Batteries


Yesterday Mike Bennett and I worked on a load tester for my nicad pack in my shop. It loads each cell one at a time with around 40 amps. Which is 1C. The goal is to find bad or failing cells when the pack is discharged down to the first drop in pack voltage. This occurs around the 5 mile mark. So I know many cells are croaking at that point.

We bought a 100 amp, 12v lead acid load tester from Harbor Freight for $25 on sale. We ended up using bailing wire as an additional load element. I’ve used bailing wire before for discharging cells. It works fine. It’s a soft wire that when it turns red, it still soft. So it stays consistent. We did some empirical testing to see how much bailing wire we would need to add to get 40 amps at around .6 volts. We came up with two 11″ pieces in parallel. We crimped on 1/4″ yellow lugs and bolted them to the large copper crimps already in the load tester. That worked great!

The meter ranges from 0-16v. As it turns out it was fairly accurate around the .5-1.5 volt range. But that was too small of a needle swing for me. So at a later date I will recalibrate the meter by 10x. It will then read 0-1.6v. Perfect for nicad testing. It has a 51 ohm 1/8 watt resistor across the meter movement. So that needs to be changed to a 470 ohm pot so it can be calibrated nicely.

We load tested a couple of cells continuously to see if the bailing wire would fail. It got hot, but never glowed. So I think the only weak link is the rocker switch moving 40 amps through it. Since having a rocker switch would require 2 hands to operate the load tester, I may very well bypass the switch. Then the load tester can just be rocked only the 2 cell terminals and that will activate the load tester. There are 250 cells to test. So speed is everything!!

This load tester could be tweaked for use on lithium too.

Ooops. Overshot a bit. Hit 107.00kw today.

Pavement dried out. The acceleration is stretching the front suspension upward
nicely.

98.44kw so far. Getting there. I bet any sagging is caused by the pack.

I’ve got the pack doing better. But now it’s raining. Anywhere from 0-10 mph I
can spin the tires right up in the rain! More testing on dry pavement will force
more current from the system. That will let me know how close to 100kw I am.

Some prototyping netted 91kw of output from my truck recently. The acceleration was unbelievable. That’s still an 80% improvement. The pack is sagging more than normal right now. So I’m correcting that as of now. I suspect that would be worth a few more kw. Looking forward to 100kw soon as the pack gets updated and the R & D progresses.

The data sheets say the factory igbt’s are rated at 400 amps. Almost there now. If I can keep the pack above 250vdc at 400 amps, then I’ll be at 100kw.

I have had an igbt/hv buss redesign going. Might have to put more priority into that since the factory igbt’s are nearly at their max current rating. Although for maybe only 10-15 seconds at a time, 400 amps may not be a threat to their longevity.

With the A123 prototype modules doing fine, a full pack would have such low impedance that 100kw would be easy to hit. Maybe too easy. Going to build about 12 more A123 modules soon.

My A123 modules got a beating at a combat robotics contest recently. They held
up fine. They were charged with a BMS on each cell. Discharge rates overlapped
my truck. These cells saw far more G-forces than normal. But I needed some
testing on the structural side of the design to make sure nothing stupid
occurred. The spot welds were of particular interest. The owner uses A123’s in
his robot. Now he’s sending 100 more for more packs to be made. Sweet. This time
however he has a different module layout he wants to try.

I used nickel as the conductor since copper is apparently next to impossible to
weld with a CD welder. I was told this and didn’t believe it. Then I tried to
for 3 days. Nope. Even as fancy as my CD welder is. Nickel is universal for cell
assembly. But not only is copper cheaper, it also is a better high current
conductor. That’s also what makes it harder to weld. It basically has no
resistance. Since I’m making over 70kw now, I have to make sure a module can
pass 100kw for the future (fingers crossed). If copper ever gets used, I’ll have
to buy/make an inverter type welder.

I just won’t be satisfied until I can spin tires on dry pavement at will!

The 312v (405v max) AC motor control system in my truck is very energy
efficient. It’s only 70kw right now. But it will be 90kw here shortly. It has
the capacity to handle way over 100kw.

I don’t have to cram very much of a pack into this vehicle to get excellent
range. If I put in a 160ah set of Thundersky cells, my range would be 256 miles.
But then I have to put up with all of the cold voltage sag as well as the sheer
volume that this pack would take up. Works for some, just not for me.

Speed is electronically limited to 72mph. That also assumes I leave the
transmission locked in 2nd gear. It’s a 5 speed manual with overdrive. Adding a
shifter makes the top speed math go to some insane top speed. Or as ACP does,
remove all of the unused spinning mass in the form of gears and syncros for yet
more of an increase in efficiency.

Something I keep forgetting to explain. The cells themselves will only weight
154lbs. They completely fit in well under 1/2 the battery box.

The 7.36 kwh pack I’m building will take my truck 36 miles at it’s current
efficiency of 200wh/mi at 4000lbs. My estimate of a 15% increase in efficiency
from losing 600 lbs, will put the range at 42 miles at 175wh/mi at 3400lbs. On
only 50 amps the truck goes 60mph. There is an owner of a truck just like mine
with a 19.2kwh Thundersky pack. He went 112 miles on a charge. That’s about 171
wh/mi. But his cells weigh 550 lbs. That’s 400 lbs more than my A123 cells. It’s
feasible that my truck could end up with a 50 miles range off of the tiny A123
pack. Since the camber on the front end was finally just now set to zero, there
could be some additional increase in efficiency. I have yet to block the grille
and add the coroplast to the chassis underside to clean up some of the
aerodynamics. Those are standard features on all ACP vehicles, old and new. All
I need for my driving requirements is 15 miles of range. With 36+ miles of range
my cycle life will be close to forever.

I will pick up some cobalt 18650 cells for spot welding and testing. After all,
since the safest chemistry available can currently be set on fire so easily, it
just makes sense to try them. Their smaller size would make for an even lighter
pack. It’s just takes some mechanical and thermal engineering to make this work.
It’s just not that difficult.

I wonder if the laser welder I used back in 80’s is available surplus somewhere?
I use to spot weld nickel ‘flags’ to stainless wire. Not unlike materials for
cell module construction.

Starting last night I drove the pack down until it was only around 200v. Then pulled it into the garage for discharge maintenance. I made up a system using six 2000w, 120vac water heater elements to discharge the pack in large groups rather than just two groups.

Here is the pack attached to all of the water heater elements. A 5 gallon bucket of water was used for cooling since an experiment with air cooling caused the elements to glow red, which is an ignition source. The water level dropped about 3/4″ when the pack was done discharging.

I used my cnc to machine 6 holes in these two pieces of 3/8″ thick plywood. They held the heater elements 3″ apart and 3″ from the sides of the plastic bucket.

After the overnight discharge was complete, I put some jumpers across about 5 cells that were still showing some voltage (not shown). Shown here is the 240vac variac and a bridge rectifier mounted to it’s heat sink. It takes me about 45 minutes raising the voltage by hand, to about 320vdc. After that, the on board charger is used to finish up the charging.

11jan2011 22648 miles, recharged pack, +4.98kwh, 11.88kwh total, no o/c time. Refilled bearing reservoir with Mobile1.
10jan2011 Flattened entire pack. Jumpered redtop cells.
10jan2011 drove deep discharge, -6.90kwh.

Watered entire pack, 22627 miles, many cells very low. 3.7 gallons H20.

Set charger to 2 minutes of constant voltage time.

10jan2011 22627 miles, watered entire pack. Many cells very low. 3.7 gallons H20.

09jan2011 Drove, deep discharge, -6.93kwh, recharged 2.77kwh, 9.70kwh total. Pack was very stiff during charging and driving.
09jan2011 Recharged flattened pack, +5.88kwh, 11.18kwh total. 20 cells appeared shorted. After 30 minutes of charging they all appeared normal. Cells are marked with a red circle on their cap.
08jan2011 Flattened entire pack, -5.30kwh, 18.9ah, 19.1miles, 11:35pm.

07jan2011 Recharge.+2.94kwh, 8.73kwh total.
06jan2011 Drove deep discharge,-5.79kwh

06jan2011 Recharge, +2.18kwh, 6.53kwh total.
05jan2011 Drove deep discharge, -4.35kwh

27dec2010 Added 10 used and tested nicad cells to pack. 251 total. Set charge voltage to 400vdc.

16dec2010 +3.86kwh (8.3ah) on recharge. 11.19kwh (35.7ah) total recharge.

15dec2010 +4.55kwh on recharge. 11.59kwh total recharge.

15dec2010 -7.33kwh (27.4ah) @ 30.1 miles, 244.3wh/mi, dry, sunny, warm. Errands, highway and city driving. Heater. SJ loop.

14dec2010 -7.04kwh @ 27.7 miles, 254wh/mi, raining, cold, dark, no no heater. SJ loop.

241 cells installed. 2 shorted green top cells removed. 24 Redtops watered. 3 clear tops watered.

27nov2010 -4.55kwh, 20.3 miles, 243wh/mi

26nov2010 -4.13kwh, 15.55 miles, 265wh/mi

This week getting more done on the spot welder has been my goal. There is a lot of details. I got more parts from Halted. The fet board in now tested and fully modified and assembled. I’ve gutted the chassis that the spot welder will be mounted into. It’s going to be a very tight fit. I’ve cut a base plate to mount the electronics onto.

Here is the modification that Fritz has everyone do to their boards. We are suppose to just use a piece of solid wire down each open path. I used copper plate to make it even more robust.

To help keep warmer parts cooler, they get mounted up off of the pcb.

With my experience with motor controllers, it seemed prudent to add a schottky diode and 10 ohm resistor to each fet’s gate pin. There is an issue with ringing on the fets. We are suppose to add a board full of schottky diodes to lower the ringing’s peak value to less than the voltage rating of the fet’s diodes. I used a pointed spiral bit for cut across each gate’s trace. The gap is just wide enough so that an 0805 sized resistor can fit across it. I used a 1/8″ endmill to remove the green solder mask to make solder pads for the new components. A very light touch was required to remove the solder mask without removing any copper. I used my Bridgeport mill for this mod.

Here the parts are mounted next onto the gate’s trace. A perfect fit on the width of the trace.

Here is 1 of 3 heatsinks with fets mounted, ready for pcb installation.

Here they are with the heatsinks mounted. The one row of parts on the right, just clears the heatsink due to the double sided foam tape thickness. I still had to space the heatsink up off of the pcb a little more so that it did not contact the delicate glass diodes. The other two rows of parts were totally clear of the heatsinks.

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