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  • On the subject on POWER!

    Hey everyone,

    I have a few questions about the electrical side of things with my robot that I am building (sorry total noob!) and although I have found some excellent information on the forum, I would like to get some of your opinions for my own specific circumstances if that's OK?

    Firstly, After looking at a few posts i noticed a good few recommendations for running a robot on 24v if you intend to put a motorised axe on it (which I do!). Potentially a nooby question but I have read the spec of my motors and they are able to run at 24V (18V Drill motors) but is it OK to run the whole robot off of 24V with the intention of adding the axe to it at a later date? and how long would be a safe run time whilst over vaulting?

    Secondly, I am intending on using a LiPo battery for the bot under the recommendation of a few people. Would you have any recommendations for brand/model of battery and charger? I have been looking at the Turnigy ones as well as the Turnigy Accucel-6. Also based upon the question above weather its 18V or 24V.

    Finally, I have looked at a good few tutorials now and I'm a little confused as to the best method of working out what thickness/gauge wire I would need to run this off of. The RoboWars Australia videos suggest 12-10 Gauge but I have read on the forum that many use 14 gauge. I have been looking at getting **this** as a starting point. I have also got **these** XT-60 connectors in mind. Can anyone shed some light?

    Again, thank you all for being so awesome and helping this noob out! It is very much apprenticed!

    Will

  • #2
    Running 18V drills on 24V (actually 22.2V for Lipo batteries) is fine, its what most builders do. The motors are likely to get quite hot if you drive the bot hard for three minutes but unless the motors are particularly low quality, the ywill be fine.

    Many UK builders like the Optipower batteries and there is some sort of discount for FRA members. I have used the Hobbyking Nanotech batteries for 4 years with zero problems but some other HK battery brands are not so good. You can get an idea of battery quality at HK just by comparing prices; for equivalent spec packs, the cheaper one will almost always be lower quality in some way. All lipo chargers will charge a range battery voltages up to a listed maximum.

    A 22.2V lipo has six cells in it (usually abbreviated to just '6S'), so a charger that will work with up to 6S packs is what you want - it will also charge lower voltage packs if you need that in the future.

    For wire gauge, think of the wiring loom as a tree with the batteries at the roots and the motors and other parts as the leaves. Start out with thicker trunk wiring and then thinner branch wiring to the motors. In your case, I'd recommend 10g from the battery to the fuse, power link and up to a main distribution point. From there 12g to the axe motor and14g to the drill motors. Using thicker wire isn't bad, it just weighs more and takes up more room.

    Comment


    • #3
      Thanks Overkill for the reply!

      That's really helped! Just one thing, what would you recommend for a switch? I looked at the Botbitz fingertech screw switch but there only rated for 40 amps. Would that be enough for a 24v robot? Would you have any recommendations for UK equivalents? I know in the UK tpunaments you are required to have a link and a switch.

      Thanks again!

      Will

      Comment


      • #4
        Rory from Team Nuts makes the best switches.

        Their the smallest, and quite cheap - whilst also in my experience having the highest ratings. After testing several brands (and blowing them up), Rorys switches are the only thing i found that meets Dystopias slightly higher demands.

        Comment


        • #5
          All my bots have passed UK tech inspections with just a removable link and no switch, so unless some events are using modified FRA rules, you don't need a switch. You DO however need a fuse on the main power line that is rated for less than the maximum current that the battery is capable of.

          I use an XT90 plug for the removable link and all the similar R/C plugs will do the same job. I like the XT series connectors as they have a shroud around the pins that helps prevent the connector coming apart during impacts and their blocky shape is easy to glue into a home made base.

          Comment


          • #6
            Awesome! Thanks a lot!

            I shall get a xt90 and a fuse set up! This might be a dumb question so bare with me ha ha, but do you specifically have to have DC fuse or will any fuse rated for a certain amount of amps do fine?

            Again thanks for your time and knowledge!

            Will

            Comment


            • #7
              Virtually any fuse will be fine ,its the current rating that is important. That said, 'midi fuses' meant for cars & boats are popular and easy to use.



              This type is available in a wider range of current ratings and you can buy commercial bases for them or easily make your own. The smaller blade fuses for cars are also OK but have lower ratings not really suitable for spinners.

              Comment


              • #8
                Is the amperage independent of the voltage ?

                Comment


                • #9
                  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).

                  Comment


                  • #10
                    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)

                    Comment


                    • #11
                      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).
                      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

                      Comment


                      • #12
                        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

                        Comment


                        • #13
                          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.

                          Comment


                          • #14
                            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; 3 June 2017, 17:51.

                            Comment


                            • #15
                              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

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