Failed 1 yr old battery??

[mr300z87]

Last weekend we met some friends out at Tices shoals a local anchor spot, in our Sea Ray Sundance 250, when we first got there I switched over to what I thought was my start battery and tried to re start the engine with out success. Switched back to both batteries and she fired up no problem. Used minimal power while out and made it home no issues. Later in the week is did some testing to find that I labeled my switch wrong and the crank condition was on the house battery. The start battery is a group 27 700 CCA marine start with a 4/2017 date and the house a group 27 600 CCA marine deep cycle dated 4/2018. With the switch on start battery only the starts engine no problem, house battery only and it won't crank the engine and volts drops to less then 7 volts when I attempted to start then back to 12 volts when stop cranking, this with a volt meter connect direct to the battery. All other accessories work on either batteryl The boat has a charger and is charging all times when in the slip. Both batteries seemed to be fully charged at time of testing. I would assume the the house battery should be able to start the engine and has failed even though it is only a year old. It is wicked hot here in NJ this weekend so I have not had a chance to go back at check the fluid level in the cells. My question is should a group 27 600 CCA deep cycle battery have enough power to start a 7.4L? Of couse I bought the battery last June and it has a1 year warranty. As always advice is always appreciated.

 

[alldodge]

start battery is a group 27 700 CCA marine start with a 4/2017
house a group 27 600 CCA marine deep cycle dated 4/2018

Start bat will start the motor
House bat no crank and drops to less then 7 volts

My question is should a group 27 600 CCA deep cycle battery have enough power to start a 7.4L?

Your motor is a 7.4

I see two issues
No bat should go down to 7 volts
A deep cycle does not allow enough amps to be drawn to crank over a BBC

So no a deep cycle should not be enough for a 7.4 to start.

IMO others may differ

 

[eric102]

Are you running the original 1990 battery charger? If so it will cook the batteries dry if left on 24/7. Only a modern smart charger can be left on all the time.

I have the 7.4 in my 1991 250 Sundancer and its always started fine on a single group 27 deep cycle. I run one deep cycle on position 1 and 2 deep cycles on position 2 which I consider the house bank.

One of the best things you can do to your boat is get a smart charger and AGM batteries which are maintenance free, charge faster, can be discharged deeper (down to 30%) and will also last longer than flooded batteries (more charge cycles).

 

[Scott Danforth]

use a cranking battery for starting

just because its a year old, doesnt mean anything. some times things die

 

[tpenfield]

Did the deep cycle battery start the engine previously? Is this a new thing or just a discovery of an existing condition?

 

[dingbat]

Merc recommends a 650 cca battery. Should have no problem with a fully charged 600 cca battery.

The question then becomes, what was the batteries charge state when you tried to start the boat?

Keep in mind, 12.7 is fully charged. 12.0 is dead for practical purposes

Dual batteries on manual switches are notorious for compromised battery states

 

[mr300z87]

Thanks for all the responses. To answer some of the questions. First both batteries seemed fully charged at time of testing. I do not know the age of the charger but it does not look to be 30 years old, next time at the boat I will get a model number and do some research. I do not recall if the deep cycle has ever been able to start the engine alone so this could be a condition I just discovered. Also of note is the starter was new merc part in 2016 and all main battery cables both pos and ground were new also in 2016 at time of engine install. I believe that the deep cycle battery has failed as stated above under major start load it dropped below 7 volts. The Start battery stayed at 12 volts during start condition. My plan is to make sure the deep cycle does not have a dry cell or two give a full charge and try again. If still fails I will replace it. I agree with dingbat it should have enough power to crank and start the engine. I can also bring it to the auto store and they could load test it as well. Have a great Sunday and stay cool if in the north east it's hot out there.