When the cells are in contact, the heat is transmitted faster between their plastic covers than from cover to cover through air. So the heat travels faster to the exterior.
Also high heat would thaw the glue, but even in the summer, if the cells are attached together (making fast heat transition) and if the EV requirements and battery performance is counted, the heat will never be enough to affect the pack.
Another issue is how do you exchange a single cell if one fails? One failed cell can ruin an entire pack. Exchanging an 18650 cell is always difficult in any 18650 battery pack, because the cells need to be spot-welded with metal strips to secure the connections. So apart from removing the spot-welded strips it is still needed to also remove the glue.
The last huge issue to consider with any battery pack is the Battery Management System (BMS). The BMS is a cell manager which controls what the pack is doing and shuts down in case it senses anything is wrong. For example, a BMS protects the battery from overcharging or undercharging…both of which can cause the battery pack to fail or worse…burst into flames. The problem with a BMS is its very hard to find a good one available that fits the size of the pack I plan to build, it is expensive, and it is hard to assemble. Most commercially available packs have a BMS and it is the recommended way to go for ultimate safety and reliability.
As an experienced DIY, I decided I can safely go without a BMS because of my knowledge. Basically I use a Cycle Analyst and a high quality smart charger for my BMS. Meaning the Cycle Analyst lets me know the voltage of the pack when I am riding and shuts down the power if I forget and lets me know if my packs voltage is dropping too low…Similarly, the smart charger makes sure my pack does not get charged over the allowed voltage. Charging cannot safely be done unsupervised on a DIY pack such as mine without a BMS…and if it is, it should be done somewhere in an outdoor barbecue for safety. Sounds ridiculous…but that is the downside to DIY. Especially for newcomers to electric bikes, a good BMS is essential for fire safety!!
Ok so now you know some of the disadvantages and problems associated with a DIY pack. The advantages are that the energy density and specific energy reaches unbeatable values for a very affordable price…especially in my case using recycled cells.
So I get the equipment (spot welder, nickel strips, BMS, battery shrink wrap, hot glue, some instruments and tools) and the 18650 cells I could bought at a good reasonable price (they came from Bosch rejected welded packs), and I started to make a 13S / 15P, 48V 42-Ah pack that later would give the very interesting performance of a max range of 300 Kilometers (186 miles) at 25-MPH on an EV made for the tests (that’s another Story). All in a pack that is below 10Kg (22.7 lbs) with this dimensions: 420mm x 144mm x 67mm.
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