Often you need more capacity than what is in a battery and this is solved by connecting several batteries in parallel. If the subsystem needs a higher voltage than a battery has, it is solved by connecting batteries in series. In this article, you will learn how to connect the battery bank.

12V battery bank

Parallel connection for increased capacity

Lead-acid batteries are heavy (12V 100Ah weighs approx. 30 kg) and this means that the largest manageable size on board a boat is approx. 60 kg, i.e. 12V 200Ah in one battery. It gives approximately 100Ah of available energy, which is too low a capacity for the electrical stuff on a boat like this. If you need a battery bank with more capacity than one battery has, then you have to connect several 12V batteries in parallel.

When connecting batteries in parallel, there are basically four different ways to go about it.

1. In sequence (Sequence)
   The charger is connected to the terminals of one of the batteries. The terminals of the next battery are connected to the corresponding terminals of the previous battery (plus to plus and minus to minus).
   This connection is not recommended because the batteries will be charged at different voltages. The total cable length from the charger to the lower battery is longer than to the upper one. This leads to higher resistance which gives lower voltage to the lower battery.
2. Diagonal
   Almost like a connection in sequence, but with the difference that the charger is connected to the negative terminal of the lower battery and the positive terminal of the upper battery. In order for the batteries to be charged with the same voltage, the cables between the charger and batteries must be the same length (L1), and the cables between the batteries must be the same length (L2).
3. Common connection point (Terminal, Post)
   Here the charger is connected to two connection points with cables that can be of different lengths. A connection point connects all positive or negative terminals. The cables between a connection point and the batteries must be the same length (L1 to the negative terminals, L2 to the positive terminals).
4. Connecting rail (Busbar)
   Here the charger is connected to two connecting rails made of coarse copper with negligible resistance and the battery terminals are connected directly to the rail. It’s recommended to connect the connection rails to the battery bank connection as in a diagonal connection.

For a clear installation, always mount two terminal blocks where the cables from the battery bank connect. It is these two terminals that other parts of the electrical system connects to.

Parallel connection of two batteries

This drawing shows two batteries connected in parallel for increased capacity to 12V 200Ah (2.4kWh).

Parallel connection of three batteries

This drawing shows three batteries connected in parallel to increase the capacity to 12V 300Ah (3.6kWh).

Parallel connection of four batteries

This drawing shows four batteries connected in parallel to increase the capacity to 12V 400Ah (4.8kWh).

This is how you can continue to increase the capacity of a battery bank. Parallel connection of more and more batteries. Check with the manufacturer how many batteries that can be connected in parallel.

Series connection for increased voltage

If you need 24V consumers and only have access to 12V batteries, you must connect 12V batteries in series. In order to achieve sufficient capacity in the battery bank, it may also be necessary to connect the series-connected batteries in parallel.

When connected in series, it is important that both batteries have the same voltage when they are connected together. Every season you need to ensure that the batteries get the same voltage again. If they have different voltages, the battery with the lowest voltage will not be charged enough. The other battery may get too high a voltage.

This drawing shows two different solutions for charging series-connected batteries.

• Separate chargers
  The left drawing uses 12V chargers connected to one battery each. In principle, it is a good solution because each battery is charged separately. If you want a solution like this, you need to use a charger that is built to charge two batteries connected in series, otherwise not.
  One problem is that a shunt cannot measure how much current is supplied to the batteries. Another problem can be that the chargers have different voltages on their negative terminal. The negative terminal of the first charger is connected to the negative terminal of the electrical system, but not the second one. If the chargers have contact between the negative on the battery side and the negative on the supply side, the chargers will short-circuit the batteries
• One charger
  The right drawing shows how to use a 24V charger, which is the usual solution. In the long term, such a connection can suffer from imbalance voltage between the batteries, which leads to problems with the charging of the batteries and that they age faster. Disconnect the batteries each season and charge them to the same voltage. Also, take the opportunity to run a reconditioning (Equalization) of the batteries then. See article Electrical system, Part 3 – Battery charger.

Parallel connection of two batteries in series

This drawing shows two batteries in series connected in parallel for increased capacity 24V 200Ah (4.8kWh).

This drawing shows the other two variations of how to connect two batteries in series in parallel.

Parallel connection of three batteries in series

This drawing shows three batteries in series connected in parallel for increased capacity 24V 300Ah (7.2kWh).

This drawing shows the other two variations of how to connect two batteries in series in parallel.

Parallel connection of four batteries in series

This drawing shows four batteries in series connected in parallel for increased capacity 24V 400Ah (8.4kWh).

This drawing shows the other two variations of how to connect two batteries in series in parallel.

Connection of battery monitoring

A battery bank may need to be supplemented with equipment for monitoring individual batteries or the entire battery bank. It is equipment that

• Measuring the charge level of the bank (shunt)
• Disconnects battery if the current is too high (relay)
• Disconnects battery when charge level is too low (relay)
• Disconnects alternator when battery bank SoC > 90% (relay)
• Balances voltage across the individual batteries in a battery bank with series-connected batteries (voltage balancer)

Measuring charge level and protecting against high current

To get a good value of the battery bank’s charge level, a shunt is needed with the help of which you can measure how much current goes to or from a battery bank. If you have batteries with BMS, they have a built-in shunt and each battery knows its charge level, and then no external shunt is needed.

To protect the battery bank/electrical system against too high current, a circuit-breaking fuse is placed on the positive connection between the shunt and the charger/consumer. This fuse must be present even if you have fuse breakers on each individual battery according to the section on low charge level protection.

The drawing below shows what it might look like with a shunt that connects both positive and negative terminals. Some shunts only connect to the negative terminal of the battery bank. In this case, the shunt also has a temperature sensor (dark blue) on the battery.

Disconnect battery if charge level is too low

To protect individual batteries against too low a charge level, a remote-controlled fuse breaker is placed between the positive terminal of the battery and the connection point for the positive terminal of the battery bank.

Keep in mind that these fuses must withstand less current than the fuse for the entire battery bank. If the entire battery bank has a fuse that is 500A, it is enough that the fuse for one battery is 500A divided by the number of batteries connected in parallel.
2 batteries connected in parallel means that the fuse must be 500A/2 = 250A.

This solution assumes that the charge level is known for each battery.

When the charge level of a battery becomes too low, an alarm is sent which causes the corresponding remote-controlled circuit breaker to disconnect the battery.

In order to connect a fuse switch like this, the battery bank must be connected with a common connection point.

Disconnect generator

If you have a charging source that charges different battery banks at the same time, e.g. the 12V generator on the engine, it is not possible to charge with the correct voltage levels if the batteries being charged are of different types.

If one battery is a lead-acid starter battery and the other an LFP battery, the alternator regulator’s maximum voltage must be set to the battery that can withstand the lowest voltage when charging. In this case, it will be the starter battery that limits the voltage. This allows us to charge an LFP battery almost fully, but not quite. In order not to load the generator unnecessarily, we build a solution that makes the generator disconnect from the LFP battery when SoC = 90%. The disconnection is done with a high-power relay and a control unit that controls the relay. The controller is networked and opens the relay when the SoC reaches 90%.

Balance battery voltage

If you have batteries connected in series, there is a risk that they will get different voltages over time. This can be solved by connecting equipment that measures the voltage across the respective battery and ensures that it is equal by drawing some current from the battery with the highest voltage.

The connection diagram shows the connection of a balancer partly in series connection, and partly in parallel connection of series-connected batteries.

In most cases, you can do without a balancer when connecting lead-acid batteries in series, just make sure that the series-connected batteries have the same voltage during installation. Check and balance the batteries at least once every season.

As usual, when connecting several batteries to a battery bank, they must be of the same type and have the same capacity and age. Also, remember to charge them fully before connecting them.

When connecting LFP batteries, balancing is needed for both parallel and series connections. The balancing is performed by the BMS equipment. Follow the battery manufacturer’s guidelines for connecting batteries and balancing.