Help! Battery/Conventer Issues

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New member
Jan 3, 2007
I recently purchased a vintage 65 shasta which i equipped with a 12v trojan deep cycle agm battery and my friend gave me a used 500/1000 inverter for off-the-grid use. The battery is rated for 100 Ah. I charged with battery with a new schumacher charger to 14.3 amps. I tested the system by plugging a shop light into the converter with a 60 watt bulb and came back 8 hours later to find the bulb off and the battery down to 10% of capacity. This seemed odd so I recharged the battery and tested the system again and in one hour the 60 watt bulb had drained the battery 20%. Since the battery and tester seem to working fine the only explanation I can think of is that the inverter itself is draining the battery. Any other ideas???
Inverters themselves require power to run.  The larger the inverter, the larger the load the inverter will need to run with nothing plugged in it.  The documentation from your inverter or the tags on it may say what the current draw is with no load on it.
I have a 400/600 inverter which has a fan....  draws one amp with no load.
Yes, there is a amp draw by the inverter itself, but it isn't enough to explain your problem. The inverter overhead should be in the 0.3-0.8 amps/hour range for that size inverter.

A 60w light bulb requires 0.5 amps/hour @ 120V. To get 0.5 amps @120V requires 5.0 amps @ 12V (5 amps x 12V = 60 watts), so your little bulb id actually drawing 5 amps plus the inverter overhead from the battery. And your 100 amp-hour battery can actually produce only about 50-60 anp-hours before the voltage falls below the minimum needed for the inverter to function (typically around 10.0-10.5 VDC). That assumes a new and fully charged battery.  An older battery may be only 50-75% of that.

How long did you charge the battery? It usually takes at least 24 hours with a typical 10A automotive style charger to get a battery to full (100%) charge. The charger puts out 10A only when the battery is very low on charge. The rate quickly tapers off to 5A and soon only 1-2 amps/hour, so getting the battery above 80% charge takes a long time.

How did you determine the battery was down to 10% charge?  Most standard battery test tables or meters  base a percentage like that on a voltage of somewhere around 11.5V or a specific gravity of about 1.07.  That level will typically be reached after about 40-50 amp-hours of power usage. The 100 a=h rating assumes the battery is run completely down. Here's the definition of battery amp-hour rating, courtesy of Optima batteries:

The Amp Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp hour rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20. Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. Such a battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.) A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts.

I suggest that you charge the battery for 24-36 hours and then test again. You should be able to get 8-10 hours before the inverter shuts off or gives its low voltage alarm (does it have one?).
After doing what Gary recommended, you can run a simple test if you have a multimeter with a 10 ampere (or more) current reading setting.
1: Disconnect the battery charger and any loads from the output of the inverter.
2: Loosen but do not disconnect the negative lead from the battery to the inverter where it connects to the inverter.
3: Connect the negative meter lead to the negative lead from the battery at the inverter.
4: Connect the positive meter lead to the negative connector post of the inverter where the negative battery lead is connected.
5: Turn the inverter on.
6: Remove the negative battery lead from the inverter and read the meter.

It should read less than 1 ampere, plus or minus a couple tenths. If more than that, the inverter is drawing more current than it should, and there is a problem with it.

Note: for steps 3 and 4, you can just use the pointed ends of the meter leads to make your connections. For step 6, if you don't have more than 2 hands, you may need help to do it. Just make sure the meter leads remain in good contact when disconnecting the negative battery lead. This is important to eliminate the possibility of any inrush current.

Good luck! Let us know what you find.   
100 AH by 12 volt = 1200 watt hours

60 times 8 = 480

It is said that you should not run the battery more than half dead.

Most inverters will shut off well before half dead
Most inverters will shut off well before half dead

That would mean they shut down at around 12.1-12.2VDC, John. I don't know of any inverters that do that! Most will run along merrily to the low 11's or less. That's  100% discharged according to the standard battery performance tables.

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