House battery voltage under draw?

[tommymsw]

I have a new (to me motor home) and an upgrade from my old one. In my old RV, I had to check the house battery voltage manually. With the new rv, I have a switch on the wall.
However, when I check the batteries while I am using them, they quickly get below 11.9 (to 11.6 or so), but later, when not in use, seem to be OK again at low to mid 12. They are BRAND new. Like one day old!

So, do they read lower when under draw? And I should only read them when they are not in use? Or is something wrong with my readout on the wall?

[pnichols]

Almot wrote:
Depending on your alternator and how it is wired, driving half a day can be better than plugging it in 20A overnight.

It certainly works very well using the Ford E450's 130 amp alternator in our Class C motorhome.

At cold start at idle the alternator can temporarily dump 70-80 amps into our 230 AH AGM deep cycle battery bank with it's voltage above 14.X volts. When we travel, the alternator's voltage slowly drops to 13.5 - 13.8 volts as the chassis and coach batteries charge and as underhood and outside ambient temperatures rise. The alternator's voltage only goes below around 13.5 volts in high outside ambient temperatures - usually from about 90 degrees F on up ... which is perfect for liquid or AGM lead acid batteries because all charging voltages should be reduced as ambient air temperature around the batteries increases. The alternator gets our coach batteries charged to where they are not accepting any current in 3-5 hours of driving - depending upon how low the coach's AGM batteries were when we started.

I can watch all this happen as we travel because I have chassis battery and coach batteries digital voltmeters on the cab dash, in addition to a digital ammeter on the dash indicating current flow into or out of the coach batteries (negative readings for current flow out of the coach batteries and positive readings for current flow into the coach batteries).

AGM batteries with their low internal resistance, combined with whatever Ford and/or Winnebago seem to have done, is working out well.

[theoldwizard1]

Almot wrote:
Depending on your alternator and how it is wired, driving half a day can be better than plugging it in 20A overnight.

Not true, especially on modern vehicles where the alternator voltage can drop below 13.5V shortly after startup.

tommymsw wrote:
I didn't realize that plugging them in would be SO much better than driving.

Read this DC-DC Battery charging

[Almot]

pianotuna wrote:

I'd jump up to between 400 and 600 watts, which, other than running an air conditioner may nearly eliminate generator use.

Yes. At least 300W solar for offgrid camping.

His camping situation wasn't made clear. "No boon docking", and then "to run the generator or plug in at a park and only rely on the solar to keep the batteries topped off from normal drain while it sat empty". If RV park has no electricity and you have to run a generator - this is the same as boon docking.

I would NOT buy bottom of the barrel off-brand controller, even for as little as 100W panel. In China deception is a perfected art. It stops working or won't work properly from the beginning and you end up with overcharged or discharged battery.

[Sam_Spade]

tommymsw wrote:

Although maybe wattage matters in this case? Or maybe the amps matter?

Yes indeed.

You need to pose your question to Winnebago or the company that makes the solar controller.....armed with the model number.

I think it's likely that the "cheap" panel they installed was matched with a cheap controller and it probably needs to be changed to.

[landyacht318]

A 10 watt panel can indeed overvoltage fully charged batteries that have and have had no load on them.


https://www.youtube.com/watch?v=z5RtZe9AW2E&list=UUoPqTkOluQsuu3RpGnxVwFw&index=9

Do not assume anything with the intents of the people who made your RV, other than that they did every single thing possible to extract maximum profit at each and every stage of the design and build.

If you replace the 10 watt panel with a hundred watt panel, you will need to install a controller to prevent overvolting the batteries. Simple charge controllers capable of handling a 100 wat panel, are basically 4 wires to install, and can be had for as little as 16$ on Amazon, Perhaps less if one really is shopping for the cheapest of the cheap.

[pianotuna]

Hi,

It is quite possible that a 10 watt panel might not have a controller.

I'd jump up to between 400 and 600 watts, which, other than running an air conditioner may nearly eliminate generator use.

[Almot]

tommymsw wrote:
Or maybe the amps matter?

Yes, they do.

Solar controllers have max current rating. For 100W panel you need controller rated to 8 or 10 Amps. It is not clear whether your 10W system has any controller, and what rating.

[tommymsw]

So one last thing.... If I replace the 10W with a 100W, do I need to change the solar inverter (or control system) that is in there already? Or just replace the old panel with the new one?

My instinct tells me that if the RV is set up for 12V solar already, that the wattage should not matter? As long as we stick with 12v, the current system should still work, (only faster I hope).

i.e. If you have one 100W panel, you should be able to add another or another without changing the charging system. The same way I can just add more outlets to the same line in my house as long as I do not exceed the maximum amperage.

Although maybe wattage matters in this case? Or maybe the amps matter?

[2oldman]

tommymsw wrote:
Not like some owner installed it. Winnebago installed it!

Understood. That's usually what 'an option' means.

[tommymsw]

2oldman wrote:

tommymsw wrote:
solar. I have the 10W and that is clearly too small (wonder why it was even an option).

Because people don't know anything about solar.

The 10W solar came as an option from the manufacturer. Not like some owner installed it. Winnebago installed it!

[pianotuna]

Nor about electricity in general.

2oldman wrote:

tommymsw wrote:
solar. I have the 10W and that is clearly too small (wonder why it was even an option).

Because people don't know anything about solar.

[2oldman]

tommymsw wrote:
solar. I have the 10W and that is clearly too small (wonder why it was even an option).

Because people don't know anything about solar.

[Sam_Spade]

BFL13 wrote:

You don't need a multi-stage charger at all. You just need a single voltage charger set to the specified Absorption Voltage for the battery being charged.

Once again.....the "experts" are talking right PAST the common owner and user on this subject.

Single voltage chargers that come with RVs typically are "cheap" and not adjustable. The average owner has no business adjusting the charging voltage anyway.

So.....for the vast majority of RV owners, a good quality 3 or 4 stage converter/charger is by FAR the best choice.

[BFL13]

Shows 225s and 210s among the three types

https://www.interstatebatteries.com/recreation-vehicles/golf-cart-batteries

Local Interstate shop says the 225s are Trojans. You see them in RV dealer stores too. Replaced their previous (Mexican) 232AH HDs a couple years ago, which replaced their USB 232 U-2200s around 2010 ISTR.