Port side starter battery dying/dead...

[Sicilian_Sonshi]

Port side starter battery dying/dead and I need to replace on my 2010 Sundancer 330.  How do I tell which battery to replace since there are 4 of them down there.  Two close to engines and two behind them.  I had the starboard starter battery replaced last year during a service (so I can exclude that one).  Any tips?  Thanks!

[wingless]

Doesn't each bank have two batteries in parallel?

My preference on battery health is the old school method.

That is fully charge the battery for 24 hours on an appropriate charger.

Then, apply a load that would discharge a new battery in 20 hours. So, for a 100Ah battery, apply a 5A load.

Run the battery on the load until discharged. Measure the run time.

The battery is considered good if the measured capacity exceeds 50% of the nameplate rating, exceeding 10 hours in this run time test.

The new method is to use a multimeter-type device that measures the AC conductance, compares it to an internal table to report good / bad as an immediate test result.

The marginal test is to measure each battery with a voltage meter. A fully charged idle battery measures about 12.5VDC.

Check ALL the electrical connections. A single bad connection will cause the boat to act like the battery is bad. ALL the connections MUST be clean, tight and sealed. Liquid rubber brush on insulation is good to maintain the connection integrity.

[Sous]

I will always replace all at once. Outside of some underling issue, once you start to experience issue with one (bank) the others are not far behind. Your experience of replacing the starboard side last year is the perfect example.

[Sicilian_Sonshi]

I agree Sous...but this issue creates a second question: I have 4 batteries (2 for each engine in parallel). I checked each battery and all the batteries are "starting" or cranking type. I thought 2 are suppossed to be starter and the other two should be deep cycle (for house electric needs). Is this normal or the best set-up? A little confusing since I want to replace all the batteries and don't want to buy the wrong type.

[Sous]

So, I'm not trying to sound mean, honestly. But do you know how it's wired? In your post you say starting battery (singular), and in the comment it's 2 per motor. I'd make sure you know how it's configured first, that's key. If in fact you have 2 in parallel per motor, then I'd suggest using dual purpose batteries since they will be serving as starting and house batteries.

[270SDASE]

If any one battery of a 2 bank system is “bad”, then the other is bad too (esp. for deep cycle). Sous is right, you need to know how you 4 batteries are configured to know which one to replace. If in dought, or too hard to figure out - replace all four and be confident of a flawless boating season for now and several years to come ($400ish from Costco for 4 deep cycle batteries, assuming core replacements). Otherwise pull each battery and have tested at a local auto parts establishment for free.