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in today's video we're going to discuss current sharing with large lithium iron phosphate battery banks we have a 48 volt battery bank here and we have different capacity packs wired together in parallel and a lot of people want to know if it's safe to do that so we're going to use my pack as a real world example of what can occur if you do not wire them up correctly also people want to know if it's safe to use batteries from different manufacturers in parallel let's say that you bought a jack appear
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battery and you have an eg4 is it safe to use it in a single system in parallel furthermore we have a grade b pack up here in parallel with brand new grade a packs over here that are a different capacity so we'll discuss all this and more so first off this is an example of bad wiring for large batteries we have five batteries in parallel and we're only using cables to connect them together so let's think about voltage drop in this situation this battery when it supplies power to the loads or it's
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being charged it's going to see the voltage drop across all of these conductors and that means that this battery will lag behind the other batteries this one will be the last to charge to 100 percent and will be the last to discharge to zero percent furthermore this battery over here will charge and discharge faster this one will be the first one to hit 100 percent out of all of these packs now how much current a battery can take in and take out is dependent on the internal resistance and the internal resistance
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is dependent on what cells you're using their age and their health and also their capacity and these two batteries in the front here have a larger capacity thus they'll have a lower internal resistance that means that they can take more of the current from over there even though there's a voltage drop but not by much this is only five percent more in capacity so let's look at the state of charge on the bms and see what we've got so first battery state of charge is 72 percent second battery is 78 percent
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third battery is 89 you see what's going on here fourth battery is 100 and the fifth battery is 100 as well so the closer you are to the charger the less voltage drop and the higher the state of charge because these were all charging up yesterday and that's a massive difference going from 72 percent state of charge to 100 state of charge but there is no current feeding from one battery into the other and this is very typical of lithium iron phosphate because of the flat charge and discharge curve for you to feed into
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this battery the voltage needs to be higher than the voltage drop across this and if the curve is very flat it takes quite a higher voltage to feed into these batteries over here also these have different cells in these packs so they will have different internal resistance values and that can also affect how the current is shared across this battery bank now let's ask the obvious question is this bad for performance it is if you have high sea rate loads and you want to push these at 1c you need these wired in a different
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configuration this is not appropriate but if you have a large battery bank and you're only pulling 0.1 c from each pack this is totally fine and you will actually pull the full capacity of this entire pack as this battery charges up this one will be 100 then this one will be 100 and then this one will be 100 they will charge and discharge at different rates and feed more or less current depending on their internal resistance but you will still pull full capacity and i've been testing this for months just to see it in action there's
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also studies that you can review about the internal resistance of mismatched capacity packs in parallel and it's totally fine and safe to do the big issue though is each pack needs to have its own over current protection device whether it's a bms with over current protection or its own dedicated circuit breaker each parallel string should have its own over current protection device and each server rack battery has its own circuit breaker but let's say we're using a battery that does not have a
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circuit breaker and you don't trust if the bms will work especially for cheap chinese lithium iron phosphate batteries what you want instead is a dedicated fuse for each battery and preferably right on the positive terminal now let's imagine that this pack is not connected to other big batteries behind me and i need to use this at a higher c rate the only way to do it is to connect all of these batteries to a bus bar with equal length conductor so let's say we put all of these batteries into a server rack
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and it has a negative bus bar on this side and a positive bus bar on this side all of these batteries need to connect to the bus bars with the equal length conductor furthermore if you have high c rate loads such as an ev powered by lithium iron phosphate cells you want to use the same cells across the whole pack you want to use the same age of cells and you want to use typically the same manufacturer of bms if you have parallel strings because that can change the internal resistance value as well but if you're using them for solar it is
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totally fine to use different manufacturer in different capacity packs in parallel and no matter what type of system in the long run you want the batteries to cycle together it is good for the performance and the health of the battery over like a decade or more if one pack is feeding more current than the other ones and they're the same capacity that one will be the first one to die because there will be elevated temperature and the degradation rate will increase now for solar you can get by with wiring your batteries in this
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manner but you will have better performance like i said if you have a bus bar and then you connect them with equal length conductor that is the best way to do it now let's imagine that we take the main supply conductor over here and we connect it over here that will reduce the current sharing problems and we won't have such a massive difference in state of charge but it's still not ideal the only way to ensure that these feed current in the same manner is to have bus bars for long term performance
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and health you always want to use bus bars with equal length conductors now let's discuss the size of conductor to put these batteries into parallel for these 100 amp hour 48 volt packs i found that 2 gauge wire works really well it barely fits on these terminals on the jack appear and i have pretty low current sharing problems considering how massive this parallel string is now let's imagine that these are six gauge wires we would have less current going in and out of this battery and he would have a hard time keeping up with the
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other batteries but if you have these batteries connected to a bus bar you can actually get by with a short piece of six gauge wire especially with like five or six of these batteries six gauge for each one is enough to trigger the over current protection device and to feed the system with all the current that it needs but please check the manual of your battery to see what size conductor they recommend personally i do not listen to them and i make it as large as i can to fit safely on this terminal in my system i have multiple packs of
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batteries connected in parallel and to ensure that there is no issues of if one battery fails if the bms fails and there's a shorted cell i do not want this battery bank feeding into this battery bank so even though these have their own individual circuit breakers i have other circuit breakers to isolate each string of batteries and each of these circuit breakers can handle the interrupt amperage that is created by these packs so for the batteries behind me they have their own circuit breaker to isolate them so for my grade b pack
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because i would say it's the most dangerous pack that i have here which is pretty safe i'm not going to have any issues with it but if a fault were to occur i would imagine it would be the grade b packs so for that pack it has its own circuit breaker that has an interrupt rating of 1500 amps and these batteries behind me are connected to their own circuit breaker right here but you'll notice that i undersized this thermal circuit breaker because i would like this to trip before the circuit breaker on the units and this one is
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rated for 150 amps even though these batteries can supply 400 amps i would want this to trip first because that would mean that the other batteries are not supplying as much current and they are taking the grunt of the work and these can pull about 200 amps max so if any of my parallel strings went down it would shut down the whole system with these circuit breakers down here and then for these batteries we have this circuit breaker but this is undersized and i need to replace this one i need to order one because this would not trip
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with the amount of current that these could actually create so yeah i need to swap this thing out pretty bad so on this system we have three parallel strings supplying this distribution bus bar and this is directly connected to t-class fuses and these have a very high interrupt current rating so no matter what if there was a failure in one of these inverters for whatever reason i know that these would trip it would be very expensive and i hope that they never and i hope these other circuit breakers would trip first but if
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everything goes to complete hell these will trip no matter what and if you have a large battery bank connected to any large device you need to use a t-class fuse the internal resistance of this whole pack is just so low that so much current could flow through here circuit interrupt current rating is in the tens of thousands of amps usually 10 or 30 000 amps and these will still disconnect if you use a circuit breaker instead of this t-class fuse and you have a dead short in this device you're going to have to extinguish a dc arc that has a
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lot of current going through it i mean that thing is going to be massive it would just melt the circuit breaker and the arc would just stay there it would just keep flowing current and that current would flow to the defective device or the short and create even more heat so you could catch things on fire and lots of bad things can occur that's why we have hardy board too whenever you mount a circuit breaker or a fuse or a device i'm having some form of flame retardant material connected to it is ideal something else to consider is the
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current carrying capacity of the conductor supplying your over current protection device you need them large enough to carry that current so that you can trip this device the conductor will never become the fuse it will not disconnect i've had very small wires and they just heat up melt off the insulation and can carry massive amounts of current and some people might be wondering why do these need to handle tens of thousands of amps of current and that's because even small cables for the first like couple milliseconds can shoot
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tons of current just for that instance before it heats up and then creates more resistance and that's why it's so crucial to use a t-class fuse because those tens of thousands of amps can occur and it can destroy your system also when you have a massive battery bank you're probably going to have multiple all-in-one systems or inverters run in parallel and these inverters are run in parallel so they have to have equal length conductors and this is very important so that they can see or sense the same voltage so that they will
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operate at the same time so when this one is done charging this one is also done charging when this one shuts down this one also shuts down and all of the terminations between these devices have to be perfect any difference of resistance will be felt by these two units something else to consider is that even though the string of cells right here is isolated from the rest of the pack with its own dedicated circuit breaker it also has its own bms and this bms has over current protection but i would not rely on just that you have to
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have a dedicated breaker for each parallel string all it takes is one bms to have a shorted fet which we've done multiple times on this channel and you will not be able to disconnect this pack through the bms because that fet will allow current to flow and then you'll have a very dangerous situation not only will the bms heat up but current can flow out of this battery and also into this battery something to realize with parallel strings is it's not that this battery is very dangerous on its own but
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having a failure in this battery being fed by other batteries in parallel is where the danger can occur if you have a shorted cell in a failed bms which is very rare but it's connected in parallel without a circuit breaker to other batteries those batteries will feed into this shorted cell and you could have a big problem this is why i do not recommend people use lead acid and lithium iron phosphate because let's say you have a shorted cell in the lead acid and then you have a low internal resistance lithium iron phosphate
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feeding current into that parallel string with the lead acid cell you're going to have a lot of heat and that battery is going to fail and it could fail catastrophically you could have you know the vents blow off the top with sulfuric acid going everywhere you have a lot of heat it could just melt through the floor um you could have a lot of things occur so i do not recommend doing that now what i said in this video does not apply to lead acid batteries lead acid batteries when you put them into parallel or series need to be the same
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age the same health the same manufacturer the same electrolyte the same on plate design everything has to be the same if not one string will fill very quickly so yeah these arguments do not apply to lead acid battery banks but with lithium iron phosphate if each pack in each string has its own over current protection i think it's fine to put different age batteries together in parallel and i actually asked some battery engineers and they agreed with me there's also literature about putting different size batteries into parallel
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and it seems safe to do as long as you have over current protection for each and every single one even in other lithium ion chemistry such as nca for a tesla ev battery pack each and every cell has its own fuse so if that one shorts or something bad happens it will disconnect itself from the parallel pack not because that cell's necessarily dangerous it technically is but all of those cells could feed current into a shortage cell and it would create a lot of heat cause thermal runaway and then it could spread to the other cells so
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yeah you have to have over current protection on each and every single parallel cell string or pack it's it's required but it seems safe to do with lithium i've been doing it for all of my systems and it works fine if you're a beginner watching this video you need to learn ohm's law you need to learn how resistance can change current flow of charged particles through a conductor and how electrons flow and what influences that all of the connections in this system can change the resistance i mean that
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will influence the voltage drop which will change how much current will flow in and out of each pack every single connection here every every single connection um so yeah you have to think in that way when you're designing these systems for long-term performance you want your batteries to cycle up and down at the same rates but like i said with solar it's not that crucial because it's at such a low c rate so it really won't influence it that much if you're speed charging a lead acid and you have lots of parallel strings then
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it needs to be perfect or else one string will fill very quickly with these lithium iron phosphate battery banks in large configurations like this i watch the current every single day to see if one battery is feeding more or less than the other ones and here they feed almost the same amount they are off by quite a bit with the state of charge but for their size they're feeding into the system in a good proportion what you don't want is one like small battery feeding tons of current into the loads and then you check the current from this
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battery going in and it's not supplying that much that means you have an issue you want it proportional to the size of batteries so that they're all feeding in and out with the same amount of current relative to their size now i'm starting to repeat myself so i think you guys get the idea um the best way to do it is bus bars in a server rack with equal length conductors but you can still get away with doing it like this and guess what i'm going to add more batteries here and i'm still going to pull full capacity
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and at this c rate and how much they're feeding it will not hurt them so if you know how to do it right and you size your conductors properly for safety so that you can trip an over current protection device you should be good to go but you need to think about this in terms of ohm's law so anyways i can talk forever but yeah thank you so much for watching and please let me know if you have any questions in the comments section below i would love to hear what you guys have to say anyways thanks for watching and i will talk to you in the
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