Batteries C Rates

Batteries - C Rates

In order to build skills beyond the level one course, we have to introduce at a few more terms used for rating marine batteries. One of the most valuable of which refers to the rates that batteries are charged and discharged, this is known as C rates. The C Rate compares the the actual current flowing to or from a battery at any given time to its quoted Ah rating, the formula for this is simply:

C Rate = Current ÷ Capacity

C Rate = A ÷ Ah


You have a 120Ah battery and a load of 12 Amps, to calculate the C rating for this situation it is simply:

A ÷ Ah = C Rate

12 ÷ 120 = C 0.1

C Rates are quoted with the letter C and then the number.

If you are given a C Rate, you can use this to work out the current flow this represents for your particular battery. The formula for this is:

A = Ah x C rate


Your battery manual states that your battery has a very high charge acceptance rate of 0.5C you have a single battery with an Ah rating of 85Ah. To calculate the charge current it's simply:

85 x 0.5 = 42.5 A

C Rates & Ah Ratings

As covered in the level 1 course, Ah refers to capacity, this is a theoretical current you can draw for one hour before the voltage falls to 10.5v, most importantly this Ah capacity figure depends on how quickly or slowly we drain the power. The reality is that if you were to draw 100 Amps from a 100Ah battery it would probably be completely flat in only 20 minutes! Read on to see why...

This is because the quoted Ah rating is based on a much slower discharge rate of 20 hours.

The quoted Ah ratings for batteries are most often based on a discharge rate of 20 hours, this is a C rate of just C0.05. So to put this in context, for a 100 Ah battery, you'd only get the quoted capacity of 100Ah if you discharged the battery at its 20 hour rate (Ah ÷ 20Hr rate in this case 100 ÷ 20) = 5 Amps for 20 hours.

Remember though, this would completely flatten the battery rendering it useless; the battery would have to be discharged to no more than 50%, so ultimately that's 5A for 10 hours.

OKAY, here is where the problem is! If you run a load that would, given enough time, discharge the battery in less time than 20 hours you will NOT get the quoted Ah capacity. In our example of a 100Ah battery we worked out that the 20hr Rate was 5 Amps, so this means that if you were to discharge at a higher current than 5 Amps, say 20 Amps, then you are running the battery 4x harder than it was originally run when the manufactures measured the Ah rating. It will run hotter and you will not get the quoted 100Ah.

Hour Rate Tables

Below is an extract courtesy of Trojan Batteries, the section of table shows the Ah rating for batteries at a range of discharge times. Note that the 20 Hr Rate column represents the standard quoted Ah ratings that will be printed on the battery; this is often the only figure quoted by battery manufactures.

Remember. -The quoted Ampere-Hour Capacity is the amount of electricity that a battery will deliver for 20 hours before the voltage falls to 10.50V. For example, a 60Ah battery will deliver a current of 3A for 20 hours - in theory. But, we should never fully discharge a lead acid battery beyond 55% to 60%.

C Rate Equivalence Table

To help understand C rates fully, and compare the battery performance for different sized banks I have drawn up a C Rates Equivalence Table, this is not a standard table, but it clearly shows C rates based on current draw for different sized battery banks. This can be useful to quickly see by how much an additional battery will effect C rates.

The table can also be used to evaluate charging requirements too, many manufacturers specify an optimum minimum C rate for charging, note that the maximum rates are limited by the absorption rates of the battery.

There is a copy of this table you can print, below the quiz on this section. Watch the video lesson below...

C Rates Equivalence Table.pdf
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