6 VDC vs 12 VDC Battery

[sluiceman]

I want to add an extra battery to my travel trailer.

Are 2 ea 6 VDC deep cycle batteries hooked up in series better than 2 ea 12 VDC deep cycle batteries hooked up in parallel?

[wolfe10]

The answer is not as simple as it might appear.

IF, and that is the big IF it truly is a deep cycle 12 VDC battery of the same capacity (amp-hrs) as the 6 VDC batteries wired in series, then no real difference.

But, true deep cycle 12 VDC batteries are not very common. The reason most go with the 6 VDC's is they are the least expensive deep cycle batteries. The reason-- hundreds of thousands are in use in golf carts!

[jbrunson]

99% of the time, the answer is yes. Golf cart batteries are higher capacity usually. If you encounter a dead cell, it will be cheaper to replace one six volt battery as opposed to replacing a 12 volt battery of the same capacity.

The simple way to evaluate your purchase is cost of battery per pound; forget about what label is on the battery, most are private label products with just a few manufacturing facilities making product for dozens of brands.

Wal-Mart usually has best price.

[Lenp]

For a bank of TWO (only two) batteries here is my opinion....

Typical 12 volt batteries ARE NOT deep cycle batteries! IF they have a "cranking amps" ratings they are really starting batteries. Most of the "deep cycle" 12 volt batteries are really starting batteries. To find a true deep cycle battery look for "amp hour" ratings versus "cranking amps". suggest Trojan T1275 for 12 volt or T105 for 6 volt.

Now for a little more technical pros and cons....

With two 12 volt batteries in parallel the charge and discharge current in each should be balanced equally between the two. Unfortunately, internal resistance of the batteries combined with (potentially) unequal cable and connection resistance will result in one battery getting charged and discharged more (or less) than the other. With two 6 volt batteries in series ALL of the charge or discharge current flows through both batteries ensuring (roughly) equal charge/discharge of each battery. Thus, 6 volts are preferred.

Now, if you have failure of one of your batteries...... with two 6 volt batteries you are simply without any batteries. However, with two 12 volt batteries you can simply remove the defective battery and continue on.

Please understand that the above only applies to a TWO battery system. For larger (4, 6, or more) batteries it gets a little more involved but the same concepts apply.

Personally I prefer a bank of four or six 6 volt batteries. Unfortunately, my current MH has room for only three house batteries so I have chosen three Trojan T1275 12 volt batteries to give me 450 amp hours of storage. Combined with 750 watts of solar and a Samsung residential 17 Cu Ft refer, we are able to dry camp for weeks with only limited generator use on cloudy (or shaded) days. Perhaps someday I will add more batteries in another compartment but for now we are good to go.

Lenp