Which Battery Size

[Master-Spud]

So some AGM batteries are on sale and I am debating if I should make the upgrade and if so to what size. Currently have a single lead acid battery group 24 that came with my boat. I have another group 24 I bought but never used from a year or 2 back for my camper. Which I could use when I do my dual battery install. Now seeing as I upgraded the stereo, with an extra 2 amps, (both about 900watts each) sub and tower speakers power consumption is gonna be up.

I won't have shore power to top things up after ever outting, and I tend to spend a week at my lake lot (no power but I do have a generator) I am adding on onboard charging system, but that will only help when back to civilization or if I lug out the generator (looking to avoid that). So the question would be, do I try the group 24's and see if that's enough. Or just bite the bullet and pick up 2 bigger ones? Group 27's big enough or just go to 30's. I know obviously this all depends on how much I use the stereo and how much I run the boat. But just looking at some rough guesses from people that have had smaller batteries and had to go bigger to keep up with their needs. Saving 50 bucks a pop off each of the batteries right now, so worth looking into.

 

[Seadeals]

Since you can't maintain them regularly, you will have more margin with a larger size. Prices increase with size but with your use a 27 would probably be worth it. I have two 24s but trailer and always charge after each outing. I need two new batteries this season. What batteries are you considering and where is the discount? It would be nice to save some dough.

 

[David Analog]

That is a mixed bag of worms. That long week is the killer. Here is just one way to look at it.
More battery capacity normally means less percentage discharge which is healthy for the batteries.
More batteries than you can properly recharge can easily go from an asset to a liability. Depleted batteries can represent a serious load on the boat charging system, a system that is limited capacity.
The wild card here is the usage. And that will vary in intensity and time and more. Everyone's usage is different. To end the speculation, which could be all over the map, measure the results under real usage conditions.
Use the two batteries you have and measure the battery voltage at key times.
Before the outing.
After running about for a day and before a stay at rest.
After the stay at rest.
At the end of the day.
A new fully charged lead acid battery, after any charger or alternator higher charge has dissipated, will be about 12.7 volts, and just a bit more for an AGM.
At no time should you ever discharge your batteries below 11.8 volts.
You'll learn a lot about the system behaviors. And from that you can design your battery system and your charging routine. It will also show you what adjustments you might have to make to protect the system. For example, on a long holiday weekend, without an available shore charge, you may not be able to use your audio system all the time for intervals in order to preserve your batteries, both in short term performance but also lifespan.
A very large solar panel with a smart control module would normally help. But it does you no good overnight. It's also limited current and you need good current to recharge deeply cycled batteries. But a charge in any form is better than no charge.
Maybe a third fully charged battery stored in the RV and used for rotation.

 

[Murf'n'surf]

Go for the 31's. With your amps, why not have that extra capacity.

Remember that these motors can not charge your batteries fully after depleting them. Can you run a loooong extension cord? Your charger probably takes 4-5 hours to complete the full cycle so running the generator kinda sucks. You'll probably be fine after a week as long as you are running the boat a lot and not jamming tunes while floating all day.

 

[Master-Spud]

Thanks for the replies, unfortunately running a long extension cord isn't really an option... Boat it anchored off shore, and no dock or anything so running a cord through the water just sounds plain dangerous lol

 

[CrankyGypsy]

just a thought to increase your options: I recently read a 2011 article that the thought of "don't mix battery sizes" is not really an issue. this was tested by Lifeline, but I can't locate the original findings. it is only an issue if the batteries are different ages or different types (internal design).

http://www.morganscloud.com/2011/10/03/mixing-battery-sizes/

 

[Bruce]

I am running two group 31 AGM batteries in battery boxes.

Sounds like you are a good candidate for solar charging.

 

[Master-Spud]

Solar is tempting, only issue I see is what the heck to do with the large panel needed when not charging, kinda of a pain to bring it to shore each time I take the boat out, and small enough to fit on the boat storage and I can't see it doing enough to make a difference

 

[Bruce]

Solar is tempting, only issue I see is what the heck to do with the large panel needed when not charging, kinda of a pain to bring it to shore each time I take the boat out, and small enough to fit on the boat storage and I can't see it doing enough to make a difference

Look at this thread https://jetboaters.net/threads/solar-charger.4938/

The $60 30 watt solar panel that I am using for the kids playground is 26.4" x 16.1" x 1" and 6.5 pounds. I believe you would be able to store a couple of them under the seats or build storage under the hood.

A pair of those with charge controllers would recharge your batteries during the week.

 

[David Analog]

just a thought to increase your options: I recently read a 2011 article that the thought of "don't mix battery sizes" is not really an issue. this was tested by Lifeline, but I can't locate the original findings. it is only an issue if the batteries are different ages or different types (internal design).

http://www.morganscloud.com/2011/10/03/mixing-battery-sizes/

Thank you for that as it was some good reading.
The shared batteries in a single bank need to be of the same chemistry and age. But they do not have to be the identical size. Out of the gate it doesn't pass the smell test. But once explained in those terms it makes perfect sense.

 

[Seadeals]

So I did upgrade my two group 24s to a new matched pair of marine maxx group 29 lead acids. I greatly appreciate all the great info on the forum including excellent recommendations for AGMs; but could not justify the extra expense of them at this point. I now have a lot more capacity.
OLD SETUP


NEW SETUP

I do have one question. Since they are both identical and assuming I have charged them back fully first, is there any reason I should not hook my BatteryMinder up to one and select the 1+2 on the battery switch to keep both maintained together? The 1500 model I used can maintain two at a time.