Battery lifetime/cost vs. state of charge

[Adelie]

Originally Posted by a64pilot

One thing we are missing is that it's virtually impossible to recharge to 100% with Solar if you discharge to 50%.
. . . .

Try again only this time show your math. There are plenty of folks that have indicated they can keep their batteries full with solar panels and no generator except in special circumstances. A majority? Don't know, didn't keep count.

As an example, James Baldwin on Atom. He has a 420a-hr bank and he keeps it charged with 100w of solar panels. Since Atom uses a 6hp outboard for power i doubt he gets more than 10 or 20 a-hr off that if any. Atom Voyages - The Sailboat Atom

Here's my math

Start with the assumptions:
A] Boat is between 23N & 23S except during summer. That means the shortest you will see sun above the horizon is about 11 hours per day at a tropic at winter solstice.
B] 400a-hr battery bank.
C] Solar capacity in watts equal to the bank capacity in amp-hours so 400 watts. That's equal to about 30amps when going full boar.
D] 50% DoD or 200amp-hr out. Recharging will be 100a-h to go back in Bulk and 100a-hr absorption.

Beyond these assumptions there are 2 scenarios.
1] Panels flat.
2] Solar tracking

1] Panels flat
Sun will be 44* above the horizon at noon so max output is 70% of nameplate which would be 21amps. Integrating the area of the sine wave from sunrise to noon gives about 75 amp-hr. This means that panels run past noon before they hit absorption charging. To make it to absorption charging by noon you would need 515w of panels or a 300 a-hr bank with the same 400w of panels.
Time for absorption also depends on condition of batteries. Batteries in good condition should get thru absorption in 5 hr or so. In mediocre condition more like 7 hours. Let's assume 6hr so you need to make 100amps in the 1st 5 hr. To do this you need 600w of panels or stay with the 400w and have a 300a-hr bank.

Ok so you aren't making it 2 or 3 months per year with flat panels where nameplate wattage is the same as battery amp-hours. But you are coming close.

2] Tracking panels.
With tracking panels you are getting full output as soon as the sun is fully up. Let's assume you only adjust the panels hourly so you are only getting 90% of nameplate wattage. That works out to 27amps. That means you are thru bulk charging in under 4 hours and you have more than 7hr for absorption plenty even if the batteries are approaching the end of their lives. Assuming 6hr absorption then you need 20a continuous (90% output) for 5 hours or 300w of nameplate capacity.

In reality not a lot of folks are going to install tracking panels, but some do.



Let's go back and examine one of the assumptions a little more. Namely the 50% DoD assumption. That shouldn't be what you draw down to every day, that should be what you drawing down to after 3 or 4 days of cloudy weather. And even in cloudy weather you should still be generating some power. So if you are down to 50% or even 80% DoD it should be after a number of cloudy days, not 1 night. In that case normal consumption is not 200a-hr per day so you should be able to get back to 100% in 2 clear days, 3 at most. Yes you will be running PSoC for that period and the battery will suffer capacity loss for it but, this will not be a daily occurrence, but an occasional one.

On the other hand if you are drawing 200 a-hr/d from a 400 a-hr bank then you need to be rethinking your bank size and by implication how much solar you have and maybe also your consumption habits.

[Pizzazz]

Let me share some real world numbers for the Southern California area. You can expect a 100W panel to produce around 25-30AH on a good day. Assume bulk charging is 95% efficient, absorption is 66℅ efficient. You will need four panels to put 100AH in bulk and another six to do the absorption charge. That is 10 solar panels. That is not practical on a sailboat. Then compare this scenario to just running the engine for two hours a day (if occasional cruiser) or the generator (if serious about cruising) at a cost of may be a couple of dollars/day.

Solar could be fantastic but in terms of cost, simplicity, looks, deck clutter, etc. nothing beats golf cart batteries+ diesel bulk charging.

Regarding the comments about only a 20-25% discharge or sizing the battery so that it is down to 50% after a few cloudy days - why don't you just go for a 10% discharge? The batteries would last forever and you can even get away with cheap automotive batteries .

[arsenelupiga]

i have factory 480 AH battery gel bank and 360 W panels.

Victron monitor set to start alarm when batteries discharged 20 % meaning 80 % left in batteries. We normally use up to 50-60 AH o/n on anchor and batteries back to full before midday.

where this talk is coming from that solars cant fill batteries full ? Maybe your controllers suck...

I have not sailed full night yet so cant say when o/n sailing.

[Adelie]

Originally Posted by kmacdonald

I recently reviewed some articles on marine batteries and charging systems by one of the best authorities on the subject. One thing that stood out in the articles was the lack of knowledge and understanding some of his clients had. This thread has confirmed his observations and then some.

How do you know he/she is one of the best authorities? Name? Article titles? URL?

[chala]

Originally Posted by Cadence

I think his reef is an absorption unit. If done right the propane is no problem and highly efficient

I have a caravan with a LPG absorption refrigerator and I do not risk driving around with a LPG flame on and unattended. I find the risk of a gas leak or something getting lit by the flame unnecessary.

[valhalla360]

Originally Posted by chala

I have a caravan with a LPG absorption refrigerator and I do not risk driving around with a LPG flame on and unattended. I find the risk of a gas leak or something getting lit by the flame unnecessary.

If you are paranoid or have a faulty system, feel free to turn it off.

The point wasn't if an absorption fridge is good but to explain why I needed such a small battery bank.

If you want to discuss the merits of absorption fridges, I suggest starting a thread on the subject.

[kmacdonald]

Originally Posted by Adelie

How do you know he/she is one of the best authorities? Name? Article titles? URL?

Because I'm so smart. Maine Sail, Compass Marine.

[Cadence]

Originally Posted by chala

I have a caravan with a LPG absorption refrigerator and I do not risk driving around with a LPG flame on and unattended. I find the risk of a gas leak or something getting lit by the flame unnecessary.

If you are driving around it probably flips over to 12V automatically, also efficient. The down side is recovery as opposed to conventional refrigeration, it is slow. I doubt that little flame is likely to start a fire unless incorrectly vented.

[chala]

Originally Posted by valhalla360

such a small battery bank.

So I see. That it is from your’s small battery bank that you draw all your expertise.

[valhalla360]

Originally Posted by chala

So I see. That it is from your’s small battery bank that you draw all your expertise.

No from understanding the physics and engineering aspects of how batteries and charging systems work along with actual experience on multiple cruising boats and rv's but if it makes you feel better to make snide remarks rather than discuss the factual data, have at it.

kmac asked for details regarding my selection. I just wanted to keep it clear why the selection was made to avoid confusion but some people seem to intentionally ignore what was clearly written in an effort to confuse the issues.

[kmacdonald]

The answer to the OP could be a simple ninth grade science project. The widespread confusion it has caused is reason for concern. I don't know how some people cross the street on their own.

[Cadence]

Originally Posted by chala

So I see. That it is from your’s small battery bank that you draw all your expertise.

That is crass.

[chala]

Originally Posted by kmacdonald

confusion

For some.

Originally Posted by Pizzazz

to keep wet cell batteries above 50% state of charge to ensure maximum life.

Is a myth.

[kmacdonald]

Chala, if you discharge to 50 percent DOD and only charge to 80 percent SOC you haven't insured maximum battery life Like I said before its not that binary.

[Stu Jackson]

Originally Posted by kmacdonald

Because I'm so smart. .............

And humble, too.