Batteries


I’ve been working like a dog on making an electric vehicle system for a guy on the east coast. It’s been assembled for days. Runs too. But it has this kind of miss to it. Almost like it’s got one bad spark plug wire. It just runs rough. I’ve not seen that kind of problem before with an AC electric vehicle system.

Also have the spot welder nearly assembled. It just needs some finishing. I hope to create a process to spot weld braided flat cable to lithium battery cells. The cable allows flexing without breakage. Mostly for our combat robot buddies. For automotive use it’s not really necessary. But it will be a great process to have under my belt.

After this system gets shipped, I have gathered all of my hand drawn individual schematics, and will create one large schematic for the entire Dolphin main board. I have almost every square inch documented. Just a little remains. But even a mostly completed schematic will be helpful in debugging. I’m still deciding what cad package to use.

Yesterday the Insight cell testing was finally completed. I put 4 chargers and 2 constant current power supplies to work individually charging 6 sticks at a time. That way the discharger was always testing a stick of cells. It took about 5 hours to do the remaining 15 or so sticks. The diode balancer didn’t work fast enough. So I just used the shorting clips from the flooded nicads to discharge each cell in a stick on those sticks that looked like it could help equalize. I found 3 sticks out of 20 that were able to sustain 6 minutes at a 50 amp (8C) discharge load. The rest fell under that time. That’s 5ah of capacity. Since nicads have a bit of a Peukert factor, then I think an 8C discharge will show a lower capacity than the nominal 6.5ah that they are rated at. My constant current discharger is so incredibly useful for testing these nicad cells.

The next step is to remove the pack from the Insight and do the same testing to it. I am not sure with 80k miles on the pack where the testing times will fall compared to the used pack. I still have to decide what is more useful, fully charging or discharging the Insight pack before I remove it. In order to duplicate the tests performed on the used cell, I would have to charge the Insight’s cells until very warm like I did the used ones.

I’ve ordered and will characterize and evaluate a minimal bms system called MiniBMS. Dimitri sells it from Florida. It’s all analog and uses a continuous loop of wire for signaling that one cell has gone too high or too low, by breaking the loop. A buzzer or any other indicator can be used to notify the driver. This single wire concept is suppose to have next to zero noise issues. I would be concerned that such a long length of wire could have inductance sufficient to create a high voltage spike when ever the loop is broken.

The only drawback is that the idle current is rated at 10ma. That uses 87.6 amp-hours per year from the pack. I consider that high. My own design uses 0.75ma at idle, or 6.57 amp-hours per year. I suspect it’s keeping an opto coupler enabled that accounts for the current draw. It won’t tell you what cell is having trouble, only that one of them is in trouble. Several loops could be used instead, breaking the pack into groups. This would at least narrow it down some. My pack will require 100 cells. So it’s important to me. It is suppose to have thermal protection on it as well to keep it from bypassing more than 1 amp.

The BMS board pictured here is $12 for each cell.

The control board pictured here is $30 for each vehicle.

The control board has an on board timer that allows an adjustable delay of the alarm enable. So if the driver momentarily loads the pack past the low voltage threshold, the alarm won’t sound. If the pack has a cell that is low for longer than the delay, then the alarm sounds. The control board uses a simple resistor and capacitor to achieve the delay.

The bms boards can fit any of the large format cells that have 6mm screws. The bms board is small and bolts to the negative terminal only. A length of wire is used to reach the positive terminal. The downside is that during assembly the board could be rotated into a neighboring cell and create a serious short circuit. So assembly has to be more carefully done than usual. More on this system when it arrives.

Lead acid battery’s are the most expensive technology in our vehicles on a $/mile basis. If a good lithium pack could be assembled, it would make for a lighter and far more efficient vehicle.

In the near term I’m building Lithium modules from ten A123 cells in parallel. That’s a 23ah module. Not much harder to make a 20 cell/46ah module. I’m guessing based on my records, that my truck could go 40 miles max on 23ah since the pack would be so much lighter than even the nicad that I currently have. 70-80% of 40 miles is 28-32 miles. That’s a reasonable range for my city driving needs. Each 10 cell module will run about $125 in just cell costs. That’s $3250 for 154lb bare pack. There still has to be a housing of some kind designed to hold the module. It will use up just one half of the battery box. Leaving the other half for another pack as funds allow. Factory packs weigh from 1400-2200lbs. Can you imagine the range and efficiency increases from loosing up to a ton of pack weight??

CAD model of 20 cell A123 module. Displaces 2 flooded BB600 nicads.

Right now the large format cells that most EV folks are using, have very poor regen capabilityof around 0.3C. On a 100ah cell, that’s only 30 amps of regen allowed. Our vehicles have up to 200a with stock settings. 250 amps with modified settings. A 10 cell module can handle 100 amps of charge/regen current for 15 minutes at a time. So I can easily see them handling 200 amps for 5-10 seconds when regen kicks in with modified software, under specific conditions. In other words they are very low impedance cells that can handle our regen needs nicely.

As for discharging, the A123 10 cell module can handle 700 amps of discharge continuously, and up to 1200 amps for 10 second bursts. Our vehicles can only discharge at 200 amps stock or 250 amps in a modified configuration. Here is the data sheet from A123 on this cell.

These cells have a very long cycle life. 1000 cycles at the max ratings listed. Double that with any care at all. The factory told Bill Dube that if a person can tolerate a loss of 50% capacity over time, then these cells can go 10,000 cycles. Most lead packs lose 50% capacity at 200-300 cycles.

For all of these reasons I’m building an industrial spot welder for making A123 modules. Not just for EV’s, but for the Combat Robotics crowd as well. They use these cells without a BMS and they still last a decent amount of time. Far less than they could, but those folks are happy with the performance and longevity.

Each bare 10 cell module will be 66mm wide x 260mm long. That’s 2.6″ x 10.24″. There has to be some kind of structure built to house the module and some stout connections designed so they can be connected together into a pack. I’m thinking of making the structure look a lot like a flooded nicad cell. In fact I wonder if a nicad cell housing could be adapted as the structure. These modules won’t need to be strapped tightly together like the large format cells require so they don’t over expand and die. These cells will never expand.

In theory, a 23ah pack would be 100 modules. This could fit into the Prizm battery tray. My truck holds 126 nicad cells in each half of the battery box. So it could hold 126 A123 modules per side, for a max of 252 modules per truck. The stock charger could not charge them up past 400v so only 100 modules would be fitted per side. A second 100 module pack could be added to the remaining side if desired, and wired in parallel.

I have a couple of dozen A123 cells and the 10,000 amp (5 ms) microprocessor controlled spot welder is under construction. I plan to make one 10 or 20 cell module to take the place of 1 or 2 nicads in my current pack, for testing.

Here are the fets temporarily mounted to the heatsinks. I say temporarily because I forgot to add some thermal grease under each fet. But I’m glad to know that my dimensions were ok, and that everything fit nicely. I used double sided foam tape on the bottoms of the 3 heatsinks. This will isolate it from the board per Fritz, and mechanically secure it to the board.

Looks just like the picture! Shot w/G1.

Here is a closeup of the mounted fets. The screw heads just barely clear between heatsinks. So make sure when you drill and tap that you center the holes vertically along the slot. Using button head screws would give more clearance. My flat washers were a bit thick as well. Shot w/G1.

Turns out that 6-32 screws fit the fets nicely so that’s what I tapped the heatsinks with, instead of 4-40. The cnc code did fine. I had to add a step that chamfered the top of each hole so that the tap would start easier. The nicest part of using the mill is for power tapping. It makes perfect threads and the mill does the work. The 6-32 tap is the smallest I’ve every power tapped with. It’s a cheap tap too, but it worked just fine!

Here they are just finished. Image shot with my G1 cellphone.

This morning I measured out the fet placement on the heat sinks, then wrote the g-code to do it on the cnc. I debugged the g-code on the cnc too. Now I can drill and tap all 18 holes in 3 heat sinks with ease. They will all be tapped for 4-40. 3mm would work too. I suppose I could drill a clearance hole and use nuts on the back side too.

I just got the huge 4 farad cap in the mail. Yesterday I found a nice Tektronics chassis at Halted that use to be some old test instrument. If it will hold all of the larger spot welder assemblies, then it will fit on my bench very nicely as it’s narrow. The chassis was designed to sit on either of two sides. I just need to start the assembly process.

If you have not noticed or heard, I’ve opened the US Electricar.net store for USE owners so they can get repairs and parts to keep their vehicles running. There are links on this blog to take you there. Enjoy!

Along with several others I’m on a spot welder project. This will be the tool I need to spot weld A123 cells together to make a nicad replacement module for my truck. I was in Merced a few weeks ago and got the chance to meet Fritz. Here is a video of the spot welder in action. It’s very sweet!!

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