The most over-looked lipo charger spec: Balance Current

If you think you know everything there is to know about Lipo chargers, then I've got a challenge for you because there is one specification of Lipo chargers, that is commonly overlooked and it's kind of a big deal, it can make your charge cycle take way way longer even if you've got a big powerful charger and a powerful power supply, that parameter is the balance charge current in this blog we're going to talk about why the balance charge current definitely should not be overlooked when you're picking which charger you want to get.

When you shop for a charger, the biggest question that I think you got to ask is how long will it take to charge my batteries. Given enough time in a very tiny weak charger could charge a very very large battery, but who wants to sit around for hours and hours, will you wait for packs to charge you want to get flying.

You're probably used to looking at parameters like the watt rating and here we're looking at the ISDT k2, which has a 500 watt rating across two channels or a 200 watt rating, or a 20 amp rating.

Now if you're not 100 percent sure how to analyze those things and figure out how long it's going to take to charge your battery is that enough for you, is 200 Watts enough? Is 500 Watts enough. But the one we're gonna talk about today, nobody thinks about it well, I say nobody. If you're one of the people who already knows about this, congratulations! But a lot of people overlook it.

So let's take a quick look at how charging works and what the balance function does so that we can understand why the balance charge rate can be such a limitation on your chargers performance. Here we're looking at a representation of a battery it's a 4 cell battery because I didn't want to have more graphics on this blog, all works the same with the 5 cell 6 cell whatever it doesn't matter it all works the same.

We're representing the state of charge of the battery, these batteries are sort of discharged, this yellow bit here represents how charged, it is it's down closer to 3 volts, it would be at 4.2 volts or full charge. Under ideal conditions after you discharge a battery, all of the cells are going to discharge exactly the same amount, and end up at exactly the same voltage. Under ideal conditions, the internal resistance of all the cells is going to be the same, so when we sort of suck current out of them, will suck current out of them equally. In reality this isn't always true, but let's just go with that. If we were to charge those cells the way that charging works is the charger pushes current into the battery through the main discharge lead, which is connected to all the cells and sort of charges them all together.

So the cells all charge at the same rate, they all reach 4.2 volts full charge at exactly the same time, and everything is wonderful. But that's not how it works in the real world. In the real world the internal resistance of all the cells is not going to be perfectly matched, and so even if you start with a perfectly fully charged battery with all cells at 4.2 volts, some cells are going to discharge to a lower voltage than others during normal use. And we can see that represented here, this cell is a little bit higher, this cell is a little bit lower, they're all at a different at state of charge.

Now if we go ahead and charge those cells through the main discharge lead, which is how charging always happens, then what's going to happen? When the lipo charge current flows into the cells through the main discharge lead, all the cells take charge at the same time proportional to their difference in internal resistance. So they'll begin to fill up. We can see here the problem, that occurs, that balancing is designed to solve. So all the cells have charged up at approximately the same rate, but the cell that was at the highest voltage is first to hit 4.2 volts. If we were to continue charging at this point, we would continue pushing current into all of the cells cause that's how charging works, and that cell would then go over 4.2 volts, then it would be in an unsafe condition charging over 4.2 volts, in general is considered unsafe. But the other cells aren't at 4.2 volts, so what can we do? what happens is that the charger continues to charge all the cells through the main lead, but in order to keep the full cells from becoming overcharged, it begins to sort suck current out of the full cells as they begin to sort of go over 4.2 volts and overflow. If you will just sucks it off like your soda is about to fizz over, and you go and you suck it down, that's what's happening with the balance lead. So the current goes in the main discharge lead, it fills up all the cells at the same time, and when one cell is getting too high, the charger pulls current out of the full cell, and continues to do it.

So now we can see these remaining cells begin to get full and when they get full, the charger will begin to subcurrent out of them to keep them from over filling coming over 4.2 volts, and then the final cell will come up to 4.2 volts, and the charging process is done.

Now that is not how most people think that balance charging works, most people think that the charger is pushing current in through the balance lead, charging each of the individual cells one at a time until they hit 4.2 volts. When each one hits 4.2 volts, it's done, and there are a few chargers out there that work that way, but that is usually not how they work, and why don't they work that way I don't actually know, maybe it's complicated to build four little chargers and it's easier to build like one big charger, then a balance routine that sort of keeps it from overcharging. I don't really know why they do it this way but this is how they do it. The problem is that discharging a cell is really slow and inefficient, at least the way that the charger is built. The way that these chargers discharge is they just run current through a bank of resistors, they're basically just converting that charge into heat. Let’s take the big onk and wire wound resisto for example, and I don't remember the watt rating of this thing but it's in maybe tens or maybe even a few hundred Watts. I can discharge a lot of current through this honkin thing. If I put a fan on it and blow a fan on it, I can discharge 7810 amps off of 4s lipo battery and it doesn't overheat or damage itself. This thing is enormous, your charger doesn't have one of those in it, your charger just has a little tiny bank of resistors and those resistors get hot and a little tiny fan blows over them. In order to keep the charger from just burning itself out it can only discharge currents so fast, and that's why when you put your charger into storage mode, if it's charging up, it can go super fast because your battery can take a lot of current. But if you're discharging down, it can only go typically 2 amps as the maximum that they'll do about 2 amps, and balancing is discharging.

So when you go shopping for a charger you don't just want to look at the watt rating and the amp rating although you do want to look at that, but you also want to scroll down and find somewhere in the specifications: the balance current, because the balance current is going to limit how fast you can charge the battery once any individual cell hits 4.2 volts. If your pack is a very big pack like 5,000 milliamp hours 6s pack, let's say that one of those cells hits 4.2 volts and it's full, and now your balance current is 500 milliamps, which is not a great spec, there are chargers out there with that. The entire charging process is now going to slow down to 500 milliamps because you can only put current in as fast as the charger can sip it off the top of the full cell, which means if you've got a 500 watt 20 amp charger, and you're charging that big old battery or cramming 500 Watts into that battery, until the point when one of the cells hits 4.2 volts and then we're sipping 500 milliamps out of the battery, and the whole charging process slows down.

So if you've ever wondered why your charging seems to get super slow right at the end of the charge cycle, this might be one reason why it's happening. There are other reasons that we won't go into.But if your charger has a very low balance current and your battery is very out of balance, or if you have a very big battery, that's a little bit out of balance, then all of that is going to cause the last little percent of your charging cycle to be very very slow.

Let's take a look at another charger: the Hota D6. I've got the D6 duo this is the D6 Pro, what's the balance current is 1600 milliamps is pretty good, any balance current over about 1 to 1.5 amps is pretty good, balance current down below 1 amp is not quite as good, obviously more is better, but you'll have to balance all balance. All of the other considerations that you're taking into account when you're shopping for a charger, but be aware that a lot of chargers don't actually list the balance current on their main product page, you have to download the manual or do your own research, you should be really careful because there are some chargers out there, no names are coming to me but I know they're out there, that have like a really impressive specification in terms of how many Watts they can crank, but a really terrible balance current, and they could take forever to charge your packs, that's maybe you want to stay away from them.

Check the full video here: