The first is that of starting - once you've spent your time in a place, you want to be able to start your engine. Having depleted your batteries while dry-camped somewhere can be a real inconvenience.
The second is that, since the demands on them are completely different, the type of batteries used for starting (vehicle) and deep-cycle (house) batteries can (and should) also be completely different.
The vehicle battery is tailored for starting demands. A very high current of relatively short duration is required for the starter motor to turn the engine over to start it. Once the engine starts, the alternator/regulator system applies a constant voltage of 13.8 VDC to the system. This is the voltage for which the various vehicle loads - such as electronics, lights, ignition system, electronic fuel injection, and such - are designed. It is also the proper voltage to get your starting battery back up to full charge, and to maintain a "float" charging level during the resulting drive until the engine is shut down and the battery ideally remains idle until next required for starting. Thus your vehicle system - from alternator to battery - is designed for this scenario. The starting battery is not designed ever to drop below 90 to 95% of its storage capacity - and each time it does, it will "come back" as less battery than before.
Now, consider the requirements of your "house" system. The battery should start out fully-charged, ie at full capacity, in Amp-Hours - the unit of volume or quantity of electricity in the battery. As you use power, the total available percentage of capacity will drop. You want to recharge as this percentage approaches 50 - although 60 is better. Thus, your house battery will vary in charge between 50-60 and 100 between cycles. This "deep breathing" is characteristic only of "Deep Cycle" batteries - and NOT starting batteries - although many slightly modified starting batteries are sold as "deep cycle!" For this job, you need a True Deep Cycle Battery.
The quasi-Deep Cycle, or "Marine," or "RV" battery is in most cases a slightly modified starting battery, still designed mainly for short-term "cranking power," with perhaps a slight capability for use between engine starts, such as running the radio and some lights for an hour or two when overnighting at a rest area or dockside. The first clue is that it's still rated, not in Amp-Hours of storage capacity, but in "Cranking Amps." A Real Deep Cycle battery will have no mention of "Cranking Amps" - in fact, drawing starter motor-sized current from it will certainly harm it, even once in a while. Thus, you will need two batteries: one for the Chassis, and one for the Living Area.
There are also different types of true Deep Cycle batteries, the main ones being the traditional Lead-Acid, and its close cousin, the gel-cel, both of which have been around for years, and a relative newcomer - one with radically different characteristics, the AGM, which will be discussed in detail later.
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Let's consider these factors one at a time:
My genset is a 600 Watt Yamaha. This power output works out to 600/120 =
5 Amps of total possible current at 120 Volts AC. Conversion to 12VDC
means that 600 Watts into the charger should produce about 600/12V =
50A DC at full song. Allowing for inefficiencies, we can rely on about
40 - 45A - if the Smart Charger (more below) deems it viable to pump
that much current into the battery. So the 25 A-Hrs I'd need to recharge
a day's battery use could be accomplished in an hour and a half. Or
three hours for two days' worth.
This tells me I should always recharge by the time my battery capacity has
dropped to 60% - the lower the battery, the higher the current the smart
charger will want to apply during the central charging stage. But,
as you will see below, with an AGM battery, since most of the genset
time goes into the actual charging phase, it's not uneconomical to do
a charge cycle every day if necessary - or even more often than that!
(With a conventional Lead-Acid battery, however, charging current must
not exceed 10% of capacity, so to use this 40A capability would mean I'd
need 400 A-Hrs of battery capacity to match that of my little genset.
Or run it for four times as long at 10A. So much for lead-acid
batteries.
(Notice here that the little 600W genset is actually too much for
chargint any less than 400 A-Hrs of lead-acid batteries. And running a
larger genset - like thye 6 KVA so common in air-conditioned
stand-alones - is a waste tantamount to flying a single nitwit political
figure around in his own personal 767!)
When charging from shore power, of course, there will always be enough
power available to supply the smart charger. - More on smart chargers
below.
Recharging is done by supplying a Charge Voltage sufficient to maintain
a Charge Current, in Amperes, suitable for the battery at that time.
This is not a simple matter, since proper battery charging has now been
shown to require three distinct stages of charging, each requiring a
different current, and therefore charging voltage. This new knowledge
has brought about the development of the Smart Charger - a
battery charger which senses the needs of the battery and applies the
current necessary in each of the three stages.
The Smart Charger's 120 VAC input current can be supplied in one of
three ways:
The safe charge rate for the ordinary lead-acid battery is about 10% of
total amp-hour capacity per hour. Thus, for example, a 100 A-Hr
A-Hr battery may be charged at 10 Amps, Thus, when you've used 20 to 40
A-Hrs of power in the house, - or when the voltage has dropped to 10.5 -
you need to apply charging current - in the three stages as shown in the
chart above. The newer type AGM battery can accept a charge rate up to
80% of capacity per hour - 80A for a 100 A-Hr battery! Needless to say,
this can radically reduce second-stage charging time. It also requires
considerably shorter first and third stages.
Remember also the "commission" - battery efficiency dictates
that you'll always have to put back more than you took out. How much
more depends on the condition and type of battery. Once again, the AGM
battery displays better charge/discharge efficiency.
Thus it will be immediately apparent that the AGM battery will show a
significant fuel saving when being charged by genset (much lower running
time), yet there is little advantage when charging with shore power.
Battery capacity can be increased in one of two ways: using a larger
battery, or using more batteries. The latter solution brings with it
another problem, however, and that is that batteries should never be
left connected in parallel. This is because all batteries have some
Internal Draw in their makeup - there are inter-plate short
circuits in even the best of batteries, and if two are connected
together in parallel, the worse of the two will drag down the better,
even when no other draw is present in the system. Thus, if multiple
batteries are to be used in a storage system, they must be connected
into the system one at a time, or the system must be divided up into
different sub-systems, each using its own battery. The batteries may be
connected in parallel, however, during charging, but this makes for a
lot of bother over switching and such, and is not a particularly
satisfactory setup. This leaves only the first choice for increasing
capacity: using a larger battery in the first place. So choice of
battery capacity is important from the beginning of the design process
for a battery storage system.
AGM batteries may be counted on to last much longer than regular
batteries. They cost more, but if you're charging with a fuel-burning
dedicated generator of any kind, they pay for their extra cost many times
over in decreased charging time requirements, longer life, and considerably
reduced maintenance - as in _none!_ They should be recharged by the
time they have given up 25 - 50% of their capacity; the sooner the
recharge, the less is the strain on longevity and efficiency.
They have a much greater tolerance for high charge
rates - they will accept as much as 80% of capacity for charge current
without ill effects. Thus your smart charger will pile the power in
over a much shorter time period. Furthermore, they require
considerably less "float" time during the last stage of charging. There
are other characteristics to AGM batteries as well, and the
manufacturer's description and spec sheet should be closely adhered to.
That picture changes, of
course, if you require heavy power, such as electric heat or air
conditioning. The power requirement of either of these dictates heavy
wattage supplied in 120 or even 240 VAC. But, if you plan to get by on
your 12V system, a 1500 Watt genset should be plenty. After all, it
doesn't pay in these days of high fuel prices to run a 7 KVA genset to
provide less than 15 percent of its capacity for battery charging. Thus,
if you are committed to air-conditioned boondocking from time to
time, it might be best to add a small genset for times when you don't
need the power of the big one, and of course charge your batteries at
the same time during periods of operating the "heavy hitter."
At today's fuel prices, it will be seen that the AGM battery is the more
economical choice by far. Numbers are given for comparison - a 100 AHr
battery should always be recharged before it's down to 60% of capacity.
Solar panels must be connected through a proper - and once again,
"Smart," charge controller to the battery. Similar to a smart
charger in operation, these sense
the battery voltage, and apply power from the panels accordingly.
Although solar panels don't put out a monstrous current, the aspect of
always making some power during daylight can add up to serious
overcharge unless a controller is in place to monitor this. Or, if
you're using more power each day than the solar output, this output
may be paralleled to that of the Smart Charger, since both are protected
from reverse current flow by the rectifier diodes.
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1. State Of Discharge
Obviously, when the battery is discharged below a certain percentage
of its total capacity, it must be recharged. How far down is an
important decision. The lower a battery is allowed to go before
recharges, the fewer charge cycles it will provide. Running a battery
completely flat - especially a conventional lead-acid type - does
irreparable damage and lowers its life expectancy considerably each time
this is done. On the other hand, if you recharge every time the battery
drops down to 95 percent, you may as well not have a battery at all.
A good compromise is 50 to 60% of capacity. Meaning a 100 AH battery
should be recharged when 40 to 50 AH have been taken out of it - and
remember, the lower the discharge percentage, the better. Batteries
maintained at a high state of charge last longer.
2. Capacities And Limits
How far you want to go with your house battery system depends on your
power budget - what will be your average daily power requirement?
My personal budget, for example, can be as high as 30 Amp-Hrs per day.
This assumes operating electronics (stereo) about 8 hours per day at 2
amps - 16 A-Hrs, and another 4-6 A-Hrs for lights after dark. Thus, my
100 A-Hr AGM battery could be down to 75% of capacity in one day - not a
bad time to recharge - and to 50% by the next day - definitely time to
recharge.
How About Charging On The Road?
The traditional way of charging Lead-Acid batteries on the road has
always been simply to connect them into the vehicle electrical system,
and let the alternator have its way. It will apply a constant 13.8V
charging voltage to them, resulting in some charging, more or less,
generally laying about 5-10 Amps into them. So they'll get some charge,
but not what a smart charger would do - it applies charging voltage
according to what it senses about battery state of charge, and may well
apply 15 to 18V to get them up to snuff. This is especially true of a
smart charger connected to an AGM battery. In this case, it's
absolutely essential to use a smart charger, so you'll need an inverter
to supply the smart charger from the vehicle's 13.8 VDC system. And
that inverter will have to be of a sufficient capacity. Meaning, a 100
A-Hr AGM battery, which may pull as much as 80A, can need as much as
80 X 12 = 1000 W of input power for the inverter. Since, in my personal
case, my genset is only 600W, I'll settle for a 500W inverter, and
chance having to charge my AGM battery at less than optimum level.
"Smart" Charging
What is a "Smart Charger?" This is a fairly recent arrival on the
battery tech scene, which senses the three stages of proper battery
charging, and applies the correct amount of current for the type of
battery, the state of discharge, and the stage in which its operating -
all automatically, of course. To do this properly yourself would
require much more patience and attention than the average RV'er has time
to provide. (Do not confuse the proper Smart Charger with the old-style
so-called "Automatic" battery chargers selling for about $50 at
Wal-Mart.)
3. Charging Rate
Every battery has an optimum rate during the replenishment stage. Exceeding
this rate
places serious strain on the battery. Overcharging a regular lead-acid
battery will have consequences in terms of gassing - excessive passing of
destructive acidic fumes and explosive hydrogen. The consequences are
lowering of battery life, danger due to explosion, and increased maintenance
requirements - adding distilled water to the cells, keeping terminals
cleaned up, etc. All of these problems are extant in any case with this
type of battery, but increase dramatically at the higher current rates of
overcharging. The charge rate is determined by the voltage applied by
the charger to the battery. The higher the applied voltage, the higher
the charge rate. (I=E/R)
The Three Stages Of Battery Charging
The initial charging stage is one
in which the battery "wakes up" to the fact that it's being charged, and
begins to take on power. Once this stage has been completed, the charge
rate should be increased to the actual charging - the second stage,
during which the maximum charging current may be applied. And finally,
the "float," or "finish" stage, during which the battery plates rid
themselves of accumulated gas bubbles, and the battery settles down
again. In the case of regular lead-acid batteries, this "float" stage -
during which virtually no power is added to the battery's capacity - is
about two hours, meaning you have an extra two hours of running your
genset just to finish the battery off after each charging session.
The following table shows typical examples of the regimen which will
be done by a Smart Charger for a 100 A-Hr battery - two types shown.
AGM: Up to 80%
More On Capacity
Obviously, the time between recharges, for a given daily discharge rate,
will increase with total battery capacity. Meaning, for example, if
your battery budget involves using 40 AH per day, a 100 AH battery will
require recharging every day. If, however, you use a 200 AH battery,
for the same daily power budget, the time between required recharges
will double to two days.
When To Recharge?
It's not easy to keep track of your battery usage - how many amps for
how many hours, etc. Consequently, "Still" battery voltage is the most
commonly-used indicator of just how full (or empty!) your battery is.
This is with the battery having sat still for a time - min. 1/2 hr. or
so - with no loads connected. Bottom line is about 10.5 Volts, at which
time it's reached the time to recharge - letting your battery run down
past the botttom of its ideal discharge curve is a real battery killer!
But how do you know how long, and at what current, to recharge your
battery to get it back up to "full?"
4. Battery Types
As mentioned above, there are two main types of battery: Lead-Acid, the
original and most common type, such as used for starting batteries in
most vehicles, (or its close cousin, the Gel Lead-Acid) and
the newer and much superior AGM - Absorbent Glass Mat battery, which
uses plate separators made of chemical-impregnated fiberglas matting.
Lead-Acid Batteries
The regular Lead-Acid
battery is the least efficient, and deteriorates most quickly, both in
terms of charge cycles, and in "fussiness" and resistance to abuse -
they must be charged and discharged within strict limits, and their fluid
levels must be carefully maintained, for example. They are also more
prone than other types to sulphate their terminals, making for
power-robbing high connection resistance. They're also messy, since
some acid fumes inevitably escape with the expelled hydrogen gassed out
during charging and discharging. Their close cousin, the Gel battery,
has eliminated or reduced some of these concerns, but still has to move
out of the way for the new champ - the AGM.
The AGM - Absorbent Glass Mat Battery
Although it
still uses Lead and Acid as its main components, this type of battery
contains the acid in absorbent pads of fiberglas between the plates. It
is completely sealed - there are no vents, and all gasses created within
the battery are contained and recycled within the battery case. (There
is a "pop-off" valve, which will release the gasses generated during the
destruction of the battery by massive overcharging!) Thus
there is never any gassing difficulty, nor is there any mess created by
acidic battery fumes escaping into the battery compartment. These
batteries will accept a much higher charging rate than regular
lead-acid, - as much as 70-80% of capacity - meaning genset times for
recharging will be considerably lower, and to top it all off, the
"finish" stage requirement is much less, meaning, once again, less
genset time for recharges. However, charging protocol is critical, and
they should only be recharged using a
properly-designed "smart" charger - one built expressly for charging AGM
batteries.
Genset Size - How Much Is Enough?
It will be seen that, if you're running on a completely 12VDC system,
and using your genset only for battery charging - and possibly the
occasional use of power tools, such as drills, blenders, grinders, etc.
- you don't need a lot of wattage. The Smart Charger, keeping a 100 A-Hr
AGM battery up, will output at most 80A X 15 V = 1200 Watts, meaning its
maximum input power would be 1500 Watts, and it will get by on less than
that. If battery charging is kept up, the replenishment stage will
likely draw less power still. Even the smallest of "suitcase" gensets,
providing 6 to 800 Watts,
will show Smart Charger Output of 35 to 50 Watts during the Charging
Stage, and of course plenty enough for the first and last. This should
be sufficient, although possibly not optimum, for a 100 A-Hr AGM
battery. For a Lead-Acid, which can only accept 10% of its total
capacity per hour, the 30 Amp capability of a 600 Watt genset means it
could maintain a 300 A-Hr unit, thus offsetting the long first and last
stages by permitting longer charge/discharge cycles for a given power
budget.
Recap: Comparison Chart
Following is a comparison of expected parameters for the two battery
types - both batteries of a 100 A-Hr capacity, replacing about 40 A-Hrs
of charge:
Charging House Batteries
House batteries may be charged in one of Four Ways:
Shore Power
The best way to maintain a battery is to replenish its discharge at a
slow and constant rate. Thus, when you're plugged into shore power,
your "smart charger" should be connected to shore power - and at the
other end, to the battery - at all times. It will
automatically take the house batteries to their optimum level at the
optimum charge rate - and "float" them at that level until disconnect
time comes along. Furthermore, it will act as a "120AC-12DC Converter,"
aiding and augmenting the battery in keeping the various 12V loads
supplied.
Vehicle System - Driving
A Smart Charger connected at all times to the house battery can be fed
by the vehicle system during driving. Thus, in the case of overnight
stops, if the battery has not had enough "shore power" time to reach
capacity by the time to move along, the charge should be completed by
the smart charger feed being switched over to an inverter fed by the
vehicle system as you drive. If the house battery requires no charge,
or once it has attained full charge, it will be "floated" by the Smart
Charger - If it needs to eat, the smart charger will detect that fact
and charge accordingly. Since the smart charger's input voltage is
120VAC, it may be fed from the chassis 13.8 VDC system through a suitably
rated inverter. Since the chassis' 13.8 is seldom the ideal voltage for
charging an AGM battery, it should never be connected directly to the
house battery in an attempt to charge while driving. This is an old
method which has become obsolete with the advent of the Smart Charger.
Genset Power
During Periods of "dry camping," as the battery runs down to the bottom
of its optimum charge range, it must be recharged with the genset.
Although it may be tempting to get the job done in a minimum running
time, the max charge rate, once again, must not be exceeded. The Smart
Charger, its power input now switched over to the genset, will charge
your house battery at the optimum charging voltages as the battery goes
through the three stages of recharging. During charging sessions, any
surplus power may be utilized for other 120 VAC uses, such as sewing
machine, power tools, etc.
Solar Photovoltaic Panels
The "cat's meow" for battery charging. No noise, no fuel cost, pretty
well totally automatic, these are the best, although initially the
most expensive, way to keep your house batteries up. A 50 Watt Panel,
pointed directly at a summer sun, will make 2-4 Amps all day. However, as
the angle of incidence varies from 90 degrees, and as the seasons
change, the power decreases, so a panel mounted flat on the roof, even
properly aligned to the sun, and on the south side of the bus, can only
be counted on for perhaps 250 Watt-Hrs (/12V=20 Amp-Hrs) on a long
summer day in Northern Climes.
When we go to winter days, southern climes, or both, the charge level
will drop considerably.
Still, any juice is better than no juice. The charge rate can be
optimized, of course, by mounting the panel so that it can be tilted up
towards the sun when parked, or by adding another panel or two, but even
a single panel can significantly reduce the amount of genset time during
dry camping.