will

  6. #16
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    OK, I see the confusion - time for a little electricity 101. A good analogy for electricity is water. In this analogy, the battery is a tank of water, the wiring is pipes of various diameters and the ESC is a tap. Something that people tend to forget is that the motors have a lot of wire in them and its quite thin compared to the rest of the wiring in a bot.

    In our analogy, the diameter of the pipes limits the maximum water flow; the wider the pipe, the more water can flow through it. As water flows out of the tank and into our piping system, it's flow rate (current) is limited by the thinnest pipe (wire) and that should be the motor. It doesn't matter if the pipe coming out of the tank is wide or if the tap (ESC) is huge, the amount of water flowing is dictated by the smallest pipe (the motor). You only run into trouble if some other part of your piping or tap is undersized. If you want to explore that further, this is a good start: https://learn.sparkfun.com/tutorials...ms-law/voltage. It might be too much information but if you can get your head around it, you are well on your way to becoming an electrical engineer

    Bringing this back into the real world, the motors with their skinny wiring dictate the maximum current requirements for the other components. As long as each ESC can handle more than the maximum current that its motor can draw, you won't have any problems. Similarly, as long as the battery is rated for more than the combined current of the motors, its going to perform well and not overheat.

    The real trick is figuring out how much current the motors are capable of drawing and how much they will draw in actual use. A scooter motor for the axe usually has a continuous power rating in watts. Using Ohm's law, you can work out the current from the power rating; Current = Watts / voltage. For a 250W motor running at 24V that is 250 / 24 or 10.4 amps. That 10.4A is the rating for continuous use; in an axe bot the motor is only running for a couple of seconds at a time, so it is safe for it to draw WAY more current without overheating. A spec sheet I have for a 270W scooter motor lists its stall current at 99A and that is probably close to what an axe motor will draw when is hits something or reaches its limits. A battery with a peak current rating of over 100A fine and a TZ85A will handle the motor as long as you don't stall the motor for more than a few seconds.

    The current draw of the drill motors is harder to work out; there is less info and more variables like gearing and wheel grip. The easiest way to work this out is to use an on-line calculator like this one: http://sparc.tools/TorqueCalc.php. Unfortunately, it doesn't have any settings for 18V drill motors so I plugged in a 16:1 Banebots gearbox and an RS775 motor, which will definitely draw more current than a drill and be your 'worst case scenario'. With a 125mm wheel and 22V battery, this simulation draws a total of 62A (for both motors) to spin the wheels, so your TZ85A controllers are well over rated. Given that the drill motors are less powerful and the gearbox is probably a higher reduction gives you an even greater safety margin. The calculator also estimates the battery AH rating for a 3 minute match. With the larger motors it estimates just under 2.2AH so your setup will need less for just the drive motors. Adding the intermittent axe motor and a 2.2AH battery will have just enough capacity for a full match as long as you don't go absolutely crazy with the axe.

    What about the current capacity? I like to work out the absolute worst case current draw and match that to the pack's peak capacity. Assuming you are spinning the wheels in a pushing match and also hitting your opponent with the axe, that's up to 161A of current bursts (remember this a is a worst case scenario!). For a 2.2AH pack the peak C rating is 73, while for a 3.3AH pack its only 49. That means a 2.2AH pack with an continuous C rating of 35 and peak or 70C is safe while a 3.3AH pack with a 25/50C rating will also work. Being cautious, I would personally pick a 3.3AH, 35C pack - I never want to be that guy with a Lipo fire .

    Sorry to ramble on for so long, I hope that answers all your questions.

  7. #17
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    Hi, sorry for the slow reply!

    Thank you so much! You have been a massive help! I will make sure you are credited in my build log xD

    All the best

    Will

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