Battery Choices

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I like dual purpose batteries, combo starting and deep cycle. I also prefer both to be the same type/size, as I don't run a house battery.

Rather than any one brand, though Excides have always treated me well, to me it is far more important to get a FRESH battery. All batteries are marked with the "date made" and I won't buy one unless it's within 2-3 months. Then I always charge it before use.

Keep it clean, always keep the eletrolyte level up there, and charge after the season and then before the next ... good to go. I could get 5 years out of one I bet, but I stagger them and replace each every two years so that there is always a newer one and the older (but proven) battery on board.

My 2004 has two combo deep cycle/starting series 29 batteries from Parker. I have two questions.

What is the advantage of the combo batteries? I will probably buy them agin because Parker used them but would like to know why they are preferred.

Second, everything I have alway read about batteries recommends to replace both at the same time because the older/weaker battery tends to drag down the newer battery. Have you heard that and are you concerned at all with staggering them every two years as you do?

My rig was two years old in Feb 06 and the batteries are strong, but I was thinking about replacing both soon just to eliminate any problems. Often I am 50 to 75 miles from a ramp and sometimes there are no other boats around. I try to eliminate any potential problems even if it means discarding batteries that would work longer. If I boated near other boaters and within a few miles of a marina, I would tend to stretch the length of service.
Wild Bill":37jikh2y said:
What is the advantage of the combo batteries?
Simple, at least for low-tech carb'd V6 outboards ... they really don't need the starting oommmpphhh that large inboards or diesel motors do. To me, it is the drain of running your depth, fish, vhf, gps, radar, lights, livewell pumps et all that puts the "discharge" into the battery. Note that some newer OBs, like high tech 2-stroke DI ones and newer 4-strokes, need more battery oommppph to make sure the computer starts up. Just always be sure and use at least the minimum your manual calls for. For example, Group 27s are more than enough for a older V6 outboard.

Starting batteries are made to handle a few large high amp draws on the plates, while deep cycle batteries are made to withstand deep slow discharges and frequent recharging. Combos, IMHO, give you the best of both worlds.

Wild Bill":37jikh2y said:
Second, everything I have alway read about batteries recommends to replace both at the same time because the older/weaker battery tends to drag down the newer battery.
That is only true "if" you run with the battery switch on BOTH and then shut off the motor(s) ... the stronger battery will equalize into the weaker one.

What I do and recommend, since today is the 11th - that is an odd number, so I'd run on battery switch #1. I key the battery used to the day and figure it averages the use on both batteries. You know ... I have seen new batteries fail, so I prefer the comfort of having an older, but known good battery, along with a new or newer one.

My brothers and I run up to 11 OBs each season and if I count my Dad too, we've been staggering battery purchases like this on our boats for a loooooooooong time (Dad almost 50 years boating, me almost 30), with zero battery issues ... less the ONE (last) time when my brother ran with the battery switch on BOTH and shut them off and they equalized ... I had to go get him ...
Thanks for the info Dale.

I have seen new batteries fail, so I prefer the comfort of having an older, but known good battery, along with a new or newer one.

That is a good point that I did not think of. Maybe I will get both batteries tested and if they are still fairly strong, buy one new one each year. If they are not both strong, I will get two new ones.
My usage system isn't quite as involved as Dales, and my hardware isn't as 'electrically strong' as the battery system on Warthog's X-Shark... Mine is pretty basic, and so far its worked well for me.

I run a pair of Optima Group 31 Blue Top AGM's in my 2520 MVSC, and when I go out, I put my battery switch on "Both". :)

When it comes time to replace the Optimas, I'll either put in another set of the same, or install a set of Lifeline (or the equivilant Cabella) AGM's.
I am absolutely sold on AGM's in a marine environment.

My OX66 doesn't require huge amounts of power like some of the newer motors do, and my major power draws are my Garmin 2010c GPS/chartplotter, Furuno FCV-600L sounder, Icom 502a VHF, and a couple of other minor items.

I've replaced almost every light on my boat with LED's, so other than the basic helm electronics, I don't have a very big power draw.
Oh... and I do carry a "jumpstart" aux battery for emergencies. :wink:

Thats my story and I'm sticking to it. :D
I have two deep cycles that I changed in May 2000, from a 1996 boat.

At the time I wasn't able to find the same marine cranking amps so next time I will plan it. I went from 690 MCA to 625.

Food for thought too, I may have swapped out good batteries in retrospect. I was prepping the boat for launch and back then I used to take them off the boat for the winter. I had checked the water levels and then cranked them. I got nothing. I figured I had damaged them charging them in the cellar over the winter. I turned them in for new ones then wondered if I had the ground connection good enough.

They are group 24 batteries and tehy are a pain to get in as I have an I/O with freshwater cooling so space is tight.

I added shorepower in 2002 and now leave them in the boat during the winter with power to them all winter. I check the water in them at least once a year while they are in the bilge.

Mine was rigged with group 27 combo batteries by Parker. They worked well for 4 seasons, but at the end of last season, there were a couple of times, one or the other couldn't crank the motor, so I had to put the switch in "both" position. Since I have GPS/chartplotter, FishFinder, and Radar, the drain can be heavy. When I anchor for the night, I turn the radar and FF off, and only use the GPS with the "anchor alarm" set. That has helped on my overnight trips.

Since I have twin F-115 motors, each motor has its own battery. I have two switches, but leave the port motor pointed at the port battery and the starboard motor pointed at the starboard battery. Nothing complicated about twins.

Just before my last CBBT trip in Nov, I bought new group 31 combo batteries. Can't remember off the top of my head which brand, but I got them at Bass Pro. The group 31s wouldn't fit in the battery trays, so last weekend I change the trays out. I posted a thread (with pics) on the Projects board concerning my upgrade to group 31 batteries.

If I have a problem with the electronics pulling down the starboard battery this year, I'll probably drop a "house battery" onto the plate below the deck and install a smart splitter for keeping it charged. Will move all the electronics to that battery. Will let you all know later in the fishing season.
Let's talk about this "BOTH" issue. Pro's and cons. My dealer said to leave the switch in "BOTH". Why or why not? Do both batteries charge when in "1" or "2"? Is the switch only for starting?

Wow these seem like basic questions, but I just realized that I never really thought about before.
rangerdog":6zkxhhj2 said:
Let's talk about this "BOTH" issue. Pro's and cons. My dealer said to leave the switch in "BOTH". Why or why not?
When running with the switch on BOTH and then shutting off the OBs, you run the risk of having the batteries equalize. Let's assume one is at 90% and the other is at 60%. It's a nice warm day and the OB fires easily, but you don't run that far and then you setup to drift or just stop and anchor. Your strong battery will discharge into the weaker battery ... so now you have 2 @ only 75% capacity, which may or my NOT be enough to start the OB, specially if you sat there for a few hours and ran the AM/FM radio and other electronics.

Do both batteries charge when in "1" or "2"?
No, if on 1 or 2 only that battery charges unless your boat and its systems have an isolation/combiner feature built in, which FWIW are now coming equipped on some of the newer OBs.

Is the switch only for starting?
I'd say for starting and charging, but I myself DO NOT put it to BOTH to start the OB unless I try it on the one I'm running on and it isn't strong enough to fire the OB off ... which has never happended to me by the way.

Also to correct a common misconception/urban boating myth out there ... you cannot "fry" a battery by charging both of them when running with the switch on BOTH.

Running on BOTH batteries, my addition to this topic:
I will run with the switch on BOTH when running offshore 10-20 miles in search of tuna, this way I know I'm kicking some charge into both batteries, since I'll be cruising along (seastate permitting) @ 4K RPMs or higher. If/when trolling, I'll still leave the switch on BOTH, but if I setup a slick or stop the boat and shutdown the motor, my practice is to immediately put the switch to position 1 or 2.

Again, I use the day of the week method for battery management ... taking the day's date, and using that battery position to match the odd or even date.
Back when I was researching marine batteries, I found this page which does a good job of defining flooded cell, gel cell, and absorbed glass mat (AGM) batteries. Hopefully it will help others who are pondering what to buy.

FWIW... I installed Optimas in my boat, but when it comes time to replace them, I'm considering switching to Lifelines as they have a higher CCA rating than the Optimas.

Here are a couple more, with some links specifically covering the AGM's... ... 0Batteries ... stid=19696
Cool. Thanks. I need to do a little more research, though relative to the isolation/combiner issue.

I'm also researching batteries, Kevin. So thanks for the links.

Question. My boat is a 2003, outfitted in April which makes my batteries 3 years old (in the boat). Do I trust them for another year? They have never been "stressed". Is there a safety "rule of thumb"?
Rangerdog, some others will weigh in.

I have some general ideas I follow. One is that the batteries have warranty periods and they generally reach or surpass them. Some people swap them out as a prevention early. I think that is an individual decision, and one that is based on what you have done with the batteries over their life, that nobody else knows about.

I have an I/O so I have to open the cover to switch batteries. I start with both and run with both. If I stop for awhile I switch to one battery, especially when I anchor. Some times of the summer I'll have my fridge on too that adds to the amp load.

Deep cycles take more abuse, combos are next and starting batteries have more limited cycles if you drain them under 50%. All of these wet cells aren't as durable as the other choices.

The only time you should use "both" is when a single battery won't crank the motor. Otherwise, the general guidance is to use position "1" on odd days and position "2" on even days. Another option is to use "1" outbound to the fishing grounds and "2" when inbound.

Best approach is to get an isolator/combiner. From a single charging wire, an isolator can independently charge 1 or 2 or 3 batteries. Whichever battery is lowest, gets the most charge. And when all three are charged, it cuts off so as to not overcharge any single battery. The problem with the "both" position is that both batteries get charged whether they need it or not. One battery may need charging, while the other is full. The full battery gets "overcharged" and only ends up boiling off the water and destroying the battery. So...if anyone told you to run in the "both" position for anything else than emergency, they are smoking something strange, or drinking too much coolaid.

Since I have twins, I point the starboard motor switch at the starboard battery (which is also the house battery to drive the electronics), and I point the port motor at the port battery. Each motor charges its own battery. When I run the electronics too long at anchor the house (starboard battery) gets drained to the point it won't start the starboard motor. That is when I put the starboard switch in the "both" position, start the motor, and then switch it back to the "1" position so it can charge its own battery.

Someday I'm going to install two insolators/combiners and then I can leave the switches alone--the digital circuitry in the isolators will automatically figure out which motor will charge which battery.

There is an electrical wizard over on who has posted a series of pics of how to rig 1,2,3 batteries with 1 or 2 motors. I'll try to find them if Kevin doesn't beat me to it.
For about $120 you can put the one/two/both argument to rest for good. As a plus you don't have to worry if it is an odd or even day. :wink: A battery managment cluster with a voltage sensative relay like the one that I use from


will insure that your starting battery is always charged and, when it is charged, apply charging current to the house battery and keep it topped off as well. You don't have to think about it. The device does it for you. In the event something bad happens you are still afforded the ability to parallel both batteries to get your motor running.

They have one for dual motor applications as well:


like most "rules", a understanding of the issue allows one to solve
things while evading some of the dictums floating about...i have 14 seasons on my 2520, and i did all the electrical work on the boat from
scratch myself in order to configure it to suit...let me debunk several

[1] never run on "both".......not long as there is 14 or so
volts in the system, nobody is dragging down anybody else.....the
problem is if left BETWEEN TRIP on "both", one battery failing, ie
below 12 volts, would/could pull on the other leading to total
discharge in the absence of any inputted charge....a better saying
is dont STORE on "both".

[2] the other problem with "both" is that the CHARGING PROTOCOL
varies with the construction and with the temperature of each battery
......thus, you definitely should buy the same TYPE of battery and
ideally the same mfr......they do NOT have to be the same size or
the same age (assuming both are sound, which your dmm should
assure you with a proper resting voltage determination)....dont mix
gels and wet or gels and simply cannot possibly charge
both of them properly; the agm/wet charging protocol will BURN OUT
the gels, and the gel protocol will seriously UNDERcharge the agm/
wet ones.......

[3] the key is what is the engine doing???.....a digital ammeter is
invaluable! WILL pay for itself, so it is free!.....if your engine
puts out 14.3 or more, it isnt suitable for gels...both agm and the
serious wet ones can be great.....agm doesnt need "equalization"
and doesnt gas.......

[4] four years is about the end of the working life for gels and most
others.......if you are following what is going on with the digital amm
eter and digital voltmeter, you will see the slide in performance in
plenty of time.

[5] the TEMPERATURE of the battery matters in the charging rx...
a battery in an engine room will charge differently from a cool
one in the cabin (this is my own setup).....

my own design is this......the diagram is basically an X.....on arm goes
to the alternator, one to the starting motor.......on the opposite end,
one arm goes to a grp 27 gel and the other to a grp 8 gel....there is
a battery switch at the "crossroads", whereby i can use either or both
to start the engine or i can charge either or both.....i carefully located
the battery switch in the COCKPIT for ease of viewing and changing...

normally, i start the engine on both and run for 15-20 minutes on that
to replace the starting /27 battery......i charges just fine early in the
trip UNTIL IT WARMS UP.....then my charging settings no longer are good
for it and are proper only for the cooler house battery in the cabin.....
once i switch it to "house" at 15-20`, i leave it there for the duration of
my cruise........

i have a three stage external regulator whose settings are adjusted to
gel (only) and to "cool gel" setup only would give good
results with gels at its current settings.....i monitor the electrical action
constantly on the dash with a three-way (toggle) meter with volts, amps,
and amp-hours (on the house battery).....i have 340 amp hours total,
of which i never use more than 1/3, ie 110 amp hours over three to four
days at anchor without any engine running or any shorepower.

the less-than-half discharge design and policy is what gives me four years
from every house battery and six to seven for the starting batteries......
gels are damaged by high voltages, especially at the END of the charging other upgrade, based on the meter readings was to go to
a high output alternator, e.g 100 amps at 200 F.....the standard alternator
was rated at 65amps, but never showed anywhere near that, ie about
25-30 amps max......the better alternator goes to full output at 850-900
rpm, so it charges when trolling .....note that the bigger alternator requir
ed rewiring the engine, but the wiring i did anticipated the increase.

i think the main cause of battery failures are

[1] the mfr "rate" their products in a fraudulent fashion throughout
this industry; your alternator probably WHEN HOT only puts out half
to 2/3 on what they claim....the battery can only be charged (by most
regulators) to 75-80% of what they are "rated"....and that is when the
battery is BRAND NEW......most guys or their mfr surrogates spec out
an electrical system naively on these numbers as given.....between
discharging beyond 50% and charging back only to 75%, they are
working within only a QUARTER OF THE BATTERY THEY THOUGHT WAS sum it up:most every recreational boat has an under
sized electrical system.....every aspect needs to be LARGER, the
wires, the fuses, the distribution panel, the batteries, the alternator,
and the metering......parker actually does a better job than most
any other mfr with this, as with everything else......when thinking
onboard electrical , bigger is unequivocally better

[2] as well as DESIGN, the other crucial feature to success is the
METERING.....imagine if you had neither any knowledge of fuel
consumption (and i dont mean a fuel meter necessarily) or of the
amount in the tank?!....that IS the situation in most electrical systems should check the fuel level in your tank every outing, and
have a low threshold to FILL....the electrical system is the SAME DEAL
.......measure the resting voltage of all batteries before each outing...
monitor what the engine is putting into them continuously as well as
what all sources are pulling out......electricity (in amp-hours) needs
to be handled just as one would the fuel (in gallons).....

if you put together the commonly found poor DESIGN, the almost routine
UNDERSIZING, and the almost universal lack of continuous MONITORING,
the widespread discontent and underperformance of recreational boat
electrical systems is totally predictable ...... and thus, it really does go
quite beyond little homiles and rule-making.....dan
FishFactory":3m7oka5o said:
A lot of good info, anyone know who makes BassPro batteries?

According to Google, it is "Victron Energy" whoever that is.
I'll try to track down the thread(s) over on TheHullTruth about running in the "both" position. Guidance was fairly strong as I remember, because the charging circuit sees one battery, that is an average of the two batteries. As long as the two batteries are almost identical in the level of charge they have all is OK. Trouble comes when one battery is beginning to fail and it drags the average down. The charging circuitry keeps charging thinking the battery needs it. However, the one fully charged battery keeps getting much more of a charge than it needs.

This situation is what precipitated the need for the isolator/combiners shown above. They automatically sense which battery needs what percentage of juice from the charging circuit, and when all batteries are full, tells the charging circuitry to cut back.

I'm sure you know your electrical stuff, but I think I'll follow the guidance of the two or three EEs over on the other site.
Since my earlier posting I have read everything I could get my hands on relative to the subject and I think when I replace my batteries, I am going to go with a pair of Optima AGM deep cycles with an isolater or perhaps a battery management cluster like Go Fish referenced.

I now understand about the "both" issue but I also understand what the "Doc" is saying and in fact he essentially practices proper maintenance by switching off of "both" 15 minutes into his cruise. His analogy to fuel status is on the money and a good way to think about it.