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?

[Boon_Docker]

Oh such a friendly place.

[mike-s]

pianotuna wrote:
First, they do NOT boil. They gas.

Look up "colloquial", you might learn something.

pianotuna wrote:
Second the link provided does not mention the capacity of the battery bank nor the type.

Second, learn to read:

Bank #1 = 220Ah Lifeline 6V AGM

Bank #2 = 125 Ah Marine Maxx Group 31 Flooded Lead Acid

BFL13 wrote:
I don't know which 6s he has, but don't forget the latest Interstate 6s are re-branded Trojan 105s at 225AH, same specs.

Citation needed, because Costco sells Interstate, and are reportedly made by Deka (too lazy to find that reference). The specs don't match the T-105s.

[Sam_Spade]

tommymsw wrote:

When they are charging I get over 14v as expected.

The solar panel is only a 10 watt.

14 V is the CHARGING voltage, not the battery voltage.

And 10 watts at 12 volts is less than one amp; barely enough to maintain a full charge with no load. Not nearly enough to do any actual charging.

Trying to figure out if things are working right with the present setup is just a waste of time.

You NEED to get those new batteries fully charged SOON. Just sitting at half a charge for very long causes them to age more rapidly.

[BFL13]

I don't know which 6s he has, but don't forget the latest Interstate 6s are re-branded Trojan 105s at 225AH, same specs. He says his are brand new, so might be 14.8.

He could get them up to speed with the generator inside four hours if he has a high enough amp charger that will do 14.4 or more. I have a friend with a 1998 MH with a 9100 but no CW for it. A 2000 could maybe have one with a CW, but not on average likely.

[pianotuna]

post withdrawn

[Almot]

You mentioned (I think) a no-load voltage somewhere. With this motorhome it is very likely that there will be something drawing current at any given time, unless you physically disconnect all the cables from the battery.

In addition to battery not fully charged at the beginning.

[mike-s]

BFL13 wrote:
You want a charger that can do 14.8 volts to get them done right,

Depends on exactly which batteries he has. Trojan specs that, Interstate/Costco/(supposedly Deka) 14.4, USB 14.7.

One size doesn't fit all, which is the problem with non-adjustable RV power converters.

pianotuna wrote:
Ten watts for 5 hours may not even equal self discharge rates on twin six volt batteries in series.

Or, it may boil them over time.

[time2roll]

10 watts would barely maintain 2x GC2 when disconnected. No actual recharge. I recommend 200 to 400 watts if you want to actually recharge.

Time to plug in or run the generator 6 to 10+ hours. Needs to run until the charging system drops down to float as a minimum. Just because you are 14+ does not indicate the battery is charged.

[pianotuna]

Ten watts for 5 hours may not even equal self discharge rates on twin six volt flooded batteries in series.

[BFL13]

In the OP:
"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!"

In a previous reply I said:
"With a pair of 6s in good shape, being drawn down from full steadily at their "20 hour rate" (of about 11 1/4 amps for 225AH), in 10 hours you will see about 11.6v when they get to 50% SOC. Remove all loads and the voltage should bounce back to about 12.1 (Trojan spec) after a couple of hours"

We now know your voltage readings are ok.

Conclusion is that those two 6s must be about 50% SOC (no idea how they got so low--another story?) so your recharge has to be a biggie.

You want a charger that can do 14.8 volts to get them done right, but on your converter, expect it to take half a day or so to get them sort of charged up. Too long for generator time unless you have a high amp charger (60 amps say) that can do 14.8.

It seems you have no solar, no shore power, and no fast charger? Up the creek with no paddle--? 😞

[mike-s]

BFL13 wrote:
in a 2000 year rig, you likely have a single voltage 13.6v converter and you will not have charged them up as you think you did

This.

In general, RV converters are cheap and intended for campers who plug in all the time. They're not configurable, so charge batteries too much or not enough. They're mostly about running 12V stuff when plugged in, battery charging is an afterthought - just good enough so the lights and fridge will work on the occasional overnight without AC power. As long as they'll keep a battery working past the warranty, manufacturer's continue to buy the cheapest ones available (e.g. WFCO).

Since the OP has 2x 6 V batteries, it behooves them to get a good charger (quality modern converter retrofit, decent solar w/controller, or good stand-alone battery charger). The increase in battery lifetime alone will pay for it.

[2oldman]

tommymsw wrote:
The solar panel is only a 10 watt.

That's almost nothing.

[tommymsw]

I checked with my meter on the batteries and it does confirm the terminal on the wall. When I turn everything off, they do seem to go up to 12.1 right away.

Maybe it is a case of them needing to be charged a few times.

When they are charging I get over 14v as expected.

The solar panel is only a 10 watt. So maybe it is not giving enough time, however I was using just some lights and ran the furnace for a couple hours (to test it). My older RV can read 12.4 for DAYS when using just the lights.

I am getting my generator fixed today (hopefully) so I can let them charge way up and I will see from there.

[pianotuna]

Hi tommymsw,

The batteries have never been fully charged. They need to get to 14.8 volts. Driving for 9 or 10 hours for a few days might get the job done.

You don't specify the wattage on the solar, nor the type of charge controller. If the controller does not have a temperature sensor on the battery bank, it is unlikely to fully charge them either.

To be able to use voltage under load you would need to put on the loads, then use a hydrometer to check the specific gravity. That may allow you to have a "much better guess" at state of charge under load. I.E. make your very own chart of what voltages are when particular loads are applied.

For loads you mention "just a few lights". If they are incandescent, the load may be 2.2 amps per light. The jars you have are tested for a 25 amp load. So six lights may be over 50% of the planed for recommended demand. It is a known fact that voltage drop is greater with twin six volt batteries in series.

tommymsw wrote:
To answer some questions...

3. I am NOT connected to shore. I do have a solar panel and it does in fact charge the batteries (reading over 14v when charging). I have also driven and charged the batteries. They do charge and seem to hold their charge when not in use.


However, if the volts do appear lower when under load, how then do you know when to stop using them? I assumed you could check them in use and know when you need to stop.

[Almot]

tommymsw wrote:

1. When I say the batteries when NOT under load go back to low 12 again, that is BEFORE they charge back up. They do charge back up to full again.
...

However, if the volts do appear lower when under load, how then do you know when to stop using them? I assumed you could check them in use and know when you need to stop.

How do you know they are charged fully when you charge them?

Voltage accurately corresponds to State of Charge only after battery has rested for (at least) 12 hours since charging, and in absence of loads. Get a multimeter and measure directly on battery contacts, not through the wires several feet long.

Solar charger reading 14V (or is it a remote voltmeter again?) - this doesn't tell much, either. Only after it had STAYED at 14.4-14.6 for an hour or two, you can be sure that it's charged more than 75-80%, though it won't be full yet.

In use voltage readings are all over, you can't rely on them. If you need to know when to stop (whether you meant to stop using energy or to stop charging) - you need to install inline amper-hour meter. When being charged with solar panel, there is no need to stop charging, good solar controller will stop by itself. Not all of them are equally good, though.