More Battery Advice

[jcollins]

My friends call me the battery killer. I am. I admit it. I just spent an hour or so looking at Deka and West Marine. (supposedly re-packaged Deka)

This is what I have : (4-5 years old)

3 group 31 Deka DC31DT batteries. Supposedly one on switch postion 1 and the other two on switch position 2.
These batteries are advertised on the Deka website as "12-VOLT MARINE/RV HEAVY-DUTY DEEP CYCLE SERVICE (INCLUDING STARTING)"
On the site it says MCA @ 32 degrees=810 and on the battery label

I'm thinking of testing the 3 and keeping the best one for the house.

Adding 2 group 31 starting batteries with MCA of 1000. I'm looking at Deka 8A31DTM SeaMate line. I am also looking at the Odyssey batteries. They are pricey but have a 2 year warranty and boast "up to 12 years" of life.

Any thoughts?

[Mariner]

I would think that a single group 31 should turn over your engine just fine, meaning that the bank with just a single battery should be your start bank, and the bank with two batteries should be your house bank.

One thing to consider, since they all charge together (I assume you select "all" while running the engine), you need to be sure that they are all at the same level of health or you will risk "cooking" your new battery, as the alternator attempts to bring the older battery up to the proper voltage.

For this reason, any time you have multiple banks with different usage patterns, I would recommend some sort of intelligent charge regulator.

[Denis]

Mariner, that is a very good point. I like to alternate battery replacement instead of buying all new at the same time. I have no idea why I do that, but you make a vey good point about charging batteries that are in a similar condition.

[jcollins]

<sigh> Yes, that's why they call me the battery killer.
Good points. I was looking through the Peninsular manual the other day. They recommend two starting batteries. This engine has never started on one battery since I have had it. I think it's time to get a load tester.

One house battery is plenty for us. We don't use much when we are on the hook and carry the Honda generator.

[Mariner]

One thing to consider when using a load tester (usually on loan from a chandlery), is that you shouldn't go by it's little color-coded "good" "ok" "bad" indicators. Determine ahead of time what CCA's you should expect from your battery and be sure to test it on a full charge, but 12 hours after you've taken it off the charger (with no continuing load, obviously). Then compare what kind of CCA's it pumps out to what you expected. The battery should still perform near it's peak even after sitting overnight. Otherwise, it's time for a replacement.

The advice to replace them together is some that I offer with some hesitation. I was advised that you shouldn't replace just one battery, but the person making that recommendation was a bit over-zealous. He recommended replacing all our batteries even though one was less than a year old. He was concerned that the year old battery would have a lower voltage than the brand new one and would cause the alternator to "cook" the brand new one. I was not concinved. The year-old battery is still in top condition and shouldn't dramatically affect a brand new one. Regardless, the captain decided both needed to be replaced..... so we've got a perfectly good 8D battery sitting around.

However, if you have one battery that is bad and one that is so-so, you should definitely replace both. The so-so one will ruin your brand new one.