Battery monitor while plugged in

[f6bits]

We’re about to head out for some dry camping. It seems that when I’m plugged in to shore power, my battery monitor reads FULL. I *think* this is the case even when the battery is near empty. Does this sound correct?

My question is, how will I know when my battery is fully charged while I’m running my generator? About how long would it take to top off a half-empty Group 27 12V battery?

[Englishman]

You are correct. If you are plugged into shore power the battery panel shows fully charged.

[Festus2]

You might want to consider purchasing a multimeter so that you can take voltage readings directly from your battery or from any other points in your RV electrical wiring. If you have a 12v outlet in your unit, you can buy a plug-in voltmeter which will give you a reading on the battery and will bypass the wall-mounted monitor.
When running your generator, you can always shut it off for a sec and then take a reading on your battery.
Are you charging your battery by means of a direct hookup to your generator or are you plugging the RV into the generator and using the inverter to charge the battery? The two will probably charge at different rates and take different times to bring your battery to a fully-charaged state. Hard to determine time if you don't know the charging rate.

[Badboy]

You will be ok I leave mine plugged in some times for a week.

[f6bits]

Festus2, I’m using the trailer’s built-in three-stage charger.

I have a meter; I’m just looking at an easy way to know when’s a good time to shut off the generator once the battery is full. I don’t want to have to keep shutting off the generator just to know “is it full yet?” “how ‘bout now?” “when are we going to get therrrrrreeeee!”

I believe that there are three gen run times during the day where we’re staying. I’ll probably run it for an hour in the morning and see how it’s doing, then another hour, depending on charge rate, in the evening. After a day or so, I should have a handle on how long it takes to charge.

[f6bits]

I never heard of a plugin voltmeter. Now I have:
http://www.amazon.com/Equus-3721-Bat.../dp/B000EVWDU0

With this in mind, watching the readout should give me some idea of where it is in the charging cycle, and the battery’s condition when I’m unplugged.

[smiller]

To add to the complication you can't really just remove the charge current and immediately look at battery voltage and expect to learn much if anything as the battery must stabilize for a period of time (longer than you will want to wait) in order to get a meaningful reading.

Short of a fancy panel that measures actual amp-hours consumed and replaced the best low-buck alternative is to check the battery voltage (with no load) before you start the generator/charge cycle to get a rough idea of state of charge. For instance, 12.0 volts represents a roughly 50% state of charge (again, with no load) so if you have 100 amp-hours of battery capacity you will need about 50 amp-hours (plus some extra since charging is not 100% efficient, say 60 amp-hours) of charging for a full recharge. If you have a 30-amp charger that's two hours, except you should add (again, roughly) 50% to that since the battery will not be able to accept the full charge rate during the entire charge cycle. So in the above example (which is admittedly simplified, but close enough for an approximation) you would need about 3 hours. Plug in your own numbers of course, but you get the idea.

[f6bits]

smiller, thanks for putting it into the amp-hour context. That should help me get a grasp of the battery’s requirements. I think that in the end, it’ll just require some experience to get a feel for how much power I’m using and how long it takes to recharge.