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Do not connect batteries in parallel

Learn how to connect batteries and which way is best if in series or parallel.

Do not connect batteries in parallel

It is important to know these aspects about lead solar batteries. It does NOT apply to lithium batteries.

1.- Avoid connecting solar batteries in PARALLEL

If you are connecting the batteries in parallel, connect all the positive terminals together, and then connect all the negative terminals together. The following formula applies to parallel circuits: (I total  = I 1 + I 2  etc.). This will provide you with extra current for the load, but without additional voltage ( total  V = V 1  = V 2,  etc.). The example shown in figure 2 provides 12V to the load with a current capacity of 3A.

The intrinsic imperfections of the materials, the small variations in the manufacture, in addition to the different physical factors that affect the batteries, mean that no two batteries are the same.

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In each connection, the terminals, the crimp, the battery screws, etc. a small resistance is created on the cable.

When we connect batteries in parallel, as seen in the image, we create a closed circuit between the batteries, which allows the passage  of unbalance currents  (due to the differences between batteries and connections) that circulate from one battery to another to try to balance them.

This parallel battery configuration provides 12V for one charge and has a current capacity of 3A.

These unbalance currents are produced permanently even if there is no consumption connected and therefore  accelerate the degradation process of the batteries.

The greater the differences between batteries and connections, the greater these unbalance currents and the greater the degradation produced.

The result is:

  • An overload of the battery closer to the current input and a bad battery charge further away.
  • The first battery will charge with higher current and voltage than the battery further away.
  • The first battery will work harder and fail prematurely

IMPORTANT : If it is essential to connect the batteries in parallel, it is imperative to make the connections well to minimize the negative effect of the parallel connection:

  • Diagonal connection:  We enter through the closest battery and exit through the furthest to achieve an equal current flow.
  • Collector type:  The output cables must have the section resulting from the sum of each branch. The lengths and sections of all the branches have to be identical to avoid different resistance in the connections.
  • Midpoint:  The output cables connect to the midpoint of the battery.
  • With DC bars:  Union of the parallels by means of DC busbars. This would be the most correct connection.

It is necessary to frequently check the imbalances produced in the batteries and carry out compensations.

2.- The batteries must be connected in SERIES 

Si va a conectar las baterías en serie, conecte el terminal positivo de una al negativo de la siguiente, y así sucesivamente. La siguiente fórmula se aplica a los circuitos en serie: (Vtotal = V1+v2, etc.). Esto le dará voltaje extra para la carga, pero sin corriente adicional (Itotal = I1 = I2 etc.). La serie ejemplo que aparece en la Figura 1 funciona a 36 V con una capacidad de corriente de 1 A.

The recommended battery connection is the serial connection.

The series connection of batteries only allows the passage of current when there is consumption. When there is no consumption, the batteries remain idle.

To obtain the set with the desired voltage,  we connect batteries in series of equal capacities to add the voltages.

Circuito de batería en serie que muestra una carga de 36 V con una capacidad de corriente de 1 A.

To obtain the desired capacity in the set we must  select the capacity of each battery (identical) according to our needs.

For example, if we need a 250Ah 12V battery we have 2 options:

  • Connect in series 2 batteries of 6V and 250Ah each to obtain the set of 12V and 250Ah
  • Choose a 12V and 250Ah battery

If we want a set of 250Ah at 24V we will connect 2 12V and 250Ah batteries in series

3.- The maximum capacity that we can acquire depends on the type of battery

  • Open lead acid monoblock batteries.- Up to about 250Ah. With 12V formats.
  • Deep cycle lead acid monoblock batteries.- Up to about 1100Ah. With 6V formats.
  • AGM monoblock batteries.- Up to about 320Ah. With 12V formats.
  • GEL monoblock batteries.- Up to about 250Ah. With 12V formats.
  • Open Lead Acid Stationary Batteries.- Up to about 5000Ah. With 2V elements.
  • GEL Stationary Batteries.- Up to about 5000Ah. With 2V elements.

When the desired capacity cannot be reached with monoblock batteries , it is necessary to  choose stationary batteries  with 2v elements and, as we have seen, capacities up to 5000Ah.

4.- Do not connect new batteries with old batteries

Connecting new batteries with old batteries is a bad investment. The set will continue to work for a while, however the old batteries will progressively damage the new ones due to the charge they produce on them and also by modifying the appropriate charging voltages of the new batteries.

Finally, when the old batteries are exhausted, they must be replaced by new ones that will have to coexist with the previous batteries with a long life and misuse, so they will continue to be a burden for the latest batteries purchased. And so, we enter a loop where we are rapidly degrading the life of new batteries by combining them with old batteries.

Which means that the batteries are not expandable.  Therefore, it is  very important to size the solar batteries well before making the purchase , since the batteries that we buy must be used until their useful life is exhausted.

5.- Perform periodic equalizations

In any battery connection, imbalances occur during charging and discharging, so it will be necessary to carry out periodic equalization charges (equalizations) to maintain the batteries.


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