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Old 13-06-2019, 03:32   #1
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Battery replacement

Hi, I'm a newcomer to boat ownership having bought my first boat a couple of months ago.
I know I will need to replace the batteries at some point soon, but how do I know when?
Can some one help with advising me what the batteries should charge up to and what is an acceptable amount of charge to lose overnight.

Sorry if my question is really basic. Total novice at the beginning of a steep learning curve.

Cheers Nicster
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Old 13-06-2019, 04:08   #2
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Re: Battery replacement

Starter batt is cheap, just do it, then to a schedule, say 3-4 years.

House bank goes off SoH state of health, residual capacity as a % of when bank was installed.

Rarely does that mean the factory 20-hour Ah rating, but a proper load test is a challenge so it's rarely done by lay owners.

Industry standard EoL is 80%, but that can be pushed to 75% or even 70% depending how long you stay away from places where replacing the bank is easy and how mission critical the bank is.

Most owners get a feel for how long it takes to get the bank drawn down, but then many just wait for the bank to truly start failing before scheduling a replacement.

______
Actually measuring capacity, only way without thousands of dollars worth of lab grade equipment is called a

20 Hour Capacity Test:

An accurate AH counter can be used, but not necessary, just an ammeter and a stop watch.

#1 Benchmarking to 100% Full is straightforward, hold Absorb until trailing current drops to endAmps. If not mfg spec'd I use .005C, .5A per 100AH.

Then equalize if appropriate, and allow the batt to rest disconnected for at least 24 hours

#2 Make certain battery temperature is between 75F & 80F

#3 Identify then prepare a .05C load = Ah Capacity ÷ 20 - small light bulbs and/or resistors can work. So 220AH rating divided by 20 hours means discharging at a constant 11A load at around 77°F.

#4 Connect an accurate digital volt meter to the positive and negative battery terminals

#5 Start DC load and a stop watch at the same time

#6 As battery voltage drops, during discharge, adjust the DC load to maintain as close to the C÷20 rate as is humanly possible

#7 Immediately stop the discharge test when battery terminal voltage hits 10.499V

#8 Note the hours and minutes of run time on the stop watch and figure your percentage of 20 hours that it ran. This is your batteries Ah capacity or state of health as a percentage of the "rated" capacity. For example if a 100Ah battery ran for 16 hours it's testing at 80% of its original rated capacity. By industry standards lead acid batteries are considered “end of life” when they can no longer deliver 80% or more of their rating. They can still work beyond this point but are considered **less safe** and can be less predictable.

#9 Once you hit 10.499V recharge the battery immediately at the 20 hour rate. Once the battery is full, follow this up with equalization if appropriate

A long slow recharge (20 hour rate) can have a slight reforming effect on batteries and can actually serve to recover some lost capacity. It is not uncommon for a battery to have **more capacity** after a 20 hour test than it had going into it.

Credit to Maine Sail, https://marinehowto.com/programming-a-battery-monitor/

Better yet read it all, and check out the rest of the site.

______
A new BM on the market Balmar SG-200 claims to automatically learn your bank's SoH over time, but we do not yet know just how well it works, might depend on the type & model batteries used.

What comprises your bank? How long ago was it installed?
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Old 13-06-2019, 04:30   #3
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Re: Battery replacement

Good morning, Nicster, and welcome to the wonderful world of boat ownership!

John has given a lot of good but very technical info above. When I was first learning my boat I bought a copy of Nigel Calder’s book on boat electrical and mechanical systems, and did A Lot of googling....another good resource is the Marine How To website by Compass Marine.

In order to get answers to your specific questions, it would help forum members to know what kind of batteries you have, what kind of charging equipment is being used, and what things you use, and how often, that put a drain on your batteries.
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Old 13-06-2019, 05:38   #4
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Re: Battery replacement

You can also test your batteries at dockside.

Unhook from shore power if your boat has that capability and just run on your boat's systems for a day or two to see how well your batteries hold up.

Take voltage readings at various times each day to get an idea how your batteries are doing.

My batteries are usually at their lowest reading in the early am which is around 11.9 volts after running a fan all night.........if I use my 120 volt AC fan thru the inverter.

I have about 12.2 volts when I use my 12 volt fan

If it's sunny during the day, my batteries are usually back up to my float setting of 13.5 - 14 volts depending on which solar controllers I have hooked up and their float setting

I only have solar. No shore power or alternator. No start battery just two 12 volt 90 ah batteries in parallel
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Old 13-06-2019, 05:40   #5
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Re: Battery replacement

Quote:
Originally Posted by Nicster View Post
Can some one help with advising me what the batteries should charge up to and what is an acceptable amount of charge to lose overnight.

And also...

Each battery maker has it's own recommendations for what is fully charged (often in the neighborhood of 14.7V but it varies a bit), how fast to charge, specific charging voltages, etc....

So start with brand name, type (flooded? AGM? etc.), and size (G27? G31? GC2?). There might be a sticker on your batteries that says when they were installed, too.

-Chris
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Old 13-06-2019, 06:25   #6
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Re: Battery replacement

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Originally Posted by thomm225 View Post
You can also test your batteries at dockside
Not "can", pretty much "must".

To design and build a setup that allows effective testing off grid would be quite difficult.

To start with, a House bank layout that allows easily taking half the bank at a time offline to be broken down to individual units.
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Old 13-06-2019, 06:38   #7
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Re: Battery replacement

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Originally Posted by john61ct View Post
Not "can", pretty much "must".

To design and build a setup that allows effective testing off grid would be quite difficult.

To start with, a House bank layout that allows easily taking half the bank at a time offline to be broken down to individual units.
I guess everyone has their own method.

Here at work, we use the battery capacity test on our 24-72 battery Leibert (Vertiv) and APC UPS Units

On the boat, I take the common sense approach.

I know about where the battery voltage should be most any time of day just from experience so if that changes then I will investigate further.

Collecting data at the dock and on short overnight and weekend cruises allows you to establish a battery base line

But to each his own. Others will want to use the best battery testing available down to the nth degree, but I'm not that guy.

I have two batteries, so if one fails I have another.
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Old 13-06-2019, 07:52   #8
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Re: Battery replacement

Quote:
Originally Posted by john61ct View Post

Credit to Maine Sail, https://marinehowto.com/programming-a-battery-monitor/

Better yet read it all, and check out the rest of the site.
john61ct...

I don't think poor nester understood a word of your garbled and shorthand post - except the link to Maine Sail's site. The only useful piece of information you have given for a longtime!

Quote:
Originally Posted by Nicster View Post
Hi, I'm a newcomer to boat ownership having bought my first boat a couple of months ago.
Cheers Nicster
Read Maine Sail and search these forums before asking again. It's a long hard climb up steep learning curve. No one post here is going to give you the answer in a form you will understand - you need a book and that hasn't been written yet.

I'm afraid Nigel Calder's book is much too technical for a newbie on a steep learning curve.
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Old 13-06-2019, 08:02   #9
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Re: Battery replacement

Pretty patronizing to assume OP is incapable of getting themselves up to speed.

OP, if you aspire to electrickery knowledge in-depth, just work your way through my post, maybe copy/paste to a working scratch doc

that can be the start of a solid knowledge base, just edit as you go gathering new information, building a list of questions.

Do a bit of googling for unfamiliar jargon and if something still isn't clear, feel free to ask Q's.

Or, if you do **not** want to learn this stuff, it is true that most boaters get by fine on intuition and common sense.

If that's your route, and the bank's not too dear just start replace it when you start to notice its performance is declining.

But to get the charge specs, we do need the specific info asked for above.
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Old 13-06-2019, 08:39   #10
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Re: Battery replacement

Maybe ask all those saying "you don't need BMs etc" claiming familiarity + intuition is enough, or that it's "better" to just replace the bank every few years.

I don't, but learning this stuff isn't hard, for those who enjoy it.

I just acknowledge the above approaches are a valid alternative, comes down to personality and preference as much as anything else.

Just like deciding for yourself where you draw the line on sharing about personal details.
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