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Old 03-14-2021, 02:40 AM   #1
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Default Batteries/Starting

Hi All,

I have a 1997 Maxum 3000 SCR with a single 496 Mercruiser and a battery switch with "1", "2" and "both". It has 3 brand new batteries (I had them installed when I bought it recently).

My issue is the boat will only start when the switch is set to "both", which as I understand it shouldn't be the case. When using either battery 1 or 2 alone, the engine cranks but won't start. The battery charger appears to be functioning correctly.

Any thoughts/suggestions would be appreciated. I haven't check the voltage coming from each battery but will do that. Is there a minimum voltage reading I should be getting from each battery in order to start the engine with a single battery?

Thanks very much,
Ken
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Old 03-14-2021, 11:48 AM   #2
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Does it attempt to start when on battery 1 or 2? You should trace the battery wires to see how they are connected and make a wiring diagram to help understand why you have this condition.
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Old 03-14-2021, 01:52 PM   #3
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Does the cranking speed vary between the Both vs 1 or 2?
If it’s spinning faster/easier on Both then I’d suspect that the batteries are tired and should be load tested to see if they are any good.

Measuring the voltage while they are at rest will not confirm wether they are good.
You’ll need a voltage measurement while cranking.

A general rule of thumb is good battery will have 12.5 to 12.7 volts at rest and will not drop to below 10v during cranking and will pop back to the static reading after the load is removed.
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Old 03-14-2021, 04:53 PM   #4
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What type of batteries were installed? You should have a Starting Battery on pos #1 and Deep Cycle Marine Batteries on pos#2....

Start and run on pos#1 and switch to pos#2 when on the hook.
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Old 03-14-2021, 11:01 PM   #5
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Thanks for all the answers. In looking at the batteries today I believe I have the following setup:

Position 1 = Two Napa marine batteries (https://www.napaonline.com/en/p/NBP8240)

Position 2 = Napa Marine deep cycle starting battery (Here's a link to the actual battery: https://www.napaonline.com/en/p/NBP8274)

The engine does attempt to start on battery 1 or 2 alone, in that it cranks repeatedly but won't start. The cranking speed seems about the same regardless of whether I'm on one or both batteries.

I measured the voltage at rest and while cranking the engine and got the following:

- Before attempting to start, all batteries were at ~ 12.6 - 12.8V.

- When attempting to start with just position 1 or 2 alone, neither positions' voltage fluctuated much while cranking (Voltage jumped around between 12.6 - 12.8V, but didn't really drop). Neither position started the engine consistently (see below).

In the "both" position the engine always started. Voltage started at 12.6, dropped to 11.9/12.0 while cranking, and went back up to 12.8V after the engine started.

Interestingly, after running the engine for a few minutes, then trying to start in position 1 and 2 alone, the engine did start once from both positions alone. I wasn't able to repeat it after that (tried many times). Unfortunately I wasn't measuring the voltage the one time it did work correctly.

The wiring basically looks like the attached diagram, with the only difference being "battery 1" is the two smaller batteries connected together.

I'm more confused now than ever......
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Old 03-14-2021, 11:29 PM   #6
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Were you measuring directly at the batteries? I wonder if you switch is worn and causing a voltage drop. Try measuring the voltage at the common terminal with all three cases; 1, 2 and both.
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Old 03-14-2021, 11:37 PM   #7
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Yes, I was measuring at the individual battery terminals. Sorry which are the common terminals I should measure at?

Thanks, this is all new to me!
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Old 03-15-2021, 12:25 AM   #8
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Quote:
Originally Posted by kt_maxum View Post
Yes, I was measuring at the individual battery terminals. Sorry which are the common terminals I should measure at?

Thanks, this is all new to me!
As shown in the picture you provided the common is the output of the switch going to the starter.
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Old 03-15-2021, 12:59 AM   #9
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Right of course! Great suggestion. I'll check that and circle back.
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Old 03-15-2021, 02:00 AM   #10
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While you are testing voltage, check the positive terminal on the coil.
Since it’s cranking and you don’t seem to have a fuel issue, that leaves spark.
Low battery voltage at the coil will not produce a hot enough spark.

Coil should have no less than 10v while cranking to produce a good spark needed to start the engine
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Old 03-15-2021, 02:59 AM   #11
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Thanks will do!
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Old 03-15-2021, 12:02 PM   #12
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When testing the voltage on the coil donít use the negative terminal for ground reference as it is not ground. Use the battery negative.
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Old 05-04-2021, 05:09 AM   #13
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Just a quick update to this post with the fix that solved the problem. I had to replace both the positive and negative cables running to the engine. Both were showing signs of corrosion and weren't able to carry sufficient amperage from a single battery to start the engine. Once replaced the engine starts right up on either battery alone.
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Old 05-04-2021, 11:22 AM   #14
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Resolved in time to enjoy the boating season.
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