The second battery in the car is a way to improve autonomy and expand functionality. Tuning type. In order for the additional battery to be of real use, it must be correctly selected, installed and connected to the vehicle's on-board network. From this material on the ABS website, you will learn: is it possible to install a second battery on a car; when it is useless; which battery to choose; how best to install; connection methods. As a bonus, four myths about this type of tuning have been dispelled.

Can a second battery be installed in a car?

Definitely yes, you can. There are mass-produced cars that are equipped with two batteries from the factory. The second battery in the car can be considered as a power bank for a smartphone. However, this type of tuning can serve more purposes than simply improving autonomy. It all depends on the tasks pursued and the chosen method of connecting the second battery to the car's on-board network.

Tasks that are not practical to solve due to an additional battery.

Task one. When the standard battery is old and fails regularly. The second power supply solves this problem only partially. In general, the situation will improve markedly. But not for long. After a few months, a worn out, battery will significantly reduce the life of the new replacement battery. The old problem will become relevant again. In such cases, it is much more expedient and more profitable to replace the standard battery with a serviceable one.

Task two. Installing a second battery when the standard battery is systematically undercharged from the generator. There can be many reasons for this. On the site ABS, they are discussed in detail in a separate article. The second battery will not solve the problem. The problem will only get worse - you will be forced to recharge two batteries from the charger instead of one. Just as often as before. But twice as long. There is no benefit in this.

Why a second battery in a car?

The power from the optional battery can be used to power the following equipment:

  • winch;
  • fridge;
  • lighting;
  • heaters;
  • audio amplifier;
  • converter 12-220 V.

The 12-220 V inverter will allow you to autonomously use a boiler, kettle, microwave, grinder, drill, hairdryer, TV. The potential is limited only by the power of the additional battery. Only devices whose power does not exceed 500 watts can work for a long time. For a short time, you can turn on consumers with a power of up to 2 kW. But the inverter must be designed for such power.

Autonomy. It is easy to calculate it, knowing the battery voltage in volts and the capacity in ampere-hours. Multiplying these two parameters, we get watt-hours. Example. A 70 Ah battery is capable of storing 840 Wh. A consumer with a power of 840 W will be able to work autonomously from such a power source for 1 hour. A 10W LED bulb will last 84 hours. A 1.5 kW electric kettle can be used about 100 times. You can listen to music at a moderate volume for a couple of days.

When several devices are connected to the same power source, their power is summed up. The autonomy of the battery is proportionally reduced.

Two batteries: advantages

The benefit of the second battery will be felt only when it is installed and connected to a specific task. If there are no such tasks, then all you will notice is the loss of money and time.

Advantages of a second battery in the car:

  • improving autonomy;
  • expanding the functionality of the car;
  • the ability to connect powerful freelance consumers;
  • confident use of household appliances with a 220 V inverter;
  • reducing the risk of being far from civilization with a discharged main battery.

If you decide to install a second battery just like that, there are pluses too. When two power supplies are connected in parallel, their inrush currents are summed up. This means a more confident engine start, good starter performance, a reserve in case of severe frost, and less problems with starting a diesel engine. The capacities of the two batteries are also summed up. But not mathematically, but with one nuance. It is described below.

Two batteries: disadvantages

Cons of installing a second battery in the car:

  • additional expenses;
  • you have to spend money on good power wires and a switching device;
  • when installing the battery in the trunk, you will have to lay wires through the passenger compartment;
  • to install a second battery, you need to allocate space;
  • the second battery is an additional load on the generator;
  • if there are problems with the generator, both batteries will have to be recharged from the charger.

The wrong choice and connection of an additional battery will soon result in the need to change both power supplies. This is money that can be spent on useful tuning, high-quality consumables or minor repairs.

Selecting a second battery

The choice of the second battery depends on the tasks. Batteries are starter and traction. The former designed for short-term output of large inrush currents and very moderate power supply for low-power consumers (music, marker lights, DVR, etc.). And most of them do not tolerate deep discharges.

Traction batteries, on the contrary, are not designed to deliver huge currents, but they calmly endure long-term loads. Deep discharges, in general, are not useful for batteries of any type, but traction batteries are definitely more reliable than starter ones in this regard. The difference between starter and traction batteries is described in more detail in a separate article on the ABS website.

Do you want more confident engine starts in winter? Buy a second starter type battery. It is desirable that both batteries have the same wear and capacity. This is not critical, but it will allow you to unlock the potential of both batteries more fully. In addition, they can be safely connected directly, without diode insulators, power switches and other devices. They will work in pairs in exactly the same way as a high-capacity battery in a smartphone.

Need to provide long-term power to powerful consumers? Buy a traction battery. It is designed for such operation, which means it will please you with a long service life. For this to happen, it is necessary to connect an additional traction battery to the on-board network according to a special scheme. It is described below.

Battery type or technology. If you choose a traction battery that will not be connected to the on-board network, then the type does not matter. It can be a calcium, gel, AGM, lithium-iron-phosphate, lithium-ion battery.

If you are going to directly connect two batteries in parallel, it is advisable to have batteries made using the same technologies. This will definitely have a positive effect on the battery life.

Additional battery capacity. It is often said that the capacities of both batteries should be the same. It is not true. This is proven below. Buy a second battery of such a capacity that will solve the problem. Otherwise, the whole point of tuning is lost.

Installing a second battery

Before installing a second battery in the car, you need to solve three problems:

  1. Space for an extra battery.
  2. Power wires.
  3. Switching.

Space for an extra battery

There are few options here. Under the hood, in the cabin, in the trunk. The most preferred first option. Especially if you are buying a starter battery and your goal is to double the capacity of the stock battery. The advantage of this arrangement lies in the length of the power wires. Since they will be short, they can be taken less thick, and the whole tuning will cost much less. It is recommended to put a traction battery in the trunk, which you will use to power freelance powerful devices. Moreover, not all cars have free space under the hood. As for the interior, for now this option is discussed below.

Power wires

To install an additional battery under the hood next to the standard battery, buy wires of the same cross section as the standard ones. If they have to be laid through the entire car (in the trunk), it is better to buy thicker wires. The length proportionally increases the resistance, and hence the voltage drop. This leads to energy loss and waste for useless and dangerous heating. To calculate the cross section, there are corresponding tables on the Internet. Knowing the length and current that will flow through the wires, you can choose the optimal thickness without risking and without overpaying.


Two starter batteries can be switched directly. Plus to plus, minus to weight. If the second battery is in the trunk, its minus can always be hooked to the body. Save on wires and simplify their routing through the cabin. A place for connecting the "mass" should be prepared. Strip to metal and ensure reliable contact.

If the second battery is traction, you will need to buy an isolating diode or a control device for switching.

How to connect a second battery?

The choice of how to connect the second battery also depends on the tasks. There are many options. Five are suggested here.

The second battery is separate from the on-board network

This option is simple. But it has many downsides. The second battery is located in the trunk and is connected only to freelance devices. For example, to a 220 V inverter. With such a scheme, you do not need to decide anything with switching. You can save on power cables and buy any traction battery. There will be no problems with the generator either.

There is only one significant downside. The second battery with this connection will not be charged from the generator. It will have to be recharged in advance from the charger. This is not very convenient.

Connecting batteries in parallel directly

The scheme involves switching the second battery directly to the regular one without any devices. If it is under the hood, then it is better to use two power cables for connection - plus and minus. If in the trunk, then you can stretch one thick wire between the pluses, and connect the minus through the "mass".

Advantages. The second battery will be charged by the generator. You don't have to charge it separately. No need to spend money on switching devices. They are expensive and not always needed. Starting currents and capacities of two batteries are summed up.


  • the second battery must be a starter;
  • the circuit is not very suitable for autonomous power supply of powerful freelance consumers;
  • if the auxiliary battery is discharged, the main battery will also be discharged, which will make it impossible to start the engine;
  • when charging from a generator, a deeply discharged battery consumes a large current (25 ... 40 A), which doubles if there are two directly connected batteries.

To get rid of all these shortcomings, more complex schemes for connecting a second battery are needed. They are discussed further.

Parallel connection via manual switch

According to this scheme, the minuses of the standard and additional batteries are connected through a common mass (car body), and a manual switch is placed in the plus gap. It's inexpensive and simple. You can fully use both batteries. Even if the second one is traction. Charging can be arranged from the generator without overloading it.

The principle of operation of the scheme. Let's say you are going on a picnic, fishing or hunting. Both batteries are charged. Before the trip, you manually disconnect the second battery from the on-board network, since it is not needed yet. If it is not fully charged, leave it in the circuit. Let it charge from the generator along the way. We arrived at the place - disconnected the additional battery, and use its energy for your needs. When he sits down, the regular one will remain charged, which will allow you to start the engine without problems.

If the extra battery is not enough, you can recharge it from the generator. In extreme cases, you can carefully use a regular battery. But be sure to monitor the voltage so that it is not completely discharged.

If after resting the second battery is completely discharged, do not rush to immediately charge it from the generator. Do not connect it to the on-board network for a couple of hours. During this time, the main battery will be charged. Then connect the second battery to the on-board network. With this campaign, the load on the generator will be moderate and it will last longer. It also makes sense to charge an additional battery at home from a charger.

Isolating Diode Installation

An isolation diode is a power component that is installed in the break in the positive line between the generator and two batteries. It is advisable to use the scheme if, for example, a winch is operated on board the vehicle. The principle of operation of the circuit. Due to the diodes, both batteries are charged from the generator, but when a powerful load is turned on, only an additional battery is activated.

There is only one drawback. Diode insulator is expensive. Particularly high quality. Therefore, if you need a second battery to power low-power devices, it is not necessary to buy a diode. It is more profitable to use simpler schemes, which are described above.

Split controller

This is a relatively simple way to connect a second battery in a car. To implement the scheme, you need a control device. They are different. There are cheap, electronic, programmable ones.

There are almost no disadvantages to this method. But there are advantages. The main advantage is versatility. You can implement different schemes and solve many problems. In this scheme, you can use batteries that differ in type, technology, capacity, age. In addition, a simple isolating controller is inexpensive.

Common misconceptions

In conclusion, consider four myths about the second battery in the car.

The stock alternator can't handle two batteries

Firstly, there will be a large load only if the generator charges deeply discharged batteries. If the standard battery is normally charged, then the generator will “not notice anything”. It is not necessary to buy a more powerful generator or install a second one. This may only be necessary if you plan to install an additional battery from the truck in the car.

Batteries must be of the same type and capacity

As for the type and technology, this is not necessary. But in some cases, it is desirable. The above describes which ones. As for the capacity, there really is a nuance here. The total capacity of two batteries does not consist of those ampere-hours that are written on the labels. An old battery may have a much smaller capacity than stated on the case. The total capacity of two batteries will be approximately equal to the capacity of a less capacious battery multiplied by 2.

Example. Standard battery 60 Ah. A second battery of 100 Ah was connected to it. In total, it will not be 160 Ah, but only 120 Ah. This logic does not work when the second battery is connected according to the first two schemes.

Batteries will discharge each other

If one battery has a strong self-discharge, then it will really discharge quickly on its own, and then it will take energy from an additional one. If both batteries are working, then, regardless of their capacities, nothing like this will happen. A less capacious battery with a more capacious one will not take a charge, since the activation of the charge does not depend on ampere hours, but on the voltage at the terminals. If you connect a discharged battery to a charged one, the first one will start charging from the second one. But not because the capacity is different. It's just that such batteries will have different voltages at the terminals. Where the voltage is lower, current flows there.

The second battery must not be installed inside the car

The main reason is toxic fumes. If you buy a classic battery with a bulk liquid electrolyte or made using AGM technology, then it really should not be installed in the cabin. If you want a battery in the cabin, buy a gel, lithium-ion or lithium iron phosphate. They are more expensive. But health is even more expensive.

Video: how to install a second car battery in your vehicle


You can install a second battery in the car. Sometimes it's just a necessity. Before tuning, decide why you need an additional power source. The choice of battery and the connection scheme depend on this.