Battery State of Charge

Tech Net Library

Battery State of Charge

Guidelines and a Useful Reference Table

by Larry Shick s/v Moira

How to Check Your Battery State of Charge

Cross-Checking State of Charge Using Battery Size, Voltage, and Amperage


The Link 2000 attempts to estimate state of charge by estimating the "efficiency" of the charging process, i.e.; how much of the input Amp-hours actually stick to the battery vs being lost as heat. The estimate of state of charge wanders further and further from reality until the bank is fully charged to the reset point.

One way to cross-check the state of charge is by using the graph 1-23 from Nigel Calder's Boat Owner's Mechanical and Electrical Manual (2nd Edition). For a given battery bank size, state of charge, and input Amperage, the bank will show a certain voltage: higher voltage reflects a higher state of charge.

The table below enables one to cross-check state of charge. The figures in the body of the table are Amperages, expressed as a percentage of the total capacity of the battery bank. If the charging process has achieved a certain voltage with LESS THAN the indicated amperage (from the body of the table), the battery bank has reached the state of charge at the top of the corresponding column.

Let's use a 400 Ah battery bank as an example. Suppose that charging has reached 13.5 V with the application of 28 Amps (7 [located at the intersection of the 13.5 V line and the 75% column] x400). The battery bank is approximately 75% charged

It will be most convenient to take the values from the body of the table into a spreadsheet and multiply them by the size of your battery bank. Print out the results for actual use.

Important Assumptions and Caveats
(1) The table assumes that the rate of charge is reasonably stable.
(2) Be aware that, as the battery bank ages, its actual capacity will fall below nominal (as-purchased) capacity. You must use the actual capacity with this table. Using the nominal capacity will cause the table to over-estimate the state of charge.

% of 'C'

50%

75%

90%

100%

14.0V

23%

12%

5.7%

1.5

13.8

20

10

4.8

1.25

13.6

17

7.5

3.6

1.0

13.5

15

7

3.3

0.87

13.4

13.5

6

2.9

0.75

13.2

10

5

2.3

0.5

13.0

7.5

3

1.3

0.25

12.8

5

1

0.4

0



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