Just Another Battery Thread

MattB92

MattB92

Active Member
Nov 9, 2019
272
Barrie, Ontario
Boat Info
1997 370 Sundancer
Engines
454 Merc's
I've tried searching all the different battery threads but none of them answer my question or the advice I am looking for.

I've owned the boat for 3 seasons and I am not sure when the batteries were replaced last. Near the end of this season when out on anchor for even just a few hours it seemed like the batteries just didn't have enough juice to start the engine. So time to replace now as a piece of mind.

Current setup: 2 Deep Cycle batteries wired in parallel. Part # 27DC-180 (Dual purpose Series for deep cycle and cranking) Didn't even know that existed - shows what I know about batteries.

My thoughts were to add a 3rd battery. Have 1 Cranking and 2 house? Would like to be able to spend a week out on the hook and not have to worry about draining the engine battery. My question is about adding the crank battery. Would you also wire this to the on board charger? Or does it only get charged by the alternator?
 
I'm also thinking of selling this boat next spring... so maybe I just do a direct replacement and no upgrade at all? Lol :confused:
 
Do you currently have a battery switch (1-2-Both-Off)?

Are you sure about your current batteries being dual purpose batteries? The "DC" in the name sounds like it's indicating deep cycles.
 
OK... so you have a switch but only one "bank", then. Meaning, you only use one "selection" on the switch. A "bank" is any number of batteries hooked together - basically making one, bigger battery.

Put two, dedicated DC's (in parallel) on, say #1 of the switch and put a dedicated starting battery on #2. Both banks will be charged by the alternator (assuming it's hooked up correctly, of course) when on "both" or (better) when selecting #1 or #2 directly. Yes, hook-up your onboard battery charger (ideally a 3-bank charger) to each battery. You can "get away" with a 2-bank, but it's not the best.
 
MattB92 said:
I've owned the boat for 3 seasons and I am not sure when the batteries were replaced last. Near the end of this season when out on anchor for even just a few hours it seemed like the batteries just didn't have enough juice to start the engine. So time to replace now as a piece of mind.

Current setup: 2 Deep Cycle batteries wired in parallel. Part # 27DC-180 (Dual purpose Series for deep cycle and cranking) Didn't even know that existed - shows what I know about batteries.

My thoughts were to add a 3rd battery. Have 1 Cranking and 2 house? Would like to be able to spend a week out on the hook and not have to worry about draining the engine battery. My question is about adding the crank battery. Would you also wire this to the on board charger? Or does it only get charged by the alternator?
Click to expand...

Depending on what you run at anchor, a week without recharging might be a tad much...

But an easy way to increase capacity a bit would be to upsize your two G27 dual purpose batteries to two G31 dual purpose batteries... if you have space for the slightly larger footprint.

The measure of capacity is amp-hours, and you can look up specs from various battery makers to compare. (FWIW, best I've found are Odyssey Extreme AGMs, and the cost a leg.)

Under normal circumstances, battery makers usually recommened you discharge to no less than 50% of capacity.Perhaps your G27s are giving you 80Ah each, 160Ah total, 80Ah usable before recharge? A pair of decent G31s could give you 100Ah each, 200Ah total, 100 usable before recharge...

There are other ways to increase capacity -- 4x 6V golf cart batteries wired in series/parallel, for example -- but that comes with additional footprint requirements.

I'd say easiest is to first measure to see what kind of space you have available, see if G31s will work...

-Chris
 
ranger58sb said:
Depending on what you run at anchor, a week without recharging might be a tad much...

But an easy way to increase capacity a bit would be to upsize your two G27 dual purpose batteries to two G31 dual purpose batteries... if you have space for the slightly larger footprint.

The measure of capacity is amp-hours, and you can look up specs from various battery makers to compare. (FWIW, best I've found are Odyssey Extreme AGMs, and the cost a leg.)

Under normal circumstances, battery makers usually recommened you discharge to no less than 50% of capacity.Perhaps your G27s are giving you 80Ah each, 160Ah total, 80Ah usable before recharge? A pair of decent G31s could give you 100Ah each, 200Ah total, 100 usable before recharge...

There are other ways to increase capacity -- 4x 6V golf cart batteries wired in series/parallel, for example -- but that comes with additional footprint requirements.

I'd say easiest is to first measure to see what kind of space you have available, see if G31s will work...

-Chris
Click to expand...

Footprint is fairly tight but I'll definitely look into those options. Again I don't plan to own this boat for much longer so might just stay with the G27's. Thanks Chris!
 
MattB92 said:
Yes - I typically just keep it on Both. I probably should of changed it to 1 or 2 when on the hook so only draining 1 of the batteries, correct?
Click to expand...
If the batteries are truly hooked up in parallel then they are essentially one big battery and only one selection on the switch (either #1 or #2) is used. Maybe you're just using the wrong terminology? If you have a separate B+ cable to each battery from the switch, then they are not hooked up in parallel. I would "assume" they are not actually in parallel (that would not be the norm).
 
Lazy Daze said:
If the batteries are truly hooked up in parallel then they are essentially one big battery and only one selection on the switch (either #1 or #2) is used. Maybe you're just using the wrong terminology? If you have a separate B+ cable to each battery from the switch, then they are not hooked up in parallel. I would "assume" they are not actually in parallel (that would not be the norm).
Click to expand...

I'll have to get down to the boat and check.. I thought they were wired in parallel

upload_2022-10-14_13-18-18.png
 
Good pic. That's not parallel. See the two, separate B+ cables? Each one of them should go to a separate post on the switch.

So... back to the beginning... :) One large (or two) DC's on one switch position and a starting battery on the other position.
 
Lazy Daze said:
Good pic. That's not parallel. See the two, separate B+ cables? Each one of them should go to a separate post on the switch.

So... back to the beginning... :) One large (or two) DC's on one switch position and a starting battery on the other position.
Click to expand...

Yea makes sense. I didn't even look at that picture till now when I started digging through my phone. I should of probably went down and confirmed everything before asking questions lol, Thanks!
 
MattB92 said:
Yea makes sense. I didn't even look at that picture till now when I started digging through my phone. I should of probably went down and confirmed everything before asking questions lol, Thanks!
Click to expand...
I have a cranking battery wired to the #1 switch and Deep Cycle to the #2 switch. When we leave the marina I turn the switch to both so they are both charging while cruising. As soon as the anchor is set I put I switch to #2. Then when it's time to pull anchor I switch to #1, start the engine and switch to both. In all honesty I don't switch the batteries myself, I just tell whichever family member is closest to the battery switch. :D
 
MattB92 said:
Footprint is fairly tight but I'll definitely look into those options. Again I don't plan to own this boat for much longer so might just stay with the G27's. Thanks Chris!
Click to expand...

I see your batteries are actually switched, not parallel. Just to add a bit of detail, that should mean you only have approx 40Ah usable capacity (1/2 of a notional 80Ah or whatever your G27s offer) before recharging when you're using only one battery for house service.

Upsizing to G31s would make that 50Ah usable capacity before recharging.

Following typical maker's guidelines to discharge no more than 50% for longest life.

-Chris
 
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