Changing batteries

Davidasailor26 · Mar 9, 2024

Any particular reason you want to switch to AGM? I have them and like them for how we use the boat, but they’re not universally better. If you don’t get back to 100% state of charge routinely they’ll suffer more than flooded. The main advantage of them is that they can take higher charge rates, but it sounds like you might not be able to take advantage of that anyway if you have a stock alternator.

I have 300AH of AGM (at 12V) with a stock 80A Hitachi alternator and haven’t had any problems yet. I see about 60A very briefly when the batteries are around 50% SOC, then it quickly settles around 40A. Your mileage may vary.

Stu Jackson · Mar 9, 2024

AGM technology is the worst for boaters ever invented.

Why? Read Maine Sail's reasons:

AGM Batteries - Making The Choice (from Maine Sail)

AGM Batteries - Making The Choice

AGM Battery Issues and the Blue Seas Dual Circuit Switch (from Maine Sail) "DARN AGM Batteries"
Darn AGM Batteries

Additional Observations on the Limits of AGMs Electrical Systems 101 - Page 3

marcham · Mar 9, 2024

Upgrades escalate quickly.

1. What alternator do you have?

2. How do you use the boat? Dock to dock, weeklong cruise at anchor, on mooring all summer long?

3. How long will you keep the boat? 1-3 years, 10 years?

4. Do you have a power monitor (ie: Victron SmartShunt) ?

The main disadvantage of AGM is if they are overcharged they will vent and then your battery is toast. A flooded battery will heat up and boil a bit, but the water can be topped up. Therefore, a good (expensive) charging system is preferred for AGM, which usually means having a smart regulator. Note that a programmable charging system for AGM will most likely also be usable for LIFEPO4 at a later date.

Most likely, if you have a stock alternator, you will be fine but the batteries would charge faster with a high output alternator and external regulator. I would test your charging system output before changing battery type. The stock alternator of most boats, by design, will undercharge the batteries.

The main advantage of AGM over flooded is higher charge/discharge rates, deeper depth of discharge and no need to be in a vented compartment (can be under the settee, etc...). This needs a high output alternator to be taken advantage of.

artboas · Mar 10, 2024

Duluthwinger said:
Hi,
My boat is a 2011Hunter 39
I am going to change out my house and starting batteries this spring.
Here are my thoughts so please let me know if I am wrong or maybe going down the wrong path.
How I'm thinking about this is that right now my house and start batteries are the flooded type and I want to change to AGM batteries.
If I just change out the flooded type to AGM everything should be OK if my charger has a AGM selection (or I will buy a new smart charger that does).
The batteries I am thinking about getting are the Trojan 6 volt AGM (4) in a series/parallel configuration.
For alternator charging the techs at Trojan said any alternator that puts out 5% to 50% of the banks AH's should be OK.
I don't know if I should somehow protect the alternator from over heating due to high amp draw to charge the new AGM house bank from a low charge.
I am going to add solar onto the boat also this spring and trying to get it all to work is giving me headaches. I am going to try doing as much as I can to try and keep the cost down.
Your thoughts are much appreciated,
Al

I highly recommend 6v AGM. Just respect the charge and discharge characteristics
I wouid never go back to flooded cells

just my opinion

Stu Jackson · Mar 11, 2024

marcham said:
The main disadvantage of AGM is if they are overcharged they will vent and then your battery is toast.

While possibly true, the only thing that can overcharge a battery is too high a voltage, and modern marine chargers are pretty capable units.

The main and CRITICAL disadvantage of AGMs on a boat is their inability to cope with PSOC - Partial State of Charge. They require recharge to full regularly.

Unless you plug in whenever you stop sailing, or you have a huge solar array (rare for monohull sailboats) it doesn't happen for cruisers. Many cruisers deal with this, but if you don't AGMs are the wrong choice.

And for most people who do plug in after weekend sailing, AGMs are a waste of money because for the way you use your boat, wet cells would be just fine (unless you have batteries in hard to reach spots; and then you just have the wrong boat

).

dlochner · Mar 11, 2024

Duluthwinger said:
Thanks for the comments I would hope to have the boat for at least 10 years ( hope I live that long).
We are hoping to cruise more with the boat that is why I will be putting solar on this spring.
The boat is on Lake Superior in Bayfield Wi so we have a lot of islands to visit and I've always wanted to go to Isle Royale and up into Canada.
The two flooded 8d batteries are in two different locations, one is a space below the floor next to the chart table and the other one is under the portside settee with the flooded start battery. I'm not sure the settee or the space below the floor is vented i will have to look to see if it is. These are a few of the reasons I was wondering about AGM batteries. I don't mind making a few changes to the charging system but if I have to change out the alternator and maybe the battery charger I'm not sure how much better the system would be over the flooded batteries.
Thanks all!

If the charger is the original charger from decades ago, a new charger is in order, regardless of the battery type. Old single stage chargers will murder batteries. A 3 stage smart charger of appropriate size from Victron or Sterling/ProMariner would be good choices. How big a charger depends on the size of the battery bank and the chemistry.

Another issue to consider, is your energy budget. How much energy will you use on average per day? From this you can calculate how big the bank needs to be and how much solar you really need.

When you say, cruising on Lake Superior, how long will you be out? How long between marina stays? What is the weather like? Lots of cloudy days means lower solar production. Lots of calm days means more motoring and better battery charging.

What are your long term plans? Are you dreaming of taking the boat out of the Great Lakes and cruising world? If so, it's important to think about the ability to scale up capacity and charging ability, especially if LFP batteries might be on the horizon.

The last consideration is space, how much space do you have for batteries? 6v golf cart batteries might be a good choice, however, space may limit this option.

jssailem · Mar 11, 2024

Duluthwinger said:
I don't mind making a few changes to the charging system but if I have to change out the alternator and maybe the battery charger I'm not sure how much better the system would be over the flooded batteries.

You have pretty much clarified the issue. Your change from racing and San Juan 24’s into cruising and the selection of a 2011 39ft sailboat suggests the charge system you have in the 39ft Hunter was “state of the art” 13 years ago. There have been a number of changes in the past 13 years. Battery charging systems evolved from dumb time based charging to smart programmable charging and monitoring.

Battery Chemistries (FLA, AGM, Lithium - Battery Chemistry - Battery Design) are evolving.

While the costs of the new batteries can put a significant dent in your pocketbook, the costs to be able to charge batteries and keep them healthy had a head start. Updates in new battery charging have extended downwards enhancing the life of all chemistries. The new charging systems can keep the old Flood Lead-Acid (FLA) batteries healthy longer. In a way new charging systems and longer life FLA power storage has driven down costs for boaters.

I know you posted a simple question. As is sometimes the case with boats. You scratch a problem and find a rabbit hole.

I approach the problems on my boat in phases. With power storage, this was to first:

solve the infrastructure to manage the charging of batteries (selecting a system that can support as many chemistries as I can afford). This meant getting a modern smart design charger (I followed the suggestions of MaineSail).
Building a solid battery power transmission system (rewiring to improve charging and safety).
Then since pocket book was a consideration, buying good true deep cycle Flood Lead-Acid batteries.

The new charging system is programmable, as such that it can be adapted to new chemistry power storage needs yet provide longer life for my FLA batteries.

artboas · Mar 13, 2024

jssailem said:
You have pretty much clarified the issue. Your change from racing and San Juan 24’s into cruising and the selection of a 2011 39ft sailboat suggests the charge system you have in the 39ft Hunter was “state of the art” 13 years ago. There have been a number of changes in the past 13 years. Battery charging systems evolved from dumb time based charging to smart programmable charging and monitoring.

Battery Chemistries (FLA, AGM, Lithium - Battery Chemistry - Battery Design) are evolving.

While the costs of the new batteries can put a significant dent in your pocketbook, the costs to be able to charge batteries and keep them healthy had a head start. Updates in new battery charging have extended downwards enhancing the life of all chemistries. The new charging systems can keep the old Flood Lead-Acid (FLA) batteries healthy longer. In a way new charging systems and longer life FLA power storage has driven down costs for boaters.

I know you posted a simple question. As is sometimes the case with boats. You scratch a problem and find a rabbit hole.

I approach the problems on my boat in phases. With power storage, this was to first:

solve the infrastructure to manage the charging of batteries (selecting a system that can support as many chemistries as I can afford). This meant getting a modern smart design charger (I followed the suggestions of MaineSail).

Building a solid battery power transmission system (rewiring to improve charging and safety).

Then since pocket book was a consideration, buying good true deep cycle Flood Lead-Acid batteries.

The new charging system is programmable, as such that it can be adapted to new chemistry power storage needs yet provide longer life for my FLA batteries.

Forum discussions about batteries are almost talking politics. You just need to be accepting of others opinions
Having said that, I’m glad I switched to AGM. On the 49, the batteries are under the floor right in the middle of the main salon
Out gassing was always a problem, and I did everything to try to minimize it. I just couldn’t deal with the corrosive smell.
My charger is Magnum, and all the settings were perfect.
Since switching, living on board has really been better.
Most evenings , I’m on a dock, and the batteries are never run flat. They should last awhile.
I know that other battery technologies work better in different scenarios. But, this was my main reason for the switch to AGM