Dual 6 volt GC2 batteries and charging characteristics

[busterbrown73]

We just returned home to Michigan from a 24 day trip around the southwest. This was the first big test of our new dual 6 volt battery bank (upgraded from a single group 24 12 volt).

As I have really no experience with charging behaviors of a serial 6 volt setup, I'm looking to see if I need to upgrade the OEM converter charger on my travel trailer. This is what I noticed during our trip.

1. Started with a fully charged 13.3 volt bank on the morning of our departure after plugged into shore power for 48 hours. The Duracell (215 amp hour) flooded batteries are brand new.

2. After using typical appliances through a dry camping night, (DC inverter for television/fan, furnace, led lights, LPG fridge, and water pump), the bank would drop to 12.4 to 12.6 volts. We would run our 3700 watt inverter generator for 5 to 6 hours during the next day to allow the converter to recharge the bank. I thought it was preforming as expected except that the converter would charge to about 12.8 to 12.9 volts. I never got back to 13.3 volts until we plugged into shore power at a full hookup campground.

3. On several legs of the journey, we would leave the dry camp location without running the generator in the morning and drive 4 or 5 hours to our next camp site. I checked the volts on arrival at the new site and my TV would barely provide a recharge (12.5 to maybe 12.6). The previous group 24 12 volt would be completely charged after just a couple of hours of running the truck. (Obviously, I have much more amp hours with the GC2s).

My concerns are:

1. Is my OEM converter the bottleneck in my coach's recharging capabilities?
2. For a dual 6-volt in-series battery bank (flooded), what should I expect as a full charge? I've read 13.3 volts as being 100% as a float charge but absorption voltage could be much higher.
3. Will a true 4 stage converter charger (bulk, absorption, float, equalize) help maximize battery usability and increase overall battery life?
4. If a new converter is needed, what size (amp) is preferred? Are converter/inverter types recommended as I see the price point goes much higher? Realistically, I would like to see a full charge after a couple hours of generator use.
5. Is a larger battery bank crippled by the inadequate charging abilities of the converter?

[BFL13]

But still no answer to Phil's question why you have to push harder. Why is "low and slow" with AGMs not just as good given more time?

[wa8yxm]

pnichols wrote:

That's a rockhound answer, but not a technical one. 🙂

Glad you liked it. I am often accused of being too technical so I tried to make it more "Blue Collar" as it were.

And yes I did leave out a whole bunch of stuff because I was answering the SPECIFIC why do you have to push harder to force amp hours back into the battery question.

Watching a re-run of Murder She Wrote. the mayor is campaigning "I was there when, I was there when. You named the new sewer line after me"

I can think of NOTHING better to name after a politician than a sewer line.

[Vintage465]

pnichols wrote:
For years, now, NO ONE has explained the technical reason(s) as to why RV deep cycle dry (AGM) lead acid batteries require voltages greater than 14 volts to charge them fully if one has plenty of time to do less-than-14-volts charging. Is there some electro-chemistry reason - maybe related to activation energies, for instance?

In other words - for us non-full-time RV'ers who have plenty of time when at home to keep our RVs hooked up so that our stock single voltage converters can be left on continuously - why should we need to spend the money and effort to replace our single-voltage converters? What's the reason(s) that we might slowly be ruining (either by shortening their service life or reducing their capacity) our AGM batteries over time if we make and keep them "fully charged" this way when not on RV trips? What does "use 14.XX volts for cycle service charging" mean? Is this talking about faster charging back to original full capacity - or not ever getting fully charged back to original full capacity unless 14.XX volts are applied?

I guess I don't believe battery degradation happens from using lower charging voltages for long periods if someone can't explain it technically. Maybe there's a lot of battery hoopla out there?

https://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/

This is an blog that goes into great(and long)detail about charging requirements that RV batteries require to be successful. There is a lot of stuff in here and you kinda have to "chew up the fish and spit out the bones" to digest the right and necessary info. The two things that made sense to me is the idea that charging a battery is like filling a tire with your home compressor. You'll never get to 80psig in your tires by using 80psig from the compressor. It'll take a long time to get to 80psig using 90psig from the compressor. But if you're at a 100-120 psig in the compressor 80psig is no big deal. Same with volts, you need to force volts in with more on the upstream side. The other thing that makes sense is how none of the ratings on chargers really mean much as far as output volts and amps. They're rated on a bench in a clinic, not installed in an RV with too small a wire going to far and not putting out enough volts in the first place. This though on chargers applies to solar and controllers too if not installed with heavy enough conductors and set up with the dip-switches to charge at a high enough output.

[BFL13]

LY has posted a number of times how his AGM needs a good jolt to get it back up to speed after some solar (low amps) recharging. He reports that it loses AH capacity if only ever recharged "low and slow". He has to do a proper deep cycle with high voltage and amps to restore it.

I have two different brand AGMs. Both spec sheets say they want higher voltage recharges in the 14s if in "cycle use" but ok at 13.6ish for "float use". ( UPS duty I assume)

They don't need destratification as such, but there must be some equivalent reason.

IMO, I suspect Phil is getting away with it from his alternator recharging erasing his sinfully low amps recharging. That does not account for not doing the 20% rate in bulk though (unless it does that too).

It could be a case where we just don't do that many cycles per year (unlike LY) so we think all is well, because the batts last for many years despite not being taken care of according to Hoyle.

[pnichols]

wa8yxm wrote:

pnichols wrote:
For years, now, NO ONE has explained the technical reason(s) as to why RV deep cycle dry (AGM) lead acid batteries require voltages greater than 14 volts to charge them fully if one has plenty of time to do less-than-14-volts charging. Is there some electro-chemistry reason - maybe related to activation energies, for instance?

Actually it has been explained many times and it is very simple.

To use a classical reference (Pushing a rock up a hill) for this discussion the "Effective weight" of the rock is what you would see if you put a scale against the rock perpendicular to the hill. IT will of course be a fraction of the actual weight of the rock but this is how hard the rock pushes back against you.

Now if the Effective weight is 13,600 pounds (13.6 thousand pounds) and you push against it with 13.6 Thousand pounds you are going no place fast. the rock will not move.

To move it up the hill you need to PUSH HARDER. perhaps 14,600 pounds will roll it up the grade.

Same with charging a battery. push 13.6 and you go nowhere. that's why it is called FLOAT. .To push amp hours into the battery YOU HAVE TO PUSH HARDER. That's all there is to it.

That's a rockhound answer, but not a technical one. 🙂

It still merely argues for faster charging through use of a higher charging voltage ... not supporting an argument for damaging charging through use of lower voltages that are sill higher than intrinsic battery terminal voltages throughout the charging process. Of course a higher charging voltage will force more current per hour through any given resistance -> EE101.

But if one has the "hours to work with" - like after an RV trip when the rig is parked back home plugged in for weeks - how can deep cycle AGM batteries be ruined by a stock converter taking a longer time to charge them fully?

I wonder how many $$$ have been spent by RV AGM battery owners replacing their "stock converters" because their converters would in no way properly treat their AGM batteries? With motorhome RV'ers who camp only a few days here and there on trips and have an alternator dumping amps into their coach's AGM batteries between camping spots, why must 13.6-13.8 volt fixed voltage converters be replaced ... other than for faster charging desired when drycamped using a generator for charging?

Of course liquid acid RV batteries present another situation. They sometimes need stirring via high charging voltages and can also have their acid boiled away via being left on fixed voltage converters putting out a too-high fixed voltage.

[mordecai81]

busterbrown73 wrote:

mordecai81 wrote:
Voltage is a tricky indicator of charge level because the batts must rest for awhile after charging to read the true level via a voltmeter. After a few hrs rest 12.7 volts is a full battery.
A typical WFCO converter/charger will never get your batts up to full charge unless you are plugged into shore power for days and days. I get around that by using a stand alone 65 amp Boondocker charger run off my gen and hooked to the batts with jumper cables. 45 minutes in the morning and then solar the rest of the day works well to get the batts full.
Before adding solar, I would run the gen 3-ish hrs a day to get the batts to about 95% charge.
True deep-cycle batts like you have can take quite awhile in the absorption stage to to get to full. Very few stock converters are programmed to hold proper absorption voltage long enough to do the job.

Any contraindications to be plugged into an inverter generator at the same time using a stand-alone battery charger on the battery bank? This may be an affordable solution for the interim (until I upgrade to LiFePO4).

Nope, the onboard converter/charger will just sit there idling while the standalone charger does the heavy lifting.

[wa8yxm]

pnichols wrote:
For years, now, NO ONE has explained the technical reason(s) as to why RV deep cycle dry (AGM) lead acid batteries require voltages greater than 14 volts to charge them fully if one has plenty of time to do less-than-14-volts charging. Is there some electro-chemistry reason - maybe related to activation energies, for instance?

Actually it has been explained many times and it is very simple.

To use a classical reference (Pushing a rock up a hill) for this discussion the "Effective weight" of the rock is what you would see if you put a scale against the rock perpendicular to the hill. IT will of course be a fraction of the actual weight of the rock but this is how hard the rock pushes back against you.

Now if the Effective weight is 13,600 pounds (13.6 thousand pounds) and you push against it with 13.6 Thousand pounds you are going no place fast. the rock will not move.

To move it up the hill you need to PUSH HARDER. perhaps 14,600 pounds will roll it up the grade.

Same with charging a battery. push 13.6 and you go nowhere. that's why it is called FLOAT. .To push amp hours into the battery YOU HAVE TO PUSH HARDER. That's all there is to it.

[BFL13]

Para 5.2.2 says do the 20% with AGMs. It also says do not Float them if you charge them more than once a month. (Not sure about that!--anyway there it is)

It does not say why to do the charging profile recommended.

https://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf

It does not say you have to recharge them from when they are low enough to do a bulk stage. It does not discuss what to do if you only ever do shallow cycles with AGMs (unless I missed that) Usually you are told with Flooded batts that you should do a deep cycle every so often if you only usually do shallow cycles.

Most people AFAIK, just buy batteries and do what is reasonable and then buy new batts when needed. That's what I do with my car. I only fuss about my RV batteries. Not logical, but how it is!

[pnichols]

For years, now, NO ONE has explained the technical reason(s) as to why RV deep cycle dry (AGM) lead acid batteries require voltages greater than 14 volts to charge them fully if one has plenty of time to do less-than-14-volts charging. Is there some electro-chemistry reason - maybe related to activation energies, for instance?

In other words - for us non-full-time RV'ers who have plenty of time when at home to keep our RVs hooked up so that our stock single voltage converters can be left on continuously - why should we need to spend the money and effort to replace our single-voltage converters? What's the reason(s) that we might slowly be ruining (either by shortening their service life or reducing their capacity) our AGM batteries over time if we make and keep them "fully charged" this way when not on RV trips? What does "use 14.XX volts for cycle service charging" mean? Is this talking about faster charging back to original full capacity - or not ever getting fully charged back to original full capacity unless 14.XX volts are applied?

I guess I don't believe battery degradation happens from using lower charging voltages for long periods if someone can't explain it technically. Maybe there's a lot of battery hoopla out there?

[BFL13]

pnichols wrote:

BFL13 wrote:

MEXICOWANDERER wrote:
Good grade of AGM can withstand discharging to 30% soc but the maintenance for recharge must be as strictly rigid as a Trappist monastery.

This is where thick plates pay for their room and board.

With our camping routine doing 50-85s (approx.) with AGMs for a few days and then a two hour drive home, the alternator and solar charging during that two hours brings the batts up enough so they won't accept the required 20% amps.

Huge PITA to recharge them to full once home. Have to rig a switch to kill the solar and alternator for the drive home, or else as now, run the batts back down once home so they will accept the 20%.

The recharge to full at proper voltage to 0.5 amps per 100AH is no problem. It is that dratted 20% rule. And the battery spec sheets don't even mention it as a requirement. Only gives a max amps no min amps.

It's always something.

Hmmm .... I guess that you don't keep your Class C at home?

I keep my Class C in the backyard plugged in all the time with a float voltage on it's AGM coach batteries and a battery maintainer on it's engine battery. The coach batteries after a day or two of this floating always arrive at an acceptance level of less than 0.5 amps.

You missed the point. Yes I recharge to full at home. The requirement hard to meet is that some (Mex says all) AGMs require a 20% charging rate to the spec Vabs prior to amps tapering during Abs.

If the batts will reach Vabs at 73% SOC with a 20% rate (eg) you have to start the recharge with them under 73%. If you get home with them at 77% then amps will start at under 20% and taper from there.

You can taper down to 0.5a full but you did not do the initial 20% in bulk (because you never did any bulk--the batteries were too full for that.)

Of course you do need a high enough amp charger to be at 20% or batter, and enough 120v intake to run that charger. With a 450AH bank, and only a 75 amp charger, I have to split the bank and do part at a time, which is another chore. Oh well.

Phil gets away with not doing all that somehow with some higher voltage alternator charging amongst his 13.8 volt 7345 stuff, and his particular AGMs last a good while. It is tempting to ignore Mex about the 20% "Lifeline rule" actually applying to all AGMs, but I try to do what Mex says if I can.

[pnichols]

BFL13 wrote:

MEXICOWANDERER wrote:
Good grade of AGM can withstand discharging to 30% soc but the maintenance for recharge must be as strictly rigid as a Trappist monastery.

This is where thick plates pay for their room and board.

With our camping routine doing 50-85s (approx.) with AGMs for a few days and then a two hour drive home, the alternator and solar charging during that two hours brings the batts up enough so they won't accept the required 20% amps.

Huge PITA to recharge them to full once home. Have to rig a switch to kill the solar and alternator for the drive home, or else as now, run the batts back down once home so they will accept the 20%.

The recharge to full at proper voltage to 0.5 amps per 100AH is no problem. It is that dratted 20% rule. And the battery spec sheets don't even mention it as a requirement. Only gives a max amps no min amps.

It's always something.

Hmmm .... I guess that you don't keep your Class C at home?

I keep my Class C in the backyard plugged in all the time with a float voltage on it's AGM coach batteries and a battery maintainer on it's engine battery. The coach batteries after a day or two of this floating always arrive at an acceptance level of less than 0.5 amps.

[wa8yxm]

philh wrote:
Brand new 12V battery on a brand new TT. Even after several days hooked to 120V line voltage, and a few very brief draws of 12 while in storage, my battery voltage is 12.3...

With plans for 6V batteries

PD 4655L Wildcat is arriving tomorrow!

Ok there are basically three classes of converters. one is (Thankfully) becomming more and more rare

1: Feroresonant like the Magnetek 6300 Converter/Charger This is, by today's standard, JUNK and is the one which is more and more rare
This unit was usually a 30 amp Converter and a 3-4 amp CHARGER the problem is it had poor regulation. the Converter side was NOT filtered and when the battery was full up.. IT JUST KEPT CHARGING and boiled them dry swiftly.

2: Single stage. IE: Parallax 7300 Not a bad converter/power supply but not a good battery charger. May well NEVER get to full. 90% yes but never full Parallex bought out Magnatek and dumped the 6300 line.
Now the 7300T (Two stage on a timer) MIGHT fill em if you unplug and plug back in to reset the timer.

3: 3 or 4 stage converters like the Progressive Dynamics WIZARD jobs
And I might add I've been looking for a better charger for 14 years.. Still looking.. (nuff said)
These start with high current. then taper off to low current but about 14.5 volts, finally they drop to 13.6 to "Float" things.

Now the Progressive Dynamics Wizard... after 20 hours of float. Will kick back up to the higher voltage for a bit just to keep things interesting.. (Also working properly) not all do that automaticall but most 3 stage offer it on the manual menu.

[BFL13]

MEXICOWANDERER wrote:
Good grade of AGM can withstand discharging to 30% soc but the maintenance for recharge must be as strictly rigid as a Trappist monastery.

This is where thick plates pay for their room and board.

With our camping routine doing 50-85s (approx.) with AGMs for a few days and then a two hour drive home, the alternator and solar charging during that two hours brings the batts up enough so they won't accept the required 20% amps.

Huge PITA to recharge them to full once home. Have to rig a switch to kill the solar and alternator for the drive home, or else as now, run the batts back down once home so they will accept the 20%.

The recharge to full at proper voltage to 0.5 amps per 100AH is no problem. It is that dratted 20% rule. And the battery spec sheets don't even mention it as a requirement. Only gives a max amps no min amps.

It's always something.

[pnichols]

wa8yxm wrote:

3 tons wrote:


Ok, lets cut through ‘my nonsense’ and do the math:

My two 6v Costco GC’s are rated at 210 amp/hrs per (other makes vary a bit)
Two times 210 = 420 - So you and I both agree Sir!!
BUT, once connected in series to arrive at 12v, the 420, is reduced by 1/2, so I’m now at 210 amp/hrs total...To arrive at 12v theres NO SUCH THING AS A FREE LUNCH ...

Now, to arrive at usable amp/hrs lets assume that for longivity’s sake the DOD should not go below about 50% - now were at about 105 usable amp/hrs...

But then again, this is just my opinion...

NO two 210 amp hour GC2' in serie is 210 amp hours. THE VOLTAGE DOUBLE the amp hours remain the same.

Two 130 (G-31) 12 volt batteries in PARALLEL the amp hours double (260) the voltage remains the same.

Battery 101

Now there is one other component where if you put 2 in series the value halves. but I won't even name it because you are already confused. I will only say the value that divides is NOT Voltage. NOR is it current.

Since the overall internal resistance of a given capacity battery bank determines how fast a battery bank can be charged using any given applied charging voltage ... another interesting question is: What battery bank combination of any given amp hour capacity results in the lowest overall battery bank internal resistance (hence fastest to charge) - two 6 volters in series or two 12 volters in parallel?

To help answer the above question, remember that two 6 volt batteries in series make for a 12 volt battery with six 2 volt cells in series ... while two 12 volt batteries in parallel make for a 12 volt battery with six 2 volt cells in series (in one battery) in parallel with another six 2 volt cells in series (the other battery).

Given "ideal" batteries constructed the same way physically and chemically, the answer should be ... "no difference in overall battery bank internal resistance as seen by the charger".

HOWEVER I doubt that - using the commonly available RV batteries in real life - the two different combinations used by RV'ers results in "the same" overall internal resistance. I think I'd put my money on probably two 12 volters in parallel maybe charging a bit faster for most of us - due to an overall lower internal resistance for the combination. :h

[philh]

Brand new 12V battery on a brand new TT. Even after several days hooked to 120V line voltage, and a few very brief draws of 12 while in storage, my battery voltage is 12.3...

With plans for 6V batteries

PD 4655L Wildcat is arriving tomorrow!