Vehicle Electrical Systems – Automotive Battery Basics

Battery Basics

Without patronizing you too much I’ll just say this about batteries.  They are an integral part to a vehicle’s electrical system.  They provide the necessary electrical power to turn the starter motor which in turn gets the motor’s alternator turning in order to provide the electrical power necessary to keep the vehicle running.  While the alternator and battery work in concert with each other, both are needed for two different reasons.  A vehicle won’t run very long on just a battery and an alternator won’t generate power unless the vehicle is already running.  A battery also acts as a power reservoir in the event the electrical demands of the vehicle exceed the power generated by the vehicle’s alternator – say for instance if you have a loud stereo or something like a winch.

In terms of pragmatics, a battery has a positive terminal (supply) and a negative terminal (ground) that tap into the vehicle’s electrical system.  The battery is directly connected to the starter motor, alternator, and fuse box on the positive side and then connected to the vehicle’s chassis and engine block on the ground side.  Quality cables are important for this which is why shortly after I got my Jeep I replaced all of the main battery cables as well as the battery terminal connections.

Battery Construction

There are three things we will be talking about as far as the construction of the battery. First, is whether or not it is sealed or vented.  Second, whether the electrolyte solution is a liquid, gel, or paste. Lastly, the type of metal and whether it is solid or mesh, and if it’s in plate form or wound in a spiral.

A standard generic automotive battery is traditionally a vented, liquid, solid plate style battery.  They are often referred to as a wet-cell lead-acid battery.  The plates are grouped in a batch known as a cell.  Each cell usually provides just over 2.1 volts and are grouped in series providing a total of 12.6 volts.  The major advantage of a wet-cell battery is they are cheap and easy to recycle.  It’s also very reliable under normal conditions.  However, in four-wheel-drive vehicles that see rough terrain, the vibrations and impacts found off-road can cause the floating plates to bang together.  These impacts cause the metal plates to break and potentially short out.  When this happens the nominal 12.6 volt battery drops down to a 10.5 volt battery.  This is sometimes called “dropping a cell.”  Once that happens the battery is useless.  Another downside to the wet-cell battery is that they are prone to leaking.  The liquid acid can poor out of the battery’s vent when the vehicle is off-camber.  Not a good idea.  Lastly, the off-gassing of the acid through the vent also means these types of batteries should NEVER be used inside a passenger compartment or inside a camper where people will be sleeping.

An easy upgrade for any four-wheel-drive vehicle is switching to something other than a wet-cell battery.  For a while there were batteries on the market that used a gel instead of liquid.  While this mitigated the chance of the liquid acid leaking out and allowed the battery to be sealed, it was still a plate style construction; and while more resistant to vibration and shock damage, they were still prone to dropping cells in four-wheel-drive vehicles.  The evolutionary path of battery construction brings us to the Absorbed Glass Matt Battery – or AGM for short (no pun intended).

AGM Battery’s big advantage: no liquid acid

The two biggest advantages of an AGM battery is that the electrolyte solution is in paste form.  There is also a layer of fiberglass mesh reinforcing the paste giving it battery holding power than the previous gel type batteries.  This fiberglass mesh also insulates the various components of cells, as well as the cells from each other, helping to mitigate internal shorts.  This makes AGM battery very resistant to vibration and impact damage.  AGM batteries come in either a traditional plate style (like the Odyssey brand AGM’s I use) or spiral wound (like the popular Optima brand). The only major advantage of plate style AGM batteries over their spiral wound counterparts is more surface area and volume which gives the battery a higher overall capacity and performance when comparing batteries of the same Group Size.

Alternative AGM construction is spiral wound.
Notice “gaps” between cells that lead to lower overall capacity.

Battery Performance

When it comes to the performance of a battery there are a lot of things to look at. One of the most common measures of battery strength is the cranking amperage of the battery. This is a measure of how much electrical energy the battery can supply for 30 seconds when under load. Basically we’re looking at how much amperage the battery can supply to the vehicle starter when the key is turned. Sometimes you might hear someone refer to the cold cranking amperage of a battery. The irony in this is that normal cranking amps is tested at freezing, 32 degrees Fahrenheit, and cold cranking amps are tested at zero degrees Fahrenheit. So both are, by relative terms, cold. Either way, when selecting a battery you want to make sure the minimum cranking amperage of the battery will suit the demands of your vehicle.

Another common factor to look at as far as battery performance is the standby time of the battery. This is usually listed in minutes. This is the amount of time a battery can supply 25 Amps before going bellow the minimum safe voltage of the battery. Standby time can also be listed another way and that is in Amp-Hours. This value can be a ratio of Amps per hour (so 100 Ah can be 100 amps for one hour, or 1 Amp for 100 hours; in theory).

So, for example, an average standard car battery is usually rated around 800 cranking amps at 32 degrees F; 640 cold cranking amps at 0 degrees F; and usually has about 100 minutes of standby time. So, in short, it can supply 25 amps for just over an hour and a half for a total of just over 41 amp-hours. This is why if you leave your headlights on it won’t take long to kill your battery. At an average of about 5 amps per headlight for a total of 10 amps, your battery will be effectively dead after just four hours. Now, does that mean your battery is dead-dead. Not exactly. That’s why if you leave your lights on they may still be on but dim and your battery cannot supply enough power to turn the starter resulting in either a sluggish attempt or the dread click-click-click of the relay telling you how much of an idiot you are for leaving your lights on – don’t worry, I’ve been there and done that more times than I’m willing to admit.

Battery Capacity

So if we measure batteries in terms of cranking amps for strength, does that mean standby time is the capacity of the battery? In a way, yes. However there’s something very important we need to talk about. Amps and Volts measure two very different things. Voltage is akin to the speed limit and Amperage is akin to the size of your fuel tank. You can go a little over or under the speed limit without much issue. You can also drive pretty far on a full tank of gas but once you’re out of gas it doesn’t matter what the speed limit is you’re not going anywhere. Now, unlink the fuel tank in our car there is a maximum and minimum safe voltage for our batteries.

Too much voltage in a battery effectively cooks the battery and bakes the electrolyte solution out of the battery. This is why older car batteries needed to be routinely topped off with distilled water because the number one enemy of a battery is heat. So for most conventional car batteries the upper threshold of a standard “12 volt” car battery is about 15 volts. That’s why if you look at your vehicle’s volt meter, if so equipped, it usually reads around 14.4 volts. This is enough power to run the vehicle as well as recharge the battery.

This chart shows the CCA capacity and Ah ratings for various Group 34 Batteries

A standard car battery is considered “dead” when its voltage drops to around 10 volts. Unlike a tank of fuel that is empty when it’s void of all fuel, a battery needs some residual power in order to be recharged. This is the minimum safe voltage necessary to recharge a battery. If a standard automotive car battery is every fully depleted down to zero volts it is rendered dead-dead and cannot, under most circumstances, be recharged. Car batteries are not like cell phones or basic rechargeable batteries we put in our flashlights. Even then, rechargeable batteries usually have a number of cycles they can be charged and depleted before they can no longer be recharged. They also, over time as a result of repeated usage cycles, lose capacity and lessen their performance. This is why the usage time of your cellphone is a lot shorter when its three or four years old compared to when it was new. In that regard they aren’t that different from automotive batteries except that the minimum voltage isn’t zero, it’s 10.

So, in summary, the safe working range of the standard automotive car battery is between 10 and 15 volts with a standard operating voltage of 12’ish volts. Most common automotive batteries are considered fully charged at 12.6 volts. This range between fully charge at 12.6 volts and 10 volts is the depth of discharge of a battery. No is where things are going to get a little weird.

Battery Depth of Discharge

Remember when I said the standby time of a battery can be measured in Amp-hours (unit of measure is Ah)? Well you can only safely use a portion of that relating to the depth of discharge rating. This is a percentage of its cycle between fully charged at around 12.6 volts and effectively discharged at around 10 volts. The depth of discharge is the number of amps that are usable between those two voltages.

So, for example, if we have a 100 Ah battery with a 10% depth of discharge, we can only use 10 amps of energy before we deplete the battery to its safe minimum voltage. Most standard automotive car batteries intended for a starting, lighting, and ignition (SLI) role have a relatively shallow depth of discharge in order to maintain that minimum safe voltage which will allow the battery to be recharged. SLI batteries usually only have about a 10-20 percent depth of discharge rating in order to maximize the cranking amperage strength of the battery as well as maximize the number of cycles the battery can handle. So if a battery is rated at 120 Ah and only has a 20 percent depth of discharge, then you can only use 24 amps of power before that battery needs to be recharge. That brings us back around to the standby time rating of the battery that we mentioned above.

Alternatively, there are more expensive and robust batteries that are known as deep-cycle batteries. In contrast to a SLI battery a deep cycle battery can usually handle anywhere from 70 to 80 percent depth of discharge. Both the chemistry and construction of a deep cycle battery allows for this greater energy capacity. The trade off is that deep cycle batteries take longer to charge and are much more fickle as far as how they are charge. This is why a true deep cycle battery isn’t well suited for under the hood of a vehicle, especially in short drive cycles where the vehicle’s alternator may not supply voltage long enough to fully charge the battery. They also require a higher charge voltage, closer to 15 volts, than most car alternators put out, which is around 14.4 volts. Also, given the fluctuations of voltage output of a vehicle’s alternator (more on that in a future article) the “dirty” power coming out of the alternator usually shortens the lifespan of the battery. That’s why people who try and install a marine grade deep cycle battery in their vehicle usually complain that it’s dead after two or three years. The vehicle’s alternator just can’t keep up. Which is where the hybrid start/deep cycle battery comes in.

The battery under the hood of my Jeep is an AGM dual-duty deep cycle starting battery made by Odyssey Batteries. Unlike a standard SLI automotive battery which is only 20% depth of discharge or a standard deep-cycle battery rated at 80% depth of discharge, my battery is rated for about 50% depth of discharge. So if a hybrid battery is rated at 120 Ah, roughly 60 Amps of power before can be safely used before the battery needs to be recharged. It’s a compromise to be sure, but one I’m willing to make. This battery gives me the rugged vibration and impact resistance of an AGM battery, but also gives me a deeper depth of discharge over a standard battery giving me some extra juice when I need it for something like winching, leaving my lights on around camp at night, or when I’m parked at the drive-in movies for a few hours. The only major trade-offs are cost (these things aren’t cheap), weight (they aren’t light), and lifespan (every battery will die eventually).

Special Note: Rated standby times for batteries, listed in minutes, (usually) factor in depth of discharge. Amp-Hour ratings (usually) do not. Sadly there is little accountability in advertising and some companies will be a little gimmicky with their advertised ratings, so always read the full description when calculating Amp-Hour ratings.This is something we will cover more in more detail in a future article on multiple battery systems.

Battery Lifespan

There are a ton of variables to consider when measuring the lifespan of a battery. As previously mentioned temperature extremes are the number one enemy of all batteries. Too hot and the electrolytes bake and the battery dries out. Too cold and they freeze causing internal damage. In either case the battery can swell causing external damage. We also covered the number two enemy of all batteries and that was vibration and impact damage. Yes, AGM batteries are more resistant to such damage, but it doesn’t mean they are indistructable. Bounce one around enough times and you can still cause internal damage. The last remaining killer of a battery is using it. Yup. No mater what using a battery does shorten its life. Eventually the battery will hit the end of its life and can no longer handle the charge-discharge cycle. For some batteries like an AGM, this is a slow gradual decline in performance. For a wet-cell battery this could be a catastrophic failure where it decides to just no longer work.

The average lifespan of a wet cell automotive battery is around four to six years. Although in most cases you can expect three to five in something that is four-wheel-drive if you take care of it. The average lifespan of a hybrid start/deep-cycle battery like I have depends a lot on how well it’s cared for. If you just drop one under the hood and leave it, you may only get two years out of the battery. However, if you charge and condition the battery on a regular basis and don’t overly discharge it, and supply it with good voltage when charging it, it may last 6 or even 7 years if you’re lucky. For instance, I put my Odyssey Battery under the hood of my Jeep in 2017. We’re knocking on the door of five years with it. Now, it did sit dormant for a while (about 2 years) which didn’t do it any favors because the battery was still aging even though I wasn’t using it. Either way, I’ve managed to maintain good life and performance out of the battery because I routinely charge and condition the battery using the Odyssey 25 Amp charger. I like to do it at least once every few months and let it sit overnight so it gets into maintain mode. I also do this with the two Odyssey brand batteries in my trailer.

Battery Group Size

The last thing I’m going to mention about automotive batteries is the Group Size. You’ll often hear batteries referred to something like a “Group 34.” Think of this just like their smaller non-automotive counterparts in flashlights. Flashlight batteries come in AAA, AA, C, or D. This is the size and shape of the battery. My Jeep calls for a Group 34 battery, so when the time comes to replace it I want to make sure I replace it with another Group 34 battery. Just like you wouldn’t put AAA batteries into a D cell flashlight, or visa-versa because the battery just won’t fit and won’t do the job you’re asking it to do.

These only represent a few of the dozens of common Automotive Battery Group Sizes
Best practice is to replace a battery with the same Group Size

Now, when you’re looking at automotive batteries you might also see a alpha-code along with the Group Size. These letters refer to the post configuration of the battery. An “R” means it’s “reverse.” Don’t ask me why automotive engineers do shit like this but they do. There are also staggered post, front post, side post, staggered-reverse, front-reverse, and side-reverse. There are also batteries that are dual-post. For instance the ones in my Jeep are a Group 34/78 type which has the top posts of a Group 34 battery and the front posts of a Group 78 battery. The overall size and shape of the two groups are identical and the only difference is wear the posts are. Some battery manufactures will make dual-group batteries to save on production. Rather than make two separate batteries of the same size and shape, they just offer one battery with both post configurations. In something as common as a Group 34 battery which is found in a TON of cars, trucks, and SUVs that’s not a bad idea for the battery manufactures or the automotive engineers. I just wish they would stop doing reverse terminals (although they do have their uses – which will get to in another article when we talk about multiple battery systems).


I know this is a ton of information to throw at you all at once, but hopefully it makes sense and you’re able to make heads and tails of batteries. If you take nothing away from this article other than knowing you should, at the very least, upgrade to an AGM style battery under the hood of your overland vehicle then my job is done.

Reasons to Upgrade to an AGM Battery

  • Sealed “maintenance free” construction eliminating potential for liquid acid leaking out
  • Paste & fiberglass Mesh construction mitigates internal damage from vibration and impacts
  • Lack of liquid acid increases temperature tolerance range to -40*F to 140*F
  • Longer service life of 3-10 years depending on care and usage
  • Faster Recharge: from depleation to fully charged in 4-6 hours depending on Group Size

In future articles we will cover alternators and power distribution systems as well as multiple battery banks like dual battery systems and house batteries. As always if you have any questions don’t hesitate to ask!

I would like to give a shoutout to ECOA Corporate Partner Odyssey Battery for their support of our Mission to Educate, Encourage, and Inspire. I have run their batteries in both the Jeep and the trailer for roughly five years now with zero regrets and zero problems. Key factor in their success has been the 12 Amp charger that I use to test them and keep them in tip-top shape.