Batteries & Charging: A Primer

For some, electricity and the workings of things electrical is a mystery. The purpose of this primer is to help you understand some simple principles that relate to lead acid (LA) batteries used in RVs.

By the end of this article, I hope you will understand a little more about electricity and how to charge your RV batteries. I will try and present this information by relating electrical terms into something that may be a little easier to understand—water.

I'm going to keep this article simple and avoid getting into too much depth, because complex details can confuse and that will defeat the purpose of this article.

Understanding Electrical Terms (The Water Analogy)

Voltage, current, and capacity can easily be represented by things associated with water, its flow, and storage in a container. Here is how the terms used for both relate:

Battery capacity can be stated in several terms, but for deep cycle batteries usually associated with RV usage, Amp. Hrs.is the normal standard. This relates to liquids by stating a container's capacity (think of it in gallons).

Voltageor pressure is the term used to describe the force to move either electrons or a fluid. Pressure for fluids can be thought of as height differences in addition to that supplied via pumps.

Wire gaugeor hose/tubing diameter determines flow through the medium and is affected by voltage/pressure and length. A larger diameter has less resistance and the easier the flow, but as distance increases, so does resistance and this reduces flow.

To describe power, we need to use two terms together. For electricity, we use Voltage and Amperage. For liquids, it's Pressure and Flow Rate. In electrical terms, power is defined by Watts (Voltage x Amperage).

How Long Should You Charge an RV Battery?

We can read a lot of information on batteries and charging them, and we always find discrepancies between the articles. These discrepancies can be due to inaccurate information, focusing on different aspects of a complex issue, or commercial bias slanted to favor a manufacturer's product.

In reading technical manuals from lead-acid battery manufacturers, several things were common to all. Lead-acid batteries prefer to be charged by a regulated constant voltage power source, and the voltage should range between 2.2 and 2.4 volts per cell @ 70° F.

The length of time a battery is being charged is the major factor in setting the voltage level. The voltage setting increases as the charge time decreases. For example, if you measure the voltage of your car battery when the motor is running, you are likely to see about 14.25 volts. This is very close to 2.4 volts/cell because charging time is relatively short and cyclic.

For usage in RVs, the charge time can vary considerably—from overnight to a month or more. For our single-stage chargers standard in most older RVs, we want the voltage high enough to charge as quick as possible, but not so high it will damage the battery if left charging for extended periods.

I've been an advocate of the middle ground: 2.3 V/cell. For a 12V battery, this is 13.8V. Set much higher than this, and if the charger is left on for days or weeks, it will overcharge the battery and "boil" it dry. The lower we set the voltage, the longer it will take to fully charge.

The "Equilibrium" of Battery Charging

I'm sure all have read statements talking about charging and when nearly charged, a good charger will cut back on the current. While the current flow does drop, it is not because of something a basic constant-voltage charger does directly.

Let's think of this like siphoning water between two containers. The magic word here is "equilibrium." Nature likes to be in balance; when it isn't, forces try to move things towards equilibrium. We've all siphoned a liquid, and the flow is always greatest at the beginning of the transfer.

It decreases as the levels in both containers become closer, and STOPS when the levels are equal. This is what is happening while charging a lead-acid battery from a constant voltage source.

If our supplying source is a charger, the capacity can be thought of as infinite. The only thing to prevent overcharging our battery is if the battery can reach the voltage of the source. If our charger is set for 13.8V, the battery can rise to this level without overcharging. For charging, the only thing that controls current flow is the voltage/pressure difference.

Can You Mix and Match RV Batteries?

Some of us have more than one battery to supply power to our coach. I'm sure you have read comments that multiple batteries should be of the same age and manufacturer. While this is the best situation, it doesn't mean that batteries of different ages, manufacturers, or even capacities can't be used with success.

If you mix them, you are likely going to sacrifice some life and/or maximum useful capacity. The worst thing that will happen is one battery will fail early and, if that failure is a conductive state, it will draw down the capacity of the remaining battery.

Going back again to the water analogy: if you have two containers, one a 1-gallon can and the other a 10-gallon can, and they were connected together, what would you observe if you started to drain them? Would the 1-gallon can be emptied before the 10-gallon can? No, the level and capacity drop together.

Tips for RV Converter/Chargers

Most older RVs use a single-stage charger with a regulated output. In older converter/chargers (like the typical ferro-resonant Magnetek), the current output for charging is quite small, being about 3-5 Amps.

Newer units have solid-state designs where the full output is available to run the coach or charge batteries, usually in the range of 30-45 Amps. By their nature, these devices have regulated outputs likely set at the factory very close to 13.8V.

LA batteries don't like to be left in a discharged condition. It is important to measure your charger's output voltage when the battery is almost fully charged. When the battery is low, it will pull down the charger's output voltage and you will get a bad reading.

A fully charged 12V battery will read very close to 12.7V if it has been at rest (no current flow in either direction) for 6 hours. If the charger voltage is properly set, you can float charge your batteries for long periods of time without needing to add water.