New Battery Question

[Jfuller678]

I have reviewed the past threads and think I know the answer but I want to make sure before I invest in two new AGM batteries.

I understand that amp hours is the single most important characteristic I need for a long lasting battery. Is this correct?

I am going to be able to place them side by side in the same battery box I currently have one battery. Should they be connected parallel or serial?

Thanks in advance. I really appreciate your help.

[NevadaGeo]

I suspect you'd need to keep to 12 volts, so parallel.

[catson4]

depends on the space you have. If you can fit a pair of 6 volt batteries then do so. The pair of 6 volt bats provide more amps hours so will last longer than a pair of 12 volt units.
If you use two 12 volt bats then they will connect LOAD to POS to POS & GROUND to NEG TO NEG .

If you use two 6 volt bats the connect them: LOAD to bat #1 POS then #1 NEG to bat #2 POS and bat #2 NEG to GROUND.

That will get'er done

[Jfuller678]

Thanks for the answers. Do you have any idea why two six volt batteries provide more amp hours than two 12 volt batteries. That seems counter intuitive.

[Ag&Au]

Originally Posted by catson4

depends on the space you have. ............................. The pair of 6 volt bats provide more amps hours so will last longer than a pair of 12 volt units. ................

That doesn't seem logical to me. Are you by chance adding the amp hour ratings of the two 6 six volt batteries together?
If so it doesn't work that way. Two 6 volt 100 amp hour batteries in series will still only give 100 amp hours at 12 volts.
If you already know this, Could you please give me an example of 2 six volt batteries that will give a higher amp hour rating than a 12 volt same type battery that takes up the same physical space. I not trying to be argumentative, just trying to see if my thinking is faulty.
Regards,
Ken

[Lumatic]

I'm a bit fuzzy so take some salt.

I have heard the plates on 6 volt batteries are thicker than 12s and better suited to deep cycle batteries.

As far as I know amp hours are amp hours.

You can pay a lot of money for a Lifeline AGM and it will last a long time or less money for a cheaper battery that won't last as long.

You can also put in a single oversize battery instead of 2 smaller ones. It's cheaper. I have a Universal AGM size 4B 200 amp hours. There is even a bigger one 8B 300 amp hours.

Some converters and AGMs do not get along. Especially Univolts.

[tallytwo1994]

Originally Posted by catson4

If you can fit a pair of 6 volt batteries then do so. The pair of 6 volt bats provide more amps hours so will last longer than a pair of 12 volt units.
If you use two 12 volt bats then they will connect LOAD to POS to POS & GROUND to NEG TO NEG .

If you use two 6 volt bats the connect them: LOAD to bat #1 POS then #1 NEG to bat #2 POS and bat #2 NEG to GROUND.

That will get'er done

What????
Oh boy. I had just started thinking I was going to get a handle on this stuff. Total confusion now. Thanks guys!

Carol

[bryanl]

The battery capacity will depend upon the weight of the batteries, not voltage.

There is no advantage to 6v over 12v in either lifespan or capacity.

There is an awful lot of misinformation, specious claim, and hyperbole regarding batteries in forums. Your best way around this is to look at specifications, warranty, retailer, and cost.

[Mexray]

I'm in the battery sales bizzz....with regard to 'flooded cell' batteries (the type you can add water to)...

Typical 6 volt, deep cycle (golf cart) batteries are are rated at about 225 AMP HRS, and when used in SERIES (pos to neg) the voltage will double to 12 volts, but the AMP HR rating remains at 225 total AMP HRS

if you are using two Group-27, deep cycle 12 volt batteries, the typical AMP HR rating is about 100-105...you would then have them hooked up in PARALLEL (pos to pos & neg to neg) so the VOLTAGE remains at 12 volts, but the AMP HR rating is doubled, or about 200 AMP HRs total...

the 6 volts are constructed with thicker plates, and have rubber separators to keep the plates in place for the rugged use a golf cart gets - and are therefore a great choice of an RV - IF - you can stand the increase in height - about another 1-1/4" higher than the 12 volt models...

The AGM type sealed batteries are totally maintenance free (except cable connections) but cost OVER TWICE AS MUCH ++ as flooded cell batteries! Many of the AGM's have AMR HR ratings about 10-15% below their flooded cell cousins (some models don't) - and another consideration, is that in typical RV use you WILL NOT GET OVER TWICE THE LIFE (in years service) of flooded cell batteries...

So before venturing into the sealed AGM world, make sure you have weighed all the figures...I personally don't believe the AGM's are cost effective for typical RV use, as batteries are many times neglected when rigs are not being used, etc. etc....your situation may be different, and you take better care of your stuff..

I'm not an AGM 'hit man', I just don't think they are worth the much increased cost, UNLESS you MUST have a sealed battery for some particular application...I sell em' both all the time, and even though I make additional $'s when selling AGM's, I always make these thoughts known, in regard to RV and some Marine use, to a prospective customer...

Oh yes, and all my valued customers get our fantastic 30-30 warranty; 30 feet or 30 minutes...

[Driver1]

Hi Mexray,

I think the 30-30 warranty is an appropriate and outstanding idea given the customers' (Me included) understanding of how to use and treat batteries. Great idea!

Personally, I went with the AGM's out of fear I'd boil over the open type and the fact that it's such a pain to get at and check mine for the acid level. Still under 1 year on the AGM's - so far no problems. Yes, I paid a lot even after careful shopping. But, most importantly, I don't worry about boiled over acid eating away my trailer and I don't worry about the batteries condition except to keep them charged.

Walt

[Jfuller678]

Thanks for all the valuable information. Someone mentioned they had a Group 27 battery that had 200 amp hours. Is such a battery widely available?

[bryanl]

re "6 volts are constructed with thicker plates" - that depends upon which battery you buy. You can also buy 12v (or even 8v) batteries with "thicker plates". All this means is that you choose the battery in the line that tends towards the 'ruggedness' (aka deep cycle) end of the cost, capacity, ruggedness trade-off for whatever voltage you need.

You can also buy sealed lead acid wet cell batteries that are "maintenance free" that are not AGM's.

If your charging system is going to boil off electrolyte, it will do that to AGM's as well as wet cell batteries. AGM's, because they are sealed, are more sensitive to overcharging and more likely to be permanently damaged as a result.

A group 27 battery has about 100 or so AH at 12v. You can't get enough lead in a battery that size to double the capacity. For any size battery, the variance in capacity is maybe ten percent or so (the capacity, cost, ruggedness trade-off again). For slightly more capacity, a T105 or group 31 battery is bigger. For a lot more, try an 8D or L16. They all provide about 22 watt hours per pound.

When a salesman starts on his spiel, look at where the money is. Does the warranty he offer match the claims? Lead acid batteries are a mature technology. There are some interesting ideas in the works but what is available to us now does not have any surprises in the mainstream. No magic bullets, no big differences, no settled advantage in brand, voltage, or whatnot. Buy from a reputable retailer who stands behind what he sells and then properly use and maintain what you get.

[Driver1]

Hi Bryanl,
Good catch on my boil over comment. I should have mentioned that I replaced my Univolt with an Intelli-Power 9245 shortly before installing the AGM's and that's a major part of why I don't worry about boil over now.
Walt

[bhayden]

Don't overlook gell cells. They are between AGM and flooded lead/acid in price (closer to lead/acid). They are very popular in the marine industry. My personal experience is they stand up better to "abuse" than any other type. Of course it's better not to let them deep discharge, over charge, sit idle for months, etc. but unless you're more concencious than most it's going to happen. One big advantage of AGM is weight savings. If you're pushing your tow limits that might be a concideration. They are very good at resistance to vibration and that combined with the weight advantage make them a favorite with the offroad crowd. I don't believe they are as good at accepting deep discharge, but maybe they make specail models for this?

[Mexray]

Be advised if you are looking at 'Gel' cell batteries - they are not tolerant of charging voltages over about 14 volts - you MUST have a charging system that is designed to charge GEL cells, has a 'GEL' cell setting, or you will burn them up!

[CharlieE]

It's over 50 years since I studied DC circuits and ohms law. But I seem to remember that two 12 vot batteries in parallel will present a circuit voltage of twelve volts. The current flow is the same any where in the circuit loop except through the two batteries in parallel. . The current flowing is a function of the load applied and the voltage. With two batteries in parallel the load current drawn from each battery will be one half of the total load drawn. That's the benefit of two batteries, they last lomger between full charge and fully discharged. Maybe I've forgotten something. CharlieE

[bryanl]

re "I replaced my Univolt with an Intelli-Power 9245" - that seems to be about one of the best things you can do for better battery health over time. I have heard some confusing discussion about AGM's and the sulfation inhibiting 'equalization bump' by the IP - some say it is too much for AGM's, other say modern AGM's have no problem with it, and I have even heard that Progressive Dynamics has a special Charge-Wizard for AGM's. All hearsay so far from my point of view.

Gel Cells seem to have gone out of favor for RV use. Mexray's view is one reason. The Az Wind Sun FAQ and RV pages have some good stuff on this.

re: "DC circuits and ohms law" - the big kicker here when it comes to batteries is named Peukert. Yes, two batteries have twice the energy capacity of one and, in general for nominal RV use, the total energy capacity of the bank is the sum of the capacities of its component batteries. Where it gets interesting is that the 20 Hour energy capacity spec is a 60 watt load for a typical RV battery. That means the ratings for a 2 battery system are centered on a 120 watt load. That is high for 'old style' lights and fans and pumps but rather low for 'new style' entertainment systems and microwaves. That means that 'old style' use will show higher apparent capacity while 'new style' use will benefit from parallel batteries (like diesel engines in modern pickups) due to reduced impedance.

[bhayden]

I looked at the AZ Wind Sun FAQ. Their objection to GEL cells is primarily that they charge at a slower rate. So what? All wet batteries and every other type I know of with the possible exception of Lithium Ion will benefit from slow charging. The whole concept of a deep cycle battery is that it's designed for a long discharge period and you need to allow a long charging cycle. In a car/Off road vehicle/Class A RV where you're cranking an engine over, running a winch, using an inverter to supply AC voltage to a washing machine then it's a bigger issue.

They also mention that in really hot climates the GEL electrolyte can lose moisture and fail due to voids. Again, heat is the enemy of all batteries. They sell batteries but don't mention in the FAQ the cost differential of GEL and AGM. Even if they last 20% longer when abused (there's no evidence they won't last as long or longer with typical treatment, my experience has been longer) the AGM aren't as cost efficient. AGM does have a big weight advantage. If you're doubling the number of batteries and using the biggest ones you can find then that's an issue.

With two 6V batteries in series if one cell dies (the usual way a battery croaks) you're dead in the water. With two 12V batteries you're back to the capacity of one battery. Charging two batteries in parallel can be a bit tricky. A battery isolator, A/B/A+B is probably a really good idea.

[bryanl]

re "Charging two batteries in parallel can be a bit tricky." - not in my experience for small banks. The internal impedance of each battery tends to balance things out, even if the batteries are not well matched.

Wiring can be a concern, though, with any battery bank. Smartgauge has an analysis of this.