cell_man
100 kW
Fellas I've now now made a prototype of my proposed termination method which uses only non conducting material apart from some m4 bolts to hold it all together. Here's a few pics. Please ignore the fact that the sections are different width. I need to get some more 5mm sheet and will ensure all sections are of course identical depth.
Here are a few pics:
I've added sections of 0.5mm aluminum at each junction to allow balance wires to be added. The alumium could be changed to either stainless steel or copper as required. I think stainless may be the best bet due to it's strength. If the little tab breaks off the pack would need to be broken down to replace it.
View attachment CIMG0342.JPG
View attachment CIMG0348.JPG
This shows the order of the pieces used. Divider is approx 2mm and the clamping sections are approximately 5mm thick. Together, with the tabs and 0.5mm balance tab insert, the total thickness is just about spot on for the 15Ah cells. Would be straightforward to adjust the termination pitch thickness as required.
View attachment 4
I've also done some tests with 2 cells connected in series using the non conducting clamps with bolts immediately at the edge of the tabs. I made a quick check of the difference an additional clamp in the centre made (by tightening a G clamp down onto it). It made a significant difference, about 0.2V additional voltage drop at that termination, so bolts through the tabs are necessary IMO My main concern with using non conducting tabs at high current is the loss of any heat sinking effect metalic terminals would have. The tabs are very thin and do actually get very hot at high discharge rates. IMO the heat will be coming from 2 sources, firstly generated by the resistance in the tab material and secondly from within the cell itself. I do not have concrete figures just yet but I feel non conducting tabs are more than adequate for constant current of circa 100A or a bit more with significanly higher peaks, but when sustained high loads are required a different approach is sensible. I'm concerned that not allowing the heat from the tabs to be adequately dissipated into the terminals may cause problems within the cell allowing core temperatures to get too high. If you think about it, the tabs do actually make a great heat sink and they are directly connected to the core of the cell that actually requires cooling the most. Maybe that is part of the reason for making them so thin
I'm also playing with putting some aluminium sheets between the cells to aid cooling. However I'm starting to think that this method cannot adequately cool the cell in the centre. I mean I'm sure it will help a bit, but due to the relatively large distance from the centre of the cell to the edge where the sheets can actually disipate the heat. I'm using quite thin aluminium sheet (0.5mm) so it will quickly adjust to the cell surface temperature but will not draw much heat out of the cell with access to cooling air. Thicker sheet will help but that defeats the point of such an energy dense cell IMO. To effectively dissipate heat from the entire surface of the cell I feel it is necessary to space some aluminum sheets apart by a small distance to allow cool air to be blown through the gap. If something like this was implemented on every 2nd cell that should be enough. If there was a honey comb sheet available, that would work well IMO. Water cooling would of course be better and would allow the cell temperature to be controlled in both directions, to get some heat into them on cold days and dissipate heat when being pushed.
Probably a bit difficult to make out what going on in these pics with the mass of cables.
View attachment CIMG0349.JPG
View attachment CIMG0350.JPG
View attachment CIMG0351.JPG
This was a previous version of the termination method which used metallic clamps and which would allow the terminals to act as a bit of a heat sink.
View attachment CIMG0333.JPG
I think I'm gonna have to configure a resistive load sufficient for say a 16S pack with at least 200A capability or better a bit more. As with everything on these cells what at first seem quite straigtforward quickly turns into a bit of a ball ache, but I'd rather make sure I've covered all the bases before sending out packs that are not properly thought out and implemented. These cells are not cheap and it would be a serious tragedy if I were to put together a pack that damaged the cells in some way. If you're gonna do something, I reckon it needs to be done right
Here are a few pics:
I've added sections of 0.5mm aluminum at each junction to allow balance wires to be added. The alumium could be changed to either stainless steel or copper as required. I think stainless may be the best bet due to it's strength. If the little tab breaks off the pack would need to be broken down to replace it.
View attachment CIMG0342.JPG
View attachment CIMG0348.JPG
This shows the order of the pieces used. Divider is approx 2mm and the clamping sections are approximately 5mm thick. Together, with the tabs and 0.5mm balance tab insert, the total thickness is just about spot on for the 15Ah cells. Would be straightforward to adjust the termination pitch thickness as required.
View attachment 4
I've also done some tests with 2 cells connected in series using the non conducting clamps with bolts immediately at the edge of the tabs. I made a quick check of the difference an additional clamp in the centre made (by tightening a G clamp down onto it). It made a significant difference, about 0.2V additional voltage drop at that termination, so bolts through the tabs are necessary IMO My main concern with using non conducting tabs at high current is the loss of any heat sinking effect metalic terminals would have. The tabs are very thin and do actually get very hot at high discharge rates. IMO the heat will be coming from 2 sources, firstly generated by the resistance in the tab material and secondly from within the cell itself. I do not have concrete figures just yet but I feel non conducting tabs are more than adequate for constant current of circa 100A or a bit more with significanly higher peaks, but when sustained high loads are required a different approach is sensible. I'm concerned that not allowing the heat from the tabs to be adequately dissipated into the terminals may cause problems within the cell allowing core temperatures to get too high. If you think about it, the tabs do actually make a great heat sink and they are directly connected to the core of the cell that actually requires cooling the most. Maybe that is part of the reason for making them so thin
I'm also playing with putting some aluminium sheets between the cells to aid cooling. However I'm starting to think that this method cannot adequately cool the cell in the centre. I mean I'm sure it will help a bit, but due to the relatively large distance from the centre of the cell to the edge where the sheets can actually disipate the heat. I'm using quite thin aluminium sheet (0.5mm) so it will quickly adjust to the cell surface temperature but will not draw much heat out of the cell with access to cooling air. Thicker sheet will help but that defeats the point of such an energy dense cell IMO. To effectively dissipate heat from the entire surface of the cell I feel it is necessary to space some aluminum sheets apart by a small distance to allow cool air to be blown through the gap. If something like this was implemented on every 2nd cell that should be enough. If there was a honey comb sheet available, that would work well IMO. Water cooling would of course be better and would allow the cell temperature to be controlled in both directions, to get some heat into them on cold days and dissipate heat when being pushed.
Probably a bit difficult to make out what going on in these pics with the mass of cables.
View attachment CIMG0349.JPG
View attachment CIMG0350.JPG
View attachment CIMG0351.JPG
This was a previous version of the termination method which used metallic clamps and which would allow the terminals to act as a bit of a heat sink.
View attachment CIMG0333.JPG
I think I'm gonna have to configure a resistive load sufficient for say a 16S pack with at least 200A capability or better a bit more. As with everything on these cells what at first seem quite straigtforward quickly turns into a bit of a ball ache, but I'd rather make sure I've covered all the bases before sending out packs that are not properly thought out and implemented. These cells are not cheap and it would be a serious tragedy if I were to put together a pack that damaged the cells in some way. If you're gonna do something, I reckon it needs to be done right