Battery Chart. (1)

Here are a few printable charts which will help you to understand the condition of your boat battery bank. The charts are not intended to be the answer to your problems. But they will give a starting point towards a better understanding. 

If you have a voltmeter, you can get a voltage reading from the battery bank. However, you will only get a reasonably accurate reading if the battery bank has been off charge for between 12 to 24 hours. 

Caveat: The reading is for the whole battery bank and not a reading for each battery. It is possible that you have a faulty battery and the only way to check is to test each battery when it is disconnected from the battery bank.

There are many different kinds of battery, including the most popular types for boats which are 'Wet Cell', 'Sealed Cell', 'Gel' and 'Advanced Glass Mat'. Whichever kind you choose does not matter, other than the purchase cost is greater for the more advanced types. If you are happy to top up the batteries every few weeks than the wet cell is the best value for money. The sealed, gel and AGM type require no maintenance. 

The voltmeter needs to be an accurate digital type as the old analog (moving needle) type can be difficult to read. You will see from the first chart that a difference of 1 volt can indicate the difference between a fully charged and a completely flat battery.

There are also two different kinds of battery construction. Automotive or starter battery and deep cycle or leisure battery. Starter batteries can deliver a large punch of power which is needed to crank over the boat engine. Leisure batteries can deliver a smaller amount of power but for longer periods. 

Caveat: The more power is taken out (discharged) of a battery the shorter its life will be.

This is called the life cycle. If a starter battery is totally discharged to the flat stage it will have a minimum number of fully charged to fully discharged cycles. (12 to 15 cycles) (150-200 cycles) If the battery is discharged to the 50% level and then recharged, the number of cycles improves. (100 to 120 cycles) (400 to 500 cycles) If the battery is only discharged to 70% point then the number of cycles improves to (130 to 150 cycles) (1000+ cycles). The figures in red are better for a leisure type of battery. The improved number of cycles are as a result of materials and construction differences. 

I have four 125ah Lead Calcium wet cell batteries on Rosie. Here is a copy of my voltage chart that I have posted next to the voltmeter which I use for reference. In a morning before I run the engine I do a quick visual check. I have some idea of the state of discharge and a feeling for the life cycle. I expect to get about 4 years of service before considering the batteries will be of reduced capacity and ready for replacement.

Batteries have another curious effect that we need to be aware of. The rate that power is discharged from a battery will also have an effect on how much power is available.

If the total capacity of your battery bank is 500ah. One might expect to get 5 ampere for 100 hours or 20 amperes for 25 hours. Unfortunately batteries don't conform to simple mathematical equations. Many batteries capacities are described using the 5ah and 20ah discharge rate.  The discharge rate will be something like 5ah for 70 hours rather than 100 hours. The 20ah discharge rate will be something like 12 hours rather than the 25 hours.

Continued in 'Battery Chart (2)'