Another Battery Question

[JusDreaming]

I think one of our battery banks is giving up the ghost. So my question is can I have one bank,(two batteries), that is 6 volt wired in series, and one bank,(two batteries), group 27s. I really don't want to replace batteries that are still doing the job

[sbrin]

Yes - I have had this arrangement for 5 years without problems.

[David M]

Yes it will work although it is not optimal for storing power. In my opinion its good enough though.

[osirissail]

I will second/triple the above answers - but add to be careful if there are two differents styles of batteries - Gell + Liquid Lead acid or any combination of the three types - gel, agm - liquid lead acid. You need to be sure that your charging system is set for the most restrictive style of battery as to charging regimen.

[JusDreaming]

what would be the advantage to using two 6 volt comapred to two group 27s. The 27s are around 100 AH so would total 200 AH. Two 6 volts would be about the same AH, correct?

[osirissail]

Not quite - according to the marine supply stores, a Group 27 is about 80 AH or for 2 would be 160 AH while two of the Trojan T-105 6 Volt batteries would supply 225 AH. It is the normally very heavy construction of the 6 Volt batteries that attract boaters while supposedly translates into longer life with deeper drawn down. The 12 Volt versions are not quite as heavily built and supposedly have to be replaced more often if drawn down to the same level.
- - But more than anything the batteries you choose are more a decision as to the space available to hold the batteries. The 6 Volt tend to be considerably taller than the 12 volt series.

[clearsea]

Originally Posted by osirissail

Not quite - according to the marine supply stores, a Group 27 is about 80 AH or for 2 would be 160 AH

Is it true that amp-hours double when batteries are in parallel?

[osirissail]

Yes - two gardens hoses will give your twice the volume of water. What makes a more important difference is the allowable draw down (removal of amp-hours) before you start to damage the battery (decrease its useful life). Thin-plate auto batteries cannot tolerate deep discharges while (aptly named) deep-cycle batteries will do better.
- - What most small boaters/sailors forget is "cost effectiveness". They think they must buy the exotic batteries at 4x to 10x the price just because they have a boat. If you are "daysailing" or only doing weekends - Walmart car batteries work fine for starting your boat engine and their $40 marine batteries work fine for house lights, etc. You can replace them up to 10x before you equal the cost of an exotic (gell/AGM) battery. A new set of Walmart batteries each season will save big bucks especially when you factor in that you do not need all those fancy chargers, controllers, monitors, and other "required" accessories for "exotics."
- - If going offshore or places where getting replacement batteries in neither convenient or possible, then the fancier batteries are called for along with all the "extras" needed to keep them up. Otherwise, use that money for beer, rum or better sails.