Final (hopefully) Chargers have been ordered

And the charger saga continues.

There seems to be a charger theme as of late. But I plan on getting it right, so I shall continue this theme for a little while, anyway.

My recently purchased six individual chargers are a temporary solution to avoid murdering my brand new batteries. Series charging my pack was doing just that - undercharging some and overcharging others - because of varying internal resistance within my pack.

According to Canadian Tire's website, the return policy for these charger is "90 days in the original packaging". So I have 82 days left.

These chargers are old school transformer based chargers. Very very inefficient. So inefficient that I am embarrassed to enter fuel log data.

During the bulk charge when batteries resistance is low:
Input power: 160W
Output power: 10A*12.8V=128W
Efficiency: 80%
Once the battery voltages go up and resistance increases, the charger output current drops significantly while input current drops much less:
Input power: 125W
Output power: 5A*13.5V=67.5W
Efficiency: 54%

Since the current slows down at higher charging voltages, most of the charging time is spent in this stage, where efficiency is quite low.

But I knew it would be like this with these chargers, and I have to keep reminding myself that this is temporary.

So....for my [hopefully] final charging solution I have ordered.....(insert drumroll).....THIS:

Six of them. 12V/10A. Configured for 14.4V max output to match my AGM battery constant voltage charging spec of 2.30-2.43 VPC. Tiny (170mmX90mmX50mm) and light (1kg). PWM controlled (instead of transformer/resistance "controlled"). I really really hope they work!

Back To Six Chargers

Last week my new Kelly 7210 charger arrived - the one with a 84.0V max charging voltage. Giddy as could be, I swapped it into the place of the old one and plugged 'er in. And VOILA! Nothing. Green light on, meaning it is "fully charged". That shouldn't be when pack voltage is at 75.2V. It should charge and then cut off to float no matter what. So I took the Electric Booger for a hard run around the block. Now the pack was at 74.2V, so I plugged 'er in again: NOTHING.

So Kelly #2 was DOA. After a few emails, Kelly wanted to send me another one. Instead, I am sending it back for refund. I don't want it anymore, which I will now attempt to explain....

After my giddiness was squashed I meticulously topped up all batteries individually with a 12V charger. Turns out that my batteries have gone WAY out of balance. Some batteries took quite a while to top up. My nifty regulators have done nothing, other than illuminating my batteries in the dark. Then I plugged in the 88.2V charger and watched voltages. Things got quite wonky very quickly.

What I have concluded is that the three odd ball AGMs in the pack have a different internal resistance than the rest, so the pack is unable to keep balanced. After topping up all batteries, these rose to about 13.8V while the rest stayed around 14.8-15.3V. Before topping up, the three odd balls were usually the way-too-high ones. So unless I purchase three more brand new AGMs to match the rest ($450), balance will always be an issue.

I decided to once again go with six individual battery chargers. Well - seven if you include the one used to keep my accessory battery happy. Unfortunately, I have already sold five of my six original chargers to guys at work. But wouldn't you know it, Canadian Tire has 10A chargers on sale for half price ($29.99), so I bought six.

Anyway, enough boring stuff. How about some pictures. Here is the rear battery pack in its final form:

Note my "Battery Pack Restraint System" in case of a crash. That's right, the original rear seat belts. I would hate to be squished by 504 lbs of lead if I crashed.

Here is the "charger array":

Today I scored a beautiful piece of black plexiglass and cut it to size to make my official rear battery cover:

It sure beats the piece of shag carpet that it replaced! About the only thing I have left to do is fasten it to the rear seat back and then the car might just be...well...done! For now, anyway. Can a project like this ever actually be finished?

Oh, and did anyone notice my tribute to Clark Griswold in any of the pictures above? I will make it easy and show a nice, clear, zoomed in picture. This is how you power a butt load of chargers with one extension cord:

Snow

Today snow landed on the lower mainland:

Now that the outside temperatures are around freezing I have noticed quite a performance reduction with the Electric Booger. With the lower temperatures and with my heater running most of the time, I have been noticing the batteries are getting weaker after long drives, where before I could commute home and they were as strong as when I left. Not a big deal - it's just noticeable, that's all.

Yesterday my wife Vanessa and I drove to some friends' house, a total of a 15 km round trip. The eBooger did it no problem, but performance definitely took a dent on the way back as we neared home. I think I could have gone another 5 km - maybe even more.

New Charger

I have been planning to switch to one 72V charger instead of six 12V chargers. Since I am using AGM batteries, I needed some sort of way to keep the batteries balanced, since I would be charging them all in series. Since even the same part number battery can be slightly different than the one beside it, what can happen over time when series charging is that they can go out of balance. When this happens, the batteries that need less charging get overcharged and the ones that need more charging get undercharged.

Flooded lead acid batteries are easy to keep balanced. All you need to do with those is overcharge them for a while and each cell bubbles away and self balances. This overcharging would quickly murder AGM batteries like I have. Instead, a balancing system is needed. So I got to work soldering together very simple battery regulators. I got the design from an EV veteran named Lee Hart on the EV discussion list (EVDL) HERE.

Each regulator uses two zener diodes to shunt current through a resistor (light bulb) once the voltage across the battery posts reaches a predetermined level, in my case 13.0V. This way, batteries that are fully charged first will waste a bit of energy through the light bulb while less charged batteries get a chance to catch up.

Here is the finished product:

And here they are working in the rear battery pack:

A quick test of all my battery regulators with the original chargers showed that they all work properly. They start shunting a bit of current at 13.0V and by 13.7V they are just barely glowing. At 15.0V they are shining nice and bright.

I also installed my new Kelly 7210 charger:

It's the silver square thing at the bottom. Compare this picture to the picture in last post.

It works awesome, except for the fact that it isn't actually the proper charger for these batteries. In my great haste to purchase a battery charger, I got a charger that is meant for flooded lead acid batteries (which my pack mostly consisted of at the time) with a max voltage of 88.2V (14.4V per battery, 2.4VPC). Unfortunately, AGM batteries require a lower constant charging voltage than flooded batteries.

When I charge, the bulk stage (when the batteries are at a lower SOC and are drawing full current) is fine. All voltages are even and slowly climb to 14.0V/battery and my regulators all start glowing nicely. But as soon as they go over 14.0V, a couple of batteries quickly shoot up to 15.2-15.5V (making my regulators glow very bright!), and after a while a couple of batteries go down to 13.8V. 15.5V is very bad for these particular AGMs because it can cause electrolyte bubbling at too high of a rate which may cause them to vent. Once AGMs vent, they quickly degrade because there is no way to fill them back up with electrolyte.

What I have been doing is plugging the charger through my timer so it shuts down after a certain amount of time. That way if I forget about it, they won't fry on me. I would hate to ruin my brand new pack! I finish the charge by babysitting it, watching each battery voltage and shutting it off when I know they are done.

So...I have ordered another 7210 Kelly charger - this time with a constant voltage of 84.0V (14.0V/battery, 2.33VPC) and a float voltage of 81.0V (13.5V/battery, 2.25VPC). Apparently it shipped this morning, so I should have it this week. My current one shouldn't be difficult to sell because it has the correct charging profile for charging lithiums. Anyone in the market for a 72V/10A (88.2V max) charger?

So far, this new charger is proving to be about 30% more efficient than my last charger setup. That means that my car is 30% more efficient now. Quite a significant different for one modification.

Control System

Finally...here is a nicely labelled picture of the Electric Booger's control system.

The battery chargers are the next thing to go once my battery charging regulators are finished and my new Kelly F7210 charger is installed. Hopefully soon!

The PWM Field Controller Is Installed

I was finally able to wire in my "proper" field control system. This system basically consists of two components:
-72V/100A Kelly motor controller (meant for small bike/scooter motors)
-Amp transducer to measure current in the armature negative cable

The LEM amp transducer that I bought is typical of most units like this: an output signal of 2.5V with zero current going through it. If current goes one way, the signal voltage increases and if current goes the other way the signal voltage decreases. I did some testing with a carbon pile to put a load on a few batteries and found that at 450A, the signal voltage is 3.9V.

This signal voltage is what I feed the Kelly motor controller to tell it how much current to give my field windings. Luckily, Kelly has provisions in the controller software to narrow down the signal voltage range to whatever you want, so I didn't have to change this signal from a 2.5-3.7V one to a 0-5V one. That would have been above my head anyway!

Here is a screenshot of the parameters I am talking about:

I have adjusted "Throttle Effective Starting Position" so that when the key is on and things are "idle", the field is using 8 amps. Since the field windings are exactly 1 ohm, this takes 8V to accomplish for a total of 64 watts. Pretty insignificant.

Why not not cut field current altogether when off the throttle? Because I have found that when freewheeling down a hill at high motor RPM and you turn the field on, a voltage is generated in the armature that confuses my Alltrax controller causing it to go brain dead for a few seconds. Keeping a small amount of current going through the field prevents this from happening.

I am still tinkering with "Throttle effective ending position" so that it is a linear relationship between field current and armature current instead of reaching max field current too early, when armature current isn't high enough yet.

Varying field current proportional to armature current has made a dramatic effect on how the throttle feels. With steady field current, the throttle felt exactly like a farm tractor, where 50% throttle = 50% motor speed. It made driving very awkward, especially when trying not to accelerate at full power. Now it feels much more like a car, where throttle position controls torque output. Much easier to drive.

Many EV enthusiasts advise to stay away from Kelly products for reliability reasons, but since I am only using this controller to less than 50%, I figure I should be okay. Makes sense to me, anyway.

As happy as this all sounds, there is a problem. During my first drive to work I found that at higher RPM and light throttle the motor developed a high speed surge which shook the entire car. It was sort of like turning the throttle on and off ten times a second.

Here's is my best attempt of an explanation of this strange phenomenon. At a given motor speed, when you increase field current, back EMF (the motor's resistance to accept current) increases, which causes armature current to drop. This drop, sensed by the amp transducer, tells the field controller to lower field current. This in turn lowers back EMF, causing the armature to increase. This oscillation happened many times per second.

What I did to remedy this was change the "Throttle up/down rate", which is basically a time delay for the field controller's response to its throttle input.

I originally had it set to 1, which was really bad. I upped it to 3, 6 and then finally 9, where it is much better. It still surges a bit at part throttle but at least it doesn't oscillate and vibrate the whole car.

There you have it. One more bit done. This weekend I am working on improving the heater now that it is getting cold outside.