When to recharge house batteries?

[jeffprupis]

Been FTing for six-months now and really enjoying the journey. We have primarily camped in State Parks with 30 or 50 amp hook-ups. We're now staying at Windy Hill Rec Area in AZ with no electric. My question concerns charging the batteries. I'm watching electric usage carefully as I see my battery level drop down to 11.6 amps. My question is how do I know when it's time to run the generator to recharge the batteries and for how long? TIA my faithful sages.

[Edward Buker]

Jeff,

I think you mean 11.6V but I am not sure. It can be a little tricky to know the battery charge state because under load the house bank and wiring will have some voltage drops. The most accurate PITA way to know would be to turn off the house load using the house battery switch and after a couple of minutes measure voltage at the battery posts. You could also just be sure most all things that could pull a significant load are just switched off and use the coach monitor panel or charge monitor panel if the voltage reading was available to sort out the true battery voltage. The message is to get an accurate reading you need to not have voltage drops playing a role and that happens with little or no current (Amps) being drawn from the bank.

Approximate state-of-charge with no load on a Lead Acid battery.

12V Lead Acid Battery

12.65V  100%
12.45V    75%
12.24V    50%
12.06V    25%
11.89V      0%

In general you do not want to deplete the battery charge as a rule below 30% (about 12.1V) repeatedly as it will shorten the battery life.

When you go into charge mode the voltage reading you are getting has to settle awhile after the charge cycle and also be under no/low load to know where the battery charge state really is. You can use the charge state on the house charger panel, 30 minutes of absorption mode if your panel has those indicators might be a good rule of thumb. Those folks that boondock more then I do may chime in here. Hope this helps.

Later Ed



   

[Bill Sprague]

Jeff,

(On edit, I see Ed types faster.  His numbers are more conservative than mine and are probably a better guide.)

There is no simple answer because the easy methods of reading the battery state of charge are not effective.  That's because the meters are all based on voltage.  The only way that can work accurately is if you completely remove the battery from the system, let it rest for awhile and then read the voltage.   12.7 volts would be "full".  11.5 volts would be "dead". 

While hooked up in a motorhome you can't completely remove all loads, so voltage reading are only an estimate.  But, 11.5 volts on the inverter gauge is pretty low.

The inverter gauge does the best it can.  It is hardwired to try to compensate for a load and give you a voltage reading.  It is also programmable with way too many choices.  Therefore, what you see is really only an estimate.   

If you want more accurate readings (and don't mind the risk of acid holes in your clothes) you need a hydrometer. 

The most expensive thing you can do is let your batteries discharge "deeply".  It does not take many cycles of that before your batteries are permanently weakened.

The cheapest thing you can do is run your Onan for an hour or two in the morning and an hour or two at night.  An hour on a 7500 Onan on a light charging load will burn about 1/2 gallon.  Two or three dollars in fuel is a lot cheaper than new batteries.

The goal of the twice a day plan is to keep the batteries between a theoretical 50% and 90%.   Below 50% shortens life.  Above 90% wastes fuel.    To error below 50% can be expensive.  To error above 90% causes no harm and only burns a little fuel.

The only proof I can offer about the success of the twice a day play is that it worked for me.  We bought our '04 Monterey new and I think the Interstate batteries had been abused at a trade show or two.  I kept them working about 3 years.  I replaced them with Trojan T-105s that were still functioning well when we sold it 7 years later.   

[Jim Edwards]

Hi Jeff I have an 05 Monterey and it originally came with an Auto Generator start (AGS) module and a xantrex inverter. The inverter was replaced by the previous owner with a magnum inverter without a ags module. The ags will generally have a setting to start the generator when the voltage drops to a set point. I added this to the magnum inverter so it also will start the generator based house battery voltage and on temperature on the interior of the coach to energize the a/c units for pets. I never had the manual for the Xantrex AGS. It is a nice way to set and forget. I still have the xantrex ags module if you have the xantrex inverter and don't have a ags module.  (just left it on the coach because the magnum ags ended up else where.) Just let me know.   

I have attached pages from the 05 owners manual re: this subject.


Best 

[jeffprupis]

Thanks. Have AGS - will need to learn how to program it. The idea of running gen 1-2 hours 2x's per day suits my needs as we do not dry camp often (yet).

[Roy Warren Co-Admin]

We do dry camp regularly.  While dry camping we use the AGS system and ensure the start sequence is set for 12.2V.  During the day, we use the 95% of charge for shut-off.  We do have a residential refrigerator and we do not use the electrical heater on the AquaHot.  In the evening, we make sure we have a 99% charge so we normally begin running the generator at 7:00PM and shut it off at 10:00PM.  This gets us through the night without any faults and the battery not going below 12.1V.  Dry camping is fun and utilizing the AGS is a good way to monitor your batteries.  In fact, I leave the AGS set when we pull into parks as we have come back to the coach numerous times in the last 16 years of FT only to find the park lost electric or a breaker popped and no one else noticed.

[LaMonte Monnell]

Having the AGS is a perfect way to keep the batteries charged as required. Just get it programmed and it will do it all for you.

I wish my coach had an AGS start system. We don't drycamp much at all.

I had to replace my two chassis batteries and the four house batteries after owning the coach 3 years. The outside of the batteries were bulging by the terminals and they were getting hot. Figured it just made sense to do all 6 and be done with one more expense......

Good luck...

[Joel Ashley]

My AGS turned out to be just too confusing and unreliable the many times I tried to get it right early on.  Even having the BCS tech set it for us, he got it wrong and the darned thing went off in the middle of the night, or wouldn't run when it should.  It just didn't seem to know a.m from p.m., no matter how I did it. 

After hours of reading and re-reading manuals and failed experiments and an over-scratched head (and middle-of-the-night, scrambling-out-of-bed cussing), I gave up and shut it off.   I would still like to have it for certain circumstances, but so far it's just been easy to monitor the voltage state and kick it on at 12.1-12.2 volts. 

Generally, I almost always end up doing that in the late afternoon when it is least likely to offend neighbors, so it's just a daily dry-camp routine, and it takes about 2 hours to get from Bulk Charge down to Float Charge.  It helps if we're parked out in the sun for solar assist, but that of course is a big variable, and I end up just doing approx. 2 hours routinely to Float Charge regardless.

Joel