Hello, This is my sodium ion battery. Uh, four cells in series. so nominal 12 Vols and I've now got two Bms's a Life Po 4 BMS for discharging. That means I can discharge down to close to 2 volts and an Nmc BMS which I use for charging.

and that means I can charge these cells up to Uh 4 volts each. Now now, this is currently fully charged. so these are about 4 volts each. The pack of four cells is at about 16 volts.

Um, we can have a look at that. So I've connected the DMM to my load output. Now the load um output is on the Life Po 4 BMS That means that I can, uh, pull current from these cells until they get down to about 2 volts each. So the pack voltage would be 8 volts positive to the positive of the pack and you can see that it's about 15 5 Vols I charged to 16 and it's sagged down a bit.

Actually, you can see now that the native voltage of the battery because I've connected this now directly to the positive and negative terminals is actually 15.7 six. But anyway, I've got a light bulb here now and I'm going to connect uh that via the load output and that lights up. that's actually a 24v uh vehicle brake light bulb. And now I'm going to get the thermal imaging camera and I hope you can see on here on the board that I'm discharging through which is the Life Po 4 BMS The right hand row of mosfets are quite warm.

Um, dare I put a 12vt bulb on 15.6 volts? Yes, I probably do. Yeah, that's pretty bright, but that's also going to pull more current through the Life Po 4 BMS And you can see that the temperature of that right hand row of mosfets. now that those are actually the charge mosfets. and I'm not charging through them on the Life Po 4 board because I'm charging using the Nmc board.

the Life Po 4 board I'm only using for discharge charging, but the charge mosfets are getting hot. They're now up to 35 now. I First noticed this issue not through the thermal imaging camera because you don't normally Point thermal imaging cameras at things, but through a voltage difference. That's the voltage actually at the battery terminals positive and negative 15.6 and this is the voltage at the positive and load out output.

Terminals And you can see that's 14.98% higher than it would allow charging. I Think the Life Po 4 allows charging up to 3.65 volts per cell, so that's 7.3 That's 14.6 volts. And of course we're above 14.6 volts. So this Life Po 4bo is saying no I don't want you to charge the cells so the charge mosfets are off, but we're not using the charge mosfets on the Life Po for board, we're discharging so we're using the discharge mosfets and those mosfets are switched on because the Life Po for board is quite happy to allow discharging as long as you don't go down below about 2.1 volts per cell.

It's the charge mosfets that are switched off, but because of the strange uh connection arrangement of the two sets of mosfets, you actually go through both of them. because the charge mosfets are switched off, those devices are actually acting like dodes. and because there's a 6vt volt drop which you can see between on both sides of this life Po4 board. That's what we're looking at.

We're looking at uh one side the left hand side on that meter and the right hand side on this meter. Um, because there's a difference in voltage of 6 volts, quite a substantial volt drop, and a current of I Don't know about half an amp at the moment going through there. There's power dissipated in these Switched Off off mosfets which are acting as diodes because of their body diode. And that's a problem because it means that if we pull any more than well, I think I Measured at 2 amps, these were 50 and uh, at 3 amps, these were more like 60 and that's getting quite warm.

Uh, we could probably go up to 3 amps, but that's about the limit and this is a 50 amp. BMS When these mosfets are switched on, their on resistance is incredibly tiny and very large currents can flow through them. But when they're switched off and you're putting current through the body diode then of course they're going to get very hot. So this is how these mosfets are wired.

They're connected, drain to drain, and then when you're charging. Now, let me get this right here. These are the charging mosfets. So when you're charging, you're going drain down to source.

So yeah, we charge in that direction. and now that the voltage on the pack is very high, it's up here, somewhere outside of the range of the Life Po. 4 BMS So these charging mosfets are switched off. So yes, we're just getting this body diode.

It's actually the body diode on these ones over here. But uh, that is the body diode. The one on here points the other way. I'll just draw it in.

actually. Okay, so it looks like that. So this mosfet is turned off. This one is still turned on because we're able to discharge through the life Po for BMS Um, but this one is turned off and so of course the current is flowing through the body diode.

and that's why this one is getting hot. The right hand column of mosfets on this BMS are getting warm now. of course, when the voltage on the pack drops to Uh 14.6 these mosfets will actually turn back on again and they can conduct in the sort of unconventional. Direction Normally you would have current flowing from drain down to source.

Uh, with the mosfet turned on and with the mosfet turned on off. No current flows that way. Current flowing the other way can happen with the mosfet turned on and you course have a very low on Resistance But if the mosfet's turned off, it can of course go through the body diode. So when uh, we get down to 14.6 this mosfet will turn on and then of course there's no problem.

Both these mosfets will be cool. Now what I really want to do is drain this down to 14.6 So I'll put a bit of a load on it, see if I can get that to happen reasonably quickly. They're quite large cells, but uh, I'll see if I can do that before the light goes and we can see uh, how when this these mosfets turn back on. These are all of course in columns.

There are four mosfets in parallel for the charge mosfets and there are four mosfets in parallel for the discharge mosfets. Um, then there isn't an issue so it's only really draining down from the high voltage up here to the top of the life Po 4 Bms's range that there's an issue. Now to my mind, there's also going to be a corresponding issue when charging through the Nmc. BMS When charging and you're down right at the very low point of the voltage, you're going to get the same issue with the Uh discharge mosfets of the other BMS which I guess are the ones on the left.

Yes! So I'm discharging the cells using the 12vt bulb. Um, it's quite bright at 15.3 volts, you can see the Uh voltage difference 14.6 there 15.3 that's actually 7 volts difference isn't it? But that's um, the voltage across the Silicon diode the body diode part of uh, these charge mosfets. Even though I'm discharging through the discharge mosfets, they're switched on and they're perfectly happy. It's the charge mosfets that are switched off that are not happy because because we're using them as diodes and now you can see that the temperature has got up to about almost 50 on that set of four mosfets.

now. I Think this problem is solvable because I'm only discharging through the life Po 4 BMS So I'm not using the charge mosfet so I don't really need them to be there I Did think of tying their Gates high, but the problem is the little controller chip. the Byd chip um has. Well, funly enough, one of the outputs is Seos and the other one is open drain.

So the point is they both pull hard low so I can't really pull the gates of the Uh mosfets. High because I'll be fighting the outputs of the chip I could cut the wires to them and find VCC and tie them high, but actually there's a much easier way and that is to Simply short them out. On the back of this board, you've got access to the the source and drain connections they on big copper areas I could simply scrape them, tin them, with solder, and stick a piece of wire across them. So yes, I think on the life Po 4 board because I'm only using its discharge function I can short across the charging mosfets and on the Nmc board because I'm only using its charging function I can actually short out all of the discharge mosfets now I do have this, uh, big 50 watt car headlight bulb and it is very tempting to put that in for a while, but this is going to pull about 5 amps and I'm not quite sure uh, what temperature these mosfets would get up to if I put 5 amps through them while they've got this 6 volts or actually more like 7 volt volt drop.

Um, so you have to multiply for power, you have to multiply. Oh, power across I s r is it? No, no power is just VI isn't it? So it's just volts which is 6 or 7 whatever you want to call it. Uh, multiplied by amps. Uh, so five amps, 7 volts.

it's going to be sort of three or four. Watts They are going to get quite hot. so there we are. 15 Vols at the battery terminals 14.3 volts.

Uh, after it's gone through the BMS that's definitely a 7 volt drop. Let's take a look at the temperature of the right hand row of mosfets column of mosfets. and yeah, that's now up at uh, what is it? 52 C So that's got quite warm, the voltage is going down, and of course the bulbs getting ever so slightly dimmer. But of course the heat has accumulated in these.

so I'm just sitting here now waiting for this to go down to about 14.6 uh, volts. Whereupon these mosfets should turn turn on because the life Po 4 BMS will say oh, that's a voltage below which I'm happy to allow charging because after all these are the charging mosfets, they're just being used as diodes here. In this discharging scenario. I've just put the 24v uh brake light bulb in parallel with the 12vt one just to try and accelerate.

Uh, the voltage decline of the battery pack. Why does this keep changing exposure? Um, because I want to try and get this down to 14.6 There should be a small step change in brightness of the bulb. This voltage is the voltage across the bulb 14.1 now and that should tick up to be the same as this 14.8 So when the transition happens, uh, you should see a slight change in brightness on these two bulbs. Now, whether I catch that I've added this second bulb in Um, at the risk of heating overheating these mosfets.

But of course, if I'm going to short them out, then I don't really need them anyway. So in this specific application of trying to make two Bms's work as a sodium ion, BMS Um, I don't actually need those mosfets, so they will be shorted out um once I get my soldering iron out. So yeah, just waiting for this uh, switch over point I'll try and catch it on camera as I say I've raised the exposure on the camera cuz I just can keep the two DM Ms uh, illuminated? uh with these two bulbs in parallel. we're now getting 60 on that row of charging mosfets which I'm not using because I'm discharging.

Okay, let's take it down another 100 molts now. I don't know that it's going to switch at exactly 14.6 volts because I'm not entirely sure what the switching voltages of the BMS chip are. and also, of course, I don't know whether these cells are in Balance because my 4S balancer hasn't arrived yet, it's been about 40 days I think despite that particular AliExpress listing saying 7-Day delivery. Yeah, didn't quite happen like that.

Now what we're waiting for here is when the voltage of the highest voltage cell whichever that one might be I don't know I'd have to measure them individually. um, drops to this 3.65 Vol Point that's when the life Po 4 BMS will say oh, that's all within the range that I can operate I'll turn off I'll turn on both my charging mosfets and my discharging mosfets and assuming these are in balance and I think it was reasonably good when I last checked. Uh, that should happen at about 14.6 volts. so let's keep an eye on it.

There is, of course hysteresis, uh, on these switching thresholds. So if the charge mosfets turn off at 14.6 volts, they may well turn back on at 14.5 Vols Uh I Remember what the hysteresis was at the top end? I Got a feeling it was 100 molts. Uh, remembering back to when I looked at the data sheet. So perhaps this will happen at 14.5 volts? Okay, there it is.

I Think that's where we've reached the point now. Now where the life po for Uh BMS is saying Okay, all cell voltages are now within the operating range. Um, so both the discharge mosfets and the charge mosfets um can be both on. They are probably now both switched on.

these. uh, charging mosfets will now probably cool down and I'll get the thermal camera on that. There is a small discrep of about 100 MTS across there, but then one Uh meter is measuring directly at the battery. the other one is measuring after the 2vt drops through the two sets of mosfets and these wires of course.

Uh, and also actually these current measuring resistors. They're very low value, but uh, that resistance is in there as well. which explains this small 100 molt discrepancy. And of course, I don't know whether my two multimeters are the same, they may not be.

There may be a slight calibration error between them, but um, yeah, so that's the point Now, where we're down within the range of the life Po for U BMS and its mosfets are both on. I'll get the thermal imaging camera on there in a moment and now you can see I mean the image is a bit indistinct. now. there're sort of hot areas everywhere, but the hottest part of this uh board.

now I can't see the indicators? Where are they? Oh, it's gone up there now. the hottest, the hot indicator, the red one. Just probably because heat rises. So no longer are these mosfets.

um, distinctly hotter than everything else. And yes, the charge mosfets are no longer the hottest part of the board. In fact, I Think the balancing circuits might be on at this point, and that would make sense because we're at the upper end of the life Po 4 uh, charging and discharging range. and that's probably probably where the balance circuits come on.

They're only 50 milliamps, so there's not a lot of current there. But and some of this heat might actually be heat that's risen up uh, from these mosfets down here and is just has just heated the board. But there we are. The two voltages are essentially the same, and the mosfets are all switched solidly on.

and therefore, we could now put far more current through this. You know these are rated at 50 amps, but I'd probably drate that to say 25. But yeah, you could probably put 25 amps through these uh BMS boards if both sets of mosfets are on. You certainly can't do that if one of the sets of mosfets are off and they're being essentially used as diode silicon dodes, right? I think I'll call it there I was going to include in this video uh, me soldering little wire links across the mosfets that I don't intend to use.

but I think this video's got long enough. So I shall just now say cheerio.