Are 2 x 6 volt batteries better than one 12v battery?

Well I guess there's a few ways to look at this and considerations.

The largest 12 volt batteries are 260Ah (approx) @ 20Hr rate. 6 volt batteries can be 420Ah as they have half the plates but twice the capacity. So one answer might be yes 6 volt is better if you need 400Ah of battery because you could use two large capacity 6 volts instead of two mid-sized 12 volts. The 6 volt battery at 400Ah is quite often an L16 case size which is tall, these batteries in a flooded battery require the height to store them and also need to be deep cycled regularly to keep the acid from becoming stagnant (see stratification).

From an electrical point of view a 12 volt battery is actually six 2 volt cells in series. There is no difference between 2 x 3 cells (2 x 6 volt) or 1 x 6 cells (12 volt). The physicall size of these cells is quite often an issue for different installations so sometimes one form factor is better than another.

Lastly, if one of your batteries fail in a 2 x 6 volt installation your voltage drops dramatically (normally 2 volts per dead cell) or if there is an open circuit (just like a fuse blowing) the voltage will be zero volts. That may render your vehicle useless. Whereas if you have two 12 volt batteries in parallel the voltage will appear normal but your capacity will be reduced by 50%. This later is harder to detect if you don't have a battery monitoring system because you can't see the capacity reduction vs. voltage.

DC 2 DC Chargers what are they?

Just like a normal mains powered charger which is AC input but a DC charger is DC input voltage.

Some of the considerations are when the charger should turn on, so ignition inputs or input charge voltages in excess of OCV are used to initiate the charging process. Additional programming or hardware design has been made in some cases for regenerative braking systems and the fast surge input power generated. In some cases, the input voltages on a start battery with regen braking are in excess of 17 volts DC, which is very high on a 12v system.

Some manufacturers include multiple charge profiles or relay control systems or low voltage disconnect features to isolate discharging equipment.

Differences between Solar Panels

Solar Modules carry approx 0.5v each so a 36 cell produces 18V which is perfect for charging 12v batteries using a solar regulator (either PWM for cost efficiency or MPPT for yield efficiency).

A 60 cell panel was originally designed for use in residential installs where higher voltages are used to string series arrays together. They are typically connected to the grid and use MPPT to convert the varying energy throughout the day into a usable power on the grid. When used in portable applications an MPPT controller can convert the 30-40 volts output by a 60 cell panel into a 12 or 24 volt battery bank.

More recently 72 cell solar panels were used when solar was used to charge 24 volt systems like in off-grid. It was previously typically done by connecting two 36 cell 12v panels in series. Later panel manufacturers series connected 72 cells to produce the same voltage output which is actually closer to 38 volt open circuit than it is the 24 volt often referred to within the mobile industry. These panels are best used in high voltage strings in commercial applications up to 1000VDC because of their size and weight. But when space is tight on a roof and there's only room for a few panels in an off-grid system you can utilise the large frame size and high power output using MPPT controllers to bring the voltage down to 12, 24 or 48 volts. These panels have been up-sized from their humble beginnings of 200W right up to 365-400W today with larger wafer sizes, they are almost 2 metres by 1 metre in size. (Sept 2019).

Cells numbers when connected in series gain higher DC output voltages and those voltages are best matched to the solar controller's maximum DC input. Do not exceed the max input voltages or input max current.

More recently half cells have been produced to help negate shading reduction. A 120 cell is very similar size and power output to a 60 cell but has cells half the size of the 60 (thus twice as many).

What is EFB?

While different manufacturers will each build and design their EFB batteries differently the general concept remains the same. EFB (Enhanced Flooded Battery or Extended Flooded Battery) takes the Maintenance Free battery one step closer to AGM batteries by either using more plates to increase Ah capacity, thicker plates to increase deep cycle ability or a combination of both. They may also be enveloping the plates which are similar to Absorbed Glass Matting between the plates or enveloping. Either way, the result is a flooded battery with characteristics more in line with the advantages of AGM and a performance heading towards that of AGM but at a lower price point.

Difference between maintenance charger and charger

In the past, the term maintenance charger referred to a small current single-stage charger that would be left on a battery to replace the small self-discharge of a battery. Over time with switch modes charging the term maintenance remained but now is more about the total amount of current available.

A small current charger can only be used to maintain a medium-sized battery but if used on a small battery a multi-stage charger is more than sufficient to charge a small battery.

So, in summary, most multi-stage chargers have a charge profile and maintenance profile. Its the current available to charge than needs to be matched to the correct sized battery to become a charge/maintainer. Otherwise if to low current on too large a battery they are really only maintenance chargers.

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