[AT] Battery Chargers

[Matthew]

This is from another list, but has a lot of good info on charging
battries.  This info is from John, credit att he bottom.

--Matthew




Batteries - one of my areas of expertise.  I putz around with EVs and with
my motorhome.

About the deep discharge batteries.  You will kill conventional wet cell
deep cycle batteries in just a few, perhaps less than 10, complete discharges.
Wound electrode AGMs (Orbitals, exides) fare a little better but not much.
Hawker's pure lead electrode AGMs can take it but they're expensive and the
largest is only about 60 ah.

I once had a set of motorhome batteries killed with ONE full discharge.  Some
vandals unplugged my MH while it was parked and the batteries fully (0 volts)
discharged before I found it.  Using standard rescue techniques (high voltage,
desulfator pulser, reverse charging, EDTA in the electrolyte, etc) I managed
to get them to accept charge but the capacity never rose to more than about
30% of their ratings.  I replaced them immediately but took them to my lab
to see just how much I could recover a completely killed battery.  Answer: not
much.

With a conventional wet cell battery, you never want to pull it below about
80% depth of discharge or about 10.75 volts.  80% DOD will give you perhaps
400 cycles.  The cycle life vs DOD goes down FAST below 80%.

You can use your vehicle's generator or alternator to partially charge the
battery but it won't work very well.  A car's system is in effect, a trickle
charger.  The voltage is too low to fully or rapidly charge the battery.
The car system will fairly quickly restore up to about 50% charge IFF the
interconnect wire is heavy enough, the alternator voltage is high enough
(14.0 at least) and you use an interconnect relay (looks like a starter relay 
but with a continuous duty coil) instead of that battery isolator thing.
Problem is, chronically undercharging the battery will kill it only a little 
slower than fully discharging it.

The correct way to charge the battery is the three stage method.  This is as
follows:

Bulk -  This stage converts the bulk of the active material on the plates.
It represents a state of charge from wherever to about 70 to 80% charge,
depending on the exact chemistry and plate construction.  This is accomplished
with a constant current charge of up to about C (C is the amp-hour capacity)
amps.  This phase ends when the terminal voltage reaches about 14.75 volts
on a wet cell battery.

Absorption - this stage converts the deeper active materials in the plates.
It takes time for the acid to diffuse out of the plates so this takes more
time.  This is a constant voltage charge regime, 14.75 volts normally.  This
stage represents the state of charge from wherever bulk ends up to about
90%.

This stage ends somewhat arbitrarily when the current drops to about 2% of
the constant current bulk charge value.  The current is dropping rapidly at 
that point so anything in the 1-10% range works fine.

Float - This stage puts the last bit in.  It is done at a constant 13.9-14
volts or thereabouts.  This is usually a timed stage and lasts a couple of
hours.  At some point during this stage the battery becomes fully charged
and gases.

At that point the charger is either removed or turned down to the
maintenance/trickle voltage of 13.7 or pulse maintenance.  The preferable
maintenance is a pulse algorithm where there is no charge current until the
voltage drops to about 13.5 or 13.6.  At that point, a low current charge is
supplied until the voltage rises to around 14 volts.  Typically the ~5 amp
pulse will last 5 seconds, repeating every few minutes.

You can operate for a few cycles in the bulk charge range.  That is, charge
the battery to the end of the bulk stage (in an hour or less if your charger
can output C amps), then discharge to 20% charge.  Every 4 or 5 cycles the
battery needs the complete cycle to prevent hardening of the sulfate
crystals
on the plates which is what ultimately kills the battery.

The above voltages vary with temperature (about 0.1v/10dC for wet cells) and
are approximately correct for 70 deg F.  The best chargers are temperature
compensated.

I do this when dry camping (no hookups to shore power) in my motorhome.  I
designed and built an engine-driven "cordless battery charger" that is
capable
of 150 amp output.  Not quite 1C for my 220 amp-hour battery pack but close
enough.  I can go for several days of 60 to 80% DOD with just an hour a
day's
charging.  My CBC has a smart digital controller I designed that does the
full
multi-stage routine as described above.  It is temperature compensated via a
sensor that clips to a battery post.  Here's are some photos of a very early
prototype using a commercially available analog 3-stage charge controller.
It
is significantly different now but I haven't gotten around to making new
photos.

http://bellsouthpwp.net/j/o/johngd/files/rv/cordless_battery_charger/

I don't know of any commercially available cordless chargers but there are
some line and vehicle engine operated ones.  These are all very expensive
but
I'll mention them anyway.

This company:

http://www.amplepower.com/

Makes a very nice line of 3 stage regulators that will turn the vehicle
alternator into a 3 stage charger.  This is normally used on a boat that is
equipped with two batteries.  One battery supplies the boat while the other
is
charged.  This is necessary because the near-15 volts at the end of bulk
charging is hard on 12 volt stuff like lamps.  It is also used on
motorhomes.
Typically the motorhome will have two alternators, one for the chassis
(regulated conventionally) and one to charge the house batteries, controlled
by this type of regulator.  I plan to go this route on my MH when I get
around
to fabricating the necessary brackets to mount a second alternator.  This
will
mostly, if not completely replace the CBC.

Xantrex.com (formerly Cruising Equipment Company) makes an analog version.
That is what is pictured in my photos.  It works but is inferior to the
Ample
Power unit because it is analog and relies on fixed timers to end the
absorption and float stages instead of actually measuring the battery
condition.  It's a LOT cheaper though.

Xantrex, IOTA, Progressive Dynamics and a few other companies make 3 stage
charger/converters designed for RVs.  They're designed to be built-in and
are
expensive.

Fortunately there is exactly one company that makes an affordable, portable
3
stage charger.  This is the Vector VEC1093 40 amp charger.  Here it is on
Amazon:

http://www.amazon.com/exec/obidos/tg/detail/-/B00009RB0T/qid=1088231265/sr=8
-2/ref=sr_8_xs_ap_i2_xgl60/103-2822418-6348618?v=glance&s=home-garden&n=5078
46
http://tinyurl.com/3h33t

I have one of these and wrote a very extensive review of it for
rec.outdoors.rv-travel a couple of months ago.  When I looked around to buy
one, every price I found on the net was 109.99 so I bought it from Amazon.
Northern Tool stocks the 20 amp version in their stores but not the 40 amp.

This charger does a textbook-perfect three stage charge, does pulse
maintenance and includes a desulfator to help recover capacity in sulfated
batteries.  Only two things I don't like about it.  It's too big (that whole
pod on the bottom serves only to contain an EMI filter) and the clips are
flimsy.

Here are some photos and data that accompanied my test report on RORT:

http://bellsouthpwp.net/j/o/johngd/files/rv/Vector%20battery%20charger/

In particular,

http://bellsouthpwp.net/j/o/johngd/files/rv/Vector%20battery%20charger/First
%20test%20cycle.jpg

Shows the charge cycle on a set of 220 ah golf cart batteries.  I recorded
the
AC INPUT amps to the charger because I didn't have a DC amps stripchart
recorder handy.  The input current rises during the bulk charge stage
because
at a constant output current and constant input line voltage, it takes more
input amps as the output voltage rises.

This pair of 6 volt batteries in series has about twice the capacity of a
single 12 volt deep cycle battery.  The bulk stage took about 4 hours here
so
it would take about half that (2 hours) for a single battery.

You can use this charger two ways.  You could plug it in at night when you
park the van.  Or you could install a suitable 120vac inverter in your van
and
plug the charger into that.  Sounds kludgy but it actually works quite well.

I have a 1000 watt inverter in my MH (Costco, $79) that powers my electric
refrigerator and my microwave when I'm dry camping and don't want to crank
the
main generator.  It also powers the Vector quite nicely.

When I take long travel (as opposed to camping) trips in my MH, moving to a
new place every night or two, I use this lashup to charge my house batteries
(225 ah) while I'm driving.  I simply open the battery interconnect relay,
flip the inverter over to the engine alternator system and let the Vector do
the charging.

That taken care of, let's look at the neon side.  Every 12 volt supply I've
tried (I like the Tech-22 ones the best) has a low voltage cutoff.  The
problem is, this doesn't work like you'd think it would because of the
battery's characteristics.

When the voltage drops to the trip point, the supply turns off.  With the
load
removed from the battery, the voltage recovers until the supply comes back
on
again.  In a few seconds, the voltage drops, the neon supply trips and the
cycle repeats.  You end up with an annoyingly flashing sign.  Now you may
WANT
flashing but this will be irregular and ragged.  Not attractive.

What is required is a latching undervoltage relay that will break the
connection on the first trip.  Such relays are available commercially but
are
expensive.  There is one in this picture:

http://bellsouthpwp.net/j/o/johngd/files/rv/Vector%20battery%20charger/First
%20test%20cycle.jpg

of my battery discharge tester.

I make a simple relay for my own use.  I take a conventional relay like the
one in the center of that photo and cut away the coil.  An Exacto knife
makes
quick work of it.  I then wind sufficient turns of heavier wire around the
armature so that the relay will pull in when the coil is hooked in series
with
the neon driver.  In other words, I convert the voltage operated relay to a
current operated one.  This is simple and easy, requiring less than 50
turns.

I connect the relay's normally open contacts in series with the coil and in
series with the neon supply.  I hook a momentary contact, normally open
pushbutton in parallel with the relay contacts.  (or just drill a hole in
the
case so I can manually push in the contacts.)

To turn the unit on, simply press the button.  That starts current flowing.
The current causes the relay to pick up, closing the relay contacts and
sealing it in even after the button is released.  When the neon supply trips
off the first time from undervoltage, the relay opens, removing all power
from
the supply.  No cycling possible.

One thing that makes the TECH-22 supply stand out from the crowd is the
quality of his dimming.  It dims smoothly right down to zero with no
flickering.  He pulse width modulates the full high voltage instead of
reducing the voltage.  The is the reason for no flickering and the
smoothness.

If you decide to dim (recommended to extend battery life), you have to take
that into account when you're winding the latching relay.  The current drawn
by the supply drops linearly with dimming.  A lot of dimming requires more
turns on the relay.

Hmmm, looks like I wrote another book.... Time to go to bed :-)

John
---
John De Armond
johngdDONTYOUDARE at bellsouth.net
http://bellsouthpwp.net/j/o/johngd/
Cleveland, Occupied TN