Boat Batteries for Dummies

[Clay Carter]

I have 2 gel cells bought in 1998 and all new electrical from previous owner on 1974 35-2.

2 years ago I had to replace my charger as they were not getting charged and became completely discharged once.

Suddenly I'm arriving to go sailing and yet again finding dead batteries... (What is more frustrating than that???)

Questions:

What is the normal life expectancy of a gel cell?

If they are trashed, should I buy 2 new gels or "AMI" (whatever that is) batteries or combination as my engine and house bank?

Thanks for the education...

Clay

 

[Geoff Johnson]

Since I overhauled my electrical system a couple of years ago, I will give you my opinion.


-Batteries last a lot longer if you do not discharge them more than 50%. As a practical matter you cannot charge them to more than 85% of capacity, so you really have only 35% to use. Therefore your batteries should be sized accordingly (within the physical limits of your boat.)

-In any event you probably should not expect batteries to last more than 6 or 7 years

-Whatever chemistry you choose (flooded, gel or AGM (absorbed glass mat)) you should have all the batteries share a common chemistry because the charging paramaters are different for each type.

-Make sure your charger and alternator voltages are set for the kind of battery you have or you will wreck the batteries. The charger should be a good modern three stage charger.

My boat has room for only two Group 24 (85 Ah) batteries which is not much. So I switched to AGM batteries, which accept more readily the higher charge rate of a high output Balmar alternator and have a very low self-discharge rate (e.g. left on the boat all winter without charging, charge dropped only 15%) - this is important to me since the boat is on a mooring. (FYI AGM batteries are essentially lead acid batteries with saturated mats holding the electrolyte - they were developed for the military, but are becoming the battery of choice IMHO since they are rugged and maintenance free as long as they are charged properly.)


I also added a high output alternator and three stage regulator. Finally, I added a dedicated starting battery on a shelf in the lazarette which can be paralleled manually in an emergency (otherwise it is never used). I now use my house bank for everything and always keep it paralleled with the "Both" switch so that I have in effect a 170 Ah battery. Some day I may hard wire the two house batteries together.

You can see the alternator setup on this thread:

http://www.ericsonyachts.org/infoexchange/showthread.php?t=736&highlight=balmar

 

[Loren Beach]

Power On, Dude

Battery capacity for our boats' many needs has been the subject of some good past threads on this forum. Go to the "Search" icon, upper third of this page in the blue banner. Click on it. Type in something like "batteries" or "Trojan" for a good start.
Not to brag, I assure you, but my prior post has a picture of our installation.
And, whtever Geoff sez, I usually agree with!

Best,
Loren in PDX

ps: we do not have our "hidden" AGM battery on the shore-power charger for the reasons he states.

 

[Geoff Johnson]

Yeah and I agree with whatever Loren says.

I also understand that AGM's don't like solar power either, at least being kept on a solar trickle charger.


PS Here's a good source of information (see the workshops) as well as a good place to buy stuff (no affiliation - just a satisfied customer)

http://www.jrenergy.com/(gqddpd45eazylrbfosgcbr55)/products.aspx?loc=0&type=0

 

[valentor]

I have to disagree with the comment about solar chargers and AGM batteries.

As batteries charge, the current supplied by the charging system is gradually reduced. To charge batteries with an intelligent regulator and a large alternator to 100% can take as long as 6 hours or more. Typically we don't run our engines for that long, so our batteries are rarely fully charged. In about two hours, a properly regulated 100 Amp alternator can restore about 90% of a 210 AH AGM bank.

A 5 Watt solar collector can deliver at most 300 milliamps of current at its peak, which of course is further reduced by night, clouds, and even the shadows cast by lifelines. The result is that it can take a solar collector many days to restore a large battery to a "full" charge. At the same time, the automatic bilge pump, radio clock, and other components consume small amounts of battery current.

I have been using a 5 Watt collector to trickle charge two 105 AH AGM batteries for more than five years and the batteries still test to 100% and show no degradation in capacity.

It's important to note that I only discharge them to 50% capacity before I recharge, and since Rogue is on a mooring, it is relatively rare that I use the AC charger.

It is true that applying current to a "fully" charged battery will harm it, but I think it's fairly rare that they are "fully" charged. In any case, the solar collector has worked very well for me over time.



Regards;


Steve

 

[Geoff Johnson]

That's good to know. I'll get my 5 watt charger out of the attic.