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Paralleling BMS protected Li-ion packs.

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    Would love some input on my take/idea/plan for permanently connecting a second battery.


      That is what I've done. I have four Lunacycle Wolf 52V batteries in parallel through Schottky diodes on the output side of the BMS.
      The charge side are just connected in parallel. I went this way as the diode will cause a voltage drop and didn't want the interference with the charge controller. I have both AC as well as solar charging.

      I recommend you use matching batteries with similar recharge cycles.
      Last edited by Gadgets4grls; 10-13-2020, 08:10 AM.


        How many kwh are you running?
        do you still use your solar panels?
        sorry just read you are using panels.
        Any changes in past year.?
        Last edited by Gr8fun; 10-13-2020, 09:34 AM.


          Originally posted by AZguy View Post
          The power that a passive switch has to deal with is quite high and switches rated for it are very expensive and on the large side of things - it better be break-before-make!

          I carry two batteries mounted to my bike and switch between them by very simply changing which [XT90s's] connector the controller is plugged into... easy peasy, takes seconds, looks very clean, they handle the power and are very inexpensive

          IMO the only good reason to parallel batteries is if you need more current than one can provide, and even then, not for me, I'd just budget a proper battery for the application

          I was working on a solid-state solution a couple of years ago and recently dusted it off because my business slowed to a crawl and I prefer to stay busy... It's non-trivial to pull off a cost effective approach... I may look into a load share solution but that's for another day...
          Hi AZguy - would a solid state solution require a heat dissipating mount point (such as the seat tube bottle mounts)? I do think there's a market for a plug-n-play dual battery box that offers ether Anderson or XT-90 connections - add a USB charging socket to further sweeten.


            Originally posted by ncmired View Post
            Hi AZguy - would a solid state solution require a heat dissipating mount point (such as the seat tube bottle mounts)? I do think there's a market for a plug-n-play dual battery box that offers ether Anderson or XT-90 connections - add a USB charging socket to further sweeten.
            Solid state will have to shed heat

            I've got a project like this on my back burner... seems every time I get back into it I get distracted by something else - regardless the critical part has always been the thermal portion of the solution and the rest is pretty easy


              Would one of the "Super Barrier Rectifier" components help, or are they just marketing hype?

              I was looking at this part, rated for 60 amps at 300 volts (up to 175C), that have a rated forward voltage drop (at full power) of 0.94 volts:

              Pricing small quantity is about $1.50 each. I've a desire to tie together equal-spec 17AH 52 volt packs, as well as a 30AH with a 17AH 52 volt. I'm guessing that I'd need to dissipate peaks ot 30 watts, maybe doable via a water bottle boss bolt and a short aluminum tab?


                I think the challenge you will have with those is heat

                If you are dissipating ~30W (let's just go round numbers at this point) and the maximum junction temperature (Tj) is 175°C and thermal resistance from junction to case (θjc) is 8°C/W then you'd have to keep the case at 175°C - 30W × 8°C/W = 175°C - 240°C = -65°C... clearly not practical

                If we work it the other way and say you can keep the case at 50°C (not easy at all - need excellent heat sink and temperatures here get to 50°C so in that case it would have to be an infinite sink but we're sticking with round numbers) then the maximum power you could dissipate would be (175°C - 50°C) ÷ 8°C/W = 125°C ÷ 8°C/W = ~15W... but that's not likely practical

                I did this from the datasheet for the part with the regular tabs (attached) and not the plastic ones like you posted the picture for since they were ready at my fingertips but I doubt you'll get much difference between them
                Attached Files


                  Take a look at this part I've attached data for that you might get you noodling on a more efficient solution

                  Attached Files


                    Thanks much, AZguy! I'd like to have extra-long range as an option for a few select ride situations.


                      I have two battery mounts on my BBSHD bike (one on rear rack, other on downtube) but opted for the bone simple solution of just putting in a couple of XT90S's

                      It's reasonably clean and and doesn't get much simpler - definitely a lot more efficient!

                      Plugged into rack battery:

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                      Plugged into downtube battery:

                      Click image for larger version

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                        Originally posted by calfee20 View Post
                        Do you have a link for that.
                        Here are two links to it:

                        Note that it seems to be "unavailable" in multiple outlets, to it MAY be a failed product.
                        Product Description Lithium Battery Shunt Connection Module, Parallel Connection Module Are you still annoyed for small capacity of your ebike battery? Do you want to get super long distance for your ebike? This is the cheap and smart solution, a simple and well designed parallel connection module will solve this problem. With our module, you can connect two batteries in parallel to have super long riding distance. Battery will be switched each other automatically according to left voltage. Parameter: Resistance: 0.65mohm/Channel Input Voltage: DC 0-60V (suiltable for 48V or less Voltage Lithium Battery) Input Current: max. 40A/Channel Output Current: max. 80A Standby Current: <4mA Grade: Military Grade Connection Diagram: 1: Positive Pole of Battery A Discharge 2. Negative Pole of Battery A+B Discharge, Negative Pole of Module Output (connect with controller power input - ) 3. Positive Pole of Module Output (connect with controller power input +) 4. Positive Pole of Battery B Discharge
                        1.Resistance: 0.65mohm/Channel
                        2.Input Voltage: DC 0-60V (suiltable for 48V or less Voltage Lithium Battery)
                        3.Input Current: max. 40A/Channel
                        4.Output Current: max. 80A
                        5.Standby Current: <4mA
                        6.Grade: Military Grade


                        • calfee20
                          calfee20 commented
                          Editing a comment
                          Someone posted that before somewhere and I pointed out that they are using XT-60's which are marginally adequate.

                        • AZguy
                          AZguy commented
                          Editing a comment
                          This overseas stuff

                          When it comes to current and power the max ratings are far too frequently just very short time maximums and have little to do with what they are truly setup to do on a continuous basis

                          XT-60's sort of lends something to this line of thinking

                        Originally posted by New Mariner View Post
                        If that piece switches between batteries, it is not a true parallel connector
                        That is true. Functionally, It looks a lot like an idea I was toying with:
                        Using solid state switches (SSS) or MOSFETs. Not considering for charging configuration, but...
                        I could see a design that turns a SSS (or MOSFETs) off or on depending on the voltages of the battery packs. I expect some hysteresis would be required as a "switched-off" battery's voltage would recover somewhat with time. As one battery's voltage drops below threshold, it is disabled and the second battery is enabled so quickly the system won't care. Seems to me the losses theoretically could be significantly reduced over diode solutions, and you'd avoid any BMS issues of paralleling 2 packs. Basically just trying to emulate an automated mechanical switch implementation.
                        One could even design the control circuit to automatically switch back and forth between the 2 battery packs to even out their voltages and discharge states over a given ride. For example, could enable one battery for X minutes of riding, then auto-switch to the second battery for Y minutes (X could = Y) of riding, and back and forth until both packs are depleted to point of being disabled. OR, a similar approach but using deltaV as the driving parameter instead of time. Of course, once one pack is depleted to desired threshold (e.g. 30V in "36volt" pack) then power would only be enabled from second battery until it, too, is depleted to threshold.
                        Last edited by jGecko; 11-09-2020, 03:10 PM.


                          For what it's worth, Louis Rossman of Rossman Repair Group posted an interesting solution. To be fair it was about how last month his unitpackpower battery burned down so I guess take it with a grain of salt whether it is a good idea, though I have seen several other reports of that company's batteries catching fire, including one sold by Doug over at cali-ebike where it burned the whole house down. So probably a defective pack rather than the bridge rectifier. Any feedback on this design?

                          BTW this would not be compatible with Luna Wolf batteries since our BMS is not set up that way, but our tech Seb found it to be a rather clever way of going about it so maybe someone might find it interesting.

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                          Last edited by paxtana; 11-09-2020, 07:46 PM.


                            I was just thinking a possible reason to do parallel batteries is if you operate in freezing conditions. If you are swapping batteries at that point the not being used one's temp will drop from not being used so when you do swap it in it may be at reduced capacity just from being cold and your now dead one will be at greater risk because a dead battery will freeze easier and experience more damage from freezing than a fully charged one. If you are running in parallel at least both will be used and the use keeps some heat in em.

                            I have been watching Louis for several years even though I don't use mac books or any other apple products. I just like watching the way he goes about troubleshooting and then the more or less rants that come up during repairs or in their own videos. I think one of his videos about his first Bafang build was the first time I ever really heard the name or any name other than Bosch or Shimano when it came to somewhat legit E bikes.

                            I had seen several videos where he mentioned a bridge rectifier but thought he was misspeaking and meant it was more of blocking diode isolater sort of setup. I don't think he shared the diagram till around the time of the fire. I guess it should work and you can find bridges in all sorts of different packages with and without heat syncs.

                            He just recently posed a video about his most recent post fire build. Its another Yuba cargo bike with a BBSHD, phaserunner, cycleanilist, and this time 4 Em3 packs with an option for a 5th. He mentions a '3 phase bridge' this time. At first I was having trouble figuring out how that would work if it was a packaged device. By packaged device I mean some sort of block that just has 5 terminals as opposed to 6 individual diodes bolted to 2 different heat syncs that become the DC outputs in the typical usage. I was thinking he would have to have the individual parts and modify the path so it was different than you would use in a normal 3 phase bridge but after drawing it out I think a package unit would be fine.

                            With the standard 'full wave bridge' which is the diamond arrow looking thing in the diagram above pin 4 at the bottom where he has the charger input would normally be the DC - out. Pin 1 at the top would be DC + out and he is still using that as the DC out to the controller. Pins 2 and 3 would be your AC in or in this case each battery. If you think of the diodes (the arrows) as check valves it all makes sense. The power can flow in the direction the arrow is pointing but it can't flow backwards, think if the line at the point as the flapper that shuts and won't let it back flow.

                            If you look at his diagram you can see how it works. Battery 1 in can flow to the output but not back to the charger or over to batt 2. Battery 2 can flow to the out but not anywhere else. Charger can flow to both batteries and the output but because of the checking nature there is no loop that would let anything cross between the batteries.

                            The 3 phase bridge just adds another set of diodes in the center connecting between 1 and 4 and they are the same direction. Your 3rd phase connection is between them and I suppose you would label 5 in this case. He must be running the 4 current packs in parallel pairs so the '3rd phase' is currently unused and is the input for a 3rd pack or set of packs that he mentions.


                              I have now read every single post in this thread and have concluded that I have done just about every thing wrong. As I strive to up my knowledge, I have opened a few doors I would have just as soon not opened because it means revisiting the drawing board.
                              A little background. I built up a titanium Fat bike with a Luna BBSHD and 52V 14Ah GA Shark Pack from Luna in early 2016. I now have 13,000 miles plus on this rig with no major issues. I no longer run rough single track but still do gravel , sand, snow, pavement, two track and easy single track. I have migrated more towards Bikepacking/Dirt touring long distance so I added battery capacity to the bike. I have my original battery with about 300 cycles one it, in parallel with a Luna Mini 52V 7Ah GA (100 cycles) plus two Unitpack Power 52 Volt 14Ah Samsung E shark packs. (20 cycles) Total online power is 2,538 WH. No fuses on discharge. I have tow chargers. A Luna 5 amp charger and a Chinese 15 amp charger. I have been using the big charger through the discharge ports with all batteries on the bike. I have used the smaller charger through one bayonet charge port on the Luna 14Ah battery.
                              98% of the time I am below 750 watts motor output but occasionally go over and seldom hit the 30 amp max battery pull. I generally cruise between 16 and 20 mph and often pull a trailer. Me and the rig go anywhere from 290lbs up to 425 lbs. Longest one day ride so far has been 130 miles into a stiff headwind with the trailer at full load. (3,354 Wh) Charged twice for 45 mi and ended at 48.8 volts. .

                              The fact that I have had zero issues is probably dumb luck. I am exploring options to keep charging with the big charger but doing so with as many precautions as I can live with at this point. I have considered making the following harness (Photo) and charging the batteries at the same time off the bike. I still want to be able to put 800 watts an hour in the tank for long distance travel.

                              I am open to other suggestions. The above harness I can make immediately and still continue to ride. I do not store the bike with the batteries on it. .


                              • paxtana
                                paxtana commented
                                Editing a comment
                                IDK about the overall design but I would make sure that's a fast blow fuse