The need for a 24-volt power source arises in a variety of situations: from upgrading a truck’s on-board network to assembling an autonomous solar power station. Two 12-volt batteries connected in series provide exactly the same 24V, which are required for the operation of powerful inverters, winches or industrial equipment. However, simply “twisting the wires” is not enough here - errors in the circuit or selection of components can lead to charge imbalance, overheating or even fire due to reverse current due to uneven discharge.

In this article we will look not only theoretical basis serial connection, but also practical nuances: how to choose batteries with the same characteristics, which protective devices must be installed, and why you cannot use batteries of different capacities or degrees of wear. We will pay special attention common myths - for example, that “a parallel connection will give 24V” (spoiler: no, this is a gross mistake).

Why does a serial connection give 24V?

When serial connection batteries, their voltages are summed up, and the capacity remains equal to the capacity of the weakest element. This is a key principle that is often confused with parallel connection (where the capacitance is added and the voltage remains 12V). For example:

  • 🔋 Two batteries each 12V/100Ah → with a serial connection they will give 24V/100Ah.
  • 🔌 The same batteries in parallel connection → 12V/200Ah.
  • ⚡ If you connect batteries with different capacities (for example, 100Ah and 60Ah), the total capacity will be limited 60Ah, and the weaker battery will be overloaded.

Physically the process looks like this: plus the first battery is connected to minus second, and the remaining free terminals (minus first and plus second) become outputs for 24V. Important: the voltage on each battery remains 12V, but relative to the general chain they add up.

📊 Why do you need 24V from two 12V batteries?
For cars (winch, inverter)
For solar power plant
For industrial equipment
For home workshop
Other

What batteries can be connected to 24V?

Not all 12-volt batteries are suitable for series connection. Critical parameters that must be the same for both batteries:

Parameter Permissible deviation Consequences of non-compliance
Capacity (Ah) ±5% A weaker battery will be overcharged or undercharged, shortening its service life.
Type (AGM, Gel, LiFePO₄, Lead Acid) Same Different internal resistances lead to uneven charging
Wear level Same (new or with the same number of cycles) The old battery will “slow down” the new one, creating an imbalance
Open circuit voltage (V) ±0.1V A difference of more than 0.2V indicates a malfunction of one of the batteries

Selection is especially critical for lithium batteries (for example, LiFePO₄). Their BMS boards (control systems) must be compatible for serial connection. Otherwise it is possible protection failure and battery failure. For lead-acid batteries, the requirements are softer, but it is still necessary to check the quiescent voltage with a multimeter.

⚠️ Attention: Never connect batteries in series with different internal resistance (for example, gel and AGM). This will lead to one battery constantly overheating and the other undercharging. Check the resistance with a tester or check the data in the passport.

Step-by-step connection instructions

To assemble a 24-volt battery from two 12-volt batteries you will need:

  • 🔧 Two batteries with the same parameters (see section above).
  • 🔌 Cables with a cross-section of at least 16 mm² (for currents up to 100A) or 25 mm² (for currents 100–200A).
  • 🔧 Terminals or lugs for crimping (preferably tinned copper).
  • 🛠️ Insulating tape or heat shrink tubing.
  • 🔋 Charge balancer (optional, but recommended for durability).

Check the voltage of each battery (should be 12.6–12.8V)

Clean the terminals from oxides (use soda solution)

Select cables according to current (see table below)

Make sure the battery is of the same type and capacity

Prepare tools: crimper, multimeter, keys -->

Procedure:

  1. Disconnect both batteries from any load or charger.
  2. Connect positive terminal of the first battery with negative terminal of the second cable of suitable cross-section. This cable becomes a "bridge" between the batteries.
  3. Remaining free terminals (minus the first battery and plus the second) will be outputs for 24V. Connect a load or charger to them.
  4. Check the output voltage with a multimeter - it should be 24–25V (for fully charged batteries).
  5. Insulate all connections with heat shrink or tape.

If you are using batteries in a system with inverter or solar controller, install fuse on the positive line at a distance of no more than 15 cm from the terminal. Select the fuse rating using the formula:

Rating (A) = (Load power (W) / 24V) × 1.25
💡

For reliability, use color-coded cables: red for positive, black for negative. This will help avoid errors when reconnecting.

Bugs that kill batteries

Even experienced craftsmen make mistakes when assembling 24-volt systems. Here are the most dangerous of them:

  • 🔥 Using undersized cables. At a current of 50A, a 6 mm² cable will heat up to 60°C, which will lead to melting of the insulation. Always take the section with a margin.
  • 🔋 Charging from a 12-volt device. If you connect a standard auto-charger (12V) to a series circuit, one battery will be recharged and the other will be discharged.
  • ⚡ Lack of balancing. Without balancer or BMS the difference in the internal resistance of the battery will lead to the fact that one of them will constantly discharge faster.
  • 🛠️ Poor contact at terminals. Oxides or weak tightening cause a voltage drop and local overheating.
⚠️ Attention: If you are using lithium batteries, never connect them in series without specialized BMSrated for 24V. Standard 12V boards will not be able to balance the cells correctly, which will lead to fire or explosion.

Another common problem is uneven wear. For example, if one battery has already served for 3 years, and the second one is new, then when charging, the old one will overheat, and the new one will undercharge. Solution: Always use batteries with the same number of cycles or buy a new pair.

What happens if the polarity is reversed?

When connected in reverse (minus to minus, plus to plus), a short circuit with a current of hundreds of amperes will occur. This will lead to:

- Instant melting of cables and terminals.

- Possible explosion of a lead-acid battery due to the release of hydrogen.

- Failure of electronic components (if they were connected to the circuit).

Charging a 24-volt battery: nuances

A 24V charger must meet several criteria:

  • 🔌 Charge voltage: 28–29.2V (for lead-acid) or 28.8–29.4V (for LiFePO₄).
  • 🔋 Charge current: no more than 20% of the capacity (for example, for 100Ah - maximum 20A).
  • 🛡️ Availability of balancing (required for lithium batteries).

If you don't have a 24-volt charger, you can use two 12-volt ones, but only on condition:

  1. Both chargers must be identical by current and charging algorithm.
  2. Each charger is connected to to his The battery is in a pair (i.e. the first - to the first 12V, the second - to the second).
  3. Charging must occur at the same time and at the same speed.

For solar systems use MPPT controller with 24V support. It automatically balances the charge and prevents overcharging. Example of compatible models:

  • 🌞 EPEVER Tracer 2210AN (20A, 24V).
  • 🌞 Victron SmartSolar 75/15 (15A, 24V).
💡

Never charge a series circuit from a 12-volt source! This will lead to uneven voltage distribution and battery failure.

Automotive applications: winches and inverters

In trucks, SUVs and campers, 24-volt systems are used to:

  • 🚛 winch (for example, Warn Zeon 12-S has a modification for 24V).
  • ⚡ Inverters power from 2 kW (for example, MUST 3000W 24V).
  • 🔦 Additional equipment: refrigerators, compressors, welding machines.

When installing in a vehicle, consider:

  1. Placement. The batteries must be securely fastened (for example, in the trunk or on the frame) and protected from moisture.
  2. Wiring cross-section. For currents above 100A (for example, for a winch), use cables 35–50 mm².
  3. Short circuit protection. Install pyrotechnic fuse (for example, MegaFuse 150A) to the positive terminal.

For campers popular scheme with charge separator (for example, Vicotron Cyrix-CT 120A), which allows you to charge a 24V battery from a 12V car generator. However, this approach requires additional DC-DC converter (for example, Renogy 20A).

Alternatives: when two batteries are not the best choice

Connecting two 12-volt batteries is not always justified. Consider alternatives in the following cases:

Situation Problem Alternative
Requires a capacity >200Ah Two 100Ah batteries will only provide 100Ah at 24V One 24V battery with a capacity of 200Ah (for example, Trojan T-105)
Limited space Two batteries take up a lot of space Compact LiFePO₄ 24V (for example, Battle Born BB10024)
High currents (>150A) Voltage drop on long cables One powerful 24V battery with low internal resistance

For solar power plants it is often more profitable to buy ready-made 24V battery (for example, Pylontech UP2500) than messing around with two 12-volt ones. This simplifies installation and reduces the risk of imbalance.

If you need mobility (for example for a portable inverter), consider lithium batteries with built-in BMS. They are lighter than lead-acid and require no maintenance. Example: EcoFlow Delta 2 (24V, 1024Wh).

FAQ: Frequently asked questions

Is it possible to connect batteries of different capacities in series?

Technically possible, but not recommended. The capacity of the common battery will be equal to the capacity of the weakest battery, and the more powerful one will be constantly undercharged. For example, if you connect 100Ah and 60Ah, you will get 24V/60Ah, but the 100Ah battery will degrade faster due to incomplete cycle.

How to check that both batteries are charging evenly?

Connect multimeter to each battery separately during charging. The voltage on both should increase synchronously (for example, 13.8V and 13.8V when charging from a 24V source). If the difference is more than 0.2V, check the internal resistance or replace one of the batteries.

What to do if one battery in a pair fails?

Replace both battery Even if the second one is still a worker, his characteristics have already changed due to working in pairs. Using a new battery with a used one will lead to imbalance and shorten the service life of the new one.

Is it possible to use a car generator to charge a 24V battery?

No, a standard generator produces 13.8–14.4V, which is not enough for a 24V system. You will need DC-DC converter (for example, Renogy 20A), which will increase the voltage to 28–29V.

How to calculate the operating time of an inverter from two 12V batteries?

Formula: (Battery capacity (Ah) × 24V × 0.8) / Load power (W) = Operating hours. For example, for two 100Ah batteries and a 1000W inverter: (100 × 24 × 0.8) / 1000 = 1.92 hours. The coefficient of 0.8 takes into account conversion losses and incomplete discharge.