Building 48V battery bricks from recycled lithium ion 18650 cells

hello this is my homemade battery solar power system i've built on my office shed here in the back garden of my house in the northwest of the united kingdom i explain all about it in my previous youtube video if you've not seen it i recommend watching that first then this video will make a lot more sense at the end of that video i threatened to make another if the system managed to go for a whole month off grid well it managed substantially more than a month off grid but it's taken me longer to get around to making this video than i thought so here's a summary of how it performed in 2020 living in good old blighty the sun in january is as good as a sieve with no holes in it so the batteries never got up to charge to run the office so it was powered 100 from our national grid electricity supply in february they started being enough daylight to get the office running off-grid all the brown areas are the times when the office was being powered from the national grid but the green areas are the times when the office was running off grid powered completely from the 12 rooftop solar panels backed up by the battery bricks in the power cabinet march is looking even more green with only the odd times when the system had to switch back to drawing power from the grid april is almost completely green and may the 19th was just annoying june ran completely off grid as did july as did august september is when the daylight starts dropping and the system had to switch back to grid on a couple of occasions october autumn's properly arrived and everything starts turning brown again november only a few occasions off-grid december i took no readings because there was nothing worth recording throughout 2020 i've been building more battery bricks adding them into the system as time and funds have allowed by the start of april there were 14 bricks online and by the end of june i had 16 bricks online the amount of energy i can store in each brick varies depending on how good the batch of cells is that i've used to build it these are quite a good batch about 2 800 2 900 milliamp hours per cell at the end of june i had 16 bricks online and from the readings i took i calculated that the system stored just under 14 kilowatt hours of power which works out at a little under of one kilowatt hour on average per brick in the previous video i explained the basics of the system so now i'm going to do a technical deep dive into how i build the battery bricks as also highlighted on the previous video messing around with mains voltages and lithium-ion batteries has the tendency to be slightly lethal or cause raging infernos of fiery death we're not handled with proper care and attention so don't do this at home and if you really must don't come complaining to me if it makes you dead or otherwise inconveniences your health right safety warning out of the way i'm going to go back to battery basics so pause the video get yourself a nice cup of tea and a biscuit for my american friends a biscuit is the proper word for a cookie okay are you ready we're going to do some nice gentle maths and physics maths and physics the cells i use in my system are called 18650s they look very much like a normal double a battery like the kind you'd have in your tv remote with a positive end and a negative end but they're a bit bigger and they're chock-full of lithium-ion rechargeable goodness i use secondhand ones recycled from old laptop batteries medical devices e-bikes that kind of thing so they've all had a useful life already but with a bit of loving care and attention they can still do more good work so first off why are they called 18650s well it's because they're 18 millimeters in diameter and 65 millimeters long and naught [Music] if you look up the voltage of 18 650s you'll find they're sometimes listed as being 3.7 volts sometimes 3.8 volts it's like make your mind a battery what voltage are you you pesky little thing settle down it's it's all quite simple an 18650 is fully charged at 4.2 volts and as it discharges its voltage goes down gradually until it reaches about 3.3 volts when it's empty 3.7 volts or 3.8 volts is what's usually called the working voltage or rated voltage it's kind of like the average of the cell as it discharges some manufacturers call them 3.7 some call them 3.8 ah you see this is obviously a highly simplified explanation but basically correct now hold on just a flipping minute mr gill you just said it's still got 3.3 volts in it how can that be empty naught volts is empty right what's going on well spotted you clever little physics padawan but perfectly good explanation there is i don't claim to be an expert at battery chemistry but as far as i understand it it's like this say your battery is at 3.3 volts and you try and suck more power out of it the poor worn out little cell goes nope and although it will give you a dribbling of power its amps are all gone and the more you try and suck power out of it the more the exhausted little things voltage plummets and if it drops down below about two and a half two volts you just end up permanently damaging it so i'm kind to my poor little second hand cells they've had a good life already and seeing as this is their retirement i don't discharge them below 3.3 volts this is the brains of the system the charge controller and inverter this model is a voltacon 5 kva 48 volt off-grid dual mppt solar charge controller and inverter there are two separate solar panel power inputs each one can handle a maximum of 145 volts but they work best between the range of 60 to 115 volts six of my solar panels face east and the other six face west and they each produce a maximum of 39 volts each in full sun so i've wired the solar panels three in series which means that when the panels are in full sun they produce a maximum voltage of 117 volts which is only slightly out of the optimum range but well within the 145 volt maximum because this is a 48 volt system the controller then converts that voltage down to 48 volts to charge the batteries i've designed it such that i can keep adding 48 volt battery bricks for as long as i've got room in the cabinet now the educationally gifted amongst you might have noticed that 4.2 volts is not 48 volts we obviously can't just connect the 18650 cells directly to the system as they are but fret ye not you might also recall from your elementary physics that if you connect cells in series positive to negative the voltages add up for example three fully charged cells series up would give you 12.6 volts going down to 9.9 volts when they're empty so let's just keep adding 4.2 volt cells and if you do you'll find you can't get to exactly 48 volts well that's just rude 11 cells gives you 46.2 and 12 cells gives you 50.4 volts i know i said earlier that the voltacon is a 48 volt system which is true but it's also programmable and you can adjust the charge voltage to whatever you want up to a maximum of 60 volts next i'll explain how i decided to configure the cells in my battery bricks but first i need to take a step back and show you this neat little app that allows you to control the system from the comfort of your pc here's how i've got mine set up there's a lot of telemetry on here but there's only three important settings to look at now the first is the fully charged voltage value which is set here this system is actually designed for charging lead acid batteries so it has two different settings for what's called bulk and float voltages but those don't apply to lithium so i've gone ahead and set both the bulk and float to the same value 57.5 volts i'll come back to why i've chosen 57.5 in a moment the next most important setting is the back to grid setting which tells the system what voltage your batteries should be at when they're empty i've set my system to 47 volts which again i'll come back to later back to the batteries when the system is on grid and the batteries are being charged up this setting allows you to choose how full you want the batteries to be before it switches from the grid and back to drawing power from the batteries when the system has switched back off grid if the solar panels are generating more power than is currently being drawn then the batteries will continue to charge even while off grid going back to the battery bricks 11 cells gives a full charge of 46.2 volts and 12 cells gives it full charge of 50.4 volts for reasons that may become clear i build my battery bricks out of 14 cells in series i stack them up like this in a zigzag fashion for ease of construction 14 cells fully charged at 4.2 volts per cell is 58.8 volts but as i mentioned only a few moments ago i'm only charging them to 57.5 volts which means each cell is only charged to 4.11 volts not 4.2 why would i not charge them up to full charge i hear you ask i will explain i hear myself reply remember these are recycled cells they've already worked hard all their lives and now they deserve a little r r in their retirement 4.11 volts is only 90 charged which puts less stress on them under charging them like this means the cells can be charged and discharged many more times before they eventually lose their ability to hold charge and remember for a significant portion of the year these cells are being charged and discharged on a 24-hour cycle so at the expense of a bit of daily power their overall lifetime can be extended significantly it's exactly the same reason that i've chosen a shutoff voltage of 47 volts if i was discharging each cell all the way to 3.3 volts then for the whole brick the shutoff voltage should be 46.2 volts but shutting them off early at 47 volts means they still have a teensy bit of power left in them doing this also helps extend their life and rechargeability to make one of my battery bricks i put eight of these zigzag strings of cells together and wire them in parallel with copper bus bars made from the cores of two and a half mil mains wiring cable the kind of thing you use when installing new sockets in your house you might recall from your elementary physics that if you put cells in parallel the current adds up not the voltage so by using eight strings i'm increasing the amount of power i can get from the brick by eight times without changing the working voltages there's no special reason why i chose eight it just makes a practical sized battery brick that isn't too heavy to lug around and each one gives me a decent amount of power i can safely pull from one of these bricks 500 watts with no significant stress to the cells that's enough maths and physics for now ah yes i know but we need to actually build these suckers 14 cells in series by 8 cells in parallel is 112 cells in a brick and they need to be held together somehow and for that we need frames a bag of 20 frames like this costs about 9 quid on ebay they're designed specifically for holding together 18650 cells which is handy they have these little interlocking wedges so they click together quite nicely the first thing to do is assemble them into the right size fourteen by eight cells each framelet is five by four so six of them makes up a fifteen by eight frame which is one row too long to demonstrate that fifteen is one more than 14 i have helpfully shown you in this video clip here after i've hacked off the unnecessary row it's on to populating the brick i used to collect old laptop batteries and recycle the cells myself but recently i've been buying my recycled cells off ebay i'm happy to let someone else crack open the laptop batteries test and grade the cells for me and they arrive in handy little cardboard boxes and i just fill up the bricks with cells the cells going into this one are mostly around 2 600 2 900 milliamp hours which isn't too bad one brick full of these will provide about one kilowatt hour of energy storage i'm making sure to get each row the right way round for the zigzag series wiring getting them the wrong way around is a great way to kill off your cells when the brick is fully loaded with cells the top frame goes on like that and there you have a 48 volt brick all ready for wiring up copper bus bars these are the thin copper rods that connect each row of eight cells together in parallel stripped out of the mains installation cable as described earlier i lay them out roughly where they need to go in between each row of cells then it's time to crack out the araldite and get them glued on a quick note about good cell safety if you rummage around on youtube you'll find plenty of videos of people torturing these 18650s to show how dangerous they are the poor little things get bludgeoned to death blow torched to death heat gunned to death they're tough little cookies but sadly they always end up either exploding their tops off or bursting into fire good to know that i shouldn't bludgeon or blow torch my cells so i won't do that however it is necessary to safeguard against short circuiting by that i mean that the positive end of a cell may accidentally get directly connected to the negative end and if that happens these little blighters deliver a stonking amount of current like 30 40 amps before getting themselves so hot they go into what's called thermal runaway in other words they set themselves on fire lithium burns very hot very quickly and once on fire you ain't putting it out you just have to wait until they burn themselves out again if you look on youtube you'll find videos of clever people who've put one of these cells in their trouser pocket with their keys and change whereupon they get shorted out and fires burst forth from their pants very spectacular very dangerous never put one of these cells in a pocket to make sure there aren't any unexpected surprises with my bricks i'm going to use fuse wire to connect the cells to the bus bars that way if anything goes wrong with the cells or the wiring for whatever reason the fuse wire will blow and the cell will be safe 18650s can safely deliver a maximum of around 10 amps before you start getting into overheating problems although these are recycled cells so i'm not going to push them anywhere near that hard i've got a test cell here that i've put into this holder and i'm going to deliberately short circuit it with fuse wire and see what happens i'm using 3 amp fuse wire which blows out at around 5 amps the fuse wire takes a second or two to blow and it's all over quite uneventful quite safe just how i like it in normal operation there'll be less than one amp being drawn from each cell well below the three amp fuse rating so that's all good all i need to do now is actually wire up the cells to the bus bars i've cracked out the arrow die and the bus bars are all glued on and it's time for my favorite bit oh how i look forward to this bit threading almost invisible strands of wire onto the bus bars with my big ham-fisted fingers it's like trying to tie your shoelaces while wearing hazmat gloves soldering every single cell all 112 of them on both sides of the brick 344 solder joints on every brick taking care not to heat up the cells too much with the soldering iron oh joy rapturous joy and at the end of all that you have a finished brick now it just needs to be charged and balanced but that's for another video i think then it can be plugged into the power cabinet along with all its friends and that's another kilowatt hour of energy storage ready for use i hope you found this video informative and entertaining take care out there you all this has been john gill

Channel: John Gill

Views: 6,164

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Id: 3M8UQU_TybA

Length: 19min 47sec (1187 seconds)

Published: Thu Jan 28 2021