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Cautionary tale for custom battery builders

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    I think many of the tool batteries the BMS is in the charger. You can get a clue by looking at how many pins there are in the connector. If there is only 3 the 3rd is some sort of a data line and the BMS is in the pack. If there are like 7 pins then the extra pins are the balance leads that connect to the BMS in the charger. My guess there is cost (to them not us) savings by having the BMS in the charger and tool rather than in every pack.

    I have not really done RC stuff lately but what voltage do they run at? I was thinking that stuff was lower volts so fewer sets compared to the 48v+ in the bikes. Keeping 5 or less groups in balance is less tricky than the teens we got in the bikes or the 7000+ cells in a Tesla.


      The biggest power LiPO batteries I have seen for RC use are 22V, but some motors for large drones can handle upwards of 96V. I figure in those cases, you have to series up those LiPO's, which do not have a built-in BMS of course. They want those batteries a small/light as possible, so those rely on external balancing.



        Originally posted by paxtana View Post

        Check this one out, caught fire while he was on it.

        Aftermarket battery, likely in parallel with a completely different battery, oh and it didn't even have a bms. Guy must have a deathwish.
        I wondered how long that guy was going to stand there holding that burning inferno...LOL! On that first vid with what was supposed to be a scooter battery...aren't a lot of those rigs pretty much cheap POS and therefore probably not equipped with the best batteries and chargers?

        It looks like there's some good battery experience responding on this thread, so I'd like to ask a question. I have a lot of automotive and motorcycle mechanical experience, but little of that transfers to these ebikes to which I'm very new at. When starting at the first of the year doing a bunch of research on ebikes, I read about batteries getting warm while under relatively hard use. I realize "hard use" can vary quite a bit. I have a 52V, 17.5AH battery for a BBSHD used on a fairly aggressive MTB ridden decently hard. On the few rides I've done, I've ridden the bike hard on technical trail on fast pedal assist and little to no throttle. The 500C display, when I can glances down at it, often shows 200 to over 400 watts. I can't tell how long this goes on, because as you know it registers quickly and seems to drop quickly while riding aggressively.

        I'm using the battery in a good, padded backpack with good extension cables routed properly. I stop and feel the battery and have never felt anything above ambient temp. I feel the motor, and it has some warmth but nothing concerning from the reports and info I see on these sites. So...apparently I take it that even though riding hard and using the motor to a good extent while riding like this, this isn't the kind of riding that really taxes a BBSHD or a good battery. I have programmed the motor to "smooth out" power delivery, so the motor isn't hammering on and off like it did to some degree before programming. What does it take to actually heat up the battery while riding and/or what kind of riding? Even charging this battery on one of those Luna advanced chargers at 3 amps, I'm not sure I can feel any real warmth. And yes...the battery is working great in use, and my multimeter checks compare to the charger readout almost perfectly. I'm also impressed that the display reflects that same battery level when I hook it up.


          Stress on the battery will be about what it's built to do, vs. what you make it do. Running a battery at its performance limit is what is really bad, because that means it will heat up. High heat is very bad for ebike batteries.

          As an example, a 52V system with battery rated to 30A max. constant current will be highly stressed if running near 1500W (52X30). The same voltage battery rated to a 20A constant output would be highly stressed at or over 1000W (52X20). In my case, I built with a controller that tops out at 40A, so my battery is built for a 60A max load. That decision was a compromise of cost/size, and will let me run at top speed, with what I consider acceptable strain on my battery. I am now building up a pair of smaller batteries for when I go trail riding. Those will be designed to run at a max of 20A, because I intend only to run at power levels comparable to 500W setups. Even tuned to run at 750W, my controller would only pull 15A constant current.

          Hmmmm ........ I guess that based on what I have learned, I am recommending the following when building/buying a battery. If you were to run your controller at max. power, your battery would ideally be at no more than 75% of its rated output. I'm curious to see if others have a similar recommendation.



            Don't forget to do your calculations at lower charge states too. A 52 volt at 100% is 58 so 1500w is only 26 amps. When you get down near your BMS cutoff of 42 you are at 36 amps.

            I would think if you really were planning on pulling near your max power most of the time you may even want to double that rating. 20 amps seems easy and 30 seems to still be reasonable but more than that really seems to not be as practical. Going to a higher voltage solves that and in most cases we already are carrying the cells they just need to be arranged differently.


              Good info...thanks. Apparently I am not stressing my 52V battery or the BBSHD with my application. I think some of the bad press on blown controllers with the BBS02 had me gun shy. I went for a 15 mile mountain bike ride yesterday in some decent ambient temps. The trail is very technical in some places, fast in others, and some good climbs. Apparently none of it is at a level that heats up the motor or battery. I used the throttle a bit in some rocky, technical spots to just keep momentum while still pedaling most of the times. I would love to have seen the 500C readout on the wattage, but there just wasn't any way to do that for the brief time it's on the display in those conditions...LOL! I would occasionally feel the motor and slip off the backpack to feel the battery...lukewarm motor and basically nothing out of the battery. I'm not a scared ninny, but I guess I just wanted to know what to expect in the long run with the motor and battery. From my off road rides so far and the info from 73Eldo and DaHose, it looks like I'm well below the threshold of any concern.


              • Dshue
                Dshue commented
                Editing a comment
                That's the same as I get from my BBS02B setup. I am going to take along a laser thermometer one of these days. I suspect that a lot of the heat people feel on the case of the motor is actually absorbed heat from the sun. I got worried once, turned out my top tube felt just as hot. But my battery in a heavily padded case always feels cool.

              I take offence at the title of this post........
              correct me if I am wrong but the explosion show was a cheap “retail” pack???

              it should be titled “ignorant cheapskate battery users beware”

              I do think there are explosions from DIY packs, but usually by the time you learn enough to build a pack you know some basics that would prevent a lot of the issues.

              the only fire I personally know of was a friend who’s dolphin pack heated up because of undersized tabs on the bottom...... that was not a “diy” battery.


                I think the fact that it was a retail pack, is even more reason for DIY builders to be EXTREMELY careful. Retail batteries are built by companies who get UL approval, have vast legal teams protecting them, and insurance to pay if their products cause damage.

                If your hand-built battery were to go oops upside someone's head, then YOU are liable. That means your home, car, paychecks, retirement and ANYTHING of value can be taken by the person suing you. Those of us building batteries always need to keep things like that in mind.



                • Upnorth
                  Upnorth commented
                  Editing a comment
                  I'm still looking for the UL label on my LUNA Wolf..............
                  Can't find it anywhere.

                • AZguy
                  AZguy commented
                  Editing a comment
                  Keep looking!

                  I doubt any of these batteries have legitimate UL listings LOL... you never know but I don't see UL (or any other safety lab) thinking highly of pretty much any of them

                Please be realistic. Not everyone lives in a litigation based society.
                Be careful. Be reasonable.
                i choose to learn and be careful. I don’t lose sleep be because I am afraid of being litigated.
                i choose fun enjoyable activities, I do my best to not impact others.

                go on and live in fear.

                The UL stamp doesn’t make it safe, I would not feel good knowing I put someone at risk but my ass is covered because of the stamp that a foreign company paid for .

                like I said I am willing to bet dollars to donuts that all of those fires in the videos are from batteries WITH a stamp of approval. And I am also willing to bet that that company can afford better lawyers than you.


                • AZguy
                  AZguy commented
                  Editing a comment
                  I'd bet those batteries were more likely built by fly-by-night chinese companies that nobody could touch from a litigation standpoint and doesn't bother with lawyers... and if they have a UL or any other any other kind of "stamp of approval" it's also bogus - probably has misspellings ;-}

                  But you never know...

                Ok, Upnorth. You go ahead and think what you want, and do what you want.

                I will continue to be very careful, and encourage others to do the same.