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Parallel adapter for doubling battery capacity

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    Parallel adapter for doubling battery capacity

    Be careful with this one. Both batteries should have identical cells and charge level (mostly the same voltage). If voltage is very different the packs will equalize quickly, potentially damaging the pack.

    When you are ready to charge, do not charge while the packs are still in parallel. Parallel charging could be dangerous because if one BMS fails it will continue to charge. Packs MUST be unplugged from eachother before charging.

    For more info:
    Last edited by paxtana; 06-13-2016, 12:06 PM.

    Can't this adapter be fitted with a pair of diodes to prevent one pack from "charging" the other pack?

    A diode on each of the positive lines prevents current from flowing back to the pack and only allowing the packs to supply power to the motor system.

    Just a thought.


      Yes this is covered in the "more info" link above.


        I have 2 packs. One is the 11AH Shark and the other is the 13AH Shark. (Obviously both were purchased from Lunacycles.) Buying this adapter ready to go, makes a lot of sense for someone with the real estate I have on my trike. Both battery packs ready to go without the hassles of swapping them out. Just attach and ride. Since the Bafang BBS02 doesn't have regen, trying to charge through the discharge port is a non-issue for both packs. Preventing one from trying to charge the other is a major concern, since they have a different capacity.

        I guess the question was to ask why the diodes aren't already included in this adapter. The cost of the diodes themselves is a minor issue, adding a couple of bucks at most to the over-all costs of the adapter itself. From a safety standpoint it would seem a good idea. For those who use a regen type of motor the diodes would prevent any charging via regen. It blocks the current that would go on the discharge side. An adapter with a diode isn't something that someone with regen would want, I suppose. For the rest I expect anyone else with 2 or more packs would jump at it.

        From a marketing standpoint as well. "We have the only parallel adapter that prevents packs charging themselves. Buy this from us and protect your packs!"


          It is still a mostly theoretical design. We have not gotten enough feedback on it and I am not convinced that particular diode is the most ideal choice, it would involve cutting each diode with wire cutters before soldering. And which diode you want depends on how much peak current you use on your ebike, which will vary a lot. Plus I'm not sure we've seen enough demand, it's a lot more than a few dollars to have a factory do a production run.

          If using more than two batteries you would also be right back to needing to make it yourself, or there would need to be another production run for each number of packs in parallel, as well as another production run for each typical current level. It could easily end of costing many thousands of dollars to get a full production line going, with an uncertain level of demand.


            I'm still working out the details on this, and have come across something that looks like it may work.

            After talking to Josh from we found this part: NIW5dDZ0s%252blJb91DOUcxrVHfQQ%3d%3d which has a 100V Maximum voltage across the unit, and a maximum of 40A per side of the diode. With the two sides on it, it can handle a maximum of 80A, which is a lot higher than my motor could even think of drawing. (It helps talking with someone that's more current on electronics than my HS electronics classes from 30+ years ago.)

            Connect one battery to the "left" anode, the other to the "right" anode, and the motor power connecter to both cathodes. The batteries are isolated from each other and the motor/controller can draw from each pack equally. The rider can still use just a single battery with this in-line if so desired. Or you can run both batteries for extended range without them cross-charging each other.

            The diode package is about 1.5" long, 1" wide and less than .5" thick.
            Cost of the diode is about $25/unit when bought individually. Bought in bulk I would guess the prices would be less. Not sure what this would work out to for a retail pricing.

            I'll be testing this in a couple of weeks, and will report back what I find here.


              Very interesting. Is there any difference other than packaging between this and using 2 separate diodes and wiring them up the same way?


                In a way. Most of the diodes we talked about had current and voltage limits that were borderline. For example, 60V but max 30A for both legs (15A/leg) in a "flat pack" design. This would require 1 diode for each battery pack, and would ultimately be more expensive than the one I linked to above. ($16-$19 per diode. so at least $32 for 2 batteries.) For those with the room for more than 2 packs, these smaller diodes wouldn't work if trying to connect 3 or 4 to the controller.

                This one, while a bit more expensive for a single device, fits the voltage and current for at least my use case. (Bafang BBS02 with 48V battery packs.) It it's all in just one set of connections. (One from each battery and then combining both outputs to your motor/controller.) The size of this isn't much more than the size of the connectors themselves. Once the whole thing is made up, wrapped in some heat shrink, and there it is.... Unlike other diode examples, this could be used to connect a 3rd or 4th battery in parallel. A 3rd diode would be needed for 3 or 4. The input for each battery on two diode sets, and their outputs feeding the inputs on the third diode. (This would be for trikes or bikes with trailers or cargo options. Probably not for DF bikes, unless you have a lot of space for batteries.)

                If the motor/controller draws no more than 40Amps, the 3rd diode should be safe from blowing either of the two legs. (And this would be the weakest spot of the whole design, I would think...) You could fuse each battery, or run a single fuse after all the packs are hooked in parallel. Remember, at this point it's all theoretical... No one has actually done this and reported on it here, yet... (Or if they have, I haven't seen it.)