Updated March 2016
Connecting caravan batteries for more power is rarely understood. This article explains what’s possible, why and how to do it.
Energy is the ability to perform work. Power is the rate at which that energy is used to perform that work.
Heaving up a 200 kg barbell through two metres in one second requires the same amount of energy as leisurely stacking two hundred 1 kg cans on a two metre high shelf. Neither requires that much energy – but the former requires a great deal more power. Battery usage is much the same. A battery used for engine starting must supply a lot of power for two to three seconds. The energy required, however, is tiny. It’s about the same as running a 12 watt globe for less than ten minutes.
For caravans and motor homes, the highest power requirement is likely to be a microwave oven (about 130 amps for 5-15 minutes via an inverter). For off-road RVs it may be a power winch (about 400 amps). Supplying such high short term power requires a battery that, like a weight lifter, can supply a lot of ‘grunt’ for a short time. Whilst this can be done by a starter battery, such batteries cannot cope with low current long term loads.
Any LiFePO4 battery as main supply in an RV will supply such power with ease. The power need can just be met by a 12 volt 200 amp hour AGM battery (although 300 plus amp hour is preferable). Some owners attempt this with as little as 200-300 amp hour conventional deep cycle lead acid batteries. They will only supply such power for a short time. Attempting this repeatedly considerably shortens their life.
Ensuring enough energy however for the typical longer term loads (e.g. lighting and TV) requires adequate amp hours. In practice LiFePO4 batteries can provide 65-75% of their rated amp hour capacity, AGMs about 60% and conventional deep cycle lead acid batteries about 50%. Discharging them more deeply will shorten their lives.
There are thus two RV battery requirements.
1. Enough power to cope with high peak loads. Given enough battery capacity this is generally assured.
2. Enough energy to cope with long term smaller loads whilst away from 230 volts etc. Increasing battery works – but only if there are adequate ways of charging. With many a solar-equipped RV, there is already adequate battery capacity, but not enough solar capacity to charge it.
Watts and watt hours
Another and commonly used measure (of both energy and power) is watts and watt hours. It really does assist to understand the concept as it removes misunderstandings about connecting caravan batteries for more power.
A watt is a measure of power used when work is done (i.e. energy is used). Electrically, watts are volt times amps. A watt hour is thus watts used for one hour. Why it helps to know this is because it is not unknown for do it yourself owners to believe that connecting their existing 12 volt batteries for 24 volts will double the power (they plan to use a 24/230 volt inverter).
Increasing energy capacity
With deep cycle or AGM batteries (as used in boats, cabins, camper trailers, caravans or motor homes) the amount of energy storable is more or less related to the weight of lead the battery contains. A 12 volt 100 Ah (1200 Wh) deep cycle battery weighs 25-30 kg. Its 200 Ah (2400 watt hour) equivalent weighs 50-60 kg. Lithium-ion batteries are much lighter but their capacity is still related to their weight.
To ease handling, (or obtain higher voltage, or higher current) batteries can be connected together. There are two main ways of doing so.
Parallel: positive to positive, and negative to negative to increase current.No matter how connected (series or parallel) the energy capacity of batteries is always the sum of their individual energy capacities. Connecting batteries in series (end-to-end) increases the total voltage. Connecting those batteries in parallel increases the total current. In and every case their total energy is the sum of each battery so connected.
Almost all conventional caravans and most motor homes have 12 volt systems. Whilst there is no benefit in doing so some owners prefer to use 6 volt batteries. To obtain 12 volts they are series-connected (positive to negative) as below. This results in the same current (as each 6-volt battery) but twice the voltage.
Series connection. If each 6 volt battery is 100 amp hour (600 watt hour), two series-connected such batteries can hold 100 amp hour at 12 volts (1200 watt hour). Pic: caravanandmotorhomebooks.com
If, as is common, more capacity is required, further pairs of so connected batteries are then connected in parallel, i.e. the relative pairs positives together, and negatives together (below).
Here, four 6 volt 100 amp hour batteries can hold 200 amp hour at 12 volts (2400 watt hours). Similar connection (but using 12 volt batteries are used to obtain 24 volts in converted coaches that have 24 volt alternators. Pic: caravanandmotorhomebooks.com
By far the most common arrangement, usually of 12 volt 100-120 amp hour batteries is that shown below. A few have only one 12 volt battery but most have two or more. If four, each of 100 amp hour are parallel connected (as below) their voltage remains at 12 volts, but capacity is 400 amp hours (4800 watt hours).
The above is a typical battery bank for a large caravan or motor home. Four 12 volt 100 amp hour batteries can hold 400 amp hour at 12 volts (4800 watt hours). Pic: caravanandmotorhomebooks.com
If connected in series the batteries must of identical capacity, The total voltage is the sum of each battery’s voltage: thus 100 Ah at 12 volts can also be obtained from two 6 volt 100 Ah batteries in series or six 2 volt 100 Ah. The energy stored is 1200 Wh regardless. (If one battery were to be of (say) 50 amp hour, the total output would be limited to that 50 amp hour.
If in parallel, all batteries must be the same voltage, but can be of different capacity. This can be taken to extremes: you can if you wish parallel connect a 12 volt 10 amp hour battery across a 12 volt 500 amp hour battery bank. The result is a 12 volt 510 amp hour battery bank. Contrary to common belief this is also totally safe to do. Like good socialists each (battery) will take according to its need, and supply according to its capacity.
As shown above, to increase both voltage and current one can parallel identical strings of series connected batteries. Here, the voltage is that one string. The amp hour capacity is the sum of all the batteries. Doing so is routine in large solar systems. They typically from 48 volts upwards.
This battery bank (at the author’s previous all-solar powered property outside Broome, WA) had 16 batteries, each 12 volts and 235 amp hour. Each level has four such batteries in series and all four rows are parallel connected.The output was thus 48 volts and 950 amp hour – or 45,120 watt hours (45.12 kW/h). Pic: successfulsolarbooks.com
Connecting caravan batteries for more power
As noted above, power is the rate at which energy is used. Batteries vary in their ability to do this. If you were to load a 100 amp hour deep cycle battery at a constant 50 amps its voltage is likely to fall below the level at which it can support that load in 15 minutes or so (it has used 25% of the available energy). If rested for a time it may well deliver that 50 amps for a further 10 minutes (another 16.66 amp hour). Eventually though it can no longer deliver energy at that high rate of power draw. Most of the remaining 50% energy is still available but at a probable 5 or so amps.
For any given type of battery, increasing capacity increases the available power. Connecting caravan batteries for more power is typically done by adding more in parallel.
The only way of increasing available power for the same battery capacity (number of amp hours) is to use a battery technology capable of doing so.
The conventional lead acid deep cycle battery is the least capable of delivering high power. AGMs are better. If their weight and bulk does not handicap a 400 amp hour bank of AGM batteries will provide all the power needed for RV use.
At present the best is the lithium-ion (LiFePo4). Any such battery used to feed an RV will have enough power to drive whatever you wish. These batteries also have a probable 20% or so more available energy capacity than any other battery as they can safely be discharged to a lower level. They do however cost far more. They need specialised installation and charging.
Batteries and their charging are surprisingly complex subjects. Caravan & Motorhome Electrics explains battery charging in depth, in jargon-free terms, yet is technically accurately.
If you liked this article you will like my books. All are technically accurate but written in plain English. Apart from that above, my other books are the all-new Caravan & Motorhome Book, the Camper Trailer Book, Solar That Really Works (for RVs), and Solar Success (for large home and property systems).