Batteries

Batteries don’t create electric power, they store it, much like the gas tank in your car holds gasoline to power it. This means you can turn on lights or run devices like fans or a TV even when not plugged into a power source. The alternator in your tow vehicle or chassis, solar panels or the converter (charger) in your RV all provide DC current to charge them. Once the batteries are charged they can give that energy back and provide power when shoe or other power is not available.

The capacity of a battery is measured in amp-hours (Ah). A 100Ah battery can power a device that draws one amp of current for 100 hours, or one that draws 100 amps for 1 hour.   There are two dominant technologies for RV batteries, traditional lead-acid and lithium iron phosphate (LiFePO3). There are two places batteries are used in recreational vehicles, as part of the 12VDC house system and in the chassis electrical. Each has different needs that influence the design of the battery that will perform best in that application. Chassis batteries need to deliver large currents for short periods of time while cranking the engine and are pretty much the same as any car or truck battery. House batteries supply lower current over a longer time and are usually referred as deep cycle batteries since they are designed to be drawn down to a lower level of charge.  There are two basic chemistries for deep cycle batteries, traditional lead-acid and the newer lithium technology.

Lead-Acid Batteries

Lead acid batteries have been dominant in RVs and automobiles for decades. They are fairly inexpensive, and well understood. Lead-acid battery cells consist of parallel lead and lead dioxide plates with the area between them filled with an electrolyte made up of sulfuric acid and water. A chemical reaction between the plates and acid causes the plates to become positively and negatively charged and when an electrical circuit is connected electrons flow from the negative to positive plate powering the circuit. As the battery discharges lead sulfate begins to coat the plates and the concentration of acid in the electrolyte drops. When power is applied to the battery from the converter or the chassis’ alternator the reaction is reversed and the battery charges back up. There are three different types of lead acid batteries – flooded, AGM and Gel-cell.

Flooded Batteries

Flooded lead-acid batteries are the most common type of RV house battery mainly due to their low cost. They’ve been around for over 150 years but have a number of drawbacks that make them less than perfect:

• They require periodic monitoring and replacement of electrolyte water.
• As the battery discharges it becomes more susceptible to freezing. A flooded battery that’s 20% full will freeze at about 20°F, while a fully charged one will not freeze until temperatures get to -90°F.
• Lead-acid batteries self-discharge at a rate of around 3-15% per month depending on temperature (higher discharge at hotter temperatures), requiring a power source to maintain their charge.
• Lead-acid battery life is impacted by the depth of discharge. Experts recommend that you not go below 50% discharge in order to maximize longevity. This means a battery rated at 100Ah only has about 50Ah that is usable.
• Voltage drops steadily as the battery discharges.
• They’re heavy. A 12V deep cycle battery will weigh 60-70 pounds.
• They produce hydrogen gas when in use and must be kept outside of living spaces in a vented area to avoid potential explosions.

AGM Batteries

In an AGM (Absorbed Glass Mat) battery the electrolyte is absorbed in a glass mat (hence the name) that separates the plates in the cells. AGM batteries are designed to be sealed and not need water to be added to them as flooded batteries do. It also means that the battery can be mounted in any orientation without spilling. AGM batteries charge faster than flooded batteries but they also require a charger that is designed for them. They’re also more expensive than flooded batteries, coming in at around double the cost. Other advantages of AGM batteries over flooded include:

• No need to monitor or replace water – they’re maintenance free.
• Higher voltage output.
• Longer lifespans – up to two times the cycle life of flooded batteries.
• Slower self-discharge rates.
• Better resistance to heat and cold.
• Better vibration resistance.

AGM batteries share the weight, depth of discharge and requirement for a ventilated area disadvantages with flooded batteries.

Gel-Cell Batteries

In gel-cell batteries the sulfuric acid is mixed with a thickening agent to form a gel.  They don’t contain any liquid and therefore share the advantages of being maintenance-free and being able to be mounted in any position with AGM batteries. Gel-cell batteries require a battery charger specially designed for them and also tend to cost a bit more than their AGM counterpoints. The lack of clear advantages over AGM technology and higher price means that gel-cell batteries are not nearly as popular as AGM in RV applications.

Lithium Batteries

Lithium batteries for recreational vehicles use similar technology to the batteries you find in cell phones and computers. The dominant chemistry for RVs is lithium iron phosphate (LiFePO4) which is a safer safer technology than what is used in most electronics in electronics which has been known to cause fires under certain circumstances. LiFePO4 batteries  have a number of advantages over lead-acid batteries:

• Charge cycle life of 2,000 cycles vs. 500-1,000 for flooded or AGM.
• 100% usable capacity vs. 50% usable – You can get 100Ah out of a 100Ah battery.
• 2-3x faster charging rates compared to lead-acid batteries.
• Less than half the weight of an equivalent lead-acid battery.
• Higher voltage across the full range of charge than lead-acid batteries.
• Since they don’t outgas hydrogen like lead-acid batteries they can be located in the living area of an RV.

Disadvantages of Lithium batteries:

• Lithium batteries cannot be charged at temperatures much below freezing (but can provide electricity down to around 0°F. Charging at low temperatures will damage them and most newer lithium batteries will not allow charging below freezing.
• Initial cost is still higher than older technologies.

Which Battery Technology is best?

Five or six years ago when a 100Ah lithium battery would sell for over a thousand dollars vs. $150 for a 100Ah flooded battery it was harder to justify the additional cost of lithium, but now with 100Ah lithium batteries selling for around $250 the longer cycle life and better performance seem to make them the best choice for most people.

We have more posts on RV electrical systems:

Overview of RV Electrical Systems

120 Volt AC Power Explained