No announcement yet.

Paralleling BMS protected Li-ion packs.

This is a sticky topic.
  • Filter
  • Time
  • Show
Clear All
new posts

  • Brendon1128
    Would paralleling batteries affect regen braking? Im running golden motor hubs which have regen. I love it because it saves wearing out my physical brakes

    Leave a comment:

  • Sebz
    commented on 's reply
    Thank you, corrected.

  • user2349
    >you can use the hole as the positive for the battery
    Can you verify this? The diagram for the 30a diode looks different than the 40a, the cathodes/anodes are swapped. It seems this comment was thus relevant for the 30a diode, not the 40a, assuming the 'Load-circuit layout' diagram in both
    Last edited by user2349; 06-12-2017, 10:16 PM.

    Leave a comment:

  • paxtana
    This should help:

    Click image for larger version

Name:	200764132932_parallelwiring.jpg
Views:	24223
Size:	50.1 KB
ID:	33489

    Leave a comment:

  • New Mariner
    commented on 's reply
    That is exactly what i want it for. Have 2 minis on the way, but that is not many ah, but didnt want to pay for 2 more. 18ah should be enough. Any instructions on how to do this?

  • Sebz
    You can use 3 pack if you like... I'm using 3 X 30q minis now

    Click image for larger version

Name:	IMG_0715.jpg
Views:	17880
Size:	222.2 KB
ID:	33267

    I made an adaptor for 3

    Click image for larger version

Name:	IMG_0713.JPG
Views:	18086
Size:	334.2 KB
ID:	33268

    Leave a comment:

  • scuzzycard
    commented on 's reply
    It does not have to be an even number of packs, just same cell type and capacity.

  • New Mariner
    Can 3 mighty minis be wired in parallel or would you have to use 4.

    Leave a comment:

  • Sebz
    commented on 's reply
    the forward voltage loss is 1 volt... it's a pretty big loss.

  • paxtana
    Yes you can use a couple mighty mini packs in parallel. yes you can use with 50 amp but I wouldn't push it much more than that

    lordbaconpancakes: I'm not sure we can advise you on that since it is mostly theoretical.

    If in doubt just make sure that you unplug the packs after using them and just charge them separately to roughly the same voltage before plugging back in parallel.

    Leave a comment:

  • New Mariner
    When using the mighty mini samsung cell packs, can they be wired in parallel and used for a 50 amp motor? Of course taking the necessary safety precautions as noted above

    Leave a comment:

  • lordbaconpancakes
    I found this Schottky diode will it work ??

    Leave a comment:

  • Sebz
    started a topic Paralleling BMS protected Li-ion packs.

    Paralleling BMS protected Li-ion packs.

    It's getting harder and harder to fit normal packs on to full suspensions bikes with small triangles and with the new small mini cubes that Luna offers are a godsend....but maybe one is not enough?

    Or you just want to get more range without having a big triangle pack so paralleling has been a question that is coming more and more frequent.

    So what is paralleling
    battery pack? it's taking 2 packs and connecting the red positive lead to the red and the black negative to the black, the output voltage stays the same but the maximum current capabilities are doubled and so is the pack capacity!

    Here is a basic diagram of 2 "AA" batteries in parallel to explain it simply:
    Click image for larger version  Name:	SerPar_02.jpg Views:	1 Size:	59.7 KB ID:	20726

    As you can see , volts remains the same and capacity doubled.

    Now can we do this on our Li-ion 18650 packs?

    To answer this, the very same pack you have is in fact a bunch of single cells put in parallel then these parallel string are put togeter in series.

    Let's take the famous 14ah Shark 52V Panasonic GA pack for example: it's a 14S4P pack, it's 4 cell in parallel (4P) giving 3.7V and 14ah, then 14 of these strings are put in series making it 52V 14ah. Then a BMS is mounted on the pack to protect it against overcharge, over current, under voltage, high temperature, short, reverse voltage, unbalance and etc...

    Now can we parallel 2 complete packs together and get more range and more current?

    Simply put yes! If both packs are new and they are identical (same voltage, same cell model, and same number of parallel strings "P") you can plug them in parallel and they would just equally share the current draw.

    Ex: 2X 14ah shark packs, they both have 30amps continuous rated BMS, tighter they make a 52V 28ah pack. So if lets say you run them at 30amps for the whole trip (quite impossible but it's an example) both pack would share equally the load, so they would both give out 15amps of current till they hit the cutout from the controller. After this trip if you then unplug them completely they should both have roughly the same low voltage as they had the same current draw. This is the ideal solution.

    The what if's? (Q & A)

    Q: What happens if they have not same voltage when I connect them together?
    A: The stronger pack will try to charge the weaker pack through the discharge port till the voltage is equal on both packs. depending on the voltage difference the current can be really high.

    Q: Isn't it good that they charge themselves like that?
    A: NO, depending on the type of cell you have (the internal resistance), the voltage imbalance between the packs and the type of BMS you have can result in problems. The internal resistance will dictate how much current can flow between the 2 packs depending on the voltage difference, and can easily go over the BMS limit for reverse voltage (remember you are charging through the discharge port). This can result in BMS damage or cell damage.

    Q: What to expect in terms of cross current if I can only get the 2 packs to about 1V close.
    A: Like I said it depends on the internal resistance of the pack, but it's kind of an advanced question. Lets keep it simple with a clear example:

    I'm personally running 2 Luna Mini cubes 6ah 52V with Samsung 30Q's, I recharged one to 58.4V and the other to 57.4V. Will it blow the BMS, will I see massive current?

    Cycle analyst on one pack and batt man on the second pack, (both are very precise compared to a professional multimeter)
    Click image for larger version  Name:	IMG_0250.JPG Views:	2 Size:	133.1 KB ID:	20727

    To measure the current between the 2, I made an adapter to match the female XT-90 on the batt so I could use the Cycle analyst shunt in between the packs. The packs are not connected to the motor. The Cycle Analyst has a Amax feature and records the highest current spike crossing the shunt. And with just 1 volts of difference I measured 1.79Amps Click image for larger version  Name:	IMG_0251.JPG Views:	1 Size:	127.8 KB ID:	20728

    After a few seconds the current was already below 1amps and after a few minutes the current was below 0.1amps and the voltage is settling at 57.7V

    Q: So is it safe to charge VIA the discharge port at 1.7amps?
    A: Yes it is, direct drive hub motors have regen braking and can send current higher than that to the battery. So It's safe for a small current.

    Q: Can I use a large pack and a small pack if they are at the same voltage?
    A: Short answer for inexperienced users, No!

    Longer answer :Connecting two banks with different capacities in parallel is technically fine since the batteries will be operating at the same voltage. Charge and discharge current will be shared, based on capacity. It is best if the batteries are of the same type and age.

    But different packs have different internal resistances. The Ohm's Law tells us that the stronger battery will discharge through the weaker battery -- you will always have current flowing

    So do I recommend it? Not really unless you know exactly what you are doing, this is my advice and you are welcome to share your experience with me...and real life test results too!

    Q: Can I charge both packs with one charger?
    A: Well that is a tough one and there are many theories on it. I did try it with total success as my packs are 100% identical and they both have the same number of charge cycles. they both reached the end voltage. But since both have a BMS that regulates the charging and balancing I do not recommend this procedure, there a tons of different BMS's and no one knows for sure how they actually react when in parallel. If someone has done specific testing on it they can PM me to add to this post.

    So for now the answer is do not do this
    unless you know exactly what you are doing.

    Q: Can I use different voltage like 48V paralleled to 52V?
    A: No, not good, should never do that! NEVER!!!!!

    Q: Can we use Lipo paralleled to Li-ion?
    A: Some ES members done this with protections, but no long term testing/results were posted, so I do not recommend this. Stick with same chemistry, voltage, configuration, BMS, # of cycles.

    Q: What can go wrong?
    A: You are playing with powerful batteries with high discharge capabilities. These packs can catch fire and hurt you badly if mistreated, paralleling just add more risk..... In proper charging, reverse polarity, one BMS failing and the other pushing on it... Anything is possible, stick with what works. If you want to experiment, take precautions.

    Q: Can we add protection?
    A: This was debated on numerous times on endless-sphere with no firm conclusions, some don't believe it's necessary some swear by it!

    First thing we could add to protect cross-current between packs is a diode on the positive lead of each pack.

    What is a diode? A diode is a
    two-terminal electronic component that conducts primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance to the flow of current in one direction, and high (ideally infinite) resistance in the other.

    Better explanation of diodes here

    What type to use, there are so many types and models!

    A diode has a disadvantage over no diodes.... First in forward direction (battery draining in the motor) there is a voltage loss normally of 0.7V between the legs of the your motor see 0.7V less than what the actual battery is supplying and on a 30amps controller that is a whopping 21w... So not a big problem really. Second it generates heat!

    Something better? YES, high power Schottky diodes (used in high switching power supplies). These Schottkys still have a little loss (about 0.6V down to 0.1V) and they have a big disadvantage in general electronic applications, they leak current in reverse voltage (when a battery wants to charge the other for example). Bad thing in electronics but in our case it's exactly what we are looking for, the small current leak (normally a lot less than one amp) can actually charge the other pack making them "balance" if it ever occurs.

    Special thanks to Todd Karin (Luna customer) for pointing out Schottky diode models and sharing his positive experience.

    This model should be good for connecting 2 packs with a 30amp BMS, one diode per battery. This particular diode has 2internal diodes in them so to use it you need to solder the 2 outer leg together for the output and the middle leg is to connect to the positive of the battery.

    I also found this model:
    It has a 40A limit but the form factor is better, you can use the hole as the load (output) and cut the middle leg and then solder the 2 outer legs to the positive of the battery for an easier and cleaner install.


    Theoretically (meaning we did not tested this), you could charge 2 batteries with one charger using diodes, the diode will prevent high current from one an other incase of a short or BMS failure.

    Diagram for charging and discharging using diodes (picture found on an old forum dated 1999):
    Click image for larger version  Name:	isolationdiodes.png Views:	1 Size:	14.3 KB ID:	20856

    One thing is for sure, a diode would help in a scenario involving potential BMS failure with shorted output, or any other unforeseen failure mode involving reverse current.

    Q:Anything else for protection?
    A: Parallel or not you could always use a fuse or a DC breaker... in case of a short....Doesn't hurt, it just add more wires and clutter to the bike...that is the price of safety. You know what they first!

    Q: What do you recommend?

    Stick with same voltage, same chemistry, same capacity and about the same cycle life left.
    Use protection like diodes, charge separately under surveillance and only parallel them once they are at the same voltage, disconnect them when not used or for storage.

    Special thanks to JPlabs for his great

    Also know why ;)
    Last edited by Sebz; 06-14-2017, 08:58 AM.