Frequently Asked Questions

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 motorhome’s 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, this type of battery for starting has more plates per size (thinner), for 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, 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 type battery technologies available, which have definite advantages for motor caravaners over the wet types for both applications, starting and deep cycling. 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 “equalization charge” regularly to get good cyclic life. The newer technology types do not require any equalization 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 “immobilize” the acid in a gel form. They have the advantage of not stratifying the normally liquid acid, and generally do not require any equalizing. 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 GELs and AGM do not give off gas when charged normally within their specs, 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, AGMs 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 motor caravan, the additional charging efficiency gained and the low self discharge of AGMs, 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 batteries come in a number of types, i.e. AGM, GEL and, ‘so called’ Maintenance Free types. Because these batteries are sealed you cannot use an hydrometer to measure their STATE OF CHARGE or SOE. They either need a very accurate DC volt meter (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 charged 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 measuring of its SOC.

“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 these “True Battery Monitors” can also give you a lot of information about what is really happening in your 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 your battery. Then when your 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: Trimetric 2020 by Bogart Engineering, Trace TM-500A by Xantrex and 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 an 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 or not charged for some time, as the acid will stratify and become weaker at the top than at the bottom. All wet batteries need an equalization 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 an 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 volt meters 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 an Amp Hour Meter ,or Battery Monitor as they are usually referred to. These not only give you very accurate voltage and instantaneous amps, but will 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 + or – 0.015 points of the specified values. This table is for a flooded battery in a static condition, no charging or discharging occurring, at 25 degrees C.     Discharging or charging will vary these voltages substantially.

Percentage of Charge	12 volt Battery Voltage	24 Volt Battery Voltage	Specific Gravity
100	12.70	25.40	1.265
95	12.64	25.25	1.257
90	12.58	25.16	1.249
85	12.52	25.04	1.241
80	12.46	24.92	1.233
75	12.40	24.80	1.225
70	12.36	24.72	1.218
65	12.32	24.64	1.211
60	12.28	24.56	1.204
55	12.24	24.48	1.197
50	12.20	24.40	1.190
45	12.16	24.32	1.183
40	12.12	24.24	1.176
35	12.08	24.16	1.169
30	12.04	24.08	1.162
25	12.00	24.00	1.155
20	11.98	23.96	1.148
15	11.96	23.92	1.141
10	11.94	23.88	1.134
5	11.92	23.84	1.127
Discharged	11.90	23.80	1.12

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