Ask PYS: Power

Useable Battery Capacity vs. Battery Monitor – Doug, on Oct 08, 2015 asked:

Can you please explain how many amp hours we have for use on our boat? We have two 198 Ah batteries. If we are not supposed to use more than 50%, does that mean we have 200 Ah to play with? Does this mean that we can run the battery to show 200 Ah used on our Link 10 monitor? I have noticed that the monitor itself is set up to indicate that we have 200 Ah. Should this be 400 or are the bar usage lights based on only 200 available hours? Or are they only showing 100 amps available for usage? Neither of us is clear on this. I hope my question makes sense.

You are right; with 2 X 4D flooded lead acid batteries your useable battery capacity is 200 Ah.

There are different approaches to setting up battery capacity on your battery monitor:

1)      Set-up the capacity to represent 100% of the useable battery capacity, in your case this is 200 Ah.

2)      Set-up the capacity to represent 100% of the total battery capacity, in your case this is 400 Ah.

By going with option 1, only the useable battery capacity, what you see on the battery monitor is what is available. Once you get to nearly 0% of capacity it is time to recharge.

If you went with option 2, all users would need to know the rule that at 50% of capacity it's time to recharge.

In my opinion, I recommend going with option 1 as it is easier. But either way works.