Recharging batteries: genset vs. chassis alternator

[jillhop]

Please bear with me--I'm new to the RV world (purchased my first RV this past summer). 🙂

I am planning a trip to Wisconsin this coming April where I will be dry camping in a hotel parking lot for 7 nights. This trip is for a dog event, and I will have access to the hotel for showers, etc, but no electrical hookups. The weather averages for the time I will be there is for low in the upper 30's, high in the upper 50's, so I am concerned about recharging my single 85AH battery after using the furnace overnight.

I've been doing some experiments to try to learn what it takes to recharge my battery here in the driveway. After fully charging the battery (12.83V) from shore power, I ran the furnace from 4pm to 8am the next morning. This depleted the battery to about 50% (12.11V). After 24 hours with the battery disconnected (no charging or use) the reading rebounded to 12.25V (about 60%).

I then ran the generator several times, taking readings 24 hours after each "run":

2 hours generator increased battery to 12.35V (70%)

Another 4 1/2 hours brought it up to 12.54V (~85%)

Another 3 1/2 hours brought it up to 12.75V (100% on my chart, but still not back to the original 12.83V charge from shore power).

So, that was 10 hours of generator use to bring the battery back to full charge from one night of furnace use. This was on a colder night (low 20's) than I'm likely to need on my trip, but still...

I'm wondering if running the truck engine (Ford V10) would charge the battery more quickly? I have been doing some research on solar, but I wouldn't be comfortable installing myself, so that would be a pretty big expenditure for me. While I do dry camp regularly, except for this trip doing so for more than 2-3 nights in one spot (without a day of driving in between) isn't likely to happen.

My coach has pretty much original equipment, so just a basic 1998 converter. I know from what I've read that I could benefit from a better converter, and probably adding a second battery would be helpful. But, if I don't have charging capacity to charge the single battery I have, I'm not sure how helpful that would be.

So, does my 7 nights of dry camping sound workable with the equipment I have now? I'm interested in any and all suggestions!

[NinerBikes]

Buy a good down comforter, problem solved. Or sleep with some warm dogs under the covers in your bed.

[Opie431]

Instead of running the furnace all night get some extra blankets and warmer pajamas.

[westend]

This is a tough crowd!

It's about the Mars/Venus thing and the geek/normal person thing.
As a male geek. I'd like to know ALL the exact requirements of my charging circuits and batteries....yesterday.

There is no way I'd like to have less than absolute knowledge of the power involved, nor could I plan to gain that in the future on some kind of disciplined time frame.

My wife told me that women are smarter than men. I'll show her. :B

[pnichols]

jillhop wrote:
I bought this particular battery because Fullriver recommends 13.6v for maintenance, which matches my crappy converter.

You're doing it the way I did it ... which is "out of sequence" from the way most all others do it.

I kept the 13.6V converter in our RV and searched for an AGM battery with a recommended float voltage of 13.6 volts. As it turns out, my AGM batteries are now in their 8th year and due to their characteristics (low internal resistance), they not only float safely on the converter's voltage but they also charge fast enough on the converter's voltage so as to match our camping style.

I wanted AGM batteries anyway for other reasons ... so I thought why throw away/replace a perfectly good $170-$190 converter? Instead I kept the converter and put the money saved into the cost of AGM batteries that played nicely with the converter.

[time2roll]

We just don't want you to be disappointed while at the show.
13.6 for float maintenance is fine. 14.4 is still better for actually getting back to 90%+ charged.

[jillhop]

Hey, cut me some slack! I'm just learning here, and trying to get through ONE trip where I'm going to need to dry camp for 7 nights. This sort of trip won't be repeated for me for at least another two years (and I'll probably have learned a lot by that time). After this trip, it will be 2-3 nights in a row without hookups at most, and probably mostly one night here or there. I bought this particular battery because Fullriver recommends 13.6v for maintenance, which matches my crappy converter. One step at a time ...

This is a tough crowd!

[MEXICOWANDERER]

Time is a great teacher. But remember the Golden Rule about AGM charging...

at a charge rate of 14. 4 volts the battery must decrease charging rate to 1% or less of total amp hour ratimg of the battery before it is fully charged. Amperage at specified charging voltage is the key with AGM. Several days of partial charging shouldn't be harmful. Several weeks is pressing it I'm afraid.

You may wish to get Fullriver"s directive regarding charging maintenance as an aid.

[BFL13]

jillhop wrote:
I may pick up a portable charger, but I don't think I'll need it. I took the coach out for about 30 minutes of driving around town this afternoon, and the battery is back up to 12.75v.

All Mex ever said was that an AGM will accept more amps at 13.8v than a Wet will, so you can get your AGM charged up faster at 13.8v than you could a Wet at 13.8v

He did not say that you could get your AGM fully charged instantly at 13.8v. It still takes the necessary amps for a long enough time (which makes AHs) to replace the AH missing from your usage. Think hours not minutes.

Be a shame to ruin that nice new AGM. You really need an AH counter like a Trimetric or the cheaper sort you can get on eBay to keep track of your state of charge, since you don't have a good grasp on the difference between surface charge voltage, loaded voltage, and resting voltage.

However, for the short time involved in this type of off-grid episode, it would be much cheaper to learn those voltages than blow $200 on a Trimetric!

[pnichols]

landyacht318 wrote:
I've got a group 31 that holds 12.89 volts ...

So do I.

I have a no longer available Group 31 Optima 90 amp hour cylindrical cell AGM 12V battery that sits at around 12.80 - 13.10 volts whenever I've charged it fully. It's acted that way since day one.

[landyacht318]

jillhop wrote:
I may pick up a portable charger, but I don't think I'll need it. I took the coach out for about 30 minutes of driving around town this afternoon, and the battery is back up to 12.75v.

This kills batteries. No way a 30 minute drive fully charged them. Voltage unless taken on a well rested battery is meaningless.

I've got a group 31 that holds 12.89 volts I'll sell you along with the brooklyn bridge

[jillhop]

I may pick up a portable charger, but I don't think I'll need it. I took the coach out for about 30 minutes of driving around town this afternoon, and the battery is back up to 12.75v.

[time2roll]

2 hours twice a day assumes 14.4 to 14.8 charging voltage.
Have you upgraded the Magnetek or have a 25+ amp portable charger to use?

[jillhop]

"Easy" was probably the wrong word. I'm hoping that with my 2 hours twice a day of generator use, I will be able to get higher than the 75% that I was able to achieve with the flooded deep cycle battery. I did a test last night with the furnace. Started with 12.75v, 12 hours of furnace use (high 30's outside temp) brought the new battery down to 12.57v (about 80% on my chart). With the battery I replaced, if I started at 100% after a 12 hour plug-in, it would be at 40% in the morning, then with 6 hours of charging via the generator (in 2 hour increments) I could only get it back up to about 75%. I'm going to charge for 2 hours tonight and tomorrow AM, and see where I end up. I'm optimistic.

[MEXICOWANDERER]

I agree with you. Fullriver is a good AGM battery with a good history of reviewers who boondock a lot. I for one would like to hear your impressions of how "easy" the battery is to recharge
Full chaege is when the battery accepts less than 1% of amp hour rating in amperes at 14.4 volts.