Re: Battery Question II

On Fri, 03 Mar 2006 23:34:05 -0800, altar nospam <altar@xxxxxxxxxx>
wrote:

On Sat, 04 Mar 2006 00:58:05 -0500, Neon John <no@xxxxxxxxx> wrote:

The bulk
stage ends at between 50 and 80% charge, depending on the battery
type. Closer to 50% for GC batteries, closer to 80% for paralleled 12
volt batteries.

Excellent post, John. It is appreciated.
Question: Vector 1093, 2 T105's. Bulk charge would take it to what
percentage of full, would you estimate?

I haven't yet collected any hard data using the Vector on paralleled
12s but subjectively, they suck the charger dry! Ditto for the 60 amp
Intellipower.

I have just your setup on my mobile power cart (GC batteries, 1500
watt inverter, charger, all mounted on a hand-truck.) that I use to
run my electric lawnmower and electric chainsaw.

GC batteries, for reasons I don't fully understand yet, let the
voltage rise early in the charge cycle because of high internal
resistance. Depending on where you set the bulk/absorption transition
voltage, in the 50-60% range. Absorption takes about 3 hours at 70
degrees regardless of the SOC at the beginning of the charge, as long
as it was below 50%, of course.

I have an E-meter/Link-10 with the serial-out option mounted in a
small box and configured as a portable test instrument. I've
data-logged charging cycles numerous times both on the mobile power
cart and when I had GC batteries in my MH, trying to figure out why
GCs behave like they do. And, of course, my electric car.

I've been meaning to seek out the Trojan engineer with the clue and
chat about this but from studying the architecture of several brands
of golf carts, my theory is this.

I think that GC batteries are all built to a fairly rigid, if
unwritten, set of specs, having primarily to do with internal
resistance and max discharge rate. I know that several different GC
makers rely on the battery impedance and the wiring to limit peak
current and therefore peak torque that the transaxle has to withstand.
I know this both because I've made careful measurements of carts at
our local dealer and because I've chatted with an EZ-Go engineer. This
was especially true back in the resistive speed controller days.

I met the local GC dealer in a round-about way. He was a restaurant
customer. He noticed some EV parts (what other restaurant has solid
state motor controllers, shunts, welding cables and such laying
around? :-) on my desk and struck up a conversation. I have the
CitiCar EV which uses GC running gear so we had a lot in common. He
mentioned that he offered a hotrodding service to GC owners - remove
the resistive controller or the low power solid state version and
install a nice fat Alltrax controller. He also mentioned that he was
having an abnormal number of transaxle failures after the controller
change.

A light went on. I hauled some test equipment down to his shop and
started measuring. I saw that the traction wiring was greatly
undersized, at least by RV and EV standards. I found that on all the
low voltage (36 and 48 volt) carts, the overall circuit resistance
plus the battery resistance limited the current to about 350 amps or
less, usually much less. Since torque is directly proportional to
amps in a series DC motor, when he slaps in that 450 amp Alltrax
controller, Bam, tons more torque to the transaxle. The gears shed
teeth faster'n a 50 year old Bronx whore! The Alltrax has a
programmable current limit. He set the current back to 350 amps and
all has been well. The customer doesn't get quite as much
acceleration but them's the breaks.

Anyway back to the question at hand. I noticed that the battery
terminal voltage sank rapidly and non-linearly with load. Several
different brands of batteries that he carried performed almost exactly
the same. Another light started to glow a little! Maybe this is by
design.

The GC batteries that Sam's Club sells have even higher internal
resistance. Not surprising, given the prices.

Another clue contributing to my theory is that Trojan sells a similar
battery to the GC line as a floor scrubber battery and yet another as
renewable energy storage. I've examined the batteries in my floor
contractor's scrubber and found them to be quite low in internal
resistance. This makes sense since the load from a floor scrubber is
moderate but constant and they need the maximum possible run time per
charge. The low resistance reduces the I^2R losses inside the
batteries that heat them and consume run time.

I'm told by people living off-grid whom I regard as having clues that
RE storage batteries are also quite low in internal resistance and for
the same reason. Sufficiently low that I know that a UL committee is
now masturbating about an RE fault current limiting standard. I can
just imagine how they'll screw that up!

Other tidbits of evidence include the fact that ordinary GC chargers
are rather "hard" voltage wise. Ideally, the charger would be a
constant current source with a voltage cap. The constant current
stage is the bulk stage, where the charger's voltage pulls down to the
battery's voltage while supplying the design current. At the cap, the
charger regulates voltage (becomes voltage "hard") and the current
varies downward as the battery charges during the absorption stage.

An ideal battery has zero internal resistance so all the current
limiting has to be inside the charger. If the battery has a high
resistance then the charger can be closer to a constant-voltage one
because the battery's resistance limits current. The typical
ferro-resonant GC charger is somewhere in the middle. A "soft"
constant-voltage device that regulates the voltage only poorly to
combine with the battery resistance to limit the max current.

My CitiCar had a 48 volt ferro-resonant GC charger in it when I bought
the car. I pulled out the monster, installed a smart solid state 72
volt charger and added batteries to match. I tried using the GC
charger to charge a string of Group 29 12 volt deep cycle batteries
that I had laying around. These batteries exhibit such low internal
resistance that the charger immediately blew the output fuse. Several
times :-( I hooked a carbon pile resistor in series with the charger
to limit the current and it worked fine.

These clues are adding up.

My CitiCar needs at least 450 amps to the motor to accelerate enough
to keep up with surface street traffic. The solid state controller
acts as a DC transformer, converting high input voltage at lower
current to the low voltage, high current the motor needs at stall. As
the motor accelerates, its terminal voltage rises and the controller
draws more and more current from the batteries. Somewhere around 45
mph, the battery current reaches 400 amps. The pack voltage has
sagged from 72 volts to under 55 where my pack voltmeter stops
indicating.

Fifty five volts is about the motor voltage at that speed. The result
is the controller can no longer supply the full amperage, the current
falls and the acceleration falls flat. It feels like a higher gear
has been selected. Because the pack voltage recovers as the current
drops, the car keeps accelerating until the motor voltage reaches
nearly 70 but at a much slower rate.

This illustrated what I already knew - that GC batteries are poorly
matched to EV use because of the high internal resistance. I chose
them strictly on price. I didn't have the $2500 to spare that a pack
of proper traction batteries would have required. I have about $700
in this pack which gets the job done. I'll kill it in a couple of
years by loading it this heavily but I can replace it several times
for that $2500, even ignoring the time value of the money I don't
spend up front.

The clues are REALLY adding up and the bulb over my head is glowing
pretty brightly at the idea of GC batteries being designed to an
internal resistance spec.

This is why I recommend against GC batteries for any RV in which there
will be significant load - lots of lights, a big inverter or both.
Multiple paralleled 12 volt deep cycle batteries work MUCH better.
Each battery exhibits low internal resistance. Parallel 2,3 or 4 of
'em and that resistance plunges.

Back to your question. This same high internal resistance limits the
charging speed. The electrochemical voltage and the internal
resistance voltage drop at the prevailing current add at the battery
terminals. Since the internal voltage drop is directly proportional
to the current, the bulk/absorption transition voltage setpoint
directly limits how many amps may be forced during bulk charging.

What started me on this quest was the first time I connected my
Cordless Battery Charger to my brandy new GC batteries. The CBC is
capable of 150 amps during bulk charging. The two Group 29s that I
replaced could absorb 150 amps until about 75% at 70 degrees. Two 29s
in parallel have about the same ah capacity as two GCs in series but
the internal resistance is much less.

When I connected the CBC to my brandy new GC batteries, it ran at full
charge for about 10 minutes and then the Absorption LED came on. The
battery voltage had already reached 14.8 and the battery had only
accepted a few ah. From about 50% DOD it took around 4 hours to reach
the float stage. I could reach that point in about an hour with the
Group 29s. What BS!! The whole purpose of the CBC was to charge my
pack as quickly as possible to minimize engine run time when dry
camping. I lived with that for a year before I recycled them into new
Group 29s.

As I mentioned earlier, I'm going to add a third Group 29 when I start
camping again this spring. That's another benefit of paralleled 12
volt systems. You can add another battery or replace a bad one at any
point in the pack's life. Each battery carries what share of its load
it can. I know that some folks say otherwise but hey, I have theory
behind me. And, ahem, I gots da data to back it up!

The polar opposite of a GC battery is the Yellow Top Optima and the
Exide Orbital AGM batteries. The Yellow Top (YT) is a paltry 55
amp-hours but it can deliver 2500 amps into a load for the better part
of a minute while sagging to about 10 volts. The Orbital is even a
tad bit better. They are the darlings of the EV drag racing guys
because of that current capability and low internal resistance.
They're also way outside my price range for my EV, given that they
both retail for over $150 ea.

By the same token, they will accept a charge similarly fast. There is
a guy on our EV mailing list named John Wayland who currently holds
the 1/4 mile world ET and speed record for a street-driven EV. He has
300 and something volts worth of Orbitals in a Datsun 1200. The
controllers put about 2500 amps to the motors for the entire quarter
mile. Back in the pits, he has a huge "dump charger" that consists of
a medium duty truck full of 12 volt deep cycle batteries wired in
series parallel to make about 350 volts. He hooks that thing up and
dumps the charge back in the car's batteries in under 10 minutes at
over 1000 amps, the limit being the capability of the dump batteries.
Then a large diesel generator charges the dump pack while he races.
Here's his website: http://www.plasmaboyracing.com/

I'm looking forward with great anticipation to the (hopefully) not so
distance future when we get hybrid powertrains for our motorhomes.
Imagine having 2-300 ah of either AGMs or wet Ni-Cads and being able
to push a button and charge either pack in minutes using the main
engine and generator. Fire that sucker up every morning for a 10 to
15 minute run at full power to completely charge the pack. I can
hardly wait.

BTW, if you can get past the cost, the REALLY hot-sh*t battery for
RVers who have high current demands is the wet Ni-Cad. These are
absolutely wonderful. The BB-600 standard Mil-spec aviation Ni-Cad
cell that is about the size of a paperback book is about 35 ah. Yet
it can deliver >3000 amps with almost no voltage sag. It can charge
as rapidly. Even a severe overcharge is harmless. In fact, a
significant overcharge is desirable. Essentially no Peukert effect.
Its life is indeterminate but spans decades. No memory, no sulfating,
simply add water every so often.

I have a 28 volt Korean war vintage fighter jet start pack that is
built from these. I bought it in the late 60s when I was in high
school. They're still going strong, showing almost their rated
capacity now. The only thing I've had to do is change the potassium
hydroxide electrolyte a couple of times. The KOH slowly absorbs CO2
out of the air which neutralizes the electrolyte. When the battery's
internal resistance starts to rise, dump the electrolyte and refill.
It doesn't enter into the electrochemical reaction so it can be
changed at any time and the SG does not change.

The only problem is cost. It'd take thousands of dollars to buy
enough Ni-Cads to provide 300 ah of 12 volts for an RV. On the bright
side, they do show up surplus every so often and they're always in
excellent condition.

Interesting factoids. Wet Ni-Cads are stored 100% discharged with
their terminals shorted. My start pack came with a jumper plug across
the output connector. The cells will get out of balance just like a
lead-acid battery.

The Ni-Cad equivalent of the equalization charge is to run each cell
to zero volts and to hold them shorted for awhile, then apply a
"commissioning charge", a significant overcharge. I've never been
able to get one but I've seen photos of the military's "balancing
fixture." It's an affair that clamps on top of the start pack and
applies a low ohm resistor across each cell. This thing is attached
and the battery left discharged and shorted for a few days. Slick.

John
---
John De Armond
See my website for my current email address
http://www.johngsbbq.com
Cleveland, Occupied TN
A foolish consistency is the hobgoblin of little minds.-Ralph Waldo Emerson
.