Batteries: 6 volt vs 12 volt, series vs parallel

[charliem]

[font size="3"]Batteries: 6 Volt vs 12 Volt

While working with a "soon to be newbie' the subject of batteries came up. It's an old topic, but one that each new owner must address. So I'll post my options here for the general public.

Each battery consists of a group of cells. Each lead acid cell is nominally 2 volts. When cells are connected in series the voltage adds. When connected in parallel the voltage remains constant but the Amp Hours (AH) add. The Camplite runs on 12 volts so the question is how do I connect cells to provide 12 volts and satisfy my hungry electronics while keeping an eye on cost, weight, and volume. Typical battery configuration are :

[/font][ol type="decimal"][li][font size="3"]Six cells in series for 12 volts. This is the common dealer supplied Group 24 "marine" battery. GP24 batteries are typically ~80 AH at 12 volts. Unless you can always plug into shore power, this is very limited.
[/font][/li][li][font size="3"]Two GC2 connected in series. Each GC2 is typically 6 volts at 225AH. Two GC2s in series yields 12 volts at 225 AH. GC2s are very affordable due to their demand in golf carts. I consider this a minimum for dry camping. [/font][/li][li][font size="3"]Two Trojan T1275s in parallel. Each 1275 is12 volts at 150AH. Two in parallel provide 12 volts at 300AH. This is a worthwhile increase over two GC2s but costs more. Two T1275s can easily be accommodated on the tongue of the larger CLs and are still light enough for single person lifting.[/font][/li][li][font size="3"]Four GC2s in series/parallel (two strings of two GC2 in series, then the two strings are connected in parallel). This configuration provides 12 volts at 450AH, but the weight and size become problematical on the CL tongue. Cost is favorable, again thanks to all you cart riding golfers.
 [/font][/li][li][font size="3"]There are other usable configurations, but they have weight and size problems on the tongue. Three GP27 batteries in parallel would produce 300+ AH, but the hookup is more complex and would require three parallel strings. Multiple parallel strings should be avoided. I don't like anything over two strings in parallel. Other large capacity batteries are available, but they are heavy, large, and expensive. Electrically the "best" combination would be a single string of six 2 volt cells, but this gets very expensive and eats up lots of tongue space.
[/font][/li][/ol][font size="3"]
So my recommendations are: go with 2 GC2s in series immediately. If you consistently run out of battery look at the two T1275 option and/or go to solar, generator, or jumper cables to the TV.
[/font]

[whoofit]

I agree on the GC2s. Had good luck with the GC2 Energizer cheapos made by Johnson Controls. They are no T105s but can be had for less than $70 ea. Been running a 24V system, two series strings in parallel, for 5 years now.

For the tongue challenged a set of AGMs side mounted under the trailer, or in a vented box in the trailer, is a good choice too. Would need to sacrifice some deep cycles to keep the costs in check. I'm finding out that I like the Group 30 AGM we purchased. I know the life won't be there because it is a combo battery but it is dead dumb simple. Not as vibration susceptible either.

A pair of these Group 30 AGMs in parallel would serve us for 10-12 days before charging. I know this says nothing about the usage profile but we do not conserve all that much while dry camping.

Have to mention the need for a means of equalization with lots of GC2s especially in parallel strings

[nmken]

Another Aspect in battery choice is the target discharge rate and reliability.  I've got Trojan 105s - I chose them for their durability and in effect not really worrying about the 50% discharge limit that I have always had for 12v batteries.  But on the flip side for 12v batteries they do discharge at a higher rate if that is something someone wants.  It would only be a concern for folks running large inverters.  As a hypothetical scenario, it would take 4 6 volt batteries to run a 1kw inverter that could drive the microwave, while a single 12v battery could do the job (albeit for a very short time!).

[tinkeringtechie]

While all the information in here checks out, I should mention that new users should just try the single group 24 12V for a while before making any upgrades. Learn to ration electricity and monitor the state of charge. If you do eventually upgrade at least you'll know how much capacity you'll need and how to take care of it. In my case I'm still using a 12V with **gasp** 55AH capacity. How could I possibly make that work? Well I use about 10AH per day, and my 30 watt solar panel puts it right back in. It's also an AGM battery so it has an extremely low self-discharge rate and very low internal resistance (easier to charge). I don't think I've ever gone below 75% SOC and we've dry camped for 7 days before. But just like many things in camping, YMMV.

[david]

I agree with all of the above information, but let me expand on the meaning of deep cycle batteries:

There are two types of deep cycle batteries: flooded cell (the type that has been used in cars for a hundred years) and AGMs.

There are some specific design features that make a "true" deep cycle flooded cell battery. These are thicker plates (so that some can slough off and still leave enough in place), more room on the bottom to accumulate the stuff that sloughs off and more room on the top for electrolyte over the battery plates. If the battery doesn't have these design features then it is a starting battery and it will fail much sooner than a true deep cycle battery.

Many, maybe most 12V batteries that are labeled deep cycle aren't made to those specifications. The battery is simply a starting battery with a deep cycle label stuck on. A sure way to tell is that these batteries have no amp hour specification, duh!!!

OTOH a sure way to get a true deep cycle battery is to buy a couple of 6V golf cart batteries- the GC2 or GC1 noted in the previous posts and wire them in series for 12V. Anyone trying to fake a deep cycle battery for the golf cart trade will quickly get run out of business. Also golf cart batteries are relatively cheap due to the hundreds of thousands of golf cars in service.

But what about the other type of deep cycle battery, the AGM? Due to their inherent design, an AGM starting battery is built the same as one for deep cycle service. And they do have some benefits as Tinkeringtechie notes above: faster recharge, lower self discharge, no adding water. The big downside is they are 2-3 time more expensive.

So my advice is to start with one Group 24, 27 or 31 AGM battery. That will cover the needs of the vast majority of campers. I have a Group 27 at about 70 AH and I have never gone below 50% discharged. A Group 31 which is about 100 AH will last about 5 days without recharging for most campers. If you need more, then pop rivet in place a couple of aluminum angles crosswise in place of the existing system and mount two golf cart batteries. That will give you 220 AH of capacity and while adhering to the maximum 50% discharge rule of thumb, should cover you for 10 days.

If you really want to camp longer or you use the furnace a lot at night or have big DC consuming appliances then use 2 golf cart batteries plus a solar panel, jumper cables to your TV or a Honda generator to periodically recharge or use 4 GC batteries and recharge less often or not at all. But you will probably be in the 1% minority of campers.

David

[fasteddieb]

Allow me to put this out there as a potential cautionary tale.

I used to own a plane with a 24v system. For a battery cart, I wired two 12v batteries in series and charged them with a 24v charger. 

One day I went to my hangar and found little pieces of black something scattered around. A little snooping revealed this:


[img style="max-width:100%;" src="https://c2.staticflickr.com/6/5331/17255717884_017b0d8ac2_z.jpg"]

Made a real mess. Was lucky in that none of the plexiglas on the plane was affected by the sprayed acid. Just some of the decals were marred a bit.

In any case, in a discussion thread someone told me it was a bad idea to charge two separate batteries in series. That differences in charge rate - or resistance - or something - could cause problems up to and including explosions.

I argued that even a 12v battery is just 6 cells in series, and that is not apparently a problem. Why should it be so for two separate batteries as long as they were the same make and model and capacity?

I don't really remember his response, but I think he was an electrical engineer and seemed to know his stuff. He still warned against it as a practice.

Anyway, I'm not the expert on this sort of thing, and maybe its done all the time with no ill effect.

Just something to file away and take for what its worth.

[daplumbr]

Well, that's a bit too exciting! I'm glad there were no witnesses!


There are some posts on the forum implying that given the same total amp-hours, cost, and output voltage, it's better to have two 6 volt batteries in series than two 12 volt batteries in parallel. Is there an electrical reason for that perference? 

[whoofit]

[p]I believe the answer is this. In series the current is to be considered the constant but the voltage across each battery will reflect the internal resistance of each battery. If the difference is great there will be an overvoltage situation on the battery with the higher resistance.

In parallel the voltage should be considered the constant and never exceeds the battery voltage to those levels providing the charger is not programmed to produce it in error. The parallel batteries divide the current, unlike the series string which divides the voltage.[/p][p]
[/p][p]The above assumes we are not at compliance with the source.. I feel parallel is safer because the batteries take what they need separately. Balancing can be more difficult though.[/p][p]
[/p][p]
[/p]

[daplumbr]

Thanks, that helps. I'd never thought through the voltage constant vs. current constant before, even though I've been paralleling and seriesing (new word) batteries for years. 

[whoofit]

Of course as you already know neither voltage or current remain truly constant by definition for the duration of the charge.

[daplumbr]

Well duh! 

[whoofit]

[quote source="/post/11120/thread" timestamp="1432119340" author="@sandroad"]Well duh!  [/quote]I know you know much more than just that, Merlin. But if my kid read that he'd gig me for being wrong! He can be oh so otherwise.... (rofl)

[ammobob]

And oh how they love to gig you when your wrong. 

[kath]

Following...

[david]

The perils of hooking up two 6V batteries in series to make 12V seems to have died. Let me drive a stake through its heart.

There is essentially no electrical difference between two batteries with 3 cells each hooked up in series to make 12V vs a single 6 cell 12V battery. Tens of thousand, maybe hundreds of thousand boaters use 2,4,6+ golf cart batteries hooked up this way and have none of the problems that fasteddie noted when he tried to charge two 12V batteries with a 24V charger.

I suspect that one or more of the cells in fasteddie's experiment were bad which when charged by a high output charger caused the current to skyrocket, heated the battery up and it was destroyed. This can happen whether you have 1,2,3,6 or 12 cells in a battery.

David