A little about me...
In 1985 I went to college for an EE degree. While in school, I discovered I liked working on computers more than I liked designing electronic circuits. Several jobs and years later we get to now and I have never worked a minute as an electrical engineer, but many years in IT and software development. Electronics has remained a constant hobby through all those years. When other kids my age were trying to figure out how to put D batteries in their flashlights, I was playing with transistors, 555 timers and TTL logic gates. I can't tell you how many electrolytic caps I have blown up or how many IC's I've let the magic smoke out of...but it's been a lot! I should be dead 12 or 15 times over from electrocution fiddling inside live TVs, heaters, microwave ovens and other things as a 10 year old!
Admittedly I am new to EV, but all the components that make up an EV have been with me for many years. When young teens were trying to convince their parents to let them get a drivers permit, I was buried in building H-bridges and motor drivers and doing things that modern cordless tools do all the time. There were 4 things on my bench in my dads basement that I considered invaluable. I found a 50 pound transformer somewhere that had like 10 secondary windings on it. It was a goldmine of voltages to me. I wish I still had that beast! My dad scrapped it out for the copper...what an idiot! I had an uber old soldering station I got from HP and an analog volt/amp/ohm meter. With those few things I knew I could invent cold fusion and artificial intelligence. Well I've eaten my fair share of preservatives and who knows what over the years so I'm partly artificial by now and I'm writing this thread so I must be slightly intelligent. I think I have come a good long way in the AI field. Cold fusion on the other hand has still got me stumped. I just can't figure out where the string cheese goes!!!
On to the battery build...
I bought a 48 volt electric scooter online from a guy in Florida. His customer service completely sucks. His follow through on orders could be magnitudes better to just get to marginally lousy and his ability to take criticism when he screws up (which is all the time) is absolutely non-existent. If you ever want to buy from hyperpowersports...just look on alibaba or elsewhere. He has cool products and the build quality of the Chinese scooters he sells are not overly bad. It's pretty much the company and the owner and his employees that make buying from him a bad experience. Before I ever had the scooter I was going to buy, I already had planned out what I was going to do to it. So far, most of those things have happened or are in process. If you have ever bought chinese products, then you know that product quality can be lacking sometimes. My greatest concern was that the frame, wheels, brakes and structure were sound. The electrical and electronics to me were 100% replaceable. The only original wires are the ones going to the key switch and all the electronics and switches have since been replaced with better stuff. Before I ever bought the scooter I had already spent $500 on 8000mah LIPO cells that I was building into 48 volt packs. Several tries later I had built packs that I was happy with and felt were going to be reliable and they have done a great job. I don't know why I never took any pictures of them. All I have is screen captures from video. The packs don't exist anymore.
I used a $40 BMS I found on ebay. They worked OK and fit on top of the 12 LIPO cells like someone had intended that this BMS should go with those 12 cells. The BMS worked OK, but I have been playing with LIPOs for one or 2 seconds now and I know what balance charging should look like. The BMS I used allowed some cells to get to 4.35 volts or to discharge past 3 volts and other cells it charged correctly. I'm certain the BMS has taken life from my LIPOs with it's lousy cell control. This is the BMS I used. You can find them on ebay and elsewhere. I don't recommend using them. They also come in more or less cells. IF they look like this, but with more rows of small components to the left of the mosfets, then it's the same board design and they are not very good.
This is the 4 original LIPO packs in the scooters battery bay. The cells are separated from each other by a thin layer of high density craft foam. Then the entire stack of cells is wrapped in more of the craft foam sheet. The cells have kapton tape touching their edges, but no tape touching any flat surfaces. I wanted to be sure that I never lifted the foil covers away from the insides of the cells by pulling off tape. The long sides of each pack has a single layer of foam. The bottom and narrower ends have 2 layers. My concern was that the these were the surfaces of the packs that would see the most abuse so I doubled the amount of foam protection. The packs inside the scooters battery bay laid on one end so I ended up adding a third layer of foam to the short sides for better protection. Once the cells were all bound together in foam and Kapton tape, I soldered together the battery leads, added balance cables for the BMS and my own 6S balance connectors. I wanted to be able to easily monitor the cells in each pack with a cell log or to balance charge via an R/C charger. I then added 2 thicknesses of 1" wide strips of craft foam over the exposed battery leads and taped them down with Kapton. I wanted to ensure that the wires and battey terminals were very secure and that there was no way they could ever short against anything. The BMS was then plugged in and battery wires soldered to them and then it too was covered in a heavy layer of kapton tape. The only exposure was the balance cables and the battery power cable. I once rode home in a heavy downpour and water got in the battery bay. I was happy to see that my batteries never got wet inside.
Here's my re-purposed 12S packs made into 20S packs before adding the power and balance cables and after they are complete. Ironically 2 20S packs take up almost the same amount of space as 3 12S packs. Those are 5S balance cables on them. My R/C chargers can all do 5 cells and 2 can do 6 cells. Also, 5S breaks up the cells into equal groups.
I looked at 20S BMs boards, but decided to go the route of mass charging this time and random balancing as needed. If I raise the deck a little, I could add BMS boards to each pack, but standing on the cell ends like they are, the balance connectors just clear the underside of the lid. I have a great idea for bulk balance charging. All I have to do is buy a single 20S BMS and then adapt it's balance connector to my 5S connectors and I can balance charge an entire pack all at once rather than 5 cells at a time. With multiple balance connectors like I have, every fifth cell connection has 2 balance wires on it. One wire is the last wire of the previous balance cable and the other is the first wire of the next balance cable. It's pretty easy to keep it straight in your head. Just do one balance cable at a time. Wherever one cable ends, the next one needs to start from that same spot. Then I take a cell log or other LIPO tester that plugs into the balance connector and try each connector. IF each balance connector shows 5 cells on the display and no errors, then I know I got it right. A miswired balance cable will show up as missing cells on the tester. Also, buy balance cables that have a black wire and the wires are silicon coated. The wires are much more flexible, less prone to breakage from bending and the insulation wont melt off. IF you ever have a short in a battery pack, the last thing you want to do is make it worse by shorting wires together when their insulation melts off. The black wire should be the negative connection of the first cell in the balance cable. I'll post a pick of the balance cables I bought later. They have a black wire on one end and a red wire on the other. It's much harder to miswire the cable with colored wires to steer you. When I built the 20S packs I did one thing different than I did with the 12S packs. The 12S packs have the wires laying across the battery terminals. This is a potential place for something to wear through or get cut or whatever and create a short. It never happened, but when I took them apart, I realized the potential. With the 20S packs, I routed all the wires down the center of the cells between the battery poles. This way only the soldered end of a wire was touching any battery pole. The battery poles were then covered by 2 layers of craft foam and taped down. This left a shallow channel down the middle of each pack for all the balance and power wires. The wires were then taped down with more Kapton and the power leads were routed out one end of each pack. The tops of the packs are not water proof. The best I could do was tape down the balance cables all laid out flat, then bend them over the other direction and tape them down again, then I went back the other direction and taped them down again. Hopefully with all those bends if water gets on the top of the packs it wont penetrate very far.
On the subject of balance cables, they are easy to add to any battery pack. The below picture is the schematic I drew of the 12S batteries, BMS and balance connectors. The right most connection on balance 1 is the negative end of the entire pack (B-). This is where the black wire in the first balance cable gets connected. Starting from the negative end of the pack (B- or the right most cell in the schematic) that's cell 1. The left most cell is the positive end of the pack and cell 12. So the first wire after the black wire solders to the connection between cell 1 and cell 2. The third wire in the cable solders to the connection between cell 2 and cell 3 and so on. Whatever junction between cells got the last wire in the balance cable should also get the black wire in the next balance cable. In the below schematic that is the connection between cell 6 and cell 7 where the little upside down "Y" is. . In the schematic I used 6S balance cables since it split my 12S pack exactly in half. In my 20S packs that wasn't possible so I went with whatever split the pack into equal sections and my R/C chargers could handle or four 5S connectors. The connection technique was exactly the same as doing a 12S pack with 2 6S balance cables except that there is 4 5S cables instead. You still start and end each cable exactly the same.
Something to note about electricity...(for dummies)
There is a common misconception that electricity flows from the positive pole of a battery or DC power source to the negative pole. This is exactly opposite of the truth. Electrons flow or move...they are what electricity is. Electrons have a negative charge to them and they flow from wherever there is a lot of electrons to where ever there is less electrons. When ever you get shocked by touching something grounded it is because you have extra electrons gathered onto your body that are looking for someplace to go. The ZAP is them flowing from you to a place that has less negative potential than you do. Watch lightning...sometimes it comes from the clouds and hits the ground. Sometimes it comes from the ground and hits the clouds (this is less likely, but happens). The direction of flow tells you where the abundance of electrons is. So then where do the electrons go? The simple answer is imagine that there are tiny buckets built to hold a single electron. The electrons fill the buckets. In the case of a battery, there is an abundance of extra electrons at the negative pole of the battery and an abundance of empty buckets at the positive pole of the battery. When you turn on your flashlight, they flow through the light bulb and make it glow and then back into the "buckets" in the positive end of the battery. When you recharge a battery, essentially what you are doing is dumping the electrons out of the buckets and pushing them them back into the negative pole of the battery where they will later flow back out and into the buckets again. For everyone who knows how electron flow really works, just roll your eyes and pretend you can endure this overly simplistic example. That's what I am doing right now!!! For the people out there that think electricity flows from plus to minus like we are commonly taught from birth...well this is for them. So all of that deep electron theory (LOL!!!) to point out one tiny thing. Notice in the schematic P- on the BMS. That is a 14 awg wire that goes to the XT60 connector. The wire going from B- to the first cell is also a 14awg wire. This is the current path out of the cells. All the other connections between the BMS and the cells are very small wires capable of maybe 1 or 2 amps. The red 14awg wire coming from the positive end of the batteries to the XT60 connector is the return path for electrons back into the batteries. Even the wire connecting the positive end of cell 12 to the BMS is a very small wire. That's because electrons flow out the negative end of the pack and into the BMS (B-) and then out of the P- and then into your motor or lights or whatever to make them do work and then back into the positive (red wire) end of the battery pack where the empty buckets are waiting to be filled. Remember that electrons flow. The "buckets" don't flow. They are containers for electrons after they have been used and since they have no electrons in them they have a positive potential to them that attracts electrons. So effectively dump out the buckets and push more electrons into the negative poles of the batteries to charge AND let the electrons flow out of the negative poles and into the positive poles to discharge. God! I think I nearly suffered an anurism writing that!