Selecting Battery Charge/Discharge Rates

Created by Bryonie Simon, Modified on Thu, 18 Apr at 11:22 AM by Bryonie Simon

When installing batteries to your system it is important that you have set your battery charge/discharge rates correctly to best optimise your system performance.

 

The battery charge/discharge rates are measured in current (A). To work out the maximum charge/discharge power of the battery you will multiply this current (A) by the BMS voltage.

 

The BMS voltage of a battery will vary between make/model/manufacturer so always refer to your batteries datasheet/manual for the correct current and voltage limits.

 

For the purposes of this article we will use the Sunsynk L5.12kWh IP65 battery with a BMS voltage of 58.4V. This will change if using a different battery make/model/manufacturer.

You set the charge/discharge current for the batteries on the inverter in the battery setup page of the settings menu.

A screenshot of a computer

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The Sunsynk 5.12/5.32kWh batteries have a capacity of about 100Ah and a 50A continuous charge/discharge current so you can set the capacity charge and discharge using these values.

 

One battery charging or discharging at 50A will discharge at 58.4V x 50A = 2.92kWh.

 

The charge and discharge current in the inverter settings is the total charge and discharge current of all of the batteries connected so 2 batteries would be able to charge or discharge at 100A, 3 batteries at 150A, etc.…

Although the batteries have a continuous charge or discharge current limit the inverter will also have its own charge or discharge current limit. This will apply no matter how many batteries are installed.

 

Please refer to the manual for the charge and discharge limit of your inverter. 

 

When selecting the charge and discharge current limits you will always be limited to the lowest current value whether that is the inverter or the batteries.

 

For example, the 3.6kW Ecco inverter has a 90A maximum charge/discharge current. Two 5.12/5.32kWh batteries have a continuous discharge of 100A. This means that the maximum charge/discharge is limited to the 90A of the inverter.

 

Other Current Limiting Factors

Your current should also be suitable for the rated current of your battery cables. The battery cables supplied with Sunsynk batteries are 25mm2 copper cables, this means that the current passed through them should NEVER exceed 122.5A. This can cause damage to the inverter and batteries and will void your manufacturer warranty. The table below has the current rating for common sizes copper cables.

Cable Cross Sectional Area (mm2)

DC Current Rating (A)

2.5

32

4

42

6

60

10

80

16

100

25

122.5

35

150

50

210

70

225

95

300

 

The fuse isolator/breaker between the inverter and batteries should also be appropriately rated for the passthrough of current. For example, if you are looking to charge/discharge your batteries at 190A then the fuse isolator/breaker should be 200A.


The recommended size for the DC breaker can be found in the manual for your inverter.

 

Similarly, when using a busbar the busbar must be rated for the appropriate DC current being sent to/from the inverter.

 

Calculating the AC current Draw when Grid/Generator Charging

In the battery setup menu you can set the charge current from a generator or the grid independently of any charging from the solar. (Pictured below).

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This is not the current that will be pulled from the grid/generator. This is the DC current being sent to the batteries. To calculate the AC current being pulled you will need to divide the battery power by the grid/generator voltage.

 

For example, for a 3.6kW Hybrid Ecco with x2 IP65 batteries with a grid charge current of 90A:

Battery power = 58.4V x 90A (DC)

=5256W

Grid current = 5256W ÷ 240V (grid voltage)

= 21.9A (AC)

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