PDA

View Full Version : Batteries


bob3nliz
11-16-2010, 03:31 AM
Is it better to run two 12v or two 6v for house. :?: Thank you bob

Chuck v
11-16-2010, 11:58 AM
Bob,

The use of two "golf cart" 6 volt batteries in series to replace two 12 volt batteries in parallel is both an improvement in overall performance & capacity, and can even be a safety enhancement.

Let me explain: the lead/acid storage cell used in deep cycle batteries stores electrical energy by converting it to chemical energy and back again. Charging the cell is accomplished by passing a current through it (from the generator of from the shore power converter...) until it reaches its full state of chemical storage. When you discharge the cell, the chemical reaction supplies a current out. The voltage of each lead/acid cell is nominally 2 volts, so a 6 volt battery had three cells in series and a 12 volt battery has six cells in series. When a cell is discharged, it has a high impedance, so the charging source will impress the largest voltage across the most discharged cell, causing it to take the majority of the charging energy in the string if it is the most discharged cell. The series arrangement naturally gives a balanced charge to all the cells in the string, and as all the cells come up to full charge the battery's terminal voltage rises to the target charge value and current stops flowing into the battery. In the case of two 6 volt batteries in series, this balance is easily maintained across the series pair. For two 12 volt batteries in parallel, it is much more difficult to ensure that all the cells are charged simply by sensing the terminal voltage of the parallel pair. For this reason, RV manufacturers all recommend that the two 12 volt batteries be the same age, vendor and type -- this is an attempt to match them to give the best charge characteristics for the 'less than optimal' configuration of parallel lead/acid cell charging.

My personal experience with 6 volt cart batteries in my travel trailer (the one that gets the most 'dry camping' use...) is that they are much better performers particularly in cooler weather. The batteries themselves fit in the original boxes, but are a bit taller so the lids fit a bit higher. They have a higher total amp-hour rating than the sum of the two 12 volt original batteries...

Now for the safety discussion. I have a friend that has a 5'er and at one time he replaced just one of his two 12 volt batteries just before a short trip. When he arrived at the campsite, he noticed a 'gurgling' sound from the battery compartment and when he opened the door he found a melting battery case and fumes!! It seems that the older battery had developed a shorted cell, and the new battery simply dumped its entire charge into the other one (they are connected in parallel of course...) and fried it. A 911 call got the fire department/hazardous materials crew on site to deal with the potentially dangerous situation. He converted to 6 volt series connected batteries after this close call...

I have been involved in large solar power systems for various remote power systems and some are using up to 240 volts -- and we ALWAYS chose storage cells that were appropriate amp-hour ratings so that ALL cells were in series and could be charged, maintained, and serviced appropriately.

Sorry for the long post, but I thought a background discussion on why having all cells in series is the superior configuration was called for here.

Chuck

bob3nliz
11-16-2010, 03:25 PM
Chuck v

Thank you very much, post very GOOD not to long

bob :D

purduepete2
11-17-2010, 08:22 AM
We converted our original 12v marine battery that came with our 2007 BT cruiser to two 6 volt golf cart batteries from Sam's Club before our last extended trip out West and were delighted with the performance. It handled the furnace on those cold nights and mornings without having to use the generator. Never had a concern about using lights and water pump and recharged while driving during the day.

Pete