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Suppose I have a solar powered plant like this one below:

schematic

simulate this circuit – Schematic created using CircuitLab

The solar panel charges the batteries and, when needed, the power is drawn from the batteries to power the load which in this case could be a DC motor or an AC motor depending on the application. For the sake of this example, let's say this is a 24V DC motor. Suppose the controller is just a switch.

By taking a look at the datasheet of some of the loads, most of them need a very high current at the start. For instance a centrifugal pump I took as example needs 78 A at the start.

From a theory point of view, if I consider the battery an ideal voltage generator, there is no problem. Furthermore, since 78 A are drawn for just a small time (suppose 1 second for the sake of argument) during the start up and then the load starts to draw the usual working current which is far lower than, 78 A, not that much extra-work is required from the battery after the startup.

However, batteries are far from being a perfect voltage generator, how can I understand if a battery is able to withstand such high current draw? A 120Ah battery in principle could easily do this since in theory it could supply 120 A for 1 hour (and in my case I just draw 78 A for a second) but I have some doubts that this reasoning could hold in practice.

Please address the following points in the answer:

  1. What is the technical parameter in the battery datasheet I should be looking at? (Max current draw??)
  2. What is an appropriate rule of thumb to address the start up current draw while not under/over sizing the battery package?
  3. Are the reasonings above correct? (at least in theory)

Thank you

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Car batteries have a high cold cranking current, as Neil says in his answer.

But they are usually a poor choice for solar storage, where deep cycle or leisure batteries are preferred - they are better able to withstand deep discharge.

If leisure batteries have a CCA (cold cranking amps) rating it is likely to be lower. They should have a max current rating instead. Or, their capacity may be stated at different discharge rates such as C/10 (which would discharge them in 10 hours). C (1 hour) and 10C (6 minutes, which may only deliver half the capacity at C/10).

C is their nominal capacity, which is often measured at a discharge rate of C/20, i.e. 5A for a 100 Ah battery.

Or the max discharge rate may be expressed as a C rate (10C) instead of a current.

Multiply this rate by the capacity to get the current.

So if you have a battery which allows 10C discharge, and you need 78A, then its capacity must be at least 7.8 Ah. And preferably several times that if you want a reasonably long battery life.

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If you are looking at 12v auto batteries, the 'cold cranking current' is the most appropriate battery parameter. This tells you what very short term current it can deliver to the starter motor without the voltage dropping so far that it can't power the ignition. This is normally well in excess of 200A even for batteries of 50Ah.

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