I'm working on batteries to run an underwater drone and a follow-boat using 3.7 V, 3400 mAh Li-ion cells. I need a voltage of around 18.5 V and around 20 Ah.

The thought-up solution was to make several battery-packs of 5s1p with each battery-pack having its own BMS, and then connecting a number of battery-packs in parallel to supply the drone and boat. The reason for not just creating a bigger pack is simply restrictions on Wh on aeroplanes. Are there any problems with this solution?

I tried to read up on other questions on this page and some people mentioned that by connecting sources in parallel they would fight each other and maybe "self destruct".

  • 1
    \$\begingroup\$ Can you elaborate on what you mean by "restrictions on Wh on aeroplanes"? \$\endgroup\$
    – Hearth
    Oct 4, 2022 at 15:33
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    \$\begingroup\$ Depends. If you plug them into an intelegent circuit with control and maybe some load balancing between them, it'll all be good. If not, the highest votlage pack will discharge into the lower voltage packs and things like that. A basic set of diodes would reduce this issue though. A good BMS design could even provide for the batteries to talk to eachother to balance the loading etc. \$\endgroup\$
    – Puffafish
    Oct 4, 2022 at 15:39
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    \$\begingroup\$ @Hearth There are regulations on the size of rechargable batteries you can take on a plane eg EASA and FAA. \$\endgroup\$
    – jonathanjo
    Oct 4, 2022 at 16:01
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    \$\begingroup\$ The cardinal rule is that you should never connect batteries in parallel until you have somehow allowed them to come to the same voltage first. \$\endgroup\$
    – user57037
    Oct 4, 2022 at 22:23
  • \$\begingroup\$ Would it be an option to make five packs that were 6p, ie 3.7V 20.4 Ah? Then these plug into a single BMS in a stack? Approximately like this but I suppose with longer wires. \$\endgroup\$
    – jonathanjo
    Oct 5, 2022 at 16:32

8 Answers 8



Doing so is dangerous.

The only practical safe way to increase capacity is to connect cells directly and permanently in parallel.

Any other approach is unsafe.

with each battery-pack having its own bms

No, no, no!

Each BMS controls its own protector switch. When one goes off line, its SoC stays the same, but the other battery continues to work. When the first battery decides it's OK to turn back on, the inrush of current from the most charged battery to the least charged battery causes damage (at best) or a fire (at worst).

For example:

  1. Battery 1 is fine, but battery 2 is hot
  2. Battery 2's BMS opens the switch
  3. Battery 1, keeps on working,
  4. After some time, Battery 1's SoC is not very different from Battery 1's
  5. Battery 2 cools off and turns on its switch
  6. Inrush charge from the most charged battery to the least charged one exceeds the limits and damages the cells.
  • \$\begingroup\$ But what if you do like @Puffafish suggest in the comment above? Wouldnt a diode solve this? Sry if im an idiot \$\endgroup\$
    – Soft_Kayak
    Oct 4, 2022 at 19:19
  • \$\begingroup\$ It can work if all batteries have their own BMS as long as there is a shared BMS keeping all the others in check. But that's probably too complicated for a hobby project. Source: I used to test industrial energy storage solutions. \$\endgroup\$
    – Mast
    Oct 5, 2022 at 10:44
  • \$\begingroup\$ > Wouldnt a diode solve this? --- Yes, but four diodes: two to steer the charging current, and two to steer the load current. It's energy inefficient, but it is safe. The diodes prevent current from flowing between the batteries. \$\endgroup\$ Oct 6, 2022 at 14:27
  • \$\begingroup\$ > as long as there is a shared BMS keeping all the others in check. --- Yes, you are correct. A so called "Array master". It is indeed available for large (room-sized) batteries. But for OP's battery? Not practical The array master alone would cost 10 times more that the OP's drone and be too big and heavy. \$\endgroup\$ Oct 6, 2022 at 14:30

As long as all batteries are the exact same voltage, then no current will flow between them. If they are very close, then a small current will flow for a brief time. But anything else (unknown battery states) is at risk of fire or worse. And Davide explains that paralleling BMSes is a bad idea as they are not designed for it.

Power diodes could be added to each input of the drone, so that one pack cannot force current into any other pack. But even if using a Schottky "barrier" diode (for their low forward voltage drop of ~0.2 Volts), 0.2V * 20 A * 2 (derating) = 8W rating minimum and that is wasted power as heat - for one pack.

Another idea may be to use a DC-DC converter on each power input. These are generally quite efficient (likely 90%). This presents a host of other issues though.

If the design can be changed such that each pack powers only one motor (and only shares the grounds and signal wires between them, then this might be ok. One pack dies, and just that motor stops.

Still another idea, is 5s1p batteries only, with all six wires exposed to the drone and no BMS at all. Then a custom microcontroller to manage balancing and protecting all of those cells simultaneously. Sounds challenging.

  • \$\begingroup\$ > Power diodes --- yes, absolutely. > DC-DC converter --- even better. All great suggestions for a land-based battery. For a drone, though, the extra size and weight makes these solutions impractical. \$\endgroup\$ Oct 6, 2022 at 14:32
  • \$\begingroup\$ I thought we were talking about a submersible drone. \$\endgroup\$
    – rdtsc
    Oct 6, 2022 at 17:49

Since this is a homemade solution, I think a good answer will be more on the practical side. But yeah, initially, you may want to wear gloves and safety glasses.

Two tips on how to merge batteries on the go safely:

  • If you charge them all up fully, you can connect them without worry.
  • If you have a voltmeter, charge each of the packs individually until the charger outputs a voltage equal to or above 21V (5*4.2V), then you can connect them together

If you want to connect them together earlier (with lower SOCs), you should make sure the others packs are discharged "a bit" as well. :) Situation becomes less clear. You might be fine most of the time because I assume your batteries will be usually at similar SOC and they are made of the same cells. But you have to be conscious of what you are doing and perform an analysis of what charge levels are safe to merge together.

The worst situation is if one battery is at very low SOC and others are high. Then the lower one could charge way too quickly and explode. The opposite situation (single high battery and others low) is not as bad because the safe discharging rate is usually a few times higher than in charging rate.

Preferably take the datasheet of your cell and look what is safe charge rate. Say the safe charge rate is 1c (3.4A for your cell). so now look at the discharge voltage curves. Remember, if you have 6 batteries, 5 of them might be feeding current to the lowest SOC one. Still, at 0.2c discharge rate, the voltage typically will be below 4V at <90% SOC.

You will notice that the single cell discharge voltage is above 4V only at above >90% SOC. Now, either find the charging voltage curve (might be tricky as many charts focus only on charging current), or just connect one of your discharged batteries to your trusty charger and measure the voltage that the charger produces. If that voltage is above 4V you are safe to connect it to the rest of the pack (assuming other batteries are below 90%). If the charging voltage is below 4V, wait for it to rise. Note how much time you needed for the voltage to increase this level. Then always make sure, before you connect batteries back together that they are charged for at least that much time.

Lastly, if you see sparks during reconnection you may want to undo and check the charge levels again.


If you do connect the lithium ion batteries in parallel after the fact, you'd need to do some load balancing to make sure the batteries have the same potential. (unbalanced batteries can have large currents between them and the voltages are not the same, there are balancing circuits and IC's that can balance batteries to connect them in parallel if they are disconnected). The batteries would also need to be the same size and have similar characteristics. There is a good guide here

  • \$\begingroup\$ "There is a good guide here" I'll have you note that that guide is about connecting cell in parallel, not batteries. Connecting cells in parallel is fine. OP is asking about connecting batteries in parallel. That is dangerous. \$\endgroup\$ Oct 4, 2022 at 16:02
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    \$\begingroup\$ It goes over both, and as I have said you need to balance them before connecting or have a circuit to balance them. You can connect a resistor and slowly balance them so they are the same potential or you could develop a better scheme \$\endgroup\$
    – Voltage Spike
    Oct 4, 2022 at 16:10
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    \$\begingroup\$ Somebody flagged this as "not an answer". it IS an answer. In this case the BMS per serial string makes it superior to series strings with no balancing. \$\endgroup\$
    – Russell McMahon
    Oct 5, 2022 at 4:16

As others have cautioned, it is best to connect individual cells in parallel, with a BMS for the entire pack. If you must connect several packs in parallel, each pack should have its own BMS, and steering diodes are highly recommended so as to isolate a pack that has failed. Many BMS boards have a Schottky (or silicon) diode to prevent reverse polarity when charging, and the voltage drop is not much of an issue. But diodes for the discharge circuit may waste too much power and cause overheating.

It might be better to use power MOSFETS with a circuit to make an active rectifier where voltage drop is determined by RdsON. However, this effectively neutralizes the diode effect because current can flow in both directions. This might be dealt with by monitoring current through each battery pack and turning off gate drive if reverse current is detected.

Here is a circuit that should work as a pack protector, although it needs additional design to detect reverse current and shut off the PMOS device so it works just with its body diode:

PMOS Pack Protector


The thought-up solution was to make several battery-packs of 5s1p with each battery-pack having its own BMS

This is entirely normal. Batteries fail, and the more batteries you have in a pack, the more the probability of failure gangs up on you. In large systems, it's completely common to have each "string" of batteries managed separately, so that if one dies, it can be cut loose and you run off the rest of them.

and then connecting a number of battery-packs in parallel to supply the drone and boat

This is where you have problems. Battery management systems are electronics giving you an output, and hardwiring outputs of electronics together is rarely a good idea. Ohm's Law says that if one output is a slightly higher voltage than the other, you get a current flow from one to the other - so not only are you not "sharing" current draw, you're actually making it worse. And if the output impedance is low (which it is for power devices), it doesn't take much voltage difference before you get big currents happening. Electronics tends to object to this, often with smoke signals.

There's a solution, of course. What you need is a power management system which takes all the various power sources and manages them to ensure it draws a balanced current from them all. This is perfectly achievable - it's how off-grid power management systems work, pushing charge into storage batteries from multiple different sources (wind turbines, solar panels, etc.). It's a lot more complicated than just wiring things up in parallel though!

The reason for not just creating a bigger pack is simply restrictions on Wh on aeroplanes. Are there any problems with this solution?

Yes, because airlines aren't completely stupid. The problem with batteries is that if they fail, there's a reasonable amount of lithium to light off, and it doesn't like being put out. That's why there are rules in place. According to the FAA there were 26 battery fires on planes in 2021, and early this year there was one which destroyed an entire plane, luckily before it left the ground. If you rock up with a lot of smaller batteries in one bag, it doesn't make it less dangerous transporting them, and standing on the letter of the law for the size of an individual battery pack isn't likely to make them say "oh yes, let's put all this on our plane anyway".

If you need larger battery packs, buy them when you get to your destination. Or use a shipping company to deliver them, where the shipping company can advise on how to send them in the same way as they'd ship other hazardous materials like explosives. Please don't put them on a plane with a couple of hundred other people.


A safe battery charger circuit was not mentioned. The charger must disconnect when it senses a full charge.
A circuit or timer to tell you that the battery voltage is getting low is needed.
A circuit to disconnect the battery from the load is needed when the battery discharges to about 3V.

Please lookup and learn about Li-Ion batteries.
3.7V per cell is the storage and selling voltage. 4.2V is a fully charged cell.


I have read through the post, all the reasonable answers. Let me disagree.

I have two, home made, 10S10P battery packs. I use them for my chain saw and an ebike. If I go for a long trip on the bike I connect two of the packs in parallel, one year and over 1000k later no problems.

In my case two battery packs, build from the same power cells, one BMS each, one 80A fuse each seem to o work happily in parallel.

Before using the batteries in parallel I have checked, with multimeter, the current that passes between them when connected in parallel, I have purposefully charged one battery to 40V and left the other at 38V. The current wasn't large, it was a year ago so I don't remember what it was but it was small enough to make me go ahead.

So from my experience, yes you can connect two battery packs in parallel.


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