From: John De Armond
Subject: Re: battery question
Date: Thu, 18 Jan 2001 21:10:03 -0500
Eric Roellig wrote:
> email@example.com wrote:
> > In article <firstname.lastname@example.org>,
> > email@example.com wrote:
> > >
> > > I'm new here...what's AGM technology and why is it more expensive?
> > >
> > AGM = Absorbed Glass Mat. The electrolyte is in gel form and contained
> > by a fiberglass mat between the battery plates.
> Steve, AGM's do >>NOT<< have a gelled electrolyte. That's why the glass
> mat is there.
> As I understand it, the electrolyte is held in place via surface tension
> with all the fibers of the glass mat. That way, the very fluid
> electrolyte can move to/from the plates as needed, yet there is enough
> surface tension to keep the electrolyte from leaking out of the glass
> matting if the case is broken open.
Also mostly correct. There IS liquid electrolyte available if you
crack a cell but not much.
> The gelling factor is one of the reasons that the gel cels have more
> restrictive load/charge currents.
> Eric, who's looking at AGM batteries closely...
My experience with AGM batteries ranges back to the early 70s when I
had to deal with a widespread deployment in a nuclear plant, a
deployment which turned out to be a disaster. My opinion is that
AGMs are OK when one of their unique characteristics is required for
the application. But I would NOT select this system for general RV
usage for the following reasons, among others:
1) AGM requires precise charge control. There is very little room
for error. Simple trickle charging will kill an AGM battery rapidly
- that's the major cause of the failure in the nuclear plant - the
equipment manufacturer implemented a simple trickle charger.
2) AGMs WILL gas when improperly charged, WILL lift the relief
valves and because there is so little electrolyte, will dry out very
rapidly. You only get one or two tries and the battery is toast.
3) In an RV environment, a multi-step charger designed for AGM
charging must be used and the batteries must NOT be charged from the
engine alternator. On face value, this advice might seem at odds
with what certain "smart regulator" mfrs say on their web sites.
I've been doing insturmented testing on the Heart Interface/Cruising
equipment Incharge/Alpha regulator for the last couple of weeks and
what I've discovered jives very nicely with my previous experience.
A simple automotive regulator will either not charge the battery
properly or will over-charge it, rapidly drying it out. There has
been a trend in the last few years for the regulator setpoint to be
increased from 13.8 volts to 14.2 to 14.4. Coupled to this is the
increasing use of temperature compensation in the alternator
regulator. This works OK as long as the battery is subject to about
the same temperature as the battery. When the battery is under the
hood, this works OK. However, when the battery is in the RV,
problems can develop. If the battery is in a heated and/or
insulated compartment, then when the temperature is low outside, it
will be overcharged as the regulator raises the charge voltage to
compensate. A wet battery will simply gas and use water. With the
AGM, the gassing rate will likely be higher than the recombiner can
handle and the battery will vent.
A smart, multi-stage charger will address some of this problem but
not all. The Cruising Equipment Incharge smart alternator regulator
really isn't that smart. First, it doesn't have a sensor to measure
the battery temperature. The instructions say to set the absorption
voltage according to a temperature table in the manual. Problem is,
the charging process heats the battery. I'm seeing a temperature
rise of around 40 deg F during the charge cycle on my Group 27 wet
batteries. If I set the voltage down to the value specified AFTER
the temperature rise, then the voltage is too low to fully charge
the battery and will force insufficient current to cause the heating
I'm trying to compensate for. With my wet batteries, I set the
voltage to the ambient temperature and let the batteries gas a
little. Not an option for an AGM.
A more serious problem is that the Incharge regulator implements
only a simple timed absorption cycle. It does NOT measure the
charge current and terminate the absorption cycle when the current
drops as it should. If I run the batteries down to about the same
level each time AND I allow the charge cycle to complete each time
(requiring 2+ hours), then no problem. However, consider what
happens with a vehicle. One gets in, cranks up and drives off.
This starts the smart cycle. If one shuts down before the charge
cycle is complete, the timed cycle starts over again on the next
cranking. If one made a series of short trips, say an hour at a
time, one would subject the battery to continuous, heavy
overcharging. With a wet battery, what happens is it gases
vigorously. With the AGM, one would have to either set the
absorption voltage to a sub-optimum value (defeating the purpose of
the smart charger) or dry the battery out.
According to the downloadable manual, at least the Heart Interface
Freedom inverter/charger/converter DOES implement the proper
absorption charge cycle using a battery temperature sensor AND a
charge current sensor. I haven't looked at the other models yet.
The Ample Power Next Step smart alternator regulator implements the
same timed absorption cycle scheme as the Heart/Cruising equipment
InCharge regulator so the same problems apply.
To safely use AGMs AND get the design life from them, one must forgo
charging the battery from the vehicle system and charge it only with
the Freedom-type smart charger. Personally, these restrictions are
not worth it in an RV context. If the AGM battery is charged
properly, it will last practically forever. I have a studio flash
system that I designed and built in the late 80s that is powered
from an AGM battery so that I can take it on location and not have
to worry about power. The battery is still working fine over 10
years later. I've taken VERY good care of the battery and I have
NOT cycled it every day like I do the batteries in my RV.
Me, I'm sticking to the good old fashioned wet batteries. I can
abuse 'em when necessary and when it is time to replace them, they
From: John De Armond
Subject: Re: battery question
Date: Fri, 19 Jan 2001 19:33:28 -0500
Eric Roellig wrote:
> One of the newer bits of information I have run across lately is thermal
> run away with regard to AGM's. Any info on that topic?
The issue with thermal runaway is that the internal impedance of any
battery goes up with temperature. If the battery is being charged
with a constant current charger, it will force more power into the
battery which is dissipated as more heat. This is a positive
feedback situation that continues until something external limits
it. IN a wet cell, one must work fairly hard because the acid may
flow freely and thus convection will help transport heat to the
case. Enough power has to be pushed into the battery to boil the
The AGM is different. Because the acid is absorbed on the glass
mat, it can't freely circulate. If a hot spot develops, the heat is
only removed by conduction. The end result is that AGMs can suffer
thermal runaway much more easily than regular batteries.
This is only a problem with high current chargers. One of the
benefits touted for AGMs is that they can be charged faster than
regular batteries. That puts them potentially closer to the edge.
If one is going to fast charge, he must use a charger with a thermal
sensor. I've never heard of any sort of battery suffer any kind of
runaway during discharge, though smaller AGMs will get fairly hot
when discharged rapidly.
> BTW: My interest (when I ignore the price factor :) ) is unloading the
> weight of the batteries off the tongue of my TT. The addition of a power
> jack and Hensley Arrow (163lbs worth!!) took my tongue weight from
> 700lbs or so around 900lbs or so. The only way I can realistically do
> that is to move the batteries to the living space.
I think the issue about hydrogen emissions and indoor batteries is
highly overblown. A simple calculation will demonstrate just what a
tiny bit of hydrogen and oxygen is produced even during vigorous
gassing. Back in my teenage days, I rigged up a large old battery
to fill balloons with hydrogen and oxygen for the purpose of making
explosions. Even when driving the battery to the threshold of
boiling, the production rate was dismal - several hours to fill a
small balloon. I would not at all hesitate to keep a conventional
battery inside if hydrogen was the only issue.
The major issue for me would be acid vapor and that is easily
addressed. Acid vapor is produced as the battery gasses. The finer
droplets can make it past the coalescers on the vent lids. It will
slowly corrode things, particularly electrical contacts. The
easiest solution is to simply vent the battery caps outside. Back
in the early 70s when I had a homemade conversion van, I kept a
large battery in the rear section to power the stereo. I simply
epoxied plastic nipples to each cap and ran small tygon tubing from
each through the floor. I have seen such caps ready made for sale,
though I don't know where to get them. This solution completely
isolates the vent stream from the inside environment.
Another option might be the recombiner caps that are available.
These contain catalyst materials that combine the hydrogen and
oxygen back to water and drip it back into the cells. I don't know
the extent of these caps' ability to stop acid vapor but I suspect
that it will be quite good.
> Question 2: What about using a solar charge controller to regulate
> alternator charge current to an AGM. I might soon have a solar panel for
> battery maintenance anyway...
The solar controllers I've seen are shunt regulators. That is, they
dissipate un-needed energy as heat. That wouldn't work for
I've spent a LOT of time analyzing the Cruising Equipment smart
regulator, including reverse engineering it to draw a schematic.
I'm not going to take the time to exhaustively describe it right
now (gotta get out of here so I can get snowed in up in the
mountains this weekend :-) but in general, here's how it works.
When the charge cycle is first started, the regulator works just
like a conventional regulator except that it has a higher,
adjustable setpoint. Since the discharged battery voltage is much
lower than the typ 14.7 (70 deg) setpoint, it drives the alternator
field fully and produces the maximum possible charge current. As
the so-called bulk charging proceeds, the battery voltage rises
until it reaches the regulator setpoint. The smart charger guys
define this as the transition from bulk to absorption charging, a
designation that is somewhat arbitrary. When the voltage reaches
the setpoint, the regulator controls the alternator to maintain
constant voltage - just like a conventional regulator - except that
the setpoint is higher. At the same time, a timer is started. When
this timer expires (or better, when the current has dropped a
designated amount, usually to > 10% of the amount of current at the
moment of bulk->absorption transition), the setpoint of the
regulator is dropped to the trickle value, typically 13.8-14 volts.
That's it, basically. The regulator contains some additional timers
to control when the regulator transitions back to absorption or bulk
charging when a load is applied but that's somewhat ancillary to the
For the specific situation when one hooks the charger (or runs the
engine) to the battery, conducts a charge cycle and disconnects, one
can implement this scheme very cheaply using off-the-shelf parts.
What you need is a Neihoff external regulator - I use one intended
for my 68 Fury - and a spring wound timer like is used to control
attic fans. Inside the Neihoff can is a little electronic regulator
that replaces the OEM mechanical regulator. It has an adjustment
pot to set the voltage, in the one I'm using, 10kohm. The procedure
is to get another pot, hook the ends in parallel with the ends of
the existing pot and wiring the wiper through the timer to the
existing pot. When the timer is engaged, both pots are in
parallel. The new pot is adjusted to produce the absorption
charging voltage. The existing pot is left as set to the normal
13.8-14.0 volts. This rig is hooked in place of the existing
external regulator or in place of the internal regulator as
To do a "smart" charge cycle, one simply cranks the engine and sets
the timer for whatever absorption period is desired. The alternator
will put out max output until the battery terminal voltage reaches
the bulk->absorption transition. It will then be regulated to that
voltage until the timer expires. Then the voltage will drop back to
the trickle voltage.
I made up this exact rig this afternoon to serve as the backup
regulator for my "cordless battery charger" in case the Cruising
Equipment regulator fails while we're camping. I've tested it and
it works practically the same as the Cruising Equipment unit. Cost
was $14 for the regulator, $1 for the pot from radio shack and $8
for the spring timer. In the case of my cordless battery charger,
with this regulator it will drive 120+ amps during bulk charging and
will charge a fully discharged Group 27 wet battery in around an
If you're interested, I'll try to get some photos up on my web page
From: John De Armond
Subject: Re: Time for new batteries- suggestions?
Date: Tue, 25 Mar 2003 19:33:52 -0500
On Tue, 25 Mar 2003 00:12:13 GMT, "Ken Tullis" <KenTullisNOSPAM@escapees.com>
>On 23-Mar-2003, "Rick" <firstname.lastname@example.org> wrote:
>> I need to replace my batteries in my 5th wheel. Four group 27's. I have an
>> inverter system so would like good power ratings. I also think I'd ike
>> sealed batteries so I don't have to worry about running low on water. This
>> is the first time I've replaced them so any suggestions would be very
>AGM if you've got the bucks BUT your old charger may not be compatable!
Good advice about the charger. Ordinary RV chargers will kill AGMs. I warned
GB at the time but he didn't want to listen. His died just about on schedule.
His new one will too.
AGMs DO require different charge regimes. Hawker has tones of technical
papers on their web site including papers on how to charge and maintain these
AGMs are good stuff - I have dozens in service in my various electric
vehicles. They'll easily last 10 years or >1000 cycles if properly cared for.
Most people will NOT go to the trouble and/or expense of properly charging
them. I use chargers of my own design. I have one set (24 volts) of
batteries going on 4 years old, used daily in my personal e-scooter, that
according to the E-meter installed in the scoot, still have >90% of the stated
I do NOT use AGMs in my motorhome. Unless there is a special need like GB's
restricted space situation, I cannot recommend them. Plain old deep discharge
maintenance-type batteries still provide the best bang for the buck. They are
very tolerant of moderate abuse (such as charging from the vehicle alternator
or dumb converter chargers) and are very cheap. If you don't like having to
put water in them, get a set of catalytic combiner caps. These recombine the
hydrogen and oxygen and return the water to the battery cells. Voila,
Even though I know how to take care of batteries, in my motorhome I mostly
don't. Why? Batteries are so cheap (<$100 for a pair of Group 27s from
Sam's) that if I get a couple of years out of them, it's not worth my time to
do anything more fancy. I charge off the alternator, using my Cordless
Battery Charger (engine driven smart 3 stage charger) or the dumb converter in
my rig. I occasionally give 'em an equalization charge. I keep the tops
clean. That's it.
I do NOT like golf cart batteries. I've had two sets. Both behaved the same.
They have higher internal impedance than a regular deep discharge battery.
This is by design. This seriously limits how much current can be withdrawn
with voltage maintained and it limits the charging rate. When I'm dry camping
I want to be able to charge the house batteries in about an hour to minimize
generator run time, something I can easily do with a pair of Group 27s in
parallel. I can run a 2kw inverter to better than 80% discharge on the pair
before the inverter trips from undervoltage. I can do neither with the golf
cart batteries. Battery capacity is fairly useless if the necessary voltage
cannot be sustained over the whole range of discharge.
A pair of golf cart batteries vs a pair of Group 27s, the cost is about the
same, as is the capacity so there is no financial incentive either way. I
just like deep discharge batteries better for RV service.
THE BEST SINGLE THING anyone can do for their system if they use their
batteries heavily is to install an E-meter (now called a Link-10 since Xantrex
took over) in their rig. This unit measures state-of-charge by tracking the
amps in and out. It does peukert and temperature compensation. It keeps
statistics on the battery. Best of all, it presents an ACCURATE "gas gauge".
With extra cost options, it can data log and start a generator at a specified
From: John De Armond
Subject: Re: Battery Charger Advice
Date: Mon, 10 May 2004 02:00:58 -0400
On Sat, 8 May 2004 19:26:57 -0400, "Jon Porter" <email@example.com> wrote:
>"phil willen" <firstname.lastname@example.org> wrote in message
>> I use the Vector with my Optima gel. Works great.
>> Found a good price on Ebay.
>Optima doesn't make a gel cell that I know of. Their batteries are
>constructed using "absorbed glass mat" between the lead plates, which is
>part of what makes them perform the way that they do. They are capable of
>taking up to a 100 amp charge rate (charge in an hour) as long as the
>voltage stays under about 15.5. Any charger metting the voltage specs is
>fine for an Optima, and I'm still using an old Magnetec 6300 series to
>charge the bank of three that I have in my Class B.
I thought I'd mention here that according to what I'm hearing in the electric
car world the Optima is in trouble. Seems like they fired all the engineers
about a year ago after the company was acquired by Allied and now they can't
make a reliable battery. One EV-oriented battery dealer said recently that he
was suffering over 40% warranty return rate when he dropped the line.
As of right now the hot high drain wound electrode battery is the Exide
Orbital. Exide just bought the company and hasn't been able to cause any harm