12 volt vs 6 volt Batteries for Trailers

How Get more Amp Hours with Two 6 Volt Batteries

Wiring two 6 volt batteries in series is a cost effective way to obtain a higher amp hour rating and extend battery life between recharges. You'll find that the amp hour rating of two 6v batteries greatly exceeds that of two 12v deep cycle batteries wired in parallel. Using 6 volt batteries and wiring in series results in a 12 volt system without sacrificing amp hour ratings. This is a great solution for travel trailers, RVs, marine applications, dump trailers, winch trailers, and other instances where long battery life is desired.

12 volt Batteries in Parallel vs 6 volt Batteries in Series

Voltage

Voltage is the common term for potential difference, or electrical potential difference. It is most often referred to simply as voltage, with the units being volts. Technically, voltage refers to the work required to move a point charge (single electron) from one point to another. The higher the voltage, the more work done to move a single electron. For most, it's actually a somewhat difficult concept to grasp and we just come to terms with the fact that a 12 volt system is 12 volt system, a 24 volt system is a 24 volt system, and that you can't interchange systems of various voltages.

Voltage is best explained using a hydraulic example, as it is often referred to as electrical pressure. A pressure difference is required for a fluid to flow from one point to another. Imagine a pipe filled with water and sealed (capped) at both ends - the pressure of the water in the pipe is at equilibrium and thus is the same at any reference point in the volume of water. However, open the pipe at one end and place a pump at the other. Water flows from one end of the pipe to the other because there is difference in pressure between the pipe outlet and the pump. Voltage is no different; it's the potential difference between two points. The greater the voltage, the greater the potential difference between those two points. In terms of electricity, electrical current serves as the fluid flowing in the aforementioned analogy.

Amperage

Amperage refers to the current flow, or rate of current flowing through an electrical circuit. The units of current is amps and it is entirely independent of time. That is, it measures the instantaneous flow of electrical current but says nothing about the total amount of current used over a period of time.

In the hydraulic analogy above, amperage can be considered the flowrate of the water through a pipe. Amp hours (discussed in more detail later) is therefore the total volume of water that flowed through the pipe, or the amount of current consumed by an electrical load (light, pump, any electrical device) over a period of time.

Ohms Law

Ohms' law explains the relationship between voltage, current, and resistance. It can be written in the following expressions:

E = IR (voltage = current x resistance)

I = E/R (current = voltage/resistance)

R = E/I (resistance = voltage/current)

Where:

E = voltage in units of volts

I = current in units of amps

R = resistance in units of Ohms (symbol Ω)

In some instances, voltage may be referenced as "V" instead of "E" in the equations, however "E" is used in most technical applications. If you substitute "V" for "E" an easy way to remember Ohms law is "VIR" (V=IR)...it's the only arrangement that can be spoken, and the rest of the equations can be derived from this. The importance of Ohms law is that voltage, current, and resistance are related mathematically.

If resistance is assumed constant for a given circuit, increasing voltage reduces amperage. On the contras, decreasing voltage increases amperage. This is one reason why 6 volt, 12 volt, and 24 volt systems are not interchangeable; it completely changes the current flow through the circuit.

Automotive Batteries and CCA

When it comes to starting an engine, cold cranking amps (CCA) is likely the most important rating of an automotive battery. A large engine theoretically requires a battery with more CCA than a small one. Likewise, a high compression engine (such as a diesel) requires a battery with higher CCA than a low compression engine. The high CCA is favorable anytime a starting motor has relatively high torque requirements - voltage is related to the speed of an electric motor while torque is directly related to the amperage drawn by the electric motor. 750 to 850 CCA rated batteries are common in diesel applications.

Cold cranking amps refers to the maximum amperes that can be drawn from a battery at 32° F for a duration of 30 seconds while maintaining a minimum 1.20 volts per battery cell ( a 12 volt battery has 6 cells at 2 volts per cell, translating to 7.2 volts total battery voltage). A cold environment is used for this rating because vehicles, especially those powered by diesel engines, typically require extended periods of cranking to start in cold weather. Note that cold cranking amp ratings have no direct relationship with the total storage capacity of a battery.

Deep Cycle Batteries

Unlike an automotive battery, a deep cycle battery is engineered to be drained and recharged frequently. An automotive battery undergoes a high current draw during starting, but this rarely last more than 10 seconds. Once the engine is started, the alternator charges and maintains the battery voltage. Thus, an automotive battery used in starting applications is rarely drained to any significant level of depletion.

Marine and RV applications are examples of instances where a battery may be significantly drained before it is recharged. A deep cycle battery is ideal for these applications as it can be discharged and recharged frequently without negatively impacting battery life. A battery has a limited number of times it can be discharged and recharged; a deep cycle battery has significantly more than a starting battery. The disadvantage of a deep cycle battery, which is of no importance in many applications where current draw is low, is that they generally have a much lower CCA rating.

Amp Hours (Ampere Hours)

With the science lesson out of the way, let's turn to the concept of amp hours. If you are shopping for batteries for a travel trailer, dump trailer, etc, amp hours is the most important rating of a battery. You are searching for the highest amp-hour rated battery available. "Amps" refers to nothing more than the instantaneous current draw of a load (motor, light, etc) completely independent of time. Amp-hours refers to the total storage capacity of a battery; how many amps can be drawn for a period of 1 hour before the battery is completely drained. For example, consider that a light in a travel trailer draws 1 amp from a 200 amp-hour battery. The light will last 200 hours before the battery is completely drained. How about a water pump that draws 5 amps? The water pump would run continuously for 40 hours before completely depleting the battery.

A greater amp-hour rating results in greater energy storage capacity. For any trailer that requires it's own power supply, the most common of which would be travel trailers/RVs, amp-hours is the single most important factor. Cold cranking amps (CCA) is an important considerations in automotive batteries (especially diesel engines) but are of no importance in applications that require the characteristics of a deep cycle battery - low current flow over an extended period of time.

6 volt batteries are available with much high amp-hour ratings across the board. 6 volts are commonly used in golf carts, which require a relatively high amp-hour rating and are the perfect match for travel trailer applications. We'll compare popular batteries on the market in the section below. The biggest, and likely only disadvantages of a 6 volt battery for such applications is that they require more time to fully charge and two batteries are required for a 12 volt system.

Battery Reserve Capacity

A battery manufacturer may use the term "reserve capacity" (RC) instead of providing the amp-hour rating of a battery. Reserve capacity can be converted to amp-hours by multiplying the RC by 0.4167. The resulting amp-hour rating implies that the battery is being depleted at a rate of 25 amps (1 hr = 25 Ah, 2 hrs = 50 Ah, etc).

Parallel vs Series

There are only two ways to wire multiple batteries - in series or in parallel. Connecting batteries in parallel doubles amp-hours while voltage remains constant (two 12 volt batteries in parallel produce double the amp-hours but remains a 12 volt output). Connecting batteries in series doubles voltage while amp-hours remain constant (two 6 volt batteries in series produce 12 volts but retain the same amp-hour rating of a single battery). You'll find that 12 volt batteries have significantly lower amp-hour ratings than 6 volt batteries on the market. In fact, two 12 volt deep cycle batteries connected in parallel will not come close to matching the amp-hour rating of select 6 volt batteries.

How to Wire Two Batteries in Parallel & Series

how to wire two 12v batteries in parallel

Wiring diagram for wiring two 12 volt batteries in parallel (remains 12v output)

To wire two 12 volt batteries in parallel, the negative terminal of the first battery is wired to the negative terminal of the second battery. The positive terminal of the first battery is wired to the positive terminal of the second battery. The output voltage of the system will remain unchanged, however you will achieve double the amp-hours (amp-hour rating of first battery + amp-hour rating of second battery).

how to wire two 6v batteries in series

Wiring diagram for two 6 volt batteries wired in series (achieves 12 volt output)

To wire two 6 volt batteries in the series, the positive terminal of the first battery is wired to the negative terminal of the second battery. The remaining negative terminal on the first battery and positive terminal on the second battery become your 12v negative (ground) and 12v positive outputs, respectively. The voltage of the system is doubled while the amp-hour rating remains that of a single battery (a pair of 200 Ah batteries will produce a total rating of 200 Ah once connected in series).

Battery Types - AGM, Gel, and Flooded

The three most common types of automotive and deep cycle batteries are flooded lead acid, gel, and AGM.

Flooded lead acid - A flooded lead acid battery is a serviceable battery consisting of lead plates (the electrodes) suspended in a diluted solution of sulfuric acid (the electrolyte). The electrolyte level is checked by removing caps at the top of the battery. Through the openings, the lead cells and electrolyte are visible. Distilled water is added to the batteries as necessary to maintain the electrolyte level. A flooded battery is considered a "wet cell" battery since the electrolyte is in liquid form.

AGM (absorbent glass mat) - An non-serviceable battery in which the battery electrolyte is stored in finely woven glass mats. The advantage of AGM batteries is that they do not self-discharge to the same extent as flooded batteries. Additionally, the lack of liquid in the battery make them suitable for applications with excessive vibration and/or vehicle movement (rock crawling, marine, etc).

Gel - An non-serviceable battery with similar characteristics to that of an AGM battery. Instead of storing its electrolyte in a vessel within the battery, sulfuric acid is mixed with silica to create a gel. This reduces many of the problems found in flooded batteries, such as electrolyte depletion, spilling, and corrosion of the lead cells. Gel batteries are resistant to vibration and do not not have to be mounted upright.

Battery Comparison

Price EA*

Voltage

Battery Type

Amp-hour Rating
(single battery)

Amp-hour Rating
(two batteries)

Total Cost
(less core)

Price per AH

$248.99

12

AGM

50 AH

100 AH

497.98

$4.98

$298.99

12

AGM

66 AH

132 AH

597.98

$4.53

$330.99

12

AGM

75 AH

150 AH

661.98

$4.41

$254.95

12

AGM

52 AH

104 AH

509.90

$4.90

$96.95

12

flooded

42 AH

84 AH

193.90

$2.31

$142.95

6

flooded

160 AH

160 AH

285.90

$1.79

$153.95

6

flooded

185 AH

185 AH

307.90

$1.66

$171.95

6

flooded

215 AH

215 AH

343.90

$1.60

$161.39

12

flooded

85 AH

170 AH

322.78

$1.89

$219.97

12

gel

77 AH

154 AH

439.94

$2.85

$165.60

6

flooded

225 AH

225 AH

331.20

$1.47

$209.48

6

flooded

260 AH

260 AH

418.96

$1.61

*Manufacturer names and model numbers have been omitted to avoid discrimination. Actual price will vary by location. Listed price is price (less tax, shipping, and core charge) at time of publishing from various dealers or MSRP. Non-standard size batteries that are not practical for trailer applications have been omitted. This list is by no means comprehensive.

There are obviously hundreds of battery options on the market and the list above represents the contenders resulting from our own research and is not a comprehensive comparison of the automotive/deep cycle battery marketplace. In any case, the best value and the highest amp-hour rated batteries from our list are both 6 volt, flooded lead-acid deep cycle batteries. The worst value and lowest value were both 12 volt batteries; the 6 volts displayed far superior amp-hour ratings across the board. For RV applications, nothing is more important than a battery's amp-hour rating as more amp-hours translates into longer usage before recharging is required.

Battery Life

A battery will not last forever and there are many factors that influence a battery's life. On average, we find that a deep cycle battery in RV and marine applications will yield a 2 - 3 year lifespan and is entirely dependent on storage and usage. Some important factors that play a role in battery life include:

Battery storage - Ideally, a battery should be stored in a cool, dry place. A battery that is sitting in a black box on the tongue of a trailer is likely to have a much shorter lifespan than a battery that is stored on a shelf in a covered enclosure. Parking a trailer in the shade or covering the battery boxes to reduce exposure to the sun may help prolong battery life.

Overcharging - Overcharging your batteries, or "cooking" them as we call it, will quickly kill their service life. Batteries in storage should be charged at a low charging rate periodically to maintain a full charge, but should NEVER be left on a charger. This includes leaving a solar panel connected to the batteries or leaving a trailer/RV plugged into a 120v source for extended periods of time.

Battery service - Flooded lead acid batteries are serviceable. The electrolyte level should periodically be checked and adjusted as necessary to maximum both battery performance (capacity) and battery life. A battery that is not serviced will need to be replaced much more frequently than one that is. Once the cells are exposed to air, i.e. not submerged in acid, they will rapidly corrode. In addition, a battery should never be allowed to be completely discharged. Batteries in storage should be checked and the voltage maintained frequently. A battery WILL discharge even without an electrical draw, i.e. not connected to anything.

Use the correct charger - Certain AGM and gel batteries may require a digital charger with the capability to recharge such battery types. Generally speaking, an analog battery charger should not be used with these batteries. Refer to the manufacturer's guidelines for recharging procedures and equipment types.

Battery voltage has absolutely no effect on the service life of a battery, thus there is no benefit in terms of longevity in selecting two 6 volt batteries over a single or even pair of 12 volt batteries. Don't be afraid to disconnect batteries that are being stored, especially if there is a draw on the batteries that cannot to turned off (smoke detect, carbon monoxide detector, for example). Lastly, be sure to maintain the voltage of stored batteries; just because there is not a current draw does not mean that the battery charge won't deplete over long periods of storage.