How to Parallel Charge LiPo Batteries (Safely)

追加: 2026-06-02

[00:00:00]This is a parallel charging board. And if you know what you're doing, it is a fantastic way to let you charge more batteries in the same amount of time.

[00:00:09]And if you don't know what you're doing, at best, it is a fantastic way to destroy all your batteries and waste your money. And at worst, it's a fantastic way to like burn your house down. And if that sounds a little bit catastrophic and like I'm just trying to clickbait you to get you to watch a video, like I don't want to overstate things, but once or twice a year someone emails me and says, "Oh my god, I burned my house down and a lipo battery was irresponsible." So like it's not likely to happen, but it does happen. And most of the time when it happens, usually it's because the user made some mistake that they could have avoided if they knew what they were doing.

[00:00:52]So, basically what I want to do in this video is I want to show you the safest way to parallel charge lithium polymer lithium ion batteries and take that risk of burning your house down from still pretty small to so that hopefully it's not even like a thing. I'm Joshua Bardwell. You're going to learn something today. Why would you parallel charge in the first place? Why wouldn't you just take your charger and plug your battery in one by one? And the answer to that question is that there is only so fast you can charge batteries safely.

[00:01:31]The general rule of thumb is 1 C. And 1 C means that you take the milliamp hours of the battery, you convert that to amps. So 850 mAh is 0.85 amps, and then that's how fast you charge. So this would be charged at 0.85 amps. We probably round that up to 1 amp just for, you know, for round number sakes.

[00:01:49]But we charge that at 1 amp. And this guy here, 520 milliamp hours. That's 0.52 amps. We'd probably charge that at about half an amp. That's the safest way to charge. Now, some people go a little bit faster. They'll charge it 2 C or maybe even a little higher than that.

[00:02:05]Uh, but the bottom line is that at a certain point, if you charge this battery too fast, it will be damaged and or light on fire. Which means that if I have a charger like this and I have a big handful of these little batteries, this charger can charge at like 15 amps, but these batteries can't take it. And that is where parallel charging comes in. Parallel charging is used when you need to charge more batteries than you have charger channels. If I had 15 charger channels, I'd just put one battery on each one. I'd charge it at half an amp or an amp or whatever the 1C rating was, and then I would go just be done at the end of that time period. But like let's say you're doing uh well you see a lot of parallel char parallel charging in classroom environments.

[00:02:47]People who are doing FPV drone classes uh they might have 60 or 70 batteries for their 10 or 15 students that they're going through in a given day or just a hobbyist like you or me who has 20 batteries. You're going to go fly and you need to get those all charged. Well, if I charge at 2C, the battery will be done charging in about half an hour.

[00:03:09]That's just a rule of thumb. 1 C is about an hour. 2C is about half an hour.

[00:03:12]So at about a half hour, two channels on my charger, that means 20 batteries are going to take me 10 10 time 10 cycles.

[00:03:20]At a half hour cycle, it's going to take me 5 hours to charge all my batteries.

[00:03:23]And the whole time I'm going to be sitting there plugging and unplugging plugging and unplugging. Who wants to do that? With parallel charging, I can get that done faster. You need to know that parallel charging significantly increases the risk of fire and or damage to the batteries. And I'm not trying to oversell it, but I do want you to understand the risk is higher. It's still manageable. It's still fine. But the risk is higher than charging batteries individually because if you're parallel charging and one battery dies, all the batteries on the parallel charge board die. One of the things about a parallel charge board is that all the batteries are connected together and they charge together and they discharge together. Which means that if one of those batteries has a dead cell or or goes to 0 volts, they will all go to zero volts. And the other thing is that if one battery lights on fire, all the other ones are right there, ready to go, and they're going to potentially light on fire as well. Now, for the longest time, I've had an analogy for what parallel charging is like. And I've told this story before, but I haven't had any visual aids. And I finally have visual aids because I sat down with, and some of you are going to object to this. I sat down with AI and I had AI make me visual aids. I couldn't handraw this, but it would look like crap. And these visual aids, I think, do a much better job. So, leave it in the comments if you object to the fact that I used AI to make these, but it is what it is. A battery is like a water tank. Here is an empty tank. That's a discharged battery.

[00:04:48]Here's a full tank. That's a fully charged battery. And the water level in the tank is kind of analogous to the voltage of the battery. So, this here, this is going to be 4.2 volts. This here is going to be whatever, 3.0 0 volts so so to speak. And so then this would be the XT60 that's used to charge the battery up. And to carry this analogy further, the volume of the tank is like the milliamp hours of the battery. So a taller tank that's narrower would be a higher voltage battery with less milliamp. And a shorter tank that's wider would be a lower voltage battery but with a lot of milliamp. We could have a 2S 10,000 mAh. We can have a 4S 850 mAh. Right? Those are two independent things. The height of the water is the voltage. The volume in the tank is the milliamp hours or the capacity of the battery. When we put batteries in parallel, it's kind of like connecting them with a pipe at the bottom of the tank. And so here are two batteries in parallel. And here they're being charged by this pipe putting water into the tanks. And you can see just intuitively if you've ever played with water and tanks, you can see that these two are going to fill up together. And most importantly, you can see that the water level between those two will always be the same. It is not possible physically in this scenario for this tank to have a water level up here and this tank to have a water level down there. The water level will always equalize between the two tanks. That is the nature of parallel connections in batteries. And so if the water is analogous to voltage, what that means is that when these two batteries are in parallel, they will always have the same voltage level. And if you think about it, if one of them somehow did have more water, what would happen is that water would flow through the pipe to equalize between them. And that's what happens with batteries. If they somehow have a different voltage level, they will exchange current until the voltage level equalizes. So let's imagine a scenario where we have one fully charged battery and one empty battery. What will happen when we connect them? Pause the video right now and think about this if you if you don't immediately sort of know the answer. What will happen is that the charged battery will immediately discharge current into the empty battery. Current or water will flow. The voltage level will drop. The voltage level in the empty one will come up. And the final state will be that both batteries will be at the same state of charge, the same voltage level. And that's one of the things that makes parallel charging riskier. When you have two batteries plugged in that are not at the same voltage level, there will be what's called equalization current between those batteries. And that equalization current can be higher than the batteries can actually handle and can damage the batteries. Let's do that in real life with these batteries. And this parallel charge board, by the way, this parallel charge board is my JB branded parallel charge board. I developed this in partnership with ReadyMade RC. And uh it's got a lot of features that I like that help make parallel charging a little bit safer.

[00:08:05]We'll talk about those a little later in the video. For now, though, we're going to bypass some of those safety features.

[00:08:11]We're going to do something slightly dangerous in the interest of science, and I'll do it so you don't have to. So, we've got this battery, and this battery is fully charged at 4.32 volts. And we've got this battery, and it is discharged at 3.7 volts. I'm going to take the fully charged battery, and I'm going to put this uh clamp meter on it, and it's going to measure how much current is flowing. I think we'll put this on the 20 amps scale. I'm having a little bit of trouble getting this meter zeroed. uh when you're in the DC mode, it's picking up magnetic fields and so it can be really easy for it to get like a false read from ambient magnetic fields. But I suspect we will see the difference when we plug this battery in pretty pretty clearly. Suffice it to say, we've got basically zero amps flowing now. And if I plug this battery in, this empty one, watch what happens.

[00:09:06]So now we've got 6 amps, 5 amps flowing.

[00:09:09]And the voltage is jumped up. I'm going to unplug this real quick because it's not really a good idea to charge this 650 mAh battery at 6 amps. That's 10 C.

[00:09:19]That's unsafe. Uh, as a rule of thumb, 1 to 2C is very safe and very conservative. 3 or 4 C is less safe, but not ridiculously like if you were really careful about it, maybe you would Okay, but it's not like inherently bad for the battery. And I'd say anything from about 5C up is potentially harmful to the battery. So that would be a bad thing for us to do. The other thing that can happen with that equalization current is in extreme cases it can actually damage the parallel charge board. Um the traces on this board can only handle so much current. Eventually they'll burn out.

[00:09:57]Now these little batteries probably aren't going to be able to give enough current. That was a pretty extreme case between um between fully charged and like okay, not fully discharged, but it only got up to six or seven amps. And these traces can handle well more than six or seven amps. You can see that the board actually has 15 amp automotive fuses on the XT60 and XT30 leads to protect against excessive current flow to protect the board against uh against burning itself out. but it can't protect the batteries from being damaged by a current flow as unless the current flow exceeded 15 amps. So, excessive equalization current is one way that batteries can damage each other when you're parallel charging. But there's another mistake you can make and I am not going to do this here on the bench because it will 100% of the time result in a fire. Okay? 98% of the time result in a fire. And that mistake happens when you connect batteries of different voltage, different cell count. So for example, here is a 6S battery and here is a 4S battery. The 6S battery has more voltage and the fully charged 6S battery has too much voltage to fit in the 4S battery. So when that equalization occurs, what happens? And I tried and I tried to get AI to give me a good image of this water overflowing the tank. And I this was the best. So AI failed me in the end. Uh but what'll happen is that the 6S battery will discharge into the 4S battery to the point where the 4S battery basically overflows and will light on fire. So what's going on here is not fundamentally different than this scenario. The only difference is that in this scenario when the batteries have equalized they are both within their safe range and our only concern is the speed of the current the amount of current flowing into the battery that could damage it. But in this case the concern is that the final stop voltage for the lower cell count battery is going to be so high that it's basically guaranteed to light on fire. So here are the steps for safe parallel charging.

[00:12:04]I'm going to run through these steps with you. Here they are on they're on screen and you can you can pause, you can screenshot, you can do whatever you want. I'm going to run through them with you and go into a little more detail about the what'ss and the wise. Before we continue with the video, I'd like to remind you that the single best way for you to help make sure that I am able to continue making content like this, which hopefully you value, is to join my Patreon. If you watch my content regularly, uh then hopefully eventually you'll come to a point where you go, "This guy's earned my support." If today is that day, join my Patreon at any level, at any dollar value. Pick whatever amount you think is fair for the amount of value you get out of my content. There's a link down below where you can click through. And if today is not the day, if you're like, I'm just want to watch the video. You've probably already skipped ahead. But, uh, yeah, keep watching the content. I'll keep making the content. And I hope that day comes. The first thing you do when you're parallel charging is you verify that all of your batteries are the same cell count.

[00:13:03]And you can see that is not the case here. Look carefully. I got some 4S, 4S, 4S, 4S, 4S, 3S, 3S. Do not mix them. One of the ways I've seen fires get started is somebody has a bunch of 6S batteries and then they've got like a 4S or two that they use for their goggles and they kind of look the same. Maybe they're even from the same manufacturer. And then somehow like the 4S gets dumped into the used pack with they're out flying. They got a they're just throwing all their batteries in their bag, you know, and at the end of the day, they dump the batteries out and one of the four S's has snuck in there by accident and they just throw them all on the parallel board. Bam, fire. Okay, so we'll double check that they're all the same cell count. If you can't physically see the cells, you can look at the side of the battery and that sometimes it's not printed. The surefire way is to look at the balance leads. The number of pins on the balance lead relates to the number of cells. It's always the number of cells plus one. So this 4S battery will have five pins. This uh 3S battery will have four pins and so on. That will always be the case. So you can always look at the balance lead. I guess in an extreme case you might have like a 12S battery that's made from two 6s in series and it might have two balance leads maybe separately. I don't know.

[00:14:21]But that's a pretty extreme case.

[00:14:22]Generally generally uh you just look at the balance leak. Um, so we're going to make sure that all of our batteries are the same cell count. The next thing I'm going to do is I'm going to check the cell voltage on the batteries and I'm going to group them by voltage. So, here's a battery and it's at about 3.7 volts. All the cells are at about 3.7 volts. This is also a good time to look and see if any of your batteries are wildly out of balance. Now, that's not necessarily related to balance uh to parallel charging, but as long as you're looking, then it's a good thing. And you might have a battery that's a little bit unhealthy. It's got a cell that's starting to die. But in this case, they're all right at about 3.6 3.66 to 3.68. That's not that much deviation.

[00:15:08]So, uh we would we would call that good.

[00:15:10]This one's at around 3.7. Okay. 3.7. We go. 4.2. Okay. I'm going to set that over here. That's 4.2. 3.78.

[00:15:19]3.79. That's a little bit higher, but it's still pretty close to 3.7. A good rule of thumb is that 0.1 volts per cell on the final voltage or just 0.1 volts for each of the cell voltages. So, here is 3.6.

[00:15:36]Okay? So, as long as we've got other stuff, I put 3.6 and 3.7 together. 3.7 and 3.8 and so on and so on. So, here's a 3.6. It's it's like 3.68. It's close to 3.7. And here is 3.78. So that's within 0.1 volts. We can group those guys together. 3.7. Great. Within range.

[00:15:57]We'll group that together. And 3.7.

[00:16:00]Okay. So all of these batteries are within the voltage range that I would feel comfortable putting on a parallel board together. You can stretch that a little bit. The more you stretch it, the more the equalization current will flow.

[00:16:13]But here's the thing about equalization current. the equalization current flows at the be when you first plug the batteries in and then very quickly drops off as the voltage is equalized. So if I like had one more battery and it was maybe at 3.6 maybe I would like put it on just with these guys I would stretch that a little but as a as a hard rule if you want a hard rule 0.1 volts per cell is the cutoff. Next I am going to verify that the parallel board is currently empty.

[00:16:40]Another mistake people make is that they'll have a battery on a parallel like, "Oh, I just got this one battery I need to charge." Boom. I plug it in. I set it on the charger. You don't have to parallel charge multiple batteries. You can just charge one. And maybe your parallel charge board was already plugged into the charger and you're charging away. And then you come back with a handful of batteries and you just start plugging in without noticing that a battery is already plugged in there.

[00:17:01]It's a different voltage. Bam. You got a fire. So, we're going to verify that the parallel charge port is empty. Nothing is plugged into it. Then we're going to take our batteries. We're going to plug them into the parallel charge board. And there is some debate as to whether you plug in the XT30, XT60, or the balance lead first. There's not a right answer here, but here's the thinking. Uh, one way of thinking is that that equalization current is going to happen and and that equalization current could be like an amp or two. And these little balance leads, they're only rated for like well, a couple amps at least sustained. Uh but the idea is that we would rather have the equalization current flow through these big wires than through these little wires. And so they say, well, you should plug the equal the XT60 or the XT30 in first. The counterargument to that has to do with one of the safety features of many modern parallel charge boards. These little dudes here, these are called polyfuses. And what a polyfuse does is when too much current flows through it, it it heats up. And when it heats up, it goes to a high resistance state and clamps down on that current. Let me demonstrate. So, here is my 3.7 volt battery. And I'm going to plug it in with the balance lead. The polyfuses are only on the balance lead, not the these automotive fuses for the main XT60. And here is my fully charged battery. And what we should see, I'm not actually sure if this is going to be enough current to trigger this, but we'll find out. What we should see is that these dudes will turn red as they heat up.

[00:18:34]Ready?

[00:18:40]Oh, they are. Yep. Look, here they go.

[00:18:41]See, they're turning red. Okay, I'm going to unplug that. So, uh, we actually have color changing paint on here. So, that's not a that's not a quality of the Polyfuse itself. It doesn't change colors. Uh, but, um, we paint them with some coloranging paint so that they turn red to indicate that they're getting hot. And that means you have a problem. You need to unplug batteries and go back and check what's going wrong. you have too much voltage flowing or current flowing. So the idea is that if you're going to have equalization current, you would rather have the equalization current go through the polyfuses because then if it's excessive, the polyfuses will clamp down and keep it safe. I I don't agree with that. And here's why. Number one, some equalization current will always flow.

[00:19:24]Okay? So we just need to let that happen. We need to let it happen in a safe manner. And I think the discharge lead is the best way to do that. Number two, the polyfuses actually are consumable items. They wear out over time. And the more times you use them, the faster they wear out. So if every time you charge your batteries, you plug into the palance lead, you get the equalization current flowing through the polyfuses, you're going to wear them out sooner. And then once they're worn out, you you basically just throw the parallel charge board away and buy a new one. Like theoretically, you could desolder these and replace them. But most people aren't going to do that. So, my advice is to plug the XT60s in first or XT30s, the main discharge leads in first and then let So, I'm double, you see, I'm double- checking the voltage here because I was just handling batteries with different charges and I might have messed up my piles. I'm just being extra careful. 3.7. Where's the Here's the fully charged 4S.

[00:20:22]Yep. And we're going to put that off to the side. Okay. So, I'm going to plug the discharge leads in first.

[00:20:28]and going to let the equalization happen.

[00:20:33]Um, you know what? For the sake of the camera, let's just do three of them on one side of the board so it's easier for you to see. Now, there's no way for you to know that the equalization current has finished. I guess you could get a clamp meter like this. Or if you want to pay like a ridiculous amount of money for a ultra premium board, Lumer actually has a parallel charge board that has like little readouts, little digital readouts that shows the amp flow in real time. Although it's like $100 for this parallel board, at which point I don't know, you might as well just buy another freaking charger. Uh but uh if you just wait a minute or two, probably by then the discharge current will have finished and you can plug the balance leads in. Now, the next thing people want to know is how do I plug the balance lead in? Because you can see here that this is a 6S balance plug, but this is only a 4S. And so, like where do I plug it in? And the answer is if you look carefully, do you see that we've got this little notch here? Okay, that means that this one here is like the zero pin. That's where that we're always going to start. So, we're going to take our balance lead and we're going to plug it in not here. We're going to start from this one and we're going to plug it in like that all the way cocked over to the side.

[00:21:54]Now, not every balance board or parallel board will have that exactly the same.

[00:21:59]So, you always got to look and you got to see where the zero is and you start there and plug it in on that side. Now, here's another place where you have to be real careful because if I accidentally plug this in, you couldn't do this if you're working with 6S batteries. There's only one way to plug them in. But if I'm working with 4S, 3S, it's possible that I accidentally plug this in here and that will make a big spark. The polyusels will trip and wild voltage will flow. So, you want to be real careful that I'm plugging all of these in correctly indexed all the way to the side.

[00:22:35]Okay. And as I'm doing it, I'm going to keep an eye on these polyfuses and I'm going to if I get a spark or anything like that, then I'm going to unplug and check what happened.

[00:22:47]Okay, so now they're plugged in. Another question people ask is, does it matter which of these balance leads you plug versus which of these XD60s? Cuz not all parallel charge boards are as like nice.

[00:22:59]You have one one balance plug for one XD30. And the answer is no, it doesn't.

[00:23:03]There is no requirement that you plug a particular balance plug in with a particular XD60. They're all in parallel, which means they're all electrically identical to each other.

[00:23:14]Um, so just whichever way makes the wiring convenient, the balance plug gets plugged in and the XT30 or XT60 gets plugged in. Once they're plugged in, you're going to charge them just like you normally would. The stop voltage will be 4.2 volts, whatever, 4.35 if they're high volts. And the only thing that's going to change is the charge rate. The rule is that when you have batteries on a parallel charge board, you take the milliamp hours and you add it up. So we have 650 plus 650 plus 520 1,820 milliamp. That is about 2 amp hours.

[00:23:53]It's 1.8 amp, but we're just going to round up. That's 2 amp hours. And that means we're going to charge at 2 amp. Or if we wanted to be a little bit faster, we could charge at 2C, which would be 4 amps. But you just take the milliamp hours of all the batteries that are plugged into the parallel charge board.

[00:24:08]You add them up, and you use that to get your C rate. Another question people ask is, is it okay to mix different milliamp hours on the same board? Here we've got a 520 and a 650. And the answer is that yes, as long as they're the same voltage, they're all 4S, they're all 3S, and they're all at the same voltage level, 3.8, 3.7, whatever it is. Yes, technically it can work, but in reality I don't like to go too extreme. Like I wouldn't put a 5,000 and a 500 milliamp hour on the same parallel charge board.

[00:24:39]And the reason is that that equalization current from the 5,000, it's going to be so extreme because that big cell can just push so much current. And so if there's even a little voltage differential, you could get extreme current flow between them. There's just too much different in the internal resistance and the C- rating of those big and small batteries that I I don't feel like it's a good idea even though technically theoretically it should work. Um I don't really have a rule of thumb for how extreme I'll go but like certainly I wouldn't go a 10x in milliamp hours. That's too much. A 2x in milliamp hours. Would I put a 1500 mAh and a 3,000 mAh on the same board? I probably would. Uh, so if you want me to give you a rule of thumb, let's say 2x or less is okay and anything more than 5x or 10x is probably too much. You just need to charge those guys separately.

[00:25:31]Earlier I said that these polyfuses are consumable. They will wear out over time and you will have to discard your board.

[00:25:39]How do you know when that's happened?

[00:25:41]This is important because first of all people don't know that this happens and the result when it happens is that your charger starts overcharging your cells.

[00:25:52]The reason for that is that when the polyuse wears out, it presents a higher resistance even when it's not tripped.

[00:26:00]And that high resistance causes voltage drop. Which means that as the charger is charging the battery, the charger perceives that cell as being at a lower voltage than it really is. And so the charger charges the cell up to what it perceives to be 4.2 volt. But the actual voltage is 4.25 or 4.3. As the poly polyfuse slowly wears out, the the final voltage of that cell that's connected to the polyfuse gets higher and higher and higher and then eventually it lights off. So, what you want to look for is after you finish charging, your charger shows 4.2 volts per cell because that's what it thinks happened. But when you pull the batteries off the parallel board, you double check them. You plug them into a cell checker and if you plug it into the cell checker and one of the cells is like 4.25, 4.3 volts and all the others are at 4.2, too. That's your sign that the parallel charge board you're using, the polyfuses are wearing out and you need to throw it away and buy a new one. You want to watch for that, though. Every time you parallel charge, when you take the batteries off the board, you're going to want to just double check their voltage and verify that all the cells are actually at 4.2 volts. If the charger starts overcharging the cells, that means your polyfuses are worn out. It could mean your charger screwed up. Like so what you would do is you would charge a battery without using the parallel charge board. And if the charger puts the battery at 4.2 volts rock solid when the board is not involved and then as soon as you plug the parallel board in suddenly it's overcharging. That's your sign. Then throw it out. There's no Don't screw around. Like you might think, well maybe only one of the channels is screwed up and I'll just use the other ones. Don't. There's no there's no room for screwing around with parallel charging. Throw it out. Get a new one. Sorry. That's how it is. Other safety guidelines for parallel charging.

[00:27:54]Here's a good one. It's printed right on the board. Never leave batteries unattended. Now, this is a rule anytime you're charging lipos, but especially when you're parallel charging because the consequences of a fire are magnified by the number of batteries that you have plugged into the charger. Never leave them unattended. And the other thing I'll say is never leave them on the charge board after the charge cycle has finished. So, somebody will start a charging cycle, the cycle will finish, the batteries are full charged, and then they'll just leave them on the parallel board and they'll go to bed. The problem with that is that if there's something wrong with one of the batteries and it just kind of it was fully charged and then the cell dies and the cell goes to 0 volts, all the other batteries will now go to 0 volts as well and they'll be killed. Okay, so that's what you need to know about parallel charging specifically in order to do it as safely as possible. But that's not all that you need to know. All of the other precautions that apply to normal battery charging apply to parallel charging even more so. So, if you need a refresher on how to keep lipos from burning down your house, safe lipo charging, I've got a video card on screen and uh link in the video description below. Uh I even set a battery on fire to show you like what happens when a battery pops off. If you've never seen it, it's pretty freaking impressive. Um, I'm going to put a link to that video as I said. And if you are interested in picking up one of these Stricks parallel charge boards, I'm going to put a link to my review of the Stricks parallel charge board, as well as product links down below as to why I think this is the parallel charge board. Well, certainly the one that I think I want to be using because I designed it. cards, links.