Battery Pack Disassembled (Frustrating Weekend)

[Mark911]

Had a frustrating race weekend recently. The event was the TWMX "Slam Fest" at Milestone Raceway here in Socal. An event tailored made for the Alta as it's basically a 5 lap dash for cash. Lots of big time players typically show up and it was loaded this year. Mike and Jeff Alessi, Tyler Bowers (last years winner), Alex Ray, Mitchell Falk, Derek Drake, and all the fast local boys. Of course, I wasn't racing but I let Matt Cerami take a shot with my Alta.

The morning till early afternoon is for practice with the race (two motos) starting around 3:00 PM. It was a very hot (95+) day but we needed to get the bike dialed in for Matt so we arrived early. The first practice session went well as Matt managed about 5 relatively hard laps before the thermal limiting light came on. I was prepared with a huge industrial size fan and plenty of water to keep battery temps in check. Between practice sessions I'd alternate with direct spraying and soaking towels wrapped around the pack. Unfortunately, the light would come on earlier and earlier after each session.

We sacrificed the last couple of sessions just to give the pack more time to cool before qualifying, at least two hours. All the while I'm trying to keep the bike reasonably charged and the pack as cool as possible. It was a full time job!

Qualifying was 30 minutes, fastest lap counted as normal. Matt couldn't even manage one hard lap without limiting. We spent most of qualifying sitting in the infield waiting for the last couple of minutes to make one last attempt. No go. Again, very frustrating.

Now granted, mines a 17 pack. But from that experience I'm sure the MXR pack probably wouldn't have done much better under those conditions. Also, I'm not 100% sure the limiting was due to pack temps as it could have been motor or inverter limiting. I'm not sure there's any way to tell, at least none commonly known. At this point I'm pretty sure it's battery related, however.

I've been telling myself I got to do something about this situation (pack heat soaking). So this week I finally tore my pack apart to study the design and decide if there's something I can do. Here's some pics and comments.

The Pack was much easier than I was prepared for to disassemble. It only took about an hour including careful deliberations not to break anything or get myself killed (The voltage and amps from this pack CAN kill you, so don't attempt this unless you know how to work with high voltage systems).

As I suspected, the cell side of the pack is composed of four identical "Modules". The specifics of the cell modules can be found in one of my earlier posts. The housing is a two piece design split horizontally mid pack. Each "half" houses two cell modules. The half's are again compartmentalized with a web or shelf about midway. Each web serves as the main structural interface for the forward module and aft module and as the main thermal path from the modules to the outside air. To improve the thermal conductivity of this relatively thin web, four "heatpipes" (two fwd and two aft) are incorporated into the web. In addition, thermal "paste" is applied quite liberally (too much?) between the module interface surface and the web. The "high power/amp" (+/-) terminations for each module are connected to the next "series" module through insulated "slip pins". The +/- slip pin receptacles on each module have tiny "fingers" which make intimate contact with the uninsulated ends of the slip pin allowing easy installation and good electrical contact (apparently). The modules are connected in series top to bottom, the final negative end needing a more conventional stranded wire routed back up to the top of the back through small hogouts in the webs. Each module is electrically and electronically connected via interconnect harnesses, again routed through small cutouts in the two webs. These allow module to module and module to host communications (via CAN), power and data exchange. There's flashable micro controllers, CAN transceivers, and digital isolators everywhere!

The nadir cover (lid) houses the I/O connector, main power connector (+/-) as well as the main contactor and 5 and 12 volt DCDC converters plus other electronics aboard the cover PCB. The nadir cover has only four connections to the cell modules, main +/-, a separate housing grd and a small 5 pin connector. As mentioned, the cover also connects the cover PCB to the main power connector (to inverter) and I/O connector. The I/O interface supplies the regulated 5 and 12 volts to whatever needs it and is the main CAN interface between the battery pack and the bike.

Initial thoughts? Nothing too surprising so far. I think an externally driven cool plate can be incorporated between the very top and bottom modules and their respective heat sink webs which at a minimum would allow much more rapid heat removal between motos and/or during charging. With the proper design and materials I think this could be done without negatively affecting existing "running condition" thermal control. While it doesn't solve the problem entirely, it will delay the onset after the pack has seen temperature.

 

[Rob41]

Wow Mark, you dove right in, good on you! Did you just void your warranty?

Can't wait to see if you're able to make some improvements. I'm sure if you did, Alta would take notice as well.

 

[Mark911]

Alta has some smart people. I'm sure they've done tons of R&D. However, it ultimately comes down to satisfying specific in-house performance goals at a certain price. The process results in lots of compromises in the production units. Whatever I do they've probably already thought about and would have resulted in significant cost uppers they simply can't absorb right now.

No MX bike I've ever bought has come with a warrantee, so no big deal to me.

 

[Fog 25]

Mark911 What does the close cooling system run thru? Is it just the electronics? I was thinking that if we could add a external radiator that would help with the cooling.

 

[Silent But Dirty]

Mark911 What does the close cooling system run thru? Is it just the electronics? I was thinking that if we could add a external radiator that would help with the cooling.

The battery is air cooled only. It will be pretty interesting to see what Mark can do with the inside of this thing.

 

[Mark911]

Kinda interesting how Alta documents certain assembly process like this Cell Module. Only two days to bond, assemble, wire weld and test. Not too bad. Each of my modules were serialized, 1961, 1962, 1963 and 1964. Up to almost 2000 units of this part number early this year!

 

[OneLapper]

Is it feasible to design a new battery case, or modify the existing case, with cooling plates that use water? I envision the battery modules sitting on these cooling plates using the conductive paste, and then the plate sitting on the copper looking heat sinks. I've done some water cooling on a large RC boat I have, it's around 5hp. The fittings, lines, pumps are commercially available. Making a 1/4" cooling plate (or smaller), and modifying the battery housing to fit them is a huge task though. I love the idea!

 

[Mark911]

I'll run some trades to see what makes the biggest difference. The copper heat sinks are heatpipes, or probably more accurately "vapor plates". In this application they help even out the local module temps by providing a better thermal path for the hotter "inner cells" to reject their heat. Ultimately, however, all that energy needs to flow through the small peripheral cross section of the shelf/web to get to the much greater surface area of the outer housing (radiator in this case). Possibly a better solution (but much more expensive) might be to use true heatpipes to move the heat directly from the module to the outer housing. You'd need "bent" heatpipes (shaped like an "L") along with the necessary room and processes/materials to thermally connect them to the inner surfaces of the housing walls. We used this configuration on satellites to get heat from inboard shelf mounted battery packs and electronics to the outboard radiator panels where the heat can be rejected to space. Of course, gravity can negatively affect the performance of heat pipes so this must be considered in the design layout. Using "bent" heatpipes in my opinion would be the next logical design iteration but it's still probably limited to around 20%-30% performance gain.

So yes, ultimately I envision a two relatively low profile one piece liquid cooled cold plates (one for each housing half) sandwiched between the two outboard modules and the housing shelf. Can't go to the inboard modules as there's no room but they'd still receive approx. 85% of the benefit as they share the same shelves. This would obviously change the station heights of both modules but a simple spacer between the top and bottom covers and the housing could accommodate this. To avoid any possibility of internal leaks, any liquid connections would be made outside of the housing using windows cut in the housing.

Now, whether to make a full on active system (onboard pump, dedicated radiator, etc) vs just using the system with a portable source of chilled water for cooling while charging between motos is another trade.

I think the answers will be more apparent after my initial analysis/simulations. Now that I have an actual cell module and housing to model I can run some FEA simulations to see where the thermal bottlenecks appear. If the limiting factor is the thermal path between the cells and the module's baseplate (ie-the module's epoxy housing thermal conductivity) the law of diminishing returns will set in rather quickly.

Of course, there's always the option of changing the actual cells to ones with higher C ratings. But as you can see from the attached pics the amount of wire welding is significant and the actual process would need to be developed (unless Alta wants to share?). Ultimately, my "dream pack" would include both.

 

[TCMB371]

Bummer to hear about what happened at this year's Slamfest. I was wondering why an Alta wasnt out there this year. Looks like you've had enough, though! Looking forward to seeing what you come up with.

For what its worth, I haven't had thermal limiting issues in a race situation yet with my '18 MXR. I'm using a misting fan with a 5 gal bucket of water to spray at the battery when i'm charging it, and the pack stays cool to the touch.

I'm racing at Milestone tomorrow so we'll see how my bike holds up.

 

[OneLapper]

I suggest using both on-board cooling and a chilled water system when in the pits or charging. I have a coolshirt system for my race car, just a simple cooler with fish aquarium tank pump that pumps iced water to an under shirt with tubes in it. The tube couplers are water tight when disconnected. You can easily unplug the tubes and connect to the chilled water for charging, then unplug and snap into an air to water radiator system with a small 12vdc pump. It would the best of both worlds. The chilled water system would be significantly better at cooling. It would be just a few seconds to switch between the two systems.

While you're in there, I would add a thermistor to measure the temps. It would be easy to make the onboard pump temperature controlled.

I have all of these components just sitting here if you need them for testing

Coolshirt System

Here's a source for the cold plates. A manifold tube system or mini channel would be better that a serpentine tube style.

Cold plates

 

[Han377]

Is it possible to let mineral oil flow through the pack instead of water?. As it doesn't conduct electricity. I have seen it on pc systems fully submerged into mineral oil. Connect it to a external radiator with a fan mounted behind the shrouds

 

[Philip]

Is it possible to let mineral oil flow through the pack instead of water?. As it doesn't conduct electricity. I have seen it on pc systems fully submerged into mineral oil. Connect it to a external radiator with a fan mounted behind the shrouds

I was going to suggest the same. Electric bike owners on Endless Sphere forums and other fill their controllers with oil.

Filling the battery with oil will be heavy, but you can do it in the pits only, then drain.

 

[Philip]

I had to look up "heatpipes".
heatpipes - Google Search

 

[Philip]

How about air cooling the inside of the battery? Just pump the air thought the pack while riding, and/or in the pits? The surface area of all these 18600 batteries is huge, so it should be a lot more effective than keeping this heat trapped inside a sealed aluminum box and blowing cold air at it.

 

[Han377]

Opening up the battery pack to let air flow though it would mean water can also enter the pack which would cause problems. Using the mineral oil you can make a closed circuit. Would be heavier indeed but with the bikes light feeling when riding. A extra 2/3 kilograms will not make much of a difference in my opinion

 

[Rix]

Impressive Mark, if the decks are symmetrical, looks like the pack is comprised of 504 cells. It would be a big big PITA, but probably the most straight forward solution would be swapping out cells with better quality units with higher C ratings which would solve the issues provided the thermo limiting wasn't from the motor or inverter like you had previously mentioned.

 

[Mark911]

I'm pretty sure the higher C rating would just delay the initial onset and after the first hard "ride/charge" cycle they'd be heat soaked as well. Typically, cells made for higher C rates are more thermally conductive, probably the reason for the improved performance of the MXR pack. However, I'm discovering that the poor and highly anisotropic heat transport properties of even the highest C-Rate 18650 cells would still have major limitations with the current thermal control system. A major pack redesign may be what's required. Remember, I'm talking about A level competition here.

 

[Rix]

I'm pretty sure the higher C rating would just delay the initial onset and after the first hard "ride/charge" cycle they'd be heat soaked as well. Typically, cells made for higher C rates are more thermally conductive, probably the reason for the improved performance of the MXR pack. However, I'm discovering that the poor and highly anisotropic heat transport properties of even the highest C-Rate 18650 cells would still have major limitations with the current thermal control system. A major pack redesign may be what's required. Remember, I'm talking about A level competition here.

I get what you are saying, for the riders that have the ability to ride with the throttle pinned all of the time in MAP4 will exceed the pack's/machine's abilities. I can safely say I am in no danger of being a part of this group.

 

[Han377]

I can say i was part of this group. And on my home track with deep sand the bike was overheating within 5 minutes. Map 4 was great as long as it lasted. After that 250f bikes passed me on the straight sections of the track. Same with Map 3

 

[Mark911]

I'm using ANSYS to model and run thermal/electrical simulations at the 1) cell level, 2) "mini" module level (14 cells) and 3) 1/2 Pack configuration. As usual, finding and/or developing material, component, and contact specifications for thermal conductivity, specific heat and in some cases the heat generation profile (both transient and steady state) for the cell(s) in my model is the biggest challenge as well as being the largest source of potential error (GIGO)! I'll post some ANSYS animations when I get a chance.

BTW, one thing I did note was that my pack's housing to module interface surface was VERY wavy (I need to quantify this). The web in question is relatively thin (approx. 4mm) compared to the surface area and it appears that the part wasn't properly machined (not supported, too much feed/speed, wrong end mill, etc). You can easily feel and see the low/high spots and how they correspond to the end mill route path. Intimate contact (thermal contact resistance) is very sensitive to surface to surface flatness and finish. Even a "good" bolted thermal interface usually assumes only 50% actual "part to part" contact. The remaining area is assumed to be convective (air) or in this case the "thermal grease" used by Alta. However, even the best thermal interface materials (TIMs) are nowhere near as thermally conductive as the aluminum used in the housing and cell module heat sink. This is a relatively basic design principle so until/if I can show otherwise, I have to believe Alta's engineers determined it to be a "non driver" in their analysis or intentionally specified this surface finish.

Also, I finally got around to doing a quick "pin-out" verification of my 17's wire harness. About 70% of the pin-outs are for fairly obvious use/signals. Another 20% are not quite as obvious but I can make a relatively accurate SWAG. The final 10% I need to study a bit more. Many (if not most) of the components on the Alta serve more than one purpose. Electrical/electronic systems are probably no different. For example, I'm pretty sure the wire connections between one of the main electronic modules provides FOC/SVM data during operation and "charger-to-battery" connection during charge. An interesting double duty if I'm correct.