There was a time when house batteries in caravans, campervans and motorhomes were a ‘nice to have’ option.
Those times are long gone. These days any RV without a battery is in the minority, especially for those who don’t stay in caravan parks or like to travel to remote places and need a reliable 12V supply.
Along with the batteries comes the matter of chargers and they too have developed to meet the evolving battery scene. We have moved from a ‘hot wire’ from the tow vehicle or motorhome base vehicle alternator to battery management systems, multi-stage regulators and external sources like solar panels.
LEAD-ACID BATTERY ESSENTIALS
Although there have been some great developments in the world of batteries, some of the basics have not changed at all. Irrespective of the name — conventional wet cell, sealed, AGM or gel cell — all are lead-acid batteries and work in roughly the same way, which is by a chemical reaction between lead plates and the electrolyte (a liquid or a gel in weak acid form that produces an electrical conducting solution). Lithium-ion (LiFePO4) batteries work a little differently but I’ll get to those shortly.
A 12V lead-acid battery typically has six cells, each with a positive and negative plate, the positive being lead dioxide and the negative being porous sponge lead. They are usually held apart by inert plastic or glass-fibre separators. Each cell is rated at 2.1V and all are wired in series giving a total voltage of 12.6V. The electrolyte used depends on the battery type.
VOLTAGES AND CONNECTIONS
Most RVs use a 12V DC system but occasionally you’ll find 24V DC power. Batteries can be connected in a parallel or series configuration. For two or more batteries, parallel means connecting the positive terminal to the positive of another and the same for the negative terminals. The voltage stays the same but the amp hour (Ah) capacity doubles, so two 12V 100Ah batteries become a 12V 200Ah source. Connecting those same batteries up in series, that is the positive terminal on one to the negative terminal on another, results in a 24V battery but with no change in the 100Ah capacity. Most RVs use 12V supply, so the 12V batteries are parallel connected.
For RVs are there are two battery types — those for starting engines and those for everything else. ‘Everything else’ includes sealed, absorbed glass mat (AGM), gel cell and lithium (LiFePO4) batteries. Except for the latter, everything else comes under the term deep cycle.
Aptly named, starter batteries are designed to deliver a high current (450–500A) for a short period of time, which is all that is required for a petrol or diesel engine to fire up. To achieve this, starter batteries have number of large thin plates which can give a fast chemical interaction with the electrolyte, allowing for the short term high current and fast recharge time as well. The thin plates are one of the reasons why a starter battery should not be used for a sustained electrical load as they will buckle and shorten battery life.
Starter battery capacity is measure in cold-cranking amps (CCA). In Australia, the battery must sustain the rated current for 30 seconds at a voltage greater than 7.2V at a temperature of -17.8 degrees. Temperature is a relevant to batteries because colder temperatures reduce battery performance noticeably. CCA varies considerably between vehicles and diesel engines are about double that of petrol.
Whist most batteries do not like temperature extremes, starter batteries are designed to handle engine heat, since many are found in an engine bay.
DEEP CYCLE BATTERIES
Sealed deep cycle batteries are much same as starter batteries — lead plates suspended in wet acid — except that thicker plates are used to withstand repeat cycling. Also known as flooded batteries, sealed batteries no longer need distilled water top ups.
A characteristic of all deep cycle batteries is thicker plates which allow repeated deeper and longer discharges. However, to have an effective service life, deep cycle batteries should only be discharged to about 50 per cent. Repeated discharges below that will lead to plate damage.
Discharge rates are measure in amp hours, and a common battery rating is 100Ah. In theory, that means a battery could deliver 50A for two hours or 20A for five hours, but it’s not that simple. For starters, a higher rate of discharge leads to a reduction in the battery’s capacity and whilst a battery may well deliver 20A for five hours, it may deliver 50A for only one hour.
A problem with sealed batteries is a lack of mechanical strength and their possible unsuitability for extensive offroad travel, which is why absorbed glass mat (AGM) batteries appeared in the 1990s. Developed for the military, they have an absorbed glass fibre mat that sits suspended in an electrolyte, making the battery much more robust. AGM batteries are fully sealed, designed to be maintenance free and withstand mechanical and corrugated road vibration, but are sensitive to heat in locations like under the bonnet of a car. Self-discharge, that is a battery losing its charge without being connected, is lowest for AGM batteries. Another asset is that AGM batteries can be discharged to about 30 per cent of their capacity without long term damage.
Different to AGM batteries, gel cell plates are held in sulphuric and phosphoric acid gel, a wax like substance which makes them very robust and resistant to spillage. A problem they have, though, is they are more sensitive to voltage spikes and should be charged at 14.4V, thus requiring a dedicated charger. Another characteristic of gel cells is a low self-discharge rate, but this changes rapidly as ambient temperatures get close to 40 degrees.
Lithium-ion (LiFePO4) batteries are very much in the news. Their prime advantages are that their output voltage remains constant even when down to 10–20 per cent of their charge and tend to be lighter and smaller than conventional deep cycle batteries. They can also be charged and discharged at a relatively high current.
However, lithium batteries are relatively expensive compared to other batteries and retrofitting them can mean changing the charger. Unlike other batteries which can be monitored as a single entity, each cell in a lithium battery must be monitored individually because if one cell is fully charged, the charging current to all cells will cease.
Starting with a few basics, older style battery chargers worked on the principal of applying a voltage greater than that of the existing battery voltage. For a 12V battery, a voltage of 14.2–14.4V was applied. As the difference between the battery and charging voltage was reduced so too did the charging current. The main problem with this was it did not fully charge most batteries.
Enter multi-stage chargers. Most ‘smart’ chargers have three stages of charging. The first is the boost mode, which is the initial bulk charge and supplies a constant charging current that increases the charging voltage as the battery voltage rises.
The second stage is the absorption mode which keeps the charging voltage constant once it reaches 14.2–14.4V and slowly reduces the charging current.
Finally float mode occurs. The charging voltage is reduced to 13.6–13.8V and the battery continues to charge slowly until capacity is reached. It depends on the battery type somewhat, as AGM and gel cell batteries will get to 100 per cent, more basic deep cycle batteries will only reach 90–95 per cent.
Some battery chargers have an equalization cycle which injects a high voltage to cause a battery to accept a further charge to get to 100 per cent. However, it does dissolve plate lead, causes gassing and is not recommended for AGM and gel cell batteries.
LITHIUM BATTERY CHARGERS
A lithium battery needs a specific charger or one that includes a cycle for lithium batteries because of differences in voltage levels. A fully charged lithium battery will have a voltage of 13.3–13.4V, whereas a lead acid battery is around 12.6–12.7V. At 20 per cent capacity, a lithium battery will have a voltage of about 13V but a lead acid will be approximately 11.8V. That small difference means a multi stage charger won’t work and may damage lithium batteries. Lithium battery chargers work on a constant voltage/constant charge algorithm to allow for fast charging without the risk of overcharging.
Complicating matters are when batteries are charged from more than one source, ie, mains charger or Anderson plug or solar panels. In the latter case a regulator is essential and in recent times integrated power system units have developed that manage all solar and battery systems, as well as having inverter output as well.
The RV industry has embraced solar panels big time. Solar panel technology is a subject in its own right, but all solar panels need a regulator to ensure connected batteries are effectively and efficiently charged.
MANAGING THE BATTERIES AND CHARGER
A charger problem is having multiple sources for house battery charging. In any caravan or motorhome, there is at least the tow vehicle/cab chassis alternator and a 240V mains charger. Additionally, there are solar panels, generator, DC-DC converters and even in some cases, wind turbines. Battery management systems (BMS) are fairly high-tech devices but put simply, they manage all the input, select the best charging method and ensure the batteries are charged properly. Outputs are managed too, mostly by having the minimum voltage selected, so that batteries are not discharged to unacceptable levels. A good BMS will have profiles for every type of battery charging.
12V batteries look like inert devices, which they are for the most part, but they can be dangerous if mishandled. Dropping tools across the terminals or between the positive terminal and any metal connected to the vehicle body should be avoided. Lead acid batteries should be stored in an upright position. If any serious 12V work is being done on an RV, the negative terminal on the house batteries should be disconnected.
Using sealed batteries in confined location seems to be up for debate. Although most manufacturers consider it safe, I’d still opt for a ventilated space where I could.
However, the verdict is much clearer for flooded batteries that aren’t sealed and from which hydrogen can escape. They should never be used in confined locations — a well vented area, like an engine bay or external bin, is recommended.