I understand that hooking up my two group 31 batteries in parallel will result in longer total battery life. (I had never heard of the Peukert effect till someone raised it on this forum, and then I did some reading. My ignorance is even vaster than I had thought.)
Here is the issue -- right now, I use one battery and keep the other as a fully-charged spare. I monitor the voltage on the battery in use -- when it gets down to 12.1, I swap the batteries and charge the other one with my generator. I can go about three days in cold (20 degree) weather on one battery -- each one is rated at 110 amp hours. (I have LED lights, and we are very careful about energy usage. And I know that I should have solar, except that we almost always camp in deep shady forests.)
So currently (no pun), I have six days worth of battery, without recharging. If I hook the batteries up in parallel, I will get more "camping days" out of the two batteries. But is there a way to predict how much more?
The downside, of course, is that if I use both batteries at once, I won't have a spare if they both run down. That is not likely, since I am pretty good about keeping track of the voltage. But funny things will happen -- a light gets left on during the day while we are gone, or whatever -- and I could be without power.
Yes, I can run the generator in an emergency, but I do not like the sound of it, even though it is very quiet. It is not silent, and our campsite usually is (because we often boondock).
First, the Peukert function as it is expressed is an exponent..
So, Big changes make bigger changes than expected and smaller changes make less change than was expected.
Without real numbers to work with, this is all SWAG (based on a whole lot of real world experience.
If you are getting 3 days out of one battery, when you charge them both and put them in parallel, you might get 6-1/2 days. Maybe not quite that much. You are way down the discharge rate curve. Is there a way to predict what you will get?
Sure, that is what the Peukert exponent is all about, but you have to have real numbers to put in the equation.
Problem: If you then try to charge those same batteries in parallel, one of them will probably not charge to full density and you will not have capacity *2. If you are lucky and careful, you can get to C*1.8.
My guess (SWAG): You are better off doing what you are doing both for the convenience and for the batteries.
This is what I go for performance and long passage cruisers.
Matt & Mary Colie
A sailor, his bride and their black dog going to see some dry places that have Geocaches in a coach made the year we married.
- I would use the two batteries in parallel like you are planning, for convenience. There is no need for a "spare" if you know your battery status.
- To keep better tabs on your battery status, I would get a battery monitor such as the Trimetrics (http://www.bogartengineering.com/products/TriMetric). Monitoring voltage isn't very easy to accurately dial in where a monitor will give you percent of full capacity as well as Amp Hrs used, real time voltage, and current.
- Since you have LED lights, I wouldn't be overly concerned about leaving a light on by accident as they will have minimal impact on your total capacity (~ 4 Amp Hrs over a 24 hour period)
Keep life simple. Forget about Peukert. He has no effect on you. You are a low-power user.
I don't think it's wise though to use one battery at a time. Charging 2 batteries in parallel will be almost twice as fast as charging them one at a time. It depends on converter type and cabling, but regardless of type, it's a lot more efficient to charge them together.