Running a DC fridge while connected to shore power

[hopkinslaw]

My boat spends much of its life safely plugged into shore power at the marina. I have a a fridge which I leave running all the time connected to DC. When running it draws about 5 amps of power. The fellow who repaired my fridge advised the fridge will last longer turned on and running rather than being switched off. An electrical expert I spoke with said it was fine to leave the fridge on when connected to shore power with the battery trickle charging. I have a ProNautic 1220P (20 amp) charger. All was fine until I read this post which is stuck to the top of this message board.

http://c34.org/bbs/index.php/topic,4352.0.html

I'm trying to come to terms with the suggestion that constantly sipping from my battery will shave off 35-40% of my battery's life.

I find it odd that:
-Experienced cruisers I have talked to have not heard that this is a bad practice
-A cursory search of the web does not come back with clear warnings that this very common practice is killing our batteries.
-the warning against this practice was buried in a post reviewing a particular charger
-the post contains no citations for the dramatic claim

Help me out guys. Are there studies documenting these concerns? Why are batteries so damned confusing?

I have ordered a pair of Firefly batteries to replace my old AGM batteries. Anyone know if they are more or less susceptible to this issue?

-David Hopkins
1990 Catalina 30 #5845
Vancouver

[mainesail]

I have ordered a pair of Firefly batteries to replace my old AGM batteries. Anyone know if they are more or less susceptible to this issue?

-David Hopkins
1990 Catalina 30 #5845
Vancouver

The actual inventor of the Firefly battery would prefer these batteries not be floated at all (his perfect world scenario). If they must be floated then Kurt prefers a maximum of 13.2V. Due to complaints from customers, not battery inventors or engineers, the spec now says up to 13.4V float (the manufacturer went against Kurt's advice to appease customers). The real answer for Firefly is no float is still the best option, if that is not amenable then 13.2V float.

Float is a catch 22 because the vast majority of "voltage regulated" chargers, solar controllers etc. almost always drop to float far too early. If we stop charging too early we will certainly lose Ah capacity due to chronic under charging. Float is a compromise that brings the batteries to near 100% SOC (depends upon voltage) but it is still negatively impacting battery life. Most boats also have parasitic loads and float becomes even more necessary. If float did not impact battery life UPS batteries (rarely if ever cycled) would ultimately last for 25+ years, but they don't because they have a "float life".

You bought very good, but also very expensive batteries, perhaps the best lead acid battery made in regards to sulfation damage resistance. If you must float them then 13.2V temp compensated and 14.4V max absorption, temp compensated, is what to use for all charge sources. Alt float can be higher because this is not a true float it is nothing more than a temporary condition, unless you plan to motor all the way to Phuket...

We also can't forget that many chargers can be re-triggered into absorption by the fridge or other DC loads kicking on and this too can negatively impact battery life. The best bet is to charge the batteries to full then set the battery charger to 13.1V float and 13.2V absorption (custom program), when dock side, and this will never allow it to get re-absorb triggered. The problem is that when you want to charge to 100% SOC you'll need to reset the program..

Leaving your boat plugged into a marina, with active DC loads, opens up a whole other can of worms all together. These issues are only getting worse by the day with more and more marinas coming into compliance with NFPA and NEC regulations for GFI protection. I replace thousands of $$$ in expensive batteries each year on boats whose owners insist on running dock-side DC loads. Marina power is definitely not as reliable as your homes power feed. As recently as September it was a 500Ah bank destroyed when a transient unplugged the owners boat and forgot to plug it back in. Owner was gone for two & a half weeks and when he got back the bank was at 3.3V and was unrecoverable.

[sailaway]

I leave my fridge on all summer. I also have a 20 amp charger with Walmart wet cell batteries. I don't have any problems my batteries usually last 6 to 8 years leave them in the boat over the winter. Unhook the negative, check  them every spring they read 12.6v. Charlie

[scgunner]

     David,

       Everybody's got their own ideas about how to do things, and they usually work for them. I bought my boat with a reefer because I like to have a cold beer when I entertain or go down to work on it. Consequently my reefer runs 24/7 and has since I got the boat in 1988. After 30 years of service it finally gave up the ghost. I replaced it with the same unit and, yes it's running right now.

        As far as batteries are concerned, I run 4 deep cycle and 1 starting, all wet cell. I replace all batteries about every 4 years not because they're failing but because I like my batteries fresh and dependable. I haven't noticed the fridge having and effect on the batteries one way or another.