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.
Let's look at some comparison figures for ~100 A-Hr batteries:
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.
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.)
Similarly, for charging on the road - from the vehicle system through
the inverter to the smart charger (13.8V - 120 - ~15), my inverter will
also have to be able to produce about 600 Watts of AC, so I'll need a
600 W inverter as well.
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:
What? Yes, it seems counter-intuitive to take some perfectly good 12VDC (13.8 actually) from the vehicle
system, change it up to 120VAC through an inverter, and then have the smart charger change it back to 12-15VDC
to charge your house batteries, but there's a good reason. That is that the voltage regulation of your vehicle system
is tailored to completely different requirements than that of your house batteries. It is set up to recharge your
starting battery as soon as possible after starting, and then to drop to a "float" level for the rest of the journey.
The smart charger is sensing the condition of your house batteries, and tailoring its output to their requirements, which
in many cases will be completely different. At the "high charge" stage, it will be supplying a higher voltage than that
of your vehicle system, and thus the regular system would be insufficient. So the smart charger needs to be supplied
with 120VAC, so that it may do with it what it will.
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.
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 House 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.
"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 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
An Excellent Battery Tutorial
Battery capacity is rated in Ampere-Hrs, or amp-hrs. A charge or discharge of 1 Amp for one hour is an Amp-Hr. So we might expect that a 200 A-Hr battery, for instance, could be relied upon to supply a 100 A load for 2 hours. And so it might - once or twice! Because, unfortunately, you have to maintain a headspace of about 60% - 70% of total capacity. Meaning that 200 A-Hr battery can only be used for 40% of that, or 80 A-Hrs. After that, dragging it down any more will drastically reduce its ability to come back on recharge. This usable power has been about used up by the time the battery gets down to about 12.3 Volts. So your usable range is from 12.7 to 12.2. 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.
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.
A good standard size of deep-cycle batteries is 240 A-Hrs. What does this mean? Well, for one thing, it means it can handle a max draw (or charge) of 20% of 240, or 48 Amps. It also means the total usable capacity is 30 - 40% of 240, or 72 - 96 A-Hrs. When fresh, they should deliver 40% of max capacity, (drawing a lead-acid down below 60% of usable power is a bad idea), or 96 A-Hrs. Since the current (amps) is through both batteries, the 960 is the total for both. (96 for one at 6V; 96 for two at 12 - the voltage, and therefore the wattage doubles, since watts is defined as the product of volts and amps)) Thus a pair of 240 A-Hr 6V batteries, fully charged, should put out 10A for 9.6 hours, or 48A for 30 mi, or anywhere in between, providing the product of the current and the time works out to 9.6 A-Hrs. - Not a whole lot of electricity, for those used to the kilowatt capacity of the on-the-grid house system. etc.
All of the above is important only in the planning stage. Once you're using whatever battery and charging system you have, the important thing is never to over-discharge your batteries. When they get down to the 76% level, recharge. If they get down to 50%, STOP using them until you recharge - whether by your gas generator, solar panels, windmill, or running your engine. Just Don't Use Them! - unless you plan on buying a whole new set in the near future!
The most important thing is not to over-discharge your batteries, not ever! And the quickest simplest way to determine their state of charge is their still voltage - a voltage reading taken after the battery has been left alone for 8-12 hrs. ie, first thing in the morning. Here's a battery charge chart: