| Understanding your caravan battery | |
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As any regular rallier knows a good battery is everything. But how many times have we left home with a fully charged battery only to find that it starts to fail within the first few hours? So we get the spare out and that is flat as a pancake. Not to worry - we can give it a bit of charge from the tea tent generator. So we connect it up and it doesn't take any charge. What on earth is going on? To try to explain how lead acid batteries work I've put together this little article. The answers to your questions almost certainly lie in here somewhere! |
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Use the hyperlinks or simply scroll down the page - it's up to you ! |
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| TYPES & RATINGS | CHARGING, DISCHARGING & TESTING | INSTALLATION & USE | CHOOSING A BATTERY | BATTERY DO'S & DONT'S |
| Do's | ||||
Battery Types
Lead acid batteries are categorised into different types according to their construction and intended use. Car batteries are designed to give very high starting currents for short periods and then to be quickly charged by the running engine. Caravan or leisure batteries are intended to give low currents for long periods of time and therefore to be considerably discharged before being recharged. This process is sometimes referred to as 'deep cycling'
These different duties are reflected in the way the batteries are constructed and the two types of battery should not be interchanged (except in an emergency), as life expectancy and overall performance may be affected.
Basic Construction
A 12-volt caravan battery is a lead-acid device consisting of 6 cells, each supplying about 2 volts. The cells are housed in a strong polypropylene case. Each cell contains several groups of positive and negative plates that interlock so that each positive plate is opposite a negative one and vice-versa. The plates are made of a plastic honeycomb coated with lead pastes. During charging and discharging the plates expand and contract and in so doing lose small amounts of their active material. To help prevent this (and short circuits) they are kept apart by special separator plates. These plates are non-conductive but porous to allow normal battery current to pass through.
Batteries are made of five basic components: |
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A resilient Plastic container |
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Positive and negative internal plates made of lead |
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Plate separators made of porous synthetic material |
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Electrolyte, a dilute solution of sulphuric acid and water better known as battery acid |
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Lead terminals, the connection point between the battery and whatever it powers |
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All the plates sit in a solution of dilute sulphuric acid. It is this acid that carries the charging and discharging currents to the plates and thence to the battery terminals. As the battery discharges the acid becomes weaker. The concentration of the acid can therefore be used to determine the state of charge of the battery. This type of battery needs to be kept upright to prevent the acid from spilling out.
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Gel types
For applications where it may be difficult to keep a battery level (e.g. on a boat) the acid liquid is replaced by a gel. These batteries are usually sealed but include a safety pressure relief valve. The idea is that any oxygen and hydrogen produced during charging will recombine back to water so the battery never needs to be topped up. For this to work it is important that the charging process is carefully controlled so there is no build up of pressure in the battery.Absorbed Glass Mat (AGM)
This is a relatively recent technology where the acid is held in an absorbent mat made of fibreglass. This mat is placed between the battery plates and helps to support them making the battery good for use in situations where vibration may be an issue. The design makes the battery spill proof and allows it to be sealed in the same way as a gel type battery. AGM batteries are particularly efficient at converting charging current into stored energy, a figure of 95% being typical. This compares to 80-85% for a flooded type and 85-90% for a gel type. This makes them particularly suited for use with solar panels and wind generators.
These last two types of battery are known as Valve Regulated Lead Acid or VRLA’s for short. In addition to the advantages mentioned above these batteries are particularly good a holding their charge. A flooded battery can lose up to 1% per day whereas a VRLA type will lose only 1-3% per month. The downside of VRLA batteries is cost. They are typically 2-3 times as expensive as a good quality flooded battery and, in my experience, don't always have a much better life expectancy.
Spiral wound
This is basically and AGM battery where the plates are not flat but wound together in the form of a spiral. This gives the plates better support and meaning they can be thinner. Thinner plates mean that more power can be packed into a given size.Carbon fibre types
These batteries are usually of the flooded type but use carbon fibre to help support the plates meaning that they can be thinner and thereby give a better power density.
OPTIMA SPIRAL WOUND BATTERIES (The leisure batteries are the yellow topped ones on the left) |
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Battery Ratings
As well as voltage batteries are rated in accordance with their capacity. This is measured in ampere-hours. The larger the rating the more power the battery can give before it becomes discharged. In simplistic terms a battery rated at 100 amp-hours will give 10 amps for 10 hours or 1 amp for 100 hours. In real terms the capacity will depend on the rate of discharge (the faster the rate the lower the capacity) and temperature. That's why manufacturers often quote a discharge time and temperature alongside the battery capacity. Also as a rule of thumb lead acid batteries should not be discharged much below 50% of their rated capacity and should never be discharged beyond 80% - see next section.
In recent years some manufacturers have been overstating the capacity of their batteries to make them seem better value. This led to the honest ones complaining and now all batteries have to be rated according to specific rules.
CHARGING, DISCHARGING & TESTING
The energy cycle
Batteries work by converting chemical energy into electrical energy. In a fully charged lead-acid battery the positive plates are made of lead dioxide and the negative of lead. These electrode plates are immersed in a solution of sulphuric acid and distilled (or demineralised) water. As the battery runs down some of the sulphuric acid is converted into water and the plates (both positive and negative) become coated in lead sulphate.
DISCHARGING PROCESS The negative plate turns from porous lead to lead sulphate as sulphate is taken from the electrolyte The positive plate turns from lead dioxide to lead sulphate as sulphate is taken from the electrolyte The electrolyte acid becomes very dilute as sulphate is removed to the plates |
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When the battery is charged this process is reversed thus converting the electrical energy back into chemical energy.
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CHARGING PROCESS The negative plate turns from lead sulphate to porous lead as sulphate returns to the electrolyte The positive plate turns from lead sulphate to lead dioxide as sulphate returns to the electrolyte The very dilute electrolyte acid becomes stronger as sulphate returns from the plates |
Lead Sulphate and the effects of Sulphation
Lead sulphate can take one of two forms namely amorphous and crystalline. The amorphous kind consists many minute grains with a very large combined surface area. This provides for good conductivity thereby allowing the sulphate to be readily converted back into lead and sulphuric acid when the battery is charged. This is just as well because lead sulphate is a poor conductor but the porous nature of the amorphous kind allows the current to get through to the plates beneath for this providing that the coating is not too thick. This is why you should never discharge a lead acid battery much below 80% of its capacity. If you do then you may find that the battery is reluctant to take much charging current because the plates are so well insulated with lead sulphate. Only if the current manages to find a way through will charging recommence. If you catch the battery quickly you may be lucky and it may recover completely. If not you're looking at buying a replacement. Do it too often and you'll seriously shorten the life of your battery.
Whenever battery is left even partly discharged for any length of time the sulphate becomes crystallised and acts as an extremely good insulator. Charging then becomes a slow and difficult or even impossible process. It is sometimes possible to reverse the initial crystallisation by a long trickle charge, possibly over several days or more.
In the worst case a crust of crystalline lead acetate forms on the plates and further inhibits the flow of current. No amount of charging will reverse this. The battery is then said to be sulphated and will need to be replaced.
The best way of avoiding the formation of these crystalline deposits is to keep the battery charged as much as possible at all times.
Basic |
The simplest chargers provide an unregulated voltage that may rise as high as 15 volts or more as the battery becomes charged. This is too high for a low maintenance battery and will result in the electrolyte gassing freely. In time this will reduce the level and may even result in the battery 'boiling dry'. Such chargers are only suitable for use under close supervision. |
Semi- automatic |
These are chargers which are designed to cut out once a certain battery terminal voltage has been reached. This voltage is about 14.4 volts for a low maintenance battery and 15.2 volts for a standard battery. These voltages are chosen to enable a full charge without significant gassing. Normally there is a selector switch so that you can set the charger according to the type of battery you have. It can take a long time to achieve a full charge with one of these. |
Fully automatic |
Arguably the best chargers are the multi-stage automatic ones.The most sophisticated work on a three stage principle. The first stage is known as the 'constant current phase' where a constant current is delivered into the battery until the terminal voltage reaches something in the region of 14.4 - 14.8 volts. The second stage is known as the constant voltage phase where the terminal voltage is held at the same level whilst the current drops. This is normally held until the current being drawn from the charger equates to a figure of .05c - where c = the amp/hour capacity of the battery. When this figure is reached the charger will then move to the float mode where the battery is held at a terminal voltage in the region of about 13.5v, although this is usually achieved by monitoring the voltage between a maximum and minimum range, giving the battery a small 'kick' when it reaches the minimum level. Less sophisticated automatic chargers (like the ones you are more likely to see in the high street) will give a good rate of charge until a set voltage is reached (~ 14.4 volts for a maintenance free battery) and then drop back into a charge maintaining mode which works just like the float mode described above. |
Pulse |
Pulse chargers work by applying a high frequency pulsed direct current rather than the low frequency or steady direct current supplied by conventional chargers. Makers claim they are effective in restoring heavily sulphated batteries. They are not yet widely available in the UK although a charger based on the pulse principle was available a while back for recharging zinc carbon and alkaline batteries (e.g. torch batteries). |
There are various ways you can charge a battery when you do not have access to mains power. These include generators, car alternators, solar power and wind power. For more details see the associated article Managing Without Mains |
The chargers fitted as original equipment to many UK touring caravans are really regulated power supplies rather than conventional chargers. They are designed not only to charge the battery but also to provide a smooth and stable 12-volt (nominal) supply with or without a battery being connected. This dual role inevitably results in some compromise. The result is that the output voltage is regulated at of 13.8 volts. This voltage was chosen because it lies just within the 14.0-volt maximum voltage stipulated by EEC regulations for 12-volt caravan equipment.
The compromise is that 13.8 volts is not quite enough to fully charge a lead-acid battery.
With this voltage the charge level will only reach about 85% of the battery's capacity. In practice this may not be an issue since when towing the tow car may be able to put in much of the remainder, especially if the journey lasts for a couple of hours or more.
If your caravan is fitted with a charger unit dating back to 1993 or earlier it may have an output voltage of only 13.4 volts. This is not enough to adequately charge the battery and you should always use a conventional charger.
Batteries which are never fully charged will always have some lead sulphate on their plates. In time this will crystallise and permanently reduce the battery's capacity. That is why it is a good idea to give your battery a full charge with a conventional charger once or twice a year.
State of Charge |
Specific Gravity |
The best way to check the state of charge of a battery is to measure the specific gravity, or strength, of the battery acid. A fully charged battery should have a specific gravity of about 1.27 (1.25 for Elecsol carbon fibre batteries). A discharged battery should have a specific gravity of 1.1 or less. Specific gravity may be checked with a simple hydrometer as available from most car accessory shops. See picture on right. Hydrometer readings should not vary more than .05 difference between cells. *Sulphation of Batteries starts when specific gravity falls below 1.225 |
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100% |
1.265 |
If you have a totally sealed (zero maintenance) battery you will not be able to use a hydrometer. Some of these batteries have a built in hydrometer but it will only give a reading for one of the six cells.
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The voltage present across the terminals of an unloaded battery also says much about its condition. If 12.7 volts the battery is likely to be fully charged, and at 12.0 volts indicates just 25%, which should be regarded as effectively discharged. (If the battery has recently been charged wait 12 hours or more before checking its voltage, otherwise the reading may be a little high) Simple multimeters are cheap to buy and invaluable at times. *Sulphation of Batteries starts when voltage measures less than 12. |
Voltage |
State of Charge |
12.7 |
100% |
Storage
Whenever a battery is put into storage (e.g. for winter) it should be fully charged first. This is not the end of the matter though as all batteries experience minor chemical reactions even when not connected to any load. These reactions are mostly caused by impurities in the lead plates. The speed of reaction depends on the spacing between the plates, their surface structure and the battery temperature. The higher the temperature the faster the reaction.
The reaction is referred to as self-discharge since its effect is to slowly discharge the battery. That is why it is so important to give a boost charge every few weeks to maintain the battery in tip-top condition.
Safety precautions
Handling a battery is not difficult as long as a few precautions are taken. The main potential hazards are from the sulphuric acid, gas formation and heavy electrical currents and sparks.
Sulphuric acid is a corrosive fluid that can irritate or burn human skin and make holes in clothes! If you do get some on your skin, wash immediately with plenty of water for several minutes. If accidentally swallowed the victim should drink copious quantities of water or milk.
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<< Be careful when using tools around batteries! Always wear protective goggles when handlingsulphuric acid >> |
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Gases are given off during the charging process although they are minute in quantity as long as the charging voltage does not exceed 14.4 volts. Above this voltage it is normally possible for bubbles to be seen rising from the plates. Trapped bubbles can persist for some time after charging has stopped. Be careful when moving such a battery as the bubbles may suddenly be released. The gases involved are hydrogen and oxygen - a highly explosive mixture! This is why batteries should always be ventilated to the outside of the caravan so that the gases can disperse. It is also why you should not put a naked flame or lighted cigarette near a battery.
Whilst you will not get an electric shock from the 12volts across the terminals of a battery be mindful that lead-acid batteries can store large amounts of energy. If short-circuited this energy can be released quickly causing large sparks and/or wiring to melt and even catch fire. Be especially careful when using metal tools near a battery, as it is all too easy to cause a short circuit. When connecting and disconnecting a battery always leave the negative terminal until last. This is especially true when dealing with cars with their metal bodies but is also good practice when dealing with caravans.
Installation
Modern caravans have a purpose built external locker in which the battery is housed.
The size of this locker may restrict the size of battery you can use. Older caravans often have an exposed battery in either the gas locker or a bed locker. This practice is no longer recommended. If yours is one of these you can buy a special box and venting kit to make it a little safer.
Always ensure that your battery is securely anchored so it will not move around during towing.
Terminal care and connections To help prevent corrosion the terminals should be lightly coated with Vaseline.Make sure the vaseline covers the join between the terminal post and the battery casing. It is possible to get minute amounts of acid seeping through there. |
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Service life expectancy
The service life of a lead-acid battery depends on how it is treated as much as the number of charge/discharge cycles it receives. Given fair treatment a good quality leisure battery can see about 300 deep cycles. This may equate to 6 years or more although 4-5 years is more typical. If you constantly flatten your battery or leave it discharged for any length of time then you may not get much more than a year from it. Allowing a battery to boil dry is another excellent way of shortening its life.
Elecsol claims its carbon fibre batteries are good for 1000 deep cycles but members' experience with them is somewhat variable. It's best not to mix them with other types of battery. |
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As a battery ages its effective capacity will normally reduce significantly so beware of this and be ready to change your battery when you notice a drop off in performance.
Using more than one battery
Many caravanned have a second battery to provide more power. The simplest approach is to have the second battery as a spare for when the first one runs down. An alternative is to use the second battery just to power the TV so that you can watch as much as you like without fear of flattening the main battery.
A third alternative is to connect the two batteries together in parallel. This has the advantage of single point charging and discharging. It also theoretically results in the available capacity being that of the sum of the two individual batteries. However you should never mix battery capacities as this may cause over charging on one battery and insufficient charge on the other. A 75 amp-hour should be connected to another 75 amp-hour and so on. Also on completion of charging you need to be mindful that one battery can discharge into the other, especially if one is in better condition than the other. This is why, when not in use, you should always break the connection between the two batteries. Whenever batteries are connected in parallel the connecting cable should be properly fused to protect it against short circuits and overloads.
You can also have a second battery in the boot of your car and charged by it (see separate article Managing Your 12-volt Supply Without Mains Charging)
Using a car battery
A number of caravanners connect up to their car's 12S socket and draw power directly from their car's battery. This is bad practice for three reasons. Firstly you may not then be able to start your car, especially if it's a diesel. Secondly car batteries are not designed for this type of use and may eventually be damaged. Thirdly the voltage drop caused by all the extra cabling can cause problems with the caravan's equipment. My advice is not to do it unless you are really stuck and even then use the power only for essential loads.
Budget versus expensive
If you rarely use your caravan away from mains power and can easily keep your battery charged up when not in use, then a budget priced battery should prove more than adequate.
Under these circumstances the battery will hardly be cycled and should last well.
If you frequently use your van away from mains power then the size and performance of the battery become more critical. Generally speaking the more expensive a battery is the better the performance it will give, always provided that it is well looked after. This means regular top up charges and never letting the plates become dry. If you can't achieve this you may be better buying a cheap battery and replacing it when it fails.
Carbon fibre batteries cost 2- 3 times the price of budget batteries but our members' experiences are mixed. These batteries seem to need more frequent topping up with distilled water, especially if the caravan's charger is left switched on. Failure to attend to this can wreck the battery quite quickly.
Gel batteries are becoming increasingly popular in the caravan world but their only real advantage is that they don't spill if tipped. Clearly this is important in motorbike or marine applications but less so in a more stable application like a caravan. Some manufacturers claim their gel batteries can be cycled more than normal ones but I suspect this is down to their detailed construction rather than the use of acid gel. On the downside gel batteries are usually more expensive and they need a special charger to prevent gassing.
Newness
The newness of a new battery is very important. The longer a battery sits on the shelf and is not re-charged the more opportunity there is for damaging sulphate to build up on the plates. Most batteries have a date of manufacture or first recharge code on them. Sometimes the month is indicated by a letter, 'A' being January and and so on The letter "i" is not used because it can be confused with the number 1. Years are usually indicated by the last one or two digits, so a code of 00 D would equate to April 2000. The first recharge date is the date the battery should first be recharged so aim to buy before this date or seek evidence that the battery was recharged then. Remember the newer the battery when you buy it the better its likely condition.
Battery size or capacity
In deciding how large a battery to purchase you need to bear in mind the size and duration of the loads you are likely to subject it to. If you always have access to mains and your caravan has a suitable 12 volt power supply you may not need a battery at all. However I would always recommend fitting one, as it will smooth the loading on the charger and give backup in the event of mains failure.
If you have an older caravan and do not watch much TV then a 60/65 amp-hour battery should be fine for weekend use. For longer periods or greater loading the daily battery requirement in amp-hours can be calculated by multiplying the average current used by an appliance (plus a 25% safety margin) by the expected daily usage. See table below.
The latest caravans seem to have more 12-volt lights and gadgets than ever before. If you intend to use them without mains backup I would recommend a battery of at least 85 amp-hours capacity. Indeed if you have a Carver/Truma mover fitted to your caravan then this size is a minimum. That is because the two heavy-duty motors can each take up to 45 amps on full load.
It's also worth noting that batteries do not perform as well during cold weather so add add a margin of around 25% if you intend to use one in freezing temperatures.
Calculating current consumption and battery charge life
The following table indicates the possible daily consumption of some of the 12-volt equipment in a modern caravan:
| Item | Typical Current |
+ 25% Safety Margin |
Possible average daily usage time (hours) | Daily battery requirement (amp-hours) |
| Caravan Mover | 60.0 | 75.0 | N/A | 7.5 per 6 minutes |
| Colour TV | 3.0 | 3.75 | 2 | 7.0 |
| Black + white TV | 2.0 | 2.5 | 2 | 5.0 |
| Tungsten-halogen light (20w) | 1.67 | 2.08 | 3 | 6.24 |
| Twin striplight (nominal 16w) | 0.9 | 1.125 | 3 | 3.375 |
| Single striplight (nominal 8w) | 0.6 | 0.75 | 3 | 2.25 |
| Reading spotlight (10w) | 0.83 | 1.038 | 2 | 2.076 |
| Downlighter (5W) | 0.42 | 0.525 | 2 | 1.05 |
| Spaceheater fan (max speed) | 1.5 | 1.875 | 4 | 7.5 |
| Water pump | 3.0 | 3.75 | 1 | 1.25 |
| Radio/CD/Tape (low volume) | 1.0 | 1.25 | 1 | 1.25 |
| TV on standby | 0.075 | 0.094 | 22 | 2.063 |
| TV booster box | 0.05 | 0.063 | 24 | 1.5 |
- Do think Safety First.
- Do regular inspection and maintenance especially in hot weather.
- Do recharge batteries immediately after discharge.
- Do keep the terminals clean and lightly greased.
- Do make sure the battery is securely mounted and vented.
Battery Don'ts
- Don't forget Safety First.
- Don't add new electrolyte (acid).
- Don't use unregulated high output battery charger to charge batteries.
- Don't disconnect battery cables when current is flowing.
- Don't put off recharging batteries.
- Don't add tap water as it may contain minerals that will contaminate the electrolyte.
- Don't discharge a battery any deeper than you possibly have to.
- Don't let a battery get hot to the touch or 'boil' violently when charging.
- Don't mix size and types of batteries.
- Don't forget that a deeply discharged battery can freeze.
The main points of this article are available for printing as a specially designed 2 page leaflet - see the members only section of this website