Battery was doing fine on the fan but the lights deadly

Fastfwd75 wrote:
The battery level indicator tells me nothing and the black water level stopped working after 2 weeks. At least I can still check on fresh and gray water.

That's typical. When you were with the salesman in the lot, it all sounded like it was built like the space shuttle. You'll find that it is built more like a cheap toaster.
I'd suggest you get an inexpensive multimeter before embarking on a long trip. The meter will be one of your best tools to diagnose and fix electrical issues in your RV.

Incandescent bulbs consume about 10 times more energy than LEDs. Plus the instantaneous voltage displayed on most panels are not a good measure of capacity. I installed a TriMetric battery monitor instead. Check my website if you want more info about that.

I recommend adding a second battery and gradually replacing your bulbs with LEDs.

Here is a video I did comparing LED vs incandescent bulb power consumption.

The battery level indicator tells me nothing and the black water level stopped working after 2 weeks. At least I can still check on fresh and gray water.

I agree the indicator panel is useless.. A 921 is what about 18 watts. 1.5 amps. Times six is 9 amps per hour

your Group 24 has what, 75 amp hours, (C/20 rate) and you can only use about 37 of them, And when you draw faster (the 20 hour rate is about 3 amps or two light bulbs) when you draw faster the batteries go down faster still, I do not have the chart handy but figure you get about 3 hours lamp time on that battery with six 921's.. This of course does not include the fan and other things that draw power.

Your panel indicator is not measuring battery voltage either, there is voltage drop in the wires between the battery and the fuse panel and that's where it is measuring at.. plus the voltage to state of charge chart is only valid for STATIC (At rest) voltages not for operating voltages.

On and that "You can only use half" applies to DEEP CYCLE, on Marine/Deep cycle, you should not use that much.

I suggest additional battery of if possible upgrade to a pair of GC-2 golf car six volt in series forming one big 12 volt battery (230 amp hour, roughly a 4-D)

The meter likely is simply showing the instantaneous charge state of the battery (converted into a row of lights rather than a number).

When the battery hasn't been used for a while and there is no load or charging source, the voltage roughly corresponds to how well charged the battery is. It's a cheap and easy solution for them to add but requires a little understanding to use in a meaninful manner. When you apply loads the voltage will drop almost instantaneously (depending on the load), so even testing with a meter at the posts doesn't work.

If you turn off the lights and it returns to a "full" charge state, the battery is likely still full or close to it.

If you apply a heavy load for a short period of time, it may show up as heavily discharged even after the load is removed but the chemistry of the battery will catch up and the charge state shown will gradually come back up over 10-30 minutes (assuming it isn't actually depleated).

A quick check found a vent fan pulls around 36watts. Assuming 6 15watt bulbs, the bulbs were pulling 90 watts or about 3 times as much, so that is likely why you see a difference in the meter.

An easier solution is to get the amp-hrs for your battery and calculate how much time you have. Divide the watts by 12 to get a rough idea of the amps each item draws and then multiply how long you expect to have it on to get an idea of how many amp-hrs of battery you need. Only assume that you can use 30-50% of the available amp-hrs.

If you do persist with the run it till it dies test, try to stop before reaching 50% discharge. Deep discharges are hard on batteries, hence the plan to only use 30-50% of the available amp-hrs.

Fastfwd75 wrote:
...The battery shows 1/3 full with everything on; completely full as soon as I turn off the lights. ....

6 bulbs would be 8.4 amps total draw. If the voltage rise is instant when you turn off the lights, I would look for a bad connection in the discharge path causing a large voltage drop to be seen by the monitor panel.

The idiot lights are only meaningful when the battery has no load and has not been charged for several hours. Even then they are a very crude indicator.

LED is the only way to go. Battery water is up? Have you upgraded your converter? If you do upgrade be careful of what you buy.

LED bulbs use about 10%-15% the power of regular bulbs. I replaced mine in the first year and saw my dry camping ability go from 1 day without power hook up to 3. The following year I upgraded my group 24 to a group 29.

Now it serves me well on my 3-4 day weekends without shore power when I have that type of camping trip planned. If I had it to do over, I'd get two group 27's and be good.

The indicator panel is useless as a state if charge indicator. The only meaningful measurement is with a voltmeter directly across the battery terminals or a hydrometer to measure the specific gravity.

A useful link:

12V Side of Life

I don't let my batteries fall below 12.00V or 50% charge.