Car Battery Terminals Crossed – What Happens If You Put The Battery Cables…

How to Wire 12V Batteries in Series Parallel (w/ Photos!)

Just so you know, this page contains affiliate links. If you make a purchase after clicking on one, at no extra cost to you I may earn a small commission.

In this tutorial, I’ll show you step-by-step how to wire batteries in series and parallel, as well as how to combine the two to create series-parallel combinations. I’ll also cover when to use series or parallel wiring.

Click on a wiring method to jump to its instructions:

How to Wire Batteries in Series

Wiring batteries in series sums their voltages and keeps their amp hours the same. It’s particularly useful for wiring two 6V lead acid batteries, or four 3.2V lithium cells, to make a 12V battery.

Series connections can also be used to wire multiple 12V lead acid or lithium batteries together to make a 24V, 36V, or 48V battery bank, which is useful in DIY and off-grid solar applications.

Parts Tools

  • 2 identical batteries — I’ll be using Chins 12V 100Ah LiFePO4 Batteries
  • 1 battery cables — the number of cables you need is the 1 less than the number of batteries you’re wiring in series
  • Screwdriver or ratchet — for tightening bolts
  • Optional: Multimeter — for checking battery bank voltage

Step 1: Connect the Negative Terminal of One Battery to the Positive Terminal of Another

Connect the battery cable to the negative terminal of one battery. To do so, use a ratchet or screwdriver to unscrew the terminal’s bolt. Thread the cable’s ring terminal through the bolt, then screw the bolt back on the terminal.

Note: Some people prefer to use black cables for series connections, others prefer red. I prefer black, but there’s no right or wrong choice. Just use the color of cable you prefer or have on hand.

Connect the other end of the battery cable to the positive terminal of another battery.

That’s it! Your batteries are now wired in series. You’ll often hear connected batteries referred to as a “string” of batteries. So now you have a series string of 2 batteries.

If you want, check your battery bank’s voltage with a multimeter. Because I wired two 12V batteries in series, I expect to measure a voltage of around 24 volts. (In reality, a 12V LiFePO4 battery’s resting voltage will usually be closer to 13-13.5 volts, so I’d expect a voltage of around 26-27 volts.) I got 26.4 volts, which is exactly in line with expectations.

Check! My two 12V 100Ah batteries are now wired in series, resulting in a 24V 100Ah battery bank.

Note: If you don’t want to bother with wiring batteries yourself, many brands offer pre-made 24V batteries and 48V batteries.

Step 2: Repeat as Needed

If your battery allows it, you can repeat the above steps to connect more batteries in series.

You can wire three 12V batteries in series to create a 36V battery bank. Once again, just connect the negative terminal of your 2-battery series string to the positive terminal of the third battery.

And, once again, you can use a multimeter to check that the voltage is around 36 volts. I got 39.7 volts, so I know my 3 batteries are correctly connected in series.

You can wire a fourth battery in series following the same steps. My batteries can handle up to 4 wired in series, so let’s do one last one for good measure.

And we’ll check the battery bank’s voltage with a multimeter, expecting a voltage of around 48 volts. I got 52.9 volts, so we’re good to go.

Done! Like I said, you can wire as many in series as your batteries allow for. LiFePO4 batteries are often limited to 4 batteries in series to protect the BMS. However, there are some that can’t be wired in series, such as the Renogy 12V 100Ah Smart Lithium Iron Phosphate Battery. Be sure to check!

How to Wire Batteries in Parallel

Wiring batteries in parallel sums their amp hour capacities and current limits and keeps their voltage the same.

Parallel wiring is useful when you want to keep your battery voltage the same — such as when you’re powering 12V devices directly off a 12V battery — while increasing runtime and current limits.

Parts Tools

  • 2 identical batteries
  • 2 battery cables — for 2 batteries you need 2, for 3 batteries you need 4, for 4 batteries you need 6. Formula: 2 (x – 1), where x is the number of batteries.
  • Screwdriver or ratchet — for tightening bolts

Step 1: Connect the Positive Terminal of One Battery to the Positive Terminal of Another

Use a battery cable to connect the two batteries’ positive terminals together. I recommend using a red battery cable for this connection.

Step 2: Connect the Negative Terminal of the First Battery to the Negative Terminal of the Other

Use a second battery cable to connect the two batteries’ negative terminals together. I recommend using a black battery cable for this connection.

Your 2 batteries are now wired in parallel. This is what people mean when they say you wire batteries in parallel by connecting positive to positive and negative to negative.

In this example, I wired two 12V 100Ah batteries in parallel to get a 12V 200Ah battery bank. Because parallel connections don’t affect voltage, there’s no way to use a multimeter to check the connection.

If you want, you can do a capacity test. That requires some extra equipment, though, so I won’t cover that here.

Note: If you don’t want to wire batteries in parallel yourself, many battery brands also sell 12V batteries in 200Ah, 300Ah, and 400Ah sizes.

Step 3: Repeat as Needed

If your batteries allow it, you can repeat the above steps to connect even more batteries in parallel.

battery, terminals, crossed, happens, cables

To connect a third, again wire positive to positive and negative to negative. This results for me in a 12V 300Ah battery bank.

To connect a fourth, repeat the connections. Now I have a 12V 400Ah battery bank.

How to Wire Batteries in Series-Parallel

You can use a combination of series and parallel connections to make a battery bank with your desired voltage and capacity. There are many different series-parallel wiring configurations you can choose from. I’ll cover the simplest in this tutorial.

Series-parallel wiring can get confusing. It pays to research and ask around for help on online forums if you’re unsure how your specific setup should be wired.

Parts Tools

  • 4 identical batteries
  • 4 battery cables — the exact number you need will depend on your wiring configuration
  • Screwdriver or ratchet — for tightening bolts
  • Optional: Multimeter — for checking battery bank voltage

Step 1: Wire Your Batteries in Series Strings of Equal Length

Decide what voltage you want your battery bank to have. For this example, I’ll go with 24 volts. I’m using 12V batteries, so that means each of my series strings needs to be 2 batteries in length.

Wire your batteries in series strings of equal length. I have 4 batteries, so I wired them in 2 strings, each of which has 2 batteries wired in series.

If you want, check the voltage of each string with a multimeter. In this case, I’d expect mine to read something close to 24 volts.

Now I have two 24V 100Ah battery banks, and I can connect them in parallel to expand their amp hour capacity.

Step 2: Wire Your Series Strings in Parallel

Wire the 2 series strings in parallel by connecting positive to positive and negative to negative.

If you want, check the voltage of your finished battery bank with a multimeter. I wired two 24V 100Ah battery banks in parallel to get a 24V 200Ah battery bank, so I expect a voltage of around 24 volts. I got 26.4 volts, which is exactly as expected.

Done! You can use these principles to wire even more batteries into different series-parallel combinations.

Note: A shorthand that people use to describe a battery bank’s wiring configuration is to list the number of batteries wired in series followed by the letter “s” and then the number of batteries wired in parallel followed by the letter “p”. For instance, I just created a 2s2p battery bank. Some LiFePO4 batteries can be wired into as big as 4s4p configurations.

How Wiring in Batteries in Series Parallel Affects Voltage Capacity

Wiring batteries in series sums their voltages while keeping their amp hour capacity the same. Wiring two 12V 100Ah batteries in series gives you a 24V 100Ah battery bank.

Wiring batteries in parallel sums their amp hour capacities while keeping their voltage the same. Wiring two 12V 100Ah batteries in parallel gives you a 12V 200Ah battery bank.

Amp Hours vs Watt Hours

Amp hours (Ah) and milliamp hours (mAh) are commonly used to describe battery capacity. However, the total amount of energy a battery can deliver is best expressed in watt hours (Wh), which is equal to a battery’s amp hours times its voltage.

Formula: watt hours = amp hours × voltage

What this means is that, regardless of how you wire your batteries together, you’re increasing the total watt hours of your battery bank. A 24V 100Ah battery bank and a 12V 200Ah battery bank both have 2400 watt hours.

24V × 100Ah = 2400Wh 12V × 200Ah = 2400Wh

You can use our battery capacity calculator to calculate the amp hour or watt hour capacity of your battery given how many batteries you have wired in series and parallel.

When to Wire Batteries in Series vs Parallel


  • You want to save money on wiring and equipment such as solar charge controllers. Series wiring increases voltage which helps keep current (amperage) low. Wire and other electrical equipment get more expensive the higher their current ratings get. That being said, electrical equipment also has voltage ratings, so be careful not to exceed these, either. Voltage ratings tend to be higher, though, and you don’t as often need to worry about them in DC electrical systems.
  • You can raise voltage while still powering your devices. Often, you’ll want to power a device directly off the battery. For instance, you may want to connect 12V LED lights or a 12V inverter to your 12V battery. Devices have acceptable voltage ranges — 12V LED lights might be able to accept 11-15V, for instance — so wiring in series is best when you can raise battery voltage without exceeding the acceptable input voltages of your devices. If you’re connecting your batteries to a solar charge controller, also check that the charge controller is rated for the higher battery voltage.


  • You want to increase runtime while keeping your battery bank voltage the same. Parallel wiring increases amp hour capacity while keeping voltage the same, meaning your battery bank will last longer while still being able to power the same devices.
  • You want to increase your battery’s max charge and discharge rates. Batteries have recommended charge and discharge rates, which are based on their amp hour capacity. For instance, most 100Ah LiFePO4 batteries have a recommended max continuous charge rate of 50 amps, and most 100Ah lead acid batteries have a recommended max continuous charge rate of 30 amps. Because these rates are based on battery amp hours, you can increase them by adding more batteries in parallel. You can estimate charge time using our battery charge time calculator.
  • You want to lower your battery’s C-rate. How fast a battery charges and discharges can be expressed as something called a C-rate. Different types of batteries have different recommended C-rates for charging and discharging, and exceeding these can shorten your battery’s lifespan or affect how many amp hours the battery actually outputs. You can lower your battery’s C-rate by expanding your battery bank’s amp hour capacity while keeping charing and discharging currents the same.

Car Battery Terminals Crossed – What Happens If You Put The Battery Cables On Wrong Sides

Automotive batteries can discharge for a host of different reasons, maybe you left the electricals on without the engine running, maybe it’s a parasitic drain eating away your charge, or maybe it’s just that, you don’t drive your vehicle as much – that can do it too.

In any case, if you cant get your car up, jump-starting the battery is a tried and tested solution that you might consider.

battery, terminals, crossed, happens, cables

Jump starting a vehicle seems like an easy job, you just clamp the connectors and transfer the charge from the good battery to the dead one.

But as simple as it may seem, just the simple mistake of not connecting the terminals right can spell the end for your car battery.

Failing to recognize which is the positive and which negative is not a common occurrence, in fact, most people do actually have it right. But for the curiosity of the reader let’s consider what would happen if the terminals were reversed.

Risks Of Using Jumper Cables In Reverse

Car batteries are a bit more complex than they might appear. When jump-starting a car there are certain caveats to charging your vehicle as the transfer of energy from one battery to another is not as simple as it seems.

Connecting the battery terminals in reverse can cause serious damage to the battery itself, the electrical components, and even to yourself. Each terminal of a car battery uses 12V of current with positive and negative orientation. The cable on the positive terminal uses 12V while the one on the negative side uses.12V.

If the positives and negatives are switched, the battery will try to compensate and make the negative 12 volts into a positive charge resulting in a huge surge of power and an enormous amount of heat to be produced. Needless to say, it will not be tolerated well by the system or the components within and depending on certain factors will incur damage that is mostly going to be severe and irreparable.

Damage To Jumper Cables

With an instant surge of electricity, the first to face the wrath and the consequence of an experiment would the jumper cables. the extreme heat produced will quickly melt the insulators on the jumper cables, clamping them forever on the battery which will be next in line as the plastic top will melt and bend.

Damage To Car Battery

The battery will be warped as the acidic fluids inside will boil from the extreme heat produced in the reaction. Additionally, it may spill to damage parts in the vicinity and you might not be able to do anything about it.

Blown Fuse/Fusible Links

Whether you have an explosion or not depending on the other factors and the condition of the batteries, you will however surely find blown-up fuses and wires in your vehicle.

Damage To Alternator

The vehicle charging the bad battery might also suffer mechanical damage. The surge in power will have a negative effect on the alternator and may even cause irreparable harm.

Physical Damage

If the battery is in poor condition or has not been used in a while, jump-starting with cables reversed can worsen the situation. The bad battery can even explode and cause damage to the people around.

Other Things To Consider When Jump Starting

Firstly look in the owner’s manual for any precautions or know-hows before attempting to jump-start. Some cars may use lugs instead of clamps while some vehicle may require certain precautions as removing fuses or turning on defroster. Some newer cars do not allow jump-starting and can even void your warranty so be sure to read the warranty conditions as well.

Do not let the two vehicles touch

Turn both vehicles off before locating the battery

Make sure the terminals aren’t dirty and if they are clean them with a dry cloth.

Verify if the voltage is correct using a multimeter, if the voltages do not match it could spell serious problems for both the batteries.

Remove or turn off accessories and electricals such as headlights, radios, and turn signals as the surge in power will surely short them out

Make sure the vehicle are in neutral.

Check if the fluids aren’t frozen or the battery isn’t warped and bent out of shape. If it is, it could lead to an explosion.

Always start with the bad battery first as it does not have a charge and is safe to operate.

Don’t leave anything under the hood when starting the vehicle

After charging the battery, drive the car for 15-20 minutes to let the alternator fully charge the battery.

How To Connect Jumper Cables Properly

With the potential risk of physical harm and the enormous costs involved in replacing and reprogramming the ECUs, operating a car battery to jump-start without proper knowledge is not a good idea and can land you heavy repairs if you are not careful with the steps.

A car battery has two terminals namely positive and negative. Each terminal can be identified using its color and the sign it uses. Connecting each terminal firmly is extremely important as a loose connection can hinder the process and not effectively charge the battery.

Now with the basics out of the way, let’s get to the very important matter that requires you to be extremely careful and a little on guard.

In electronics, the positive terminal is always connected first in order to avoid a high voltage meet of the semiconductors. When connecting the jumper cables always remember to first clamp cables from the positive terminal of the battery, which is usually red in color and is marked by a positive sign, and only after properly connecting should you begin to connect the cables from the negative terminal, usually represented in black color with a minus (-) sign. Also, be wary of any metal object touching both terminals of the battery simultaneously.

To better understand the implications of not connecting the positive first, let us assume, one has connected the positive terminal first.

When the negative terminal is connected, the whole chassis including the metal bolts holding it in place become grounded, upon which placing the positive terminal, connects the spanner with any common ground resulting in some sparks, splashes in a best-case scenario, and a dead battery or even an explosion of the unit in a nightmarish one.

Connecting the positive terminal first is recommended to ensure the spanner is not able to cause any damage even if it has met the chassis.

On the contrary, it is recommended to start with the negative terminal while disconnecting the cables.

Can Get Electrocuted While Operating A Car Battery?

The current from a 12VDC system is not massive enough that it can harm you, in fact, one might not even be able to detect such low voltages as the current needs to overcome the resistance of and go through. To have any effect on you, the current from the battery will have to penetrate the skin in order to electrocute you.

High voltage sources of current independently are not dangerous, Although there are a few other things that you should look out for when dealing with your car battery such as an acid leak, sparks plug arcing, jumper cables.

How to Clean Battery Corrosion (and What Causes It)

Don’t be shocked if you lift the hood of your car and catch sight of some corrosion around your battery terminals. Battery corrosion is a normal part of battery life that can be caused by typical wear and tear. But just because it’s normal doesn’t mean you should ignore it. In fact, corroded battery terminals are a common cause of reduced battery life and electrical problems in vehicles.

Why? According to the Universal Technical Institute, “Corrosion on or around your battery’s surfaces can lead to increased resistance within the circuit, which can disrupt the electrical current.”

Not only will this shorten the lifespan of your battery, but it also can cause damage to the electrical systems within your vehicle. Keep reading for a step-by-step guide on how to clean battery corrosion.

What Is Battery Corrosion?

What is corrosion and how do you know if your battery has it? As your battery runs, the sulfuric acid releases hydrogen gas. The gas then mixes with the air around it. The chemical reaction that takes place as hydrogen gas collides with the air, moisture and salt causes corrosion.

Corrosion is fairly easy to spot: It’s a white, blue or greenish powder typically surrounding one of the battery terminals, posts or cables. It has a granular, powdery texture.

What Causes Battery Corrosion?

Car battery corrosion can happen for a number of reasons. In addition to the normal release of hydrogen gas, some of the most common causes of corrosion are:

  • Age. Car batteries typically have a lifespan of three to five years. They become more susceptible to corrosion as they reach their expiration date.
  • Overheating. Batteries that are overcharged or overheat due to higher temperatures are more likely to develop corrosion. That’s why corrosion risk is highest in the summer.
  • Leaking fluid. If your battery is cracked or damaged, battery acid can leak from the casing and cause corrosion around the battery terminals.

How to Clean Battery Corrosion: Step-by-Step

Step 1: Start with safety. The powdery buildup around your battery’s terminals is caustic and can damage your skin and eyes. Wear heavy-duty gloves and eye protection while handling battery corrosion, and immediately wash away any corrosive material that gets on skin or clothing.

Step 2: Disconnect the battery. Starting with the negative terminal, carefully release the cable from the battery. Safely position the cable away from the terminal, these things are built to deliver the cable directly to the battery and are susceptible to ‘slipping’ back into place. Next remove the positive terminal connection.

Pro Tip: Before disconnecting your battery, use a battery memory saver to save stored data and protect your car’s electrical system. Be sure to reference your vehicle’s owner’s manual for specific information on using a battery memory saver.

Step 3: Inspect the battery cables. Once the battery is disconnected, take a moment to inspect the cables. Is there fraying or corrosion where the cable connects to the terminal? Is the insulation dry or cracking? Damaged cables need to be replaced.

Step 4: Remove the battery from the vehicle. It’s possible to clean corrosion from a battery while it’s still in the vehicle, but the safest method for you, your battery and your vehicle is to remove it from the car and place it in a shallow bucket or pan to collect the corrosive material you’ll be washing away.

Step 5: Start cleaning. Now it’s time to neutralize and remove the car battery corrosion. Use a wire brush or scraper to remove any solid, powdery corrosion from around the terminals and dirt from the top of the battery casing. Brush the corrosion away and let it fall into the pan below.

Step 6: Neutralize. You have a couple of options to fully remove and neutralize the remaining corrosion:

  • Battery terminal cleaner is a commercially available product designed to clean and neutralize corrosion from your battery. It’s a spray-on solution that changes color as it reacts with corrosion.
  • Baking soda and warm water make for a good neutralizing solution to clean battery corrosion. Make sure to mix your solution, dip a rag and wipe corrosion away rather than dumping the solution over the battery top. This is to prevent solution from leaking into the battery cells and neutralizing the sulfuric acid inside.

Pro Tip: Don’t forget to clean the terminal ends that connect your battery to the cables. You can dip the ends in baking soda and water solution, or use a commercial battery terminal cleaner.

Step 7: Dry and polish. Using a microfiber cloth, dry the battery casing, posts and terminals. Use a terminal cleaning brush to remove any debris or coating from the terminals that may interfere with the connection.

Step 8: Replace and reconnect. Return the battery to its tray inside your engine and reconnect the terminals. This time, start by securely attaching the positive terminal to the cable, then finish with the negative terminal. Replace the battery hold downs.

How to Prevent Battery Terminal Corrosion

While corrosion is a normal occurrence, there are steps you can take to prevent or slow it.

Protect. After a thorough cleaning, coat your battery terminals with dielectric grease or battery terminal protector. Apply a healthy coat to prevent corrosion in the future.

Avoid under or overcharging. If you notice corrosion on your battery’s positive terminal, it’s a sign that your battery may be overcharging, which can be due to a faulty voltage regulator.

Corrosion that appears on the negative battery terminal is a symptom of undercharging. This can happen if you’re taking short drives and your electronic system is drawing a significant amount of battery power for onboard electronics.

In either case, it’s a good idea to bring your vehicle in on a regular basis to check for electrical faults.Routine maintenance on all systems — including your car’s electricals — is important for the health and longevity of your vehicle. A trusted technician can help keep your car on the road for years to come.

This content is for educational purposes only, please reference your product manual for specific information.

How-to Wire Two 12-Volt Batteries to Make 12 or 24 Volts

See all 3 photos 3 photos Ron Rollings

Depending on How They’re Wired, Two 12-Volt Batteries Yield a 12-Volt System with Double the Amps or a Efficient 24-Volt System with Twice the Cranking Speed

Cars, trucks, RVs, and motorhomes run dual 12-volt batteries for various reasons. Depending on how you wire a two-battery 12-volt system, the result can be a 12-volt system or a 24-volt system—or even both 12 volts and 24 volts.

Dual-battery use examples include:

Race car ballast: Two trunk-mounted batteries can supply critical ballast, especially in classes where the rules prohibit dedicated ballast.

Drag-only car without an alternator: If you’re not running an alternator but using a modern high-output ignition system and other current-hungry devices (such as an electric water pump, an electric fuel pump, trans-brakes, nitrous solenoids, and delay boxes) in a drag car, one battery can be dedicated to just the ignition system, while the other feeds the remaining current consumers.

See all 3 photos 3 photos A battery isolator separates the auxiliary battery from the primary battery, ensuring extra loads will not drain the battery. Powermaster PN 194, shown here, has a 200-amp rating and (as of June 2020) goes for 143.99 at Summit Racing.

High-end show cars with high-zoot sound systems: A separate battery dedicated just for the sound system and isolated from the rest of the electrical system might be needed if the car sits in a parking lot with the speakers blasting out the big vibes for an extended time period. A setup like this would include two batteries, a mechanical marine dual-battery selector switch, and a battery isolator. This lets the alternator recharge both the main battery and the auxiliary battery when the car is running, but when shut down, the auxiliary battery used to power the sound-system discharges.

Hard cranking with high compression and lots of advance: Running multiple batteries in parallel generates more cold-cranking amps during crank, though voltage is still 12 volts. Today’s performance batteries and starters are so efficient that this isn’t usually an issue; before you do anything, first check for excessive voltage drop or bad grounds on the starter cables, not a weak battery or starter.

See all 3 photos 3 photos In basic form, here is the difference between wiring two 12-volt batteries in series versus in parallel. The parallel circuit still generates 12 volts but doubles the amperage output. The series circuit yields a 24-volt system, but the amperage does not change. 24 volts to the starter and solenoid makes it crank twice as fast as 12 volts.

Ultimate cranking power: When even two 12-volt batteries in parallel can’t get the job done, or if you’re the AAA emergency service vehicle that must, by hook or by crook, start anything, it’s time to juice 24 volts by wiring the batteries in series. Connect the positive terminal of one battery to the negative terminal of the other battery (see illustration). This supplies more cranking power than even two 12-volt batteries wired in parallel. Intermittent-duty starters can handle 24-volts, well, intermittently.

Full-time 24volts: If it’s good enough for a jet fighter, it’s good enough for my car. Besides, I just drool over all those trick parts at the surplus store. Trouble is, a full 24-volt conversion in a car may be impractical in the real world. Constant-duty car accessories won’t withstand 24 volts for long (if at all), and 24-volt substitute equivalents for everyday car parts might not be practical or available. (But if two 12-volt batteries are wired to yield a 24-volt system, you can still use a 12-volt alternator to charge them.)

Dual 12/24-volt system: Yet another wiring twist is to use a Littelfuse (formerly Cole Hersee) continuous-duty solenoid to create a series/parallel distribution circuit that generates 24 volts under crank, then defaults back to 12 volts to power everything else under normal running conditions.

Littelfuse Inc., Chicago, IL, 773.628.1000, Powermaster Motorsports, W. Chicago, IL, 630.957.4019 (sales) or 630.849.7754 (tech), Summit Racing Equipment, Akron, OH, 800.230.3030 (U.S.) or 330.630.3030 (outside U.S. ),

Does wiring two 12-volt batteries together make 24 volts?

See all 3 photos 3 photos Ron Rollings

Leave a Comment