Hello, at the beginning I'd like to thank all the contributors in this place. (and sorry about my english in advance)
According to DIY powerwalls which are build on PCBs, I saw couple different designs (on aliexpress, etc) but none off them was 100% compliant with my requirements So I decided to design and build my own solution. After couple tries and tests I designed "Battery Sandwich v1.1", which looks like this:
(I've put just a half of 18650s to show on the pics how it looks inside).
One set to assemble 8 batteries 18650 looks like this:
(You need just standard battery holders and make holes in that, in place where the screw comes throw).
Advantages - in short sentences:
- It's very easy and fast to assemble, no soldering nor welding as well, just a screwdriver
- Very solid and stable. Special spacers under the PCBs make them good positioning and stability
- Easy to maintenance (to disassembly, to replace some 18650s, to rebuild etc). Important especially when you're building from recycled/salvage batteries.
- Each of 18650s is connected to bus bar (to connector at the center of pcb) via fuse. I'm using axial, glass fuses. They are not so great, but not terrible. Easy to replace if the fuse blow up.
- Scalability, you can build i.e. 7s8p batteries, but also big powerwall packs with it as well.
- Build-in balancer. One set, one PCBs has two ICs of passive balancer with power of 0,5A each. Balancer is active from 4.2v and drop voltage to 4.19v, each one is switchable (on/off) by jumper on PCBs. During assembling/disassembling switching off to prevent short circuit on the screw.
- The whole design is simply - no complex ICs, no programmable logic. Easy and simple as like each of the best solution should be.
Design considerations:
- Fuses. I don't like fuses based on PCB traces or so called 'tesla' fuses. Both solutions are pretty unpredictable and could start the fire. Besides 'tesla fuses' are often made so wrong (on the yt there are many videos where the fuses are made and each of them is different in length, which make them working on very different working parameters). Cheap, glass axial fuses are kind of average of its quality, but I think they are good enough.
- The connection spring/leaf. Made from steal, nickel plated. 5mm in wide. So they make good connection to top and bottom side of 18650s (if of course the whole set is properly assemble).
- The balancer. Active above 4.20v (and releases at 4.19v). Interesting thing is if you building big packs, which are build i.e. from 80 x 18650 (all of them in parallel) you also increasing the power of balancer, because you are using 10 sets (10 pcbs with balancer), so you will get power of balancer: 10 x 2 x 0.5 = 10A. Despite the power of balancing is significant, still the additional protections is recommended. Another interesting thing with the balancers in the big packs is that they never be active on the same voltage (4.20v), i.e. one of them will active at 4.20v but another on the 4.21v. It's kind of good because there is no need to all of the balancers should active at the same time. i.e. if the voltage is 4.2 some of them will be active, but the rest of them will turn on only if the voltage is still rising.
- the "plus" wire is connected to "minus" side of the sandwich by the screw. There is no dangerous during the mounting, because there is no power until is connected by the jumper (on the "plus" side). After the wire is connected, it's protected by the fuse. Besides there is enough space (between the screw inside and the 18650 batteries) that there is no risk to short the circuit, but overall the heatshrink on 18650s is a must. The plastic nippels (sorry, I don't know is it called) on the end of the screw protect against accidentally short circuit. But, if you even make in some way the short circuit throw the screw it will blow only one fuse no matter how big is your module (rest of the balancers will work).
- The balancer generates heat by the large resistors on PCB, but space from pcb to the bottom of 18650s is aprox. 5mm, so the heat does not affect on the single cells (during the balancer is active).
- Nominal power. I'm still before tests, but based on my calculation one set (8 of 18650 batteries) should handle continuous 16A at least with no power losses (or minor ones - the fuses cause some minor power losses for sure), which I believe is applicable for most of DIY powerwalls based on recycled/salvage 18650s. This gives 2A per 18650 battery. In details: The PCB is design to handle 2A per one 18650s, the PCB traces are wide moreover this could be improved in project by 2oz copper PCB if needed. The bus connector is made from 1mm width brass (tinned). I didn't found what is the rating given by the vendor, but i believe it could handle much more power then that. (Currently I'm working on the bus bar, these ones shown on the picture are 2x4mm^2 copper). The connection leaf are made from steal (nickel plated). The steal has poor conductivity (comparing to copper), but it should handle 2A each as well. Improving this could be much costly (beryllium-copper leaf would be needed especially made for this project).
- I've try to design this project coherent with all good practices, simple and inexpensive as much as possible.
So... what do you think? If someone would like to test it I have couple sets and I can send it (in Europe for sure, the rest of world - I don't known what is the costs of delivery so I'll check individual).
More pictures:
According to DIY powerwalls which are build on PCBs, I saw couple different designs (on aliexpress, etc) but none off them was 100% compliant with my requirements So I decided to design and build my own solution. After couple tries and tests I designed "Battery Sandwich v1.1", which looks like this:
(I've put just a half of 18650s to show on the pics how it looks inside).
One set to assemble 8 batteries 18650 looks like this:
(You need just standard battery holders and make holes in that, in place where the screw comes throw).
Advantages - in short sentences:
- It's very easy and fast to assemble, no soldering nor welding as well, just a screwdriver
- Very solid and stable. Special spacers under the PCBs make them good positioning and stability
- Easy to maintenance (to disassembly, to replace some 18650s, to rebuild etc). Important especially when you're building from recycled/salvage batteries.
- Each of 18650s is connected to bus bar (to connector at the center of pcb) via fuse. I'm using axial, glass fuses. They are not so great, but not terrible. Easy to replace if the fuse blow up.
- Scalability, you can build i.e. 7s8p batteries, but also big powerwall packs with it as well.
- Build-in balancer. One set, one PCBs has two ICs of passive balancer with power of 0,5A each. Balancer is active from 4.2v and drop voltage to 4.19v, each one is switchable (on/off) by jumper on PCBs. During assembling/disassembling switching off to prevent short circuit on the screw.
- The whole design is simply - no complex ICs, no programmable logic. Easy and simple as like each of the best solution should be.
Design considerations:
- Fuses. I don't like fuses based on PCB traces or so called 'tesla' fuses. Both solutions are pretty unpredictable and could start the fire. Besides 'tesla fuses' are often made so wrong (on the yt there are many videos where the fuses are made and each of them is different in length, which make them working on very different working parameters). Cheap, glass axial fuses are kind of average of its quality, but I think they are good enough.
- The connection spring/leaf. Made from steal, nickel plated. 5mm in wide. So they make good connection to top and bottom side of 18650s (if of course the whole set is properly assemble).
- The balancer. Active above 4.20v (and releases at 4.19v). Interesting thing is if you building big packs, which are build i.e. from 80 x 18650 (all of them in parallel) you also increasing the power of balancer, because you are using 10 sets (10 pcbs with balancer), so you will get power of balancer: 10 x 2 x 0.5 = 10A. Despite the power of balancing is significant, still the additional protections is recommended. Another interesting thing with the balancers in the big packs is that they never be active on the same voltage (4.20v), i.e. one of them will active at 4.20v but another on the 4.21v. It's kind of good because there is no need to all of the balancers should active at the same time. i.e. if the voltage is 4.2 some of them will be active, but the rest of them will turn on only if the voltage is still rising.
- the "plus" wire is connected to "minus" side of the sandwich by the screw. There is no dangerous during the mounting, because there is no power until is connected by the jumper (on the "plus" side). After the wire is connected, it's protected by the fuse. Besides there is enough space (between the screw inside and the 18650 batteries) that there is no risk to short the circuit, but overall the heatshrink on 18650s is a must. The plastic nippels (sorry, I don't know is it called) on the end of the screw protect against accidentally short circuit. But, if you even make in some way the short circuit throw the screw it will blow only one fuse no matter how big is your module (rest of the balancers will work).
- The balancer generates heat by the large resistors on PCB, but space from pcb to the bottom of 18650s is aprox. 5mm, so the heat does not affect on the single cells (during the balancer is active).
- Nominal power. I'm still before tests, but based on my calculation one set (8 of 18650 batteries) should handle continuous 16A at least with no power losses (or minor ones - the fuses cause some minor power losses for sure), which I believe is applicable for most of DIY powerwalls based on recycled/salvage 18650s. This gives 2A per 18650 battery. In details: The PCB is design to handle 2A per one 18650s, the PCB traces are wide moreover this could be improved in project by 2oz copper PCB if needed. The bus connector is made from 1mm width brass (tinned). I didn't found what is the rating given by the vendor, but i believe it could handle much more power then that. (Currently I'm working on the bus bar, these ones shown on the picture are 2x4mm^2 copper). The connection leaf are made from steal (nickel plated). The steal has poor conductivity (comparing to copper), but it should handle 2A each as well. Improving this could be much costly (beryllium-copper leaf would be needed especially made for this project).
- I've try to design this project coherent with all good practices, simple and inexpensive as much as possible.
So... what do you think? If someone would like to test it I have couple sets and I can send it (in Europe for sure, the rest of world - I don't known what is the costs of delivery so I'll check individual).
More pictures:
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