6 Volt Marine Battery Conversion

I got into a discussion yesterday about how to convert a 2 battery, 12 volt house battery bank into a 2 battery, 6 volt battery bank utilizing golf cart style batteries. I thought that I’d share it with you here.

And the first question is, why would you want to do this?

A few years back the golf cart people discovered that they could get more run time on electric golf carts by using 6 volt batteries connected in series (which provides 12 volts) than they could by connecting 12 volt batteries in parallel. Recently boaters and RV’ers have discovered the same thing and the 6 volt battery combo has seen a recent growth in popularity.

Before we continue let’s look at a couple of the basic rules of electricity as they apply to battery configurations.

Connecting batteries in parallel (positive to positive and negative to negative) results in the voltage remaining as one battery however the capacity (amp hours) will be the sum of both batteries.

Connecting two 12 volt / 110 amp hour batteries in parallel will result in a 12 volt bank with a capacity of 220 amp hours.

Connecting batteries in series (positive in and negative to positive connecting the batteries with negative out) results in the voltage being the sum of both batteries and the capacity remaining as one battery.

Connecting two 6 volt / 220 amp hour batteries in series will result in a 12 volt bank with a capacity of 220 amp hours.

Hmmmm, what’s the difference you might ask?

Well here it is. 12 volt batteries are comprised of 6 cells and 6 volt batteries are comprised of 3 cells. What this means is that 6 volt batteries have larger cells with plates that are more robust and each cell has an increased electrolyte volume. Because of this they can easily withstand longer and deeper discharges than 12 volt batteries of equal capacity. 6 volt batteries also have a smaller footprint (7.1×10.25”) that a comparable Group 27 12volt, battery (6.75×12.75”) however 6 volt batteries are slightly taller (11.25”) as opposed to their 12 volt counterpart (9.5”). Be sure to factor in this height difference when considering this change. The sizes listed above are an aveage but they should be close.

Also, if you have the larger 4D or 8D batteries fitted, and if you refer to the battery size chart on an earlier posting, from a size and weight standpoint alone, the advantage of the 6 volt deal becomes obvious.

Here’s how you do it.

6 volt batteries wired in parallel

The above diagram shows the two 6 volt (each@220ah) batteries wired in series and connected to bank 1 of the battery master switch. A separate starting battery is connected to position 2 of the switch. This gives us 12 volts and 220 amp hours capacity.

Heres another popular combination if your serious about some extended off shore jaunts.

four 6v batteries

Do the math. On this one we have two pairs of 6 volt batteries (@220 ah) each wired in parallel  and with the pairs wired in series we have a bank that produces 12 volts and a capacity of 440 amp hours.

Waking Up (charging) Dead Marine Batteries

Just in case you’ve been a little negligent in battery maintenance over the winter and your batteries won’t charge try this…… Plug the boat in and turn the battery switch to “Both” to connect both banks if you have more than one. Then turn on a passive DC appliance such as the cabin lights and put a load on the system with the charger energized. Leave it for a couple of hours and this just might kick start the battery charger. Once the charger starts you can turn the lights off and let the battery charge as you normally would.

Since a dead battery has no internal resistance and offers little or no load on the charger, loading the system in this way might just be enough to get the charger going. Most chargers need to see a small load in order to initiate.

By the way if this doesn’t work you’re going shopping.

Marine Deep Cycle Battery Discharge Voltage Levels

Battery State of Charge

100% = 12.7     Volts

90% =  12.5      Volts

80% =  12.4      Volts

70% =  12.3      Volts

60% =  12.2      Volts

50% =  12.0      Volts

40% =  11.9      Volts

30% =  11.7      Volts

20% =  11.5      Volts

10% =  11.3      Volts

Most experts will agree that for best service life the maximum recommended discharge level for Deep Cycle batteries is 50%. If you go below this I wouldn’t be too concerned but keep this in mind for maximum longevity. Use your battery monitor to keep on top of this. Just make sure that when taking voltage readings that all 12 volt appliances are turned off.

Marine Deep Cycle Battery Size Chart

A short while ago I gave a presentation on marne battery and charging systems to a group at a local marina. One item that we discussed was battery groups and sizes and to that end I compiled a chart of the sizes most common to marine applications. I have since been asked to post that information here.

Batteries Size Codes

Group 24       60-85AH     11X7X10”       50-55 lbs

Group 27     85-100AH     13X7X10”       60-65 lbs

Group 29     100-110AH   13X7X10”       65-70 lbs

Group 31   105-115AH     13X7X10”       70-75 lbs

4D             210-225AH       21X9X9”       135-140lbs

8-D           225-255AH     21X11X10”      160-175 lbs

6 Volt GC   200-240AH   10X7X11.5”      60-70 lbs

I would like to add that this information is averaged and that may be accurate to within about 6mm (1/4″) or so on size and that the capacity may be more or less 10%.

Battery size codes Eg: Group 27, 4D etc. refer to the physical size of the battery only and it is not indicitive of the capacity.

You will also note that Groups 29-31 are all listed at the same size. Again some may be slight larger and some slightly smaller.

At the bottom the 6 volt battery is labelled GC. This stands for Golf Cart and being 6 volt, 2 batteries must be wired together in series in order to produce the 12 volts that is commonly required for marine applications. When you compare a 6 volt battery’s space requirements as apposed to their current capacity the advantage becomes very clear. Another note about 6 volt batteries, that is if you’re considering going this route is that while they have a slightly smaller size foot print and they are taller than their 12 volt counterparts and this needs to be considered in the planning.



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