Before the advent of the house battery, there was a period when appliances in most caravans and motorhomes relied on the alternator and starting battery from the engine for their power needs. This was fine when underway as the alternator would typically produce enough power to handle the requirements of both the engine and appliances but it left a gap when parked.
Unless plugged into a regular source like a 240V at a caravan park the trailer or motorhome body was only powered by the starting battery and as 12V loads increased, that scenario was recognised as not being practical as it often lead to flat or damaged starter batteries so fitting a battery in a caravan or a second, sperate battery in a motorhome became common.
Initially spare starter batteries were used for this purpose but the shortcomings of that idea, mostly batteries that “died” well before their expected time lead to the scenario we know today for starter and “house” (deep cycle/AGM/Lithium) batteries – two quite differently designed batteries for two quite different battery loads.
Starting batteries are designed to provide a very high current (typically around 350 – 400 amps at 12 volts) for a very short period of time, just long enough to turn the engine over and provide enough spark to get the engine running; not more.
To get such a high current out of a lead-acid battery requires the lead plates to have a very large surface area in the surrounding electrolyte. In conventional battery design that means a considerable number of very thin plates, kept apart by small separators. It’s those thin plates that mean the battery cannot withstand constant deep discharge and recharge cycles as the plates can buckle as the massive amount of current is drawn from them.
Deep cycle batteries are built with much thicker plates which makes them less ideal for short term high currents as they have less surface area for the electrolyte and plate to react and produce current but are much more suited to long term constant loads and long life thanks to the more robust nature of those thicker plates. In an emergency situation, both battery types could be used in alternative roles. For example, with a flat starter battery, there is no reason why a deep cycle battery could not be used to crank the engine but any long term use will shorten the battery’s life.
Starter batteries are rated in Cold Cranking Amps (CCA), that is – the amount of electrical current (amperes) that a battery can deliver in 30 seconds at 0 Deg F (-18.5 deg Celcius) and at the same time maintain a minimum voltage of 7.2V. The rating is determined at a very cold temperature might sound a bit odd but it takes more energy to start a very cold engine and batteries tend to perform better in a warm climate.
There is a belief that the higher the CCA figure the more power a battery can deliver and the longer it’s life. That’s not strictly true, especially in warm climate countries. Apart from being more costly, the higher the CCA rating, the more plates get added into a battery which affects the size and thickness of materials used for separators and plates. That may result in shorter battery life because of an engine constantly idling and/or vibration.
Determining the CCA battery rating for a vehicle is probably best left to the engine manufacturer. There are a considerable number of variables – engine size, ambient temperature, accessory load and even the consistency of the engine oil. Some things are not quite as obvious as they look, for instance, a high revving four-cylinder engine might draw the same starting current as a slow revving V8. Unless there’s a problem, replacing a battery with the same CCA rating is usually the best way to go.
MEET THE AUTHOR
Malcolm Street began caravanning in the early 1970s, first in a Viscount and later in a York, the former towed by a Holden Kingswood. Malcolm has RV’d extensively across Australia, New Zealand and Britain. He became an RV journalist in 1999. Each year, he reviews around 40 caravans and motorhomes in Oz and NZ. Yes, he’s a well-travelled bloke with no shortage of campfire opinions about how a given caravan could be better put together.