Another newbie signing on for the first time!

Re: Another newbie signing on for the first time!

Once again, another welcome,,,

A couple of notes. RVers tend not to have a very good understanding of batteries in my experience. They tend to think of batteries as something you "pour electricity into, and then out of" rather than the chemical factories that they really are.

Next, all loads, (and ergo all purchase choices) derive from expected loading. It is not nearly as simple as saying, " we are going to run a TV, maybe a lap top, and a few lights, etc" . What you really NEED to know is how big a load, for how long and how often. For example, a 1 kwh load over the course of a day, every day, is different than the same load 1 day, and then idle for 6. Or to complicate further, a 1 kwh load over 1 hour is different than the same 1 kwh load over the course of a 24 hour day.

Before you buy anything, spend as much time estimating your loads as best you can, understanding the basic rule of off grid living: The loads are always bigger than you think, and they always grow. Also understand that RVs almost never have "ideal" solar orientation so that any and all calcs need to understand that.

The rule of thumb that I like to use is: Name plate rating/2 (to account for all cumulative system loses) multiplied by 4, the four representing the average hours of good sun, one can reasonably expect on a daily basis, averaged over the course of the year. You can adjust your number accordingly, but seldom much more than 4, and in an RV that total number is likely to be optimistic.

So, a 100 watt PV calc. might look like this: 100/2=50*4=200 wh/day. (and once again, not to press the point, the RV calc is likely to be worse.

Batteries and charging capacity should be well matched as others have suggested, shooting for 10% of Ah capacity is a good goal. So a 100 watt panel might put out ~ 5 amps, enough to mate with ~ 50 ah of battery. Ideally, the batteries should be sized to draw a maximum of of ~25-30% on any given cycle. A great misnomer about "deep cycle" batteries is that you can routinely draw them down further "with out harm". The truth is that the cycle life of the battery is directly related to the depth of discharge during each cycle.

I would suggest that you read and understand the following links:

http://www.batteryfaq.org/

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

The final rule of solar, is that most people over estimate their solar harvest, while at the same time underestimate their loads, leading to short battery life, and unsatisfactory performance.

Just as an FYI, we live off grid, with 400 watts of PV, 450 ah of battery (12vdc). We routinely draw ~ 5-800 wh/day, and can go 3 days before we draw the batteries down below 75% SoC. On an average day, we more than keep even, and after three days of no sun, we can get fully charged with a perfect day.

Hope this helps, and once again, welcome to the forum.

Tony

Re: Another newbie signing on for the first time!

actually 300w of pv is good for 220ah of batteries at 12v. the mppt controller will transform the voltage and current to better optimize power from the pv. take the wattage and divide by an arbitrary max operating voltage like 15.5v and this equates roughly to 19.35a minus a bit for controller operations and other such losses. if this is even 17a this would be 17a/220ah=7.72% which is acceptable. if you double your battery bank then you will have trouble.

in general most batteries are good to have a charge rate of 5%-13% of battery capacity and 10% is recommended by trojan. doubling the battery capacity would take the charge rate down to about 3.86% and the batteries will not charge properly. if it were my circumstance if i did anything i would add more pv because pvs are not aimed optimally at the sun most of the time when mounted flat on trailer tops. during times of the sun being on the horizon be it sunrise/sunset or a winter sun the pv output will be greatly compromised when mounted flat. snow and dirt also do not fall off as readily when mounted flat and that adds to the problem.

do not listen to those others telling you you need more in battery unless it is because of your load requirements over the day. batteries should not be drained below 50% state of charge to preserve battery life and under this circumstance you may need to double the capacity to preserve battery life. at that point you need to more than double your pv capacity to properly charge that bigger battery bank.