Part 4 - What Type of Batteries Should I Use?

Should you go with 6v batteries or 12v?  Should you choose Flooded Lead Acid (wet cell) batteries or AGM (Absorption Glass Mat)?  Should you choose a typical car or marine starting battery, or a deep cycle battery?  These choices are discussed below.

12v or 6v?

I have two 6v golf cart batteries in my bank, and they perform well, even at 6 years old.  For those who are unfamiliar, the idea is to wire two 6v batteries together in series, which turns the two into one big 12v battery.  The main reason I bought them was, at the time, the cost per amp hour was cheaper than any deep cycle 12v batteries I could find.  Just based on the reading I have done over the years, I suspect 12v deep cycle batteries would perform just as well for me, assuming proper maintenance.  Others may disagree.

In making your own decision, here are some facts to consider:

• 12v batteries can be added individually to your battery bank.  In other words, you could have three 12v batteries wired parallel in your bank, whereas with 6v batteries you must have an even number of batteries since pairs of them must be wired together in series to achieve 12v.  This could be a factor to you if you want to fill up all available physical space with as much amp hour capacity as possible.  For example, maybe you have space for three batteries but not four.  You could use three 12v batteries, but only two 6v.  I suppose you could use two 6v and one 12v battery in your bank, but many advise against mixing battery types and capacities in the same bank.  One reason for this could be different charging recommendations for different batteries.  If you have a set of batteries that are supposed to be charged at 14.8v mixed with a battery that is supposed to be charged at 14.4v, you could be shortening the life of one or more of those batteries by charging at a different voltage than recommended.  There are probably other reasons as well.  Some also advise against mixing older and newer batteries.  Others disagree.  I have no practical experience with this and have not researched it so will not comment on it. I only mention it so you can do further research if you are considering this.

• If a 12v battery fails, it can be disconnected from the battery bank and the remaining batteries in the bank will still provide power to your rig.  If a 6v battery fails, on the other hand, it and its paired battery must both be disconnected from the system, since disconnecting only the failed 6v battery would leave an odd 6v battery in a 12v system, which will not work.  So if you only have two 6v batteries in your system to start with, you would be left with no power even if only one of those batteries failed. While this may seem to be a big disadvantage to pairs of 6v batteries, another thing to consider is the fact that 6v batteries contain half as many cells as 12v batteries.  If one cell fails, your battery has failed. Twice as many cells in a 12v battery means twice as many chances for that battery to die.  But then again, if you have two 12v batteries the only way to lose both of them is for a cell to go bad in each one.  With two 6v batteries, you only need one bad cell between the two to lose the pair.  Which one is more likely to fail?  I suspect the 6v pair, but not by as great a margin as it might seem at first glance.  The good news is, for batteries that have been used and maintained properly I think it's fairly unusual for them to fail before they're just plain worn out.  Anyway, my head is spinning now, so let's move on.

• 6v golf cart batteries are heavy duty, full "deep cycle" batteries that typically are manufactured with heavier plates than 12v batteries.  For this reason, many people believe they are more robust than 12v batteries.  However, others claim that 12v batteries that are maintained properly will last just as long as a 6v battery.

 

• Other factors:  If you are using 12v batteries wired in parallel, then you would add together the amp hour capacity for each battery to determine the capacity for the entire bank of batteries.  If, on the other hand, you are using pairs of 6v batteries where each member of a pair is wired together in series (which is the only way they can work in a 12v system), then the capacity for each pair will be the same as the amp hour rating of the lowest capacity battery in the pair.  This is because wiring two batteries together in series adds their respective voltages together but does not add the amp hour rating, and the pair is only as good as its weakest member.  When the weak member is dead, for the purposes of a 12v electrical system, the pair is dead, even if the stronger member still has power.  But I would avoid mixing batteries with different capacities.   Examples:

1. Your battery bank consists of two 12v batteries wired in parallel, and each battery has a capacity of 105 amp hours. In this case your battery bank has 12v and its capacity is 210 amp hours.
2. Your battery bank has two 6v batteries wired in series, and each battery has a capacity of 220 amp hours.  In this case your battery bank has 12v and its capacity is 220 amp hours.
3. Your battery bank has two 6v batteries wired in series.  One has a capacity of 220 amp hours and the other has a capacity of 180 amp hours.  The battery bank then has a voltage of 12v and an amp hour capacity of 180.

If you aren't quite sure what wiring in parallel and series means, a good resource for that is this article at solarray.com. 

Wet Cell or AGM?

Beyond the 6v vs. 12v question, there is also the matter of the "type" of battery to use.  Should you use Flooded Lead Acid batteries (wet cell), Gel Cell, or AGM (Absorption Glass Mat)?  I'm not going to go into the details of all these beyond saying that wet cell batteries are probably the best value for your money  if you maintain them properly and don't misuse them.  The maintenance is in keeping them topped off with distilled water, keeping terminals clean, periodically charging them and periodically equalizing* them.  Most recommend checking water level once per month.  If you run them dry, your battery life will be shortened, possibly drastically shortened.  A good read on battery maintenance is this one at Trojan's website.

Charging flooded lead acid batteries is fairly straightforward, but different kinds have different recommended charge settings, so manufacturer's recommendations should be followed to get the most out of your batteries. 

Gel Cells are typically not used due to their problematic sensitive charging needs so most who are looking for maintenance free batteries seem to go for AGM. 

AGM batteries are much more expensive than flooded lead acid, but require little maintenance and are not required to be mounted upright, as wet cells are.  There is nothing in them that can spill.  They are practically immune to freezing.  They have a very low "self discharge" rate, meaning you can leave them on a shelf for much longer than wet cell batteries without them going dead.  They are pretty much "connect and forget."  But you pay a steep price for that.

Starting or Deep Cycle?

Absolutely, without a doubt, you want to avoid "starting" batteries and go with deep cycle.  Deep Cycle batteries are designed for the kind of use they will get in an RV cabin, e.g., being slowly drained down as much as 50% before being recharged again.  Starting batteries are designed to supply power for high loads over a short amount of time, such as to power a starter motor, and then be immediately recharged fully.  They will work in a pinch as an RV house battery, but will be damaged by repeated use in this way and will die an early death as a result. 

Also, if a battery has a "Marine" label on it, that does not make it a deep cycle battery.  Some batteries are marketed as hybrids, supposedly for use as starting batteries as well as deep cycle use.  I would avoid these.  Go with full deep cycle batteries.

For a much more extensive look at batteries and everything you never wanted to know about them, see Northern Arizona Wind & Sun's battery FAQ and the Battery Council International's lead acid battery page.

Ok, now you've got your batteries figured out as well as the amount of time you need them to power your rig before being recharged.  But if you're boondocking longer than that amount of time, you need a way to charge them up again.  That's where solar power comes in.  Continue reading with Part 5, The Solar System.


* Equalization a feature available in better charge controllers.  It is the process of taking fully charged wet cell (flooded) batteries and for several hours applying an otherwise excessively high rate of voltage.  For my batteries this is about a half volt higher than their recommended normal maximum charging voltage, or 15.3v instead of the recommended 14.8v, plus or minus according to temperature.  Many battery manufacturers recommend that flooded lead acid batteries be equalized periodically, like maybe once a month or so.  The reason for this is over time sulfate crystals tend to solidify on the battery plates.  This reduces battery efficiency and capacity.  In addition, stratification of the electrolyte mixture can occur in which the concentration of acid is stronger at the bottom of the battery than at the top.  Individual battery cells begin to drift apart in terms of their respective charges.  Periodically equalizing the battery gets the mixture boiling which eliminates stratification and gets the sulfates back into the mixture and off of the plates, and can reduce cell charge inequalities which can restore efficiency and battery capacity.