Battery question

[Steve Adams]

Hi all,

We recently replaced our, and I never get this right, house batteries. There are 4 of them. I'm not sure if you call those the coach or chassis batteries.

Regardless, today, on a whim, I decided to check the voltage with our built in monitor with the coach powered up (120VAC at pole) and with power disconnected. Powered up the voltage read 13.4 volts, disconnected they read 11.7 volts. At the time I had just about everything off except one 12VDC light, a heater going and the microwave clock.

I realize that the built in monitor is probably not the best tester but that's a significant enough hit that there must be a problem.

What could possible drop the battery voltage almost 2 volts?

Thanks, Steve

PS. I have kept the battery water level up to snuff with a semi-automatic waterer so I don't feel that I would have a battery problem already.

[Gerald Farris]

Steve,
You said that you were running a heater. Is this a 120V AC heater that you are running off of the inverter?  

Gerald

[Steve Adams]

Gerald, yes, it is one of the perfectoe baseboard heaters. It just happened to be on when I did the test.

Steve

[LEAH DRAPER]

Steve
For future reference, the "house" batteries are the 'COACH' batteries,  the "chassis" is for 'ENGINE' and it's supporting features.  Chassis batteries are what you 'start' the engine with.

[Gerald Farris]

Steve,
The heater is drawing so much current from the batteries that you will have voltage readings on a monitor panel that are actually substantially below what your batteries would read with no load.

The heater will probably draw 150 amps DC or more from your batteries, and your battery bank is not large enough to sustain that kind of current draw. So the bottom line is that you should never run a 120V heater on your inverter, because a few minutes of operation will deplete you house batteries, and running your house batteries down below 12V will shorten their life.

Try running the test again with the heater unplugged, and you should see very different results, and if not you have bad batteries or a poor connection somewhere, probably at the batteries.

Gerald

[Joel Ashley]

Steve-

Gerald and Leah are absolutely right.  That high wattage heater will quickly run down your coach (house) battery bank off the inverter, and is one appliance that shouldn't be used that way, and during your test you probably just forgot it was on.

Usually your house batteries consist of four 6 volt deep-cycle ones, designed to hold up against regular discharges and recharges, and each offers more amp-hours than a 12 volt deep-cycle coach battery.  Starting (chassis) batteries aren't built to cycle like that.  

The four 6 volt batteries are in two sets;  each set of two are wired together serially (serial: one added to the other) as a pair to provide 12 volts total.  Then the two sets are wired in parallel (two 12 volt systems side by side), to provide plenty of 12 volt amp-hours of total service.

Look at it this way, volts X amps = watts, so amps = watts/volts.  That is true for both DC (12v.) and AC (110v.).  The only thing that stays the same in either voltage source for an appliance load is the watts.  So if your heater or a hair drier is say, 1500 watts, it will draw maybe 14 amps plugged into 110v. current (1500w /110v = 14a).  But if that 110v. current is being inverted from a 12v. bank of batteries, your socking them with 125 amps (1500w/12v = 125a).  Ouch!  The voltage left to push that kind of current will quickly drop.

If you know the wattage of each of your devices and appliances, you can calculate how much draw each will put on your battery bank when on the inverter.  For example, the digital clock on the microwave won't take much juice, but nukeing food with the 1000-1200w appliance will.

-Joel