The Clipper Manual: House Electrical

Engineering Your "House" Battery System

Battery Selection A: What Is "Deep Cycle"?

The 12 Volt Supply for the house system should be completely separate from that of the chassis - ie, the "engine" system. This is for a number of reasons.

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:

Battery TypeMax Current DrawService TimeRecharge Time
Chassis, or Starting300 Amps5 sec BurstsShort term blast, then long
"Marine" or "RV" "Deep Cycle"300 Amps/some low current5 Sec Bursts/ a few hoursLong float
Real deep cycle1-20 amps (20% or A-Hr Capacity)Hours at a timeUntil recharged

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.

Deep-Cycle Service

Once you're parked, and the engine and generator are off, the battery supplies a relatively low total current - it should not exceed 10% of capacity for the regular lead-acid type - for the various house loads - such as electronics and lights - and it may be taken down to as low as 50% of its energy storage capacity before being recharged - although this is an absolute maximum; the more you "breathe off the top," the longer the battery will last. These loads are preferably 12 VDC, although some use of an inverter to supply 120 VAC for regular appliances may be allowed for. As the battery gives up energy, its level of charge capacity is lowered. At some time during this discharge cycle, you will plug into shore power, or start your genset to recharge the house battery back up to full capacity. How long this will take will depend on four factors:

  1. The % state of discharge
  2. The capacity of the battery
  3. The charging rate (in amperes)
  4. The type of battery

Let's consider these factors one at a time:

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.


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.

"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.)

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:

  • Shore power 120 VAC
  • Genset 120VAC
  • Vehicle power (engine running) - feeding the charger through an inverteras you drive. (13.8VDC to 120VAC Smart Charger input.)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.

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 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.

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.

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.

Series Connecting

There are advantages to connecting a pair of 6V batteries in series instead of using a larger 12V unit. For one, you can incresase capacity by using 2 large 6 volters. But remember that, although the system voltage is thus doubled, the maximum current capabilities are not! Thus, for example, a pair of lead-acid 6 Volt batteries of 200A-Hr capacity, although they'll now supply 12V instead of 6, will still only provide 200 A-Hrs for the pair. And their max current will also be the same as for either one but not both, since the current is the same throughout in a series circuit. So, in this example, the max current draw would still be ~20% of capacity, or 40 Amps, either charging or discharging. Their individual capacities in watt-hrs double, since their voltage does, but not their current capability.

Figuring Total Capacity

The capacity of a battery system is calculated in Amp-Hours, or Watt-Hours. Since a Watt is simply the amp-volt product, Watt-Hours is Amp-Hours multiplied by voltage.


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.

Adding More Capacity

When batteries are connected in parallel, any parasitic loss in either will drag down the other. This is not a major consideration when charging, but it is when they're not. Thus, if you choose to add capacity with another battery (in the case of a single battery), or series pair, you should add a battery switch which will allow you to choose between either or both 12 V battery sets. For charging, you can hook them both together, but be sure to break the connection after charging, choosing one or the other for use during the discharge cycle. When one pair gets down to the 12.2 volt minimum, you can then switch to the other. Such battery switches are common in marine applications, and are available at most RV and marine supply stores.

When To Recharge?

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:

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 onlybe recharged using a properly-designed "smart" charger - one built expressly for charging AGM batteries.

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.

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.

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."

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:

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.

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. Note, however, that the smart charger must be fed 120VAC until it has completed the proper charging cycle. Thus, if you're making a short drive with low batteries, charging should not be attempted on vehicle power - wait until you get somewhere where you can plug into shore power, or run your genset long enough to allow the charger to finish its work.

  • 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 winter day in southern climes. 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.

    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. 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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