12 volt rechargeable batteries

Batteries come in many shapes and sizes — and types. They can be made from various materials, and that combination of materials is what we call a battery’s chemistry. And batteries with a particular chemistry may also be constructed in different ways that affect their performance.

Some chemistries allow the battery to be recharged; we’ll look at a few of those most suitable for typical 12 volt recreational vehicle (RV – motorhomes, campervans, caravans, 4WDs, camper trailers, etc.) usage.

A battery consists of one or more cells. The cell’s chemistry dictates its nominal voltage – the voltage that a charged cell will supply when lightly loaded. The cells are usually connected in series, their voltages adding to produce the overall battery voltage.

For example, lead-acid batteries have a nominal cell voltage of 2.1 V, and six cells are connected in series to give a nominal battery voltage of 12.6 V. This is why lead-acid batteries are always a multiple of 2 volts – a 6 volt battery consists of three cells, a 12 volt battery has six cells, and a 24 volt battery has twelve cells. But you’ll never see a 1.5 V lead-acid battery, because each cell is 2 V and you can’t have three quarters of a cell.

Similarly, “lithium” (LiFePO4) batteries consist of 3.2 V cells. Four of them gives 12.8 V, which makes it feasible to use them in “12 volt” installations. And eight LiFePO4 cells produce 25.6 V, suitable for “24 volt” setups.

Lead-acid

Lead-acid batteries are the oldest type of rechargeable battery, having been invented in the 1800s.
Many improvements have been made since then, although the basic chemistry (lead reacting with sulphuric acid) remains unchanged.

Starting

This is the good old-fashioned car battery, consisting of lead plates sitting in a bath of sulphuric acid electrolyte, and are a type of flooded lead-acid (FLA) battery, also known as wet batteries.

Although modern car batteries are mostly “maintenance free” (no need to regularly top up the electrolyte), they remain very heavy and have a low energy density — they do not store much energy for their size.
So why do we still use them? Other than being low cost, they have the significant advantage of being able to supply the large cranking current (hundreds of amps) required to start an engine.

Lead-acid batteries may be permanently damaged if allowed to discharge too far, and regular deep discharge cycles will reduce their lifetime. How much discharge is “too far” depends on the battery’s construction – it’s one of the trade-offs that battery designers need to consider. Car batteries are designed to provide a short, sharp cranking current, and then be immediately recharged by the running engine, not for deep discharge.

Deep-cycle

In a typical RV setup it’s usually desirable to be able to more fully discharge the battery, especially when charging may be intermittent, whether charging while driving the vehicle and then stopping for some time, or charging from solar panels and maybe having to wait a few days before being able to charge again in cloudy conditions.

Batteries designed to be largely discharged between charging cycles are referred to as deep-cycle batteries.

“Largely discharged” is a little ambiguous. As a rule, deep-cycle lead-acid batteries are designed to withstand regular discharge cycles up to 50% (i.e. they are discharged down to 50% capacity before being charged again). Some deep-cycle batteries are designed to recover from discharges of up to 80%, but even so will have a longer lifetime if discharge cycles are limited to 50%. In general, the less a lead-acid battery is discharged, the longer it will last – and this rule is true even for deep-cycle batteries.

Although deep-cycle flooded lead-acid batteries are available, they are seldom used in RVs. There are a number of reasons for this, such as:

  • flooded batteries (even “maintenance free” ones) must remain upright
  • low vibration resistance due to the lead plate construction (although that doesn’t prevent their use in 4WDs)
  • potential for sulphuric acid fumes (not only bad for people, also affects nearby electronics such as chargers)
  • require regular maintenance to achieve a long life, due to electrolyte loss during deep cycling
  • high internal resistance, limiting charging rate

The last point is important if your battery is charged intermittently (by solar, for example); you want it to charge as quickly as possible to take best advantage of every charging opportunity.

Non flooded lead-acid batteries are usually referred to as valve regulated lead-acid (VRLA) or sealed lead-acid (SLA) batteries. They are referred to as “valve regulated” because they are sealed except for a one-way pressure release valve, required in case too much gas is produced, which can happen if the battery is overcharged.

Although “maintenance free” flooded lead-acid batteries are sometimes incorrectly referred to as VRLA batteries, most resources agree that there are two main types of VRLA battery: gel and AGM.

Gel

Gel lead-acid batteries are constructed in a similar way to flooded lead-acid batteries, except that instead of the electrolyte being liquid sulfuric acid, it’s thickened to form a gel.

Gel batteries can be mounted on their side. They doesn’t release hydrogen or sulfuric acid fumes during normal operation, so are suitable for a confined space. They do not require maintenance, are suitable for deep-cycle operation, and have a long life. They can also me made quite small (with a correspondingly lower capacity) and are suitable for applications such as backup power for alarm or lighting systems.

However, gel batteries are expensive, and do not perform well in low temperatures. They also typically have a high internal resistance. This means that they have a long recharge time (although usually not a long as for a deep-cycle flooded lead-acid batteries) and are less suitable for high current (high power) operation – they are better suited to lower current usage.

AGM

Absorbent glass mat (AGM) lead-acid batteries are a more recent invention than gel batteries.

In AGM batteries the lead plates are sandwiched between glass mats which hold the electrolyte.

Like gel batteries, AGM batteries can be mounted on their side, do not release fumes (unless overcharged), and can be designed for deep-cycle operation. But they also have some advantages over gel batteries:

  • less expensive
  • shorter recharge time
  • higher current and power delivery
  • better low temperature performance
  • more rugged, less affected by vibration

These advantages have meant that AGM batteries have largely taken over from gel batteries for high-capacity (100 Ah or more) deep-cycle applications, such as typical RV usage.

Care and feeding

To maximise the service life of any deep-cycle lead-acid battery, including AGM:

  • do not overcharge it (a smart charger will take care of this)
  • use appropriate charging voltages (specify the correct battery chemistry when using a smart charger)
  • do not use more than 50% of the rated capacity (always leave a minimum 50% charge)
  • keep it charged when not in use (use a smart or trickle charger to provide a “float” voltage)

Lithium

Lithium batteries are a relatively recent development, and that development is very much still ongoing.

Lithium-ion (or Li-ion) batteries are rechargeable. There are many types of lithium-ion battery, with different chemistries, construction methods, form factors and of course characteristics.

Among the best known are lithium-ion polymer (LiPo) batteries, commonly used in mobile phones.

All lithium-ion batteries have a high energy density (they can store a lot of energy in a small space) and are low weight, especially compared with lead-acid batteries, but of course the energy density depends on the battery type. As a general rule, the lithium-ion chemistries with the highest energy densities also present the highest safety risks and may require careful handling — damage can cause fire and even explosion.

Lithium-ion batteries (like all batteries) degrade over time, and this degradation also varies by battery chemistry.

These safety and lifetime considerations, in addition to cost, have meant that one lithium-ion chemistry has started to make significant inroads into the 12 volt deep cycle RV battery market: LiFePO4.

LiFePO4

Lithium iron phosphate (LiFePO4) batteries have become an appealing alternative to AGM deep-cycle lead-acid batteries for a few reasons:

  • lower weight
  • can be discharged by 80% or more of rated capacity (compared with only 50% for AGM) with little degradation, meaning that LiFePO4 batteries have 60% more usable capacity than equivalently rated AGM batteries
  • intrinsically safe
  • high peak current
  • fast charging
  • higher output voltage, which may allow lighting to shine brighter and fridges to run more efficiently
  • more charge cycles (longer lifetime) if used and charged correctly

The only real disadvantage of LiFePO4 compared with AGM is higher cost, especially when you consider that when replacing AGM batteries with lithium it is also necessary to replace any associated battery chargers, unless they already have a “lithium” setting.

However, the fact that LiFePO4 batteries have more usable capacity than AGM batteries of the same rating, and that they are likely to last longer than the AGM batteries, make the higher cost less of a consideration.

For example, if you have 2 x 100 Ah AGM batteries, your usable capacity is only 100 Ah in total, because you should always leave the AGM batteries at least 50% charged. By comparison, a single 100 Ah LiFePO4 battery has a usable capacity of 80 Ah, because it can be safely discharged to 20%. The usable capacity is only 25% less. So even if the lithium battery costs twice as much per battery, you might only need one battery and your overall cost is the same. Then given that the lithium battery is likely to last longer, you’d come out ahead over time. But you may still need to replace your battery chargers, so it may not be as simple as that.

Certainly LiFePO4 batteries are worth serious consideration for any new deep-cycle 12 volt installation.

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