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F R E Q U E N T L Y   A S K E D   Q U E S T I O N S

What kind of battery should I purchase for my Motorhome?

How do I know if my SEALED batteries are charged?

How do I know if my WET batteries are charged?

Why do I need a Solar Controller/Regulator?

How do I know what size Deep Cycle Batteries I need?

What is the difference between connecting up batteries in series or parallel?

How many Solar Panels do I need for my Motorhome?

How much power does a Solar Panel actually produce?

What effect does shading have on a Solar Panel?

Schematic of a 24/24V DC System

What's the difference between MPPT and PWM Solar Controllers?


What kind of Battery should I purchase for my Motorhome?

If you are thinking of purchasing conventional, wet type batteries, most Motorhomes will require two.
One for starting and one for powering the Motorhomes domestic lighting, refrigeration, pumps and accessories.

     For the best performance and life, the starting battery needs to be the type that allows high cranking amps with reserve
     for starting. If you are purchasing Wet Type Batteries, or Flooded Acid Type as they are sometimes called, they have
     more plates per size (thinner) than a Deep Cycle Battery to give higher cranking ability. They are not suitable for
     powering the domestic requirements, as they are not designed to be deep cycled and recharged. They will fall apart
     internally in a short time if used for this service and are not a good investment in the long term.
     For the domestic requirements in a Wet Type Battery, a true Deep Cycle Battery is required for good life and reliability.
     These types of wet batteries have fewer (but thicker) plates and heavier duty separators for cyclic use, deep discharging
     and charging. They will give good service if charged and maintained properly. If charged properly, they will require 
     water regularly.

     There are some newer types of battery technologies available, which have definite advantages for motor caravaners
     over the wet types for both starting and deep cycling applications. For one, they do not require any maintenance at all,
     other than charging correctly (i.e. you never have to add water). They do not discharge themselves like wet batteries if
     left standing. For instance, they can be left for 6 months or more when fully charged and still be able to start your
     engine or power your Motorhome requirements. Conventional wet batteries will discharge themselves as much as 5% or
     more per week, whereas the newer technology types discharge themselves less than 1% per month! The wet Deep
     Cycle type will require an Equalisation Charge regularly to get good cyclic life. The newer technology types do not
     require any equalisation to achieve long life and good cyclic ability. These new technology batteries are generally
     referred to as VRLA (Valve Regulated Lead Acid), Sealed Type or Recombination Batteries. There are two main types, 
     GEL and AGM.

     GEL Batteries are exactly as they are called. Their electrolyte or acid is combined with silica and other chemicals to
     "immobilise" the acid in gel form. They have the advantage of not stratifying the normally liquid acid, and generally do 
     not require any equalising. They are quite efficient compared to wet batteries but require a very careful charging
     regimen, and will give good long life if charged and discharged within their prescribed regimen. They do not have, for
     their size, as  high a cranking capability for starting, and cannot be charged as quickly, or accept charge as well as wet
     type or AGM. GEL types tend to be more expensive than either wet or AGM batteries.

     AGM stands for Absorbed Glass Mat and are sometimes referred to as Starved Electrolyte Batteries. They have specially
     constructed plates, which have been wrapped with a strong, micro-porous glass mat, and the electrolyte is held tightly 
     to the plates. These plates are also compacted within the battery, which makes them very strong and rugged. There are
     a number of different varieties of this type of battery, but the better ones have the ability to operate in both a deep cycle
     mode with very high cranking ability for starting, and also have the ability to accept charging quickly and efficiently.

     In other words, they are a true dual purpose battery, and you do not have to buy two different types of batteries which
     require different charging regimens to properly keep them charged. Another plus is that they are a very safe battery as
     they are allowed to be carried on aircraft and they are not classified as Dangerous Goods. Both GEL and AGM do not
     give off gas when charged normally within their specifications, so are not dangerous like wet batteries when charging
     and discharging. They can safely be mounted anywhere inside, and do not have to be upright.

     All being equal, AGM's will give better service, longer life and cost less to operate in the long run. Additionally, if you
     are considering putting Solar charging on your Motorhome, the additional charging efficiency gained and the low self
     discharge of AGM's ensures they will more than pay for themselves in the long run.

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How do I know if my SEALED (SLA) Batteries are charged?

     SLA (Sealed Lead Acid) batteries come in a number of types: AGM, GEL and 'so called' Maintenance Free types.
     Because these batteries are sealed, you cannot use a hydrometer to measure their STATE OF CHARGE or SOC. They 
     either need a very accurate DC Voltmeter (digital preferred), or a true Ampere Hour Meter, which is by far the most
     accurate and easily understood way. If you are going to use a Voltmeter, you will need one that has a resolution and
     accuracy of one tenth of a Volt or better. Your battery will also need to be in a steady state, that is, not to have been
     either charge or discharged for at least 6 hours previous, for reasonable accuracy. This Steady State ,for all practical
     purposes, is hard to achieve in the real world. Most installations require a battery input most of the time, like powering
     gas alarms, monitoring systems, or the memory back-up for a car type stereo. Also, all forms of charging must be  
     disconnected during this period. Disconnecting the battery, in most cases, is really not practical for the measuring of its

     Ampere Hour Meters, or True Battery Monitors as they are sometimes referred to, are really the only practical way of
     telling accurately whether an SLA battery has really been charged enough. Most of the True Battery Monitors can also
     give you a lot of information about what is really happening in you electrical system. Because they count the real Amp
     Hours that your electrics have used out of your battery, you will know how much you will need to replace. Most give you
     Amp Hours removed and % left in you battery. Then when you battery is being charged it will count up to 0 and will let
     you know that all the power you have removed has been replaced. If they are programmed correctly (which is very easy
     in most of the newer units), they will automatically calculate for the efficiency of your battery type and give you a very
     accurate answer as to SOC. In our experience, the advent of Battery Monitors will make living with a DC system
     powered by batteries a pleasure and are a real necessity if you want your system to be reliable and your investment in
     batteries long lasting.

     Recommended makes of battery monitors that we have tested and used are:
          -   Bogart Engineering Models: TriMetric 2020 and 2025-RV, and PentaMetric
          -   Trace TM-500A by Xantrex
          -   XBM Battery Monitor by Xantrex

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How do I know if my WET/FLOODED Acid Batteries are charged?

     The best way to determine if a Wet Type Battery is charged, is to use a hydrometer. This, however, is not always
     convenient, for I have noticed that most batteries are located in locations difficult to access in order to do this properly.
     Using a hydrometer is messy, and can be dangerous, although it is a very accurate way of determining the State of
     Charge of a Wet Type Battery.
     There are some precautions - the reading will not be accurate if you have just topped up the water, or if the batteries
     have been stationary and not charged for some time, as the acid will start to stratify and become weaker at the top than
     at the bottom. All wet batteries need an equalisation charge regularly for this reason, besides bringing up the weak or
     lazy cells.
     Below is an accurate table of % of charge in relationship to voltage and specific gravity in 12 & 24 VDC Wet Battery
     Systems, as measured by a hydrometer. To make determining charge state easier, Expanded Scale Voltmeters and
     Ampere Hour meters, or Battery Monitors, are the most convenient, especially the Ampere Hour meter. Expanded Scale
     Voltmeters are only accurate if the batteries have been static for some time, meaning no charging or discharging taking
     place for 6 hours or more. This is not always convenient or easy to achieve. A much more convenient and very accurate
     way is to use and Ampere Hour meter, or Battery Monitor as they are usually referred to. These not only give you very
     accurate voltage and instantaneous Amps, but will also show most importantly the percentage of battery charge
     remaining, how many Amp Hours have been used up, and when the battery is fully charged. Some also tell you how
     many days since the battery was fully charged. Lowest and highest battery voltage are also included in some units for
     keeping a check on how the battery is being charged and discharged. Some have total Amp Hours used, like an
     odometer in an automobile, to measure total battery life.

     Specific gravity values can vary 0.015 points off the specified values. This table is for a flooded battery in a static
     condition, no charging or discharging occurring, at 25C. Charging or discharging will vary these voltages substantially.

Percentage of Charge
12 Volt Battery
24 Volt Battery
Specific Gravity

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Why do I need a Solar Controller/Regulator?

     The main function of a Solar Controller or Regulator is to fully charge a battery without permitting overcharging. If a
     Solar array is connected to Lead Acid Batteries with no overcharge protection, battery life will be compromised. Simple
     controllers contain a relay that opens the charging circuit, terminating the charge at a pre-set High Voltage and, once a 
     pre-set Low Voltage is reached, closes the circuit and allows charging to continue. The most sophisticated controllers
     have several stages and charging sequences to assure the battery is being fully charged. The first 70- 80% of battery
     capacity is easily replaced. It is the last 20- 30% that requires more attention and therefore more complexity.

     How Controllers Work and Available Options

     The circuitry in a controller reads the voltage of the batteries to determine the State of Charge. Designs and circuits
     vary, but most controllers read voltage to reduce the amount of power flowing into the battery as the battery nears its
     full charge.

Features that can be included with controllers are:

     - Reverse Current Leakage Protection
               by disconnecting the array or using a blocking diode to prevent current loss into the Solar modules at night

     - Low-Voltage Load Disconnect (LVD)
               to reduce damage to batteries by avoiding deep discharge

     - System Monitoring
               analogue or digital meters, indicator lights and/or warning alarms

     - Over-current Protection
               with fuses and/or circuit breakers

     - Mounting Options
               flush mounting, wall mounting, indoor or outdoor enclosures

     - Systems Control
               control of other components in the systems - standby generator or auxiliary charging system, diverting array
               power once batteries are charged, transfer to secondary batteries

     - Load Control
               automatic control of secondary loads, or control of lights, water pumps or other loads with timers or switches

     - Temperature Compensation
               utilised whenever batteries are placed in a non-climate controlled space. The charging voltage is adjusted to the
               temperature. Recommended on most systems

     - Central Wiring
               providing terminals to interconnect system wiring

     Some systems require all of these functions, others require only one or a certain combination. We can help you select a
     unit to meet your specific needs.

     Sizing a Controller

Charge controllers are rated and sized to the systems they protect by the array current and voltage. Most common are 12 Volt, 24 Volt and 48 Volt controllers. Amperage rating run from 4.5 Amps to 60 Amps.

For Example: if one module in your 12 Volt system produces 3.5 Amps and four modules are utilised, you produce a
total of 14 Amps of current at 12 Volts. Because of light reflection and the Edge-of Cloud effect, sporadically increased current levels are not uncommon. For this reason, increase the controller Amperage by a minimum of 25%, bringing the minimum controller Amperage to 18.7 Amps. Looking through our products, you'll find a 20 Amp controller, as close a match as possible to this. There is no problem with going to a larger controller, besides possible additional cost. If you think your system may increase in size, additional Amperage capacity should be considered at this time.

The chart below shows the average effective output to expect per day from summer to winter, with older technology PWM controllers, and newer MPPT (Maximum Power Point Tracking) style controllers. While in good solar conditions the difference is not so great, in winter and marginal light the MPPT controllers can produce about 35% more from the same solar source.

Solar Output Chart

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How do I know what size Deep Cycle Batteries I need?

     Sizing of Deep Cycle Batteries is critical to the performance of electrical items. Insufficient capacity results in systems
     failure, poor battery performance and shortened battery life. Remember that a battery only stores power. Charging
     capacity with the number of hours is equally as critical to good battery performance. Periodically ensure your battery is
     fully recharged. Establish the power consumption of each accessory (in watts or amps), the number of operating hours
     you will use it and the electrical systems voltage

Watts Volts = Amps          Amps x Hrs in use = Amp / Hrs

Hrs / Day
Amp / Hrs / Day













Cabin lights






TOTAL 26.59 AMP / HRS PER DAY     

     For this example, the total usage is 26.59 Amps between charging periods, if charged every day. For safe assurance
     and longevity of battery life multiply the usage (26.59 Amps) x 5 ~ 6 = 132.95 ~ 159.54 Amp / Hrs. Therefore, you will
     require a minimum of a 130 Amp Hr battery.

What is the difference between connecting up batteries in series or parallel?

When you connect your batteries in either series or parallel, you don't get double the Voltage AND double the Capacity (Ahrs) In series you get double the voltage, and in parallel you get double the capacity

When connecting batteries in series, you get double the Voltage at the same Capacity. For example, if you have 2x 6V260Ahr batteries, when you connect these in series, you would end up having 12V power with 260Ahr capacity.
When connecting batteries in parallel, you get double the Capacity at the same Voltage. For example, if you have 2x 12V130Ahr batteries, when you connect these in parallel, you would end up having 12V power with 260Ahr capacity.
Another way of thinking about this is if you have 2x pipes that are 6 units long and 260 units in diameter. If you put both these pipes into series (one after the other) then they are a total of 12 units long, but their diameter doesn't change.
Another way of thinking about this is if you have 2x pipes that are 12 units long, and 130 units in diameter. If you put both these pipes into parallel (one beside the other) then they are a total of 130+130 in diameter, but their lengths doesn't change.
So if you look at batteries like this, if you put 2x batteries in series, you will add the Voltage together, where the Capacity (Ahrs) doesn't change.
So if you look at batteries like this, if you put 2x batteries in parallel, you will add the Capacity (Ahrs) together, where the Voltage doesn't change.


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How many Solar Panels do I need for my Motorhome?

Solar modules and Motorhomes with batteries are a natural match

     Since batteries are charged when travelling, Motorhomes normally depend mostly on the vehicle's alternator for the
     primary power source. Power to charge the battery bank is also provided through a battery charger when plugged into
     mains power. However, for those who like to spend days, weeks or longer not travelling and not plugged in, Solar
     Panels can mean freedom. And because a Solar array can put as much power into you batteries during an hour of
     bright sun as a small gasoline generator, it can also mean reclaiming peace and quiet. As well, the Motorhomes
     existing battery bank and fused box make the transition to Solar a smooth and economical one. While most Motorhome
     systems utilise 1 to 2 modules, it is important to analyse your power needs. Just as with all Solar systems, you need to
     consider the Wattage of appliances and lights you are powering, as well as the average hours used each day. Unlike
     most other systems however, Motorhomes travel through different regions of climate, park at varying angles to the sun,
     sometimes in shade and sometimes not. People with similar vehicles can have very dissimilar power usage and  
     patterns of travel. These factors should all be considered when deciding on which, and how many, Solar Panels you
     need. If you have any question about how to go about this, please give us a call.

Buy now and Save later
     Your Solar charging system will pay for itself several times over by increasing battery life and reducing the amount of
     time you spend on camp-ground hook-ups and generator maintenance. Batteries that are Deep Cycled too many times,
     or sit idle for several months, can be permanently damaged. Solar modules provide a daily maintenance charge for
     your batteries and eliminate this problem. By recharging every day, the depth of discharge is reduced, and your battery
     life and performance are greatly improved.

Keeps batteries charged, Summer or Winter
     Solar Panel output is dependent on light intensity and exposure time in the sun. You'll be amazed at how much power 
     your system provides on a bright day. And even in cloudy weather, your panels will produce power although at lower

So, How many Solar Panels do you need?
     The more Solar Panels you can fit onto your roof, the better your system will be. This does not need to be done all at
     once, but can be spread over time. If you have no heating or cooling loads (i.e. refrigeration is running on gas when
     freedom camping) then you could start with a single 90 Watt or 120 Watt Solar Panel. See how you go. Are you running
     out of battery power on a daily basis and having to start the engine to boost you batteries? Then you need more Solar 
     Panels. As long as you initially install a Solar Regulator large enough to take additional panels, and leave sufficient
     extra cable length up top on the roof, you can easily add more Solar Panels when needed.

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How much power does a Solar Panel actually produce?

     While on the subject of using Solar Panels for power, it might be a good idea to explain how much power can really be
     produced by a Solar Panel to charge batteries and how that equates to usable power to power your loads.

     A good 85 Watt  Solar Panel mounted flat on the roof of a Motorhome will produce an average of 255 Watts per
     Summers day charging into batteries. This would be the usable power available from the batteries to run appliances,
     after all losses including regulators, and the efficiency of batteries. In Winter it would be a little more than half that

     Now, how does that equate to the Amp Hour storage in the batteries or what your loads are. If we take the figure
     of 255 Watts per day and divide that by the nominal charging voltage of 14 VDC for a 12VDC system, we could have
     replaced approximately 20 usable Amp Hours into our batteries. That would be enough for a very basic electrical
     If, for instance, though you have an average 80 to 110 Litre fridge, which requires about 40 Amp Hours per day
     (running 24 hours) you would need at least 2 x 85 Watt Solar Panels to sustain the fridge only.
     In other words, a good installation would normally require 2 x 120 Watt Solar Panels to power both a DC fridge and
     other essential electrics, if there was no other charging source to keep the batteries well charged.
     In Winter, the above system would run at a deficit, and another 120 Watt Solar Panel would be needed in order to be
     completely independent. If might also require a bit of extra electrical conservation.
     You can increase the average output of Solar Panels per day, especially in Winter, by aiming more at the sun. That is
     not as difficult as it sounds. A number of Motorhomes do this, and get almost twice the power in Summer, and 3 times
     the amount of power out of their Solar Panels in Winter, especially in the South Island. Most Motorhomes also have
     provision to charge their house batteries while driving, and if you drive for a few hours every few days, that will help
     put some Amp Hours back into the house batteries, especially if the system has been set up correctly. Most Motorhomes
     also have a 240 VAC charging system, and if this is a good Deep Cycle Battery charger, it will fully charge the batteries
     over night. This would, of course, require going to a Camp-ground and plugging in. It could be that the washing need to
     be done to, as a good excuse. Some Motorhomes also carry Generators to supplement their power usage, and also for
     emergency. With the new type of Inverter-Generators and a larger charger, this can be a reasonably efficient and quiet
     way to charge the house batteries when you do not have enough Solar Panels or you have lots of rainy days.

     The size of a battery for an alternative power system is an important decision too. Too little storage will cause the
     battery to be too deeply discharge often and will drastically shorten its life. A rule of thumb here is to take you average
     consumption per day in Amp Hours and multiply that by at least 4 or even better 6 times, to get good battery life. In
     practice, this means that if your average daily consumption is about 50-60 Amp Hours per day, then you would need a
     200 to 360 Amp Hour battery system for a reliable system and reasonable battery life.

     Totally alternative power is possible, and practical (no LPG or Generators), if one is willing to spend enough money on
     Solar Panels, Batteries and a large Pure Sinewave Inverter.

     One such installation is our own converted 9 meter Hino Bus. It is all alternative power, meaning everything is electrical
     operated including all of our cooking, except for barbecuing, which is wood power of course for flavour. Even the Solar
     Panels are raised electrically by the push of a button. The only other charging source is a very large heavy duty 24 VDC
     alternator with a very smart 3 stage regulator which will produce 70 Amps with the Hino engine idling, and up to 200
     Amps it needed when running. We have had this system operating for 6 years now, and would do it all the same again
     if we did another bus, but would probably include a built in Inverter-Generator in case we stay somewhere for more
     than a week and it rains every day!

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What effect does shading have on a Solar Panel?

     Solar Panels are very sensitive to shading. Unlike a Solar Thermal Panel, which can tolerate some shading, many
     brands of Solar Panels cannot even be shaded by the branch of a leafless tree.

     Shading obstructions can be defined as soft or hard sources. If a tree branch or roof rack is shading from a distance,
     the shadow is diffused or dispersed. These soft sources significantly reduce the amount of light reaching the cell/s of a
     Solar Panel. Hard sources are defined as those that stop light from reaching the cell/s, such as a tree branch, bird
     dropping, or the like, sitting directly on top of the glass. If even one full cell is hard shaded, the voltage of the Solar
     Panel will drop to half of its unshaded value in order to protect itself.

     Partial shading even one cell of a 36 cell Solar Panel, such as our AAS-85W, will reduce its power output. Because all
     the cells are connected in a series string, the weakest cell will bring the others down to its reduced power level.
     Therefore, whether half of one cell is shaded, or half of a row of cells is shaded, the power decrease will be the same 
     and proportional to the percentage of area shaded, in this case 50%.

     When a full cell is shaded, it can act as a consumer of energy produced by the remainder of the cells, and trigger the
     module to protect itself. The module will route the power around that series string. If even one full cell in a series string
     is shaded, it will most likely cause the module to reduce its power level to half of its full available value. If a row of
     cells at the bottom of a module is fully shaded, the power output may drop to zero.

     The best way to avoid a drop in output power is to avoid shading whenever possible.

Example of full-cell
shading that can
reduce Solar Panel
output power to

Example of full-cell
shading that can
reduce Solar Panel
output power by


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Features of AA Solar System


     1.   Simplified electrical, utilising existing 24 VDC system
     2.   Efficient direct charging from either systems charging to the other
     3.   All batteries always charged
     4.   Less expensive
     5.   Starting and house loads isolated when not charging
     6.   Redundant starting
     7.   Provision for necessary 12 VDC loads
     8.   Auto switch between shore side and inverter for 240 VAC system

     Possession of AA Solar Systems drawings does not constitute right of use or right of copying.

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Updated September 2013