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).
Thanks in advance for your advice!
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You run the gen to recharge the 'used' battery......
So what would be different if you ran gen to recharge batteries as needed?
Personally I don't see the benefit in changing out one battery for the other than recharging it.......I'd use both in parallel and recharge as needed with gen.
Old-Biscuit wrote: You run the gen to recharge the 'used' battery......
So what would be different if you ran gen to recharge batteries as needed?
Personally I don't see the benefit in changing out one battery for the other than recharging it.......I'd use both in parallel and recharge as needed with gen.
100ah battery lasts 20 hrs at 5 amps rate or 10 hrs to 50%
Same battery lasts 5 hrs at 14 amps rate or 2.5 hrs to 50%
Doubling the battery capacity makes that 14 amps rate the same as a 7 amp rate with one battery. So using rough proportions instead of doing it right, we get:
7 amps is 2 of 9 amps diff while time diff is 15 hrs, so 2/9 x 15 = 3.3 so 7 amps time is WAG 20 -3.3 = 16.7 hrs or 8.5 hrs to 50%
Soooo, by doubling and keeping the 14 amps rate, you would get 8.5 hrs to 50% instead of just twice 2.5 = 5 hrs
So the improvement is 3.5/5 = 70% by using both together instead of one and then the other. In this particular case only.
But the improvement will vary for each rate so it is hard to say what the daily AH usage difference would actually be. Got to be better at any rate to double up the bank though.
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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.
Good Luck
Matt
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)
The peukert effect does not have much of an impact if you are under the 20 hr rate. So in your case if your amp draw is below 5.5 amps you won't see much difference.
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.
Charging times depend a lot on how you do it in each case and if you have two chargers and the charging rates.
If you parallel the bank for 220AH and use one 35 amper it will take approx 180 min to do a 50-90.
If you parallel them for a 220ah bank and use both chargers at once to get 70amps then it will take about 130 min. Not half the time, more than that.
-doubling the charging rate does not halve the time.
-doubling the bank at same rate doubles the time at low charging rates but not at high charging rates.
-ie you can't get better time with those 35ampers (high rate on 110ah) by doing each 110 separately but at the same time instead of doing both with both where now it is 70 on 220)
If it were the same two batteries but you had two 10 ampers instead of two 35s, the story changes because of the low rates.
Can't say what would happen with the OP's gen time without knowing his usual charging rate on one battery and if he has two chargers.
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