spinningmagnets wrote:Among "serious" battery pack builders (who might own a spot-welder) the plastic holders get a lot of distain, but...with the cells being free? I can see the appeal, especially for a stand-up scooter that can be folded and then carried on a train or bus.
For a pack that is higher voltage, but low amps? I think this isn't a bad solution. The big sellers like Luna and em3ev don't have anything to worry about, nobody will sell these due to liability concerns, but there are many plans posted in several public forums (makezine, instructables, this thread, Drkangel, etc). 98% of battery pack customers just want to buy a finished pack, no garage building.
You are using higher pack-amps from this than I would, but other than that?...if I had access to free laptop cells, I could see me doing this (ie, beef up the series connections to keep them as cool as possible). Great pics, thanks for posting...
The only issues with battery holders that I have run into are...in order of most common problem first to least common problem last.
1. Keeping the cells from coming out of the holders
2. Populated with cells, a 20S2P battery holder pack is fairly heavy and glue joints get stressed. IE: reinforce glue joints as needed.
3. Cells that originally tested good going bad
4. Too much current draw per cell making them warm up.
5. insufficient current handling capacity in the battery holder.
6. accidentally popping a cell into a holder backwards
So let me explain a little...
1. My battery holders are packed to gether so there are no gaps between them. As a result I have never found a battery completely dislodged from it's holder, but I have a found quite a few that had shifted the 1/16" gap between cells in opposing holders. IE: they never actually come out of the holders, just shift a tiny bit while still maintaining full electrical contact. I usually push all the cells into each holder all the way down. If I pull out a pack after it has been in use for a month or so, just about all the cells have shifted a tiny bit. I wouldn't use battery holders without a way to keep them in the holders or the cells will slowly come out from bumps and vibrations.
2. I have had just about all the packs have a glue joint failure of one degree or other. Originally I used super glue between each 4S battery holder as the only thing holding the assembly together. That worked for several months, but eventually even though the packs can't move around or shift inside the battery box, I noticed that a few glue joints had failed. I reglued the joints back together with super glue and then added strips of 1/8" plywood to the tops and bottoms of all the packs that are glued down with flooring glue designed for tile or wood attached to concrete. It stays a little flexible, but when cured, stays stuck in place super well too. Gluing to concrete is not easy! This stuff is pretty much overkill. I have had no further glue joint failures. I think since my packs are long and narrow that they could flex down their lengths a little and that caused the glue failures between 4S modules. Now with the added plywood strips, they can no longer flex. IT has been a very long time since I had a joint failure.
3. Every used cell that originally went into my scooter was tested by me to make sure it met a minimum criteria for "good enough". However, over time, several cells have died and no longer take a charge. Out of 240 cells that number is less than 12. I randomly open the battery box and connect up a celllog to each balance connector. I quickly find which cells are going weak/died, pop them out and replace with other used/good cells. I forget exactly when I started using the used cells on my scooter, but I'm pretty close to a year now and that's all that have gone bad. I'd say not bad at all and I ride 10 miles to and from work every day on my scooter. If the weather is remotely decent, my car stays home.
4. I've never had a significant problem here, but then at most I'm running 2C continuous per cell. In the summer, I've ridden home in the early evening with it in the high 90's and just poured on the throttle every chance I got. Then when I got home I immediately checked the battery temperatures and the laptop cells were air ambient temp or 5-10F warmer. If a cell dies, that one cell gets a good bit warmer and enough to darken the color of my labels in each battery slot. I use a brother labler. The text on the label strip is created via heat application. That's why the labels get darker if they are exposed to heat. Still...that's 10 or so that are darker, but still readable. If I was really pulling hard (5C) on these cells or the battery holders, I would think I would see evidence of it everywhere. IE: black labels all over the place, distorted/melted plastic from heat due to over loading the solder legs on each holder or hot cells. The truth is, none of that has happened except darker labels where a cell died.
5. The spring clip touching the battery poles could handle 15 amps no problem, but the solder leg underneath is much smaller and can handle 6 or 7 amps at most. I overlapped them from each half of the pack and then added some solder to "beef them up more". It's never been a problem, but then I'm not exceeding the amperage of those solder legs either. I considered adding a small piece of copper wire to the legs, but it just hasn't been necessary at 2C per cell. If I were to use high discharge cells, first off they wouldn't be scrounged laptop cells and I would have built a properly welded pack, not used battery holders so it's a mute point, but yeah these battery holders wont hold up to 5C continuous discharge closed up inside a battery box without them showing evidence of over heating.
6. Early on, I labelled every battery slot for correct cell orientation. That was a lot of little tiny bits of label...40 per battery holder times 7 holders. It took hours to do. Despite my correctly labeling everything, somehow, I have randomly stuck a cell in backwards. I think it has happened 4 or 5 times. I usually notice immediately and so that cell has had a dead short with it's opposite side neighbor for a couple of seconds...no harm done. I check the voltage and everything is fine and the battery never warms up even a tiny bit. However, once and thank god...only once...I put a cell in backwards and didn't notice. I ran with that cell in backwards for several months before I did a random pack check-up. There was a single cell that was dead flat and the label behind it was black. The plastic battery holder showed no evidence of melting or distortion from heat. I'm guessing that cell got to around 150F and ran out of juice. The voltage of it's neighbor on the other side of the pack was much lower than the rest of the cells in the pack since it was carrying the load that two parallel cells should do, but was undamaged. I noted the position of that cell just in case it might fail later, but it never has. LOL! I guess that's proof of being able to run an LG laptop cell at 4C continuous.
On a side note about laptop cells. My old load tester could draw 24 amps. I would hook it up to my spare 20S2P battery holder and load test 40 cells at once. That works out to a little less than 5C per cell. The batteries would get pretty warm, but would still run the load for long enough to show around 2000mah or better of discharge assuming all "good enough" cells. This discharge rate was way over what I have ever run them at in my scooter, but they took it and kept going. My new halogen bulb load is about 4.6 amps per bulb and at 82 volts that means 4 bulbs in parallel or 18.3 amps continuous or 9 amps per cell. That's better than before so I'm not pushing the cells so hard, but they take it just fine with a little less heating up for the load test where I am weeding out the weak cells. I don't think I would want to run laptop cells at effectively 3C continuously, but for a load test it's OK.