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Old 04-03-2010, 03:18 PM   #1
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Question 12V Inverter to Charge APC Battery Back Up

OK this question is some what related to RV electrical systems.

In the tractor I drive I have added a DVD player for the TV. I also have converter box for the TV for the digital TV signals. There is also a powered set of speakers with a sub woofer that is for laptop computers but I have it plugged into the headphone jack on the TV.

The reason for the APC battery back up is the tractor engine will start and stop when set to keep the systems up (air pressure, water temperature and the batteries). It also has a climate control system so the engine does not have to run all the time (saves fuel, cut down on pollution) to heat and cool the cab.

The problem is when it goes to start it shuts off power to everything except the lights when it starts just like any vehicle so the only draw is the starter. When it does the inverter shut off and with in seconds it comes back on but I have to turn on the TV and the converter box for on air signals or the DVD player. Not a real problem because each has a remote but the DVD player will not come back on unless I unplug it from the power strip, all appliances are 115V and run off the inverter with no problem. I have to leave the DVD player unplugged for several minutes. Some of the time it starts where it left off.

What I have tried to do is use the inverter to charge the APC and have the power strip plugged into the APC for when the tractor starts and not loose power to everything. It should only be on the APC for under a minute while starting. But what happens is the APC clicks the whole time while being used on the inverter and will start to beep then shut down even while there is power from the inverter. It’s as if everything is running off the APC all the time and the inverter is not charging it.

So my thoughts are can an inverter (300W) charge an APC of (200W output 15A 120V charge) for the short time power is cut?

Is the inverter charging the APC while not needed or is it just running on the battery in the APC till it goes dead? Is that the clicking I hear?

I can not go to a larger inverter unless it will plug into a power supply socket (15 or 20A max) (lighter plug), we can not hard wire them into the tractors.

Thoughts ideas?
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Old 04-03-2010, 05:54 PM   #2
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Hi Blue74,
Based on the OP, what comes to mind first is the APC unit is not getting enough power from the inverter to meet the output demand of your appliances. Once plugged into the inverter, the appliances are running off the APC battery. The input to the APC is recharging the battery.
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Old 04-03-2010, 07:27 PM   #3
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A little more info.

Everything will run off the inverter for hours with no problems. Something else I thought of is that the inverter is putting out the power in a form that the APC does not see it and thinks the line power is off and uses the battery.

If I can not get it working in the tractor I will use it at home.
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Old 04-03-2010, 11:24 PM   #4
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You should replace the APC UPS with an AGM battery. Would be similar in size and weight, would carry the inverter while the engine was starting and would charge the battery when engine running. The inverter and APC might not be real compatible because of the modified sine wave output of the inverter. You could maybe just run 12 vDC from the accessory jack to the APC batteries and not use the 120 vAC input to the APC.
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Old 04-04-2010, 06:42 AM   #5
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Blue74:

Quote:
The inverter and APC might not be real compatible because of the modified sine wave output of the inverter.
John hit the nail on the head. I experienced these exact symptoms when I tried running a UPS off a modified sine inverter (I've tried a Xantrex Xpower 1000 and a Freedom 2500 inverter). The problem is the UPS does not measure the modified sine wave output of the inverter correctly and it switches back and forth to battery until the battery dies.

As John mentioned, you can put a small AGM battery in parallel with the inverter and that would carry the load while the engine is starting. The only problem I can see with this method is possible under or overcharging of the AGM battery because of the output voltage from the truck. (AGM batteries to not tolerate overcharging).

Another solution would be to use a true sine wave inverter like the Xantrex Prowatt SW 600. Sine wave inverters used to be much more expensive, but this one is about $150 at several vendors.

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Old 04-04-2010, 10:36 AM   #6
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Thanks John Hilley and Luv2go.

That is what I suspected and it looks like I need to came at it from the other direction and make a consent 12V source and not an 115V source. So I think next time I am home I will pull the APC from the truck and use it at home, mission failure.

Luv2go that switching back and forth must be the clicking I hear. I think your idea for use of a true sine wave inverter is the way to go on my next try. Using the AGM battery, which is in the APC, would require some kind of charger or a 12V circuit to control the charge to the battery and I would have to feed the inverter off it all the time to have reliable power. Not sure if a switch over circuit could be made to switch that fast to not loose power.

Thanks for the help.

This talk of sine wave has me wondering because I installed a 1500W Road Pro inverter and wired it as per directions on the toy hauler section. I know it works but now wonder if I should use it to power my computer when dry camping.

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Old 04-04-2010, 11:34 AM   #7
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Here is what I think is going on. It sounds like you run a small inverter from the 12V plug to power your devices. In most cases that should work fine. Even if you get a better inverter you will most likely have the same problem. When you engine is started it momentarily sends all power to the starter and you loose 12V or reduce the voltage for a short period of time. That is normal. Your UPS sees the 120v grounded and ungrounded source fail. That tells the UPS their is a fault and it shuts down and keeps trying to sense the neutral so it can restart. There are some UPS devices that do not do that, but very few. Here is what I would do. Put a 12V battery under the hood where you can. If you need to remove the battery and connections to use another tractor or something use alligator clips for the wiring connections and rubber bungee to hold the battery in place. Run a minimum of #10 wire from your tractor battery + to a 30A breaker, (they are small) and run from the breaker to your new battery +. Run a #10 wire from the chassis to your new battery -. This should facilitate charging while the engine is running. Run your inverter from the new battery and use another fuse or breaker based on the inverter size even if it has a fuse on it. Place this fuse/breaker/switch right on the + post or as close as possible. You would not need a UPS for this configuration. If it were me, I would put in a battery isolator/switch to stop any unnecessary draw from the tractor battery. Make the new battery a deep cycle and if you can afford it either a 12V AGM or 2-6 volt golf cart batteries. Of course the two 6 volt would be configured differently and you would need a considerable amount of room for them. I would use #8 wire myself and if you go to a bigger inverter you would have to increase the wire size accordingly. I just installed a 3000 Watt inverter using 4/0 welding cable. Good luck!
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Old 04-04-2010, 02:41 PM   #8
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When you start the truck the battery voltage drops below the inverter cut out voltage, that's why it's shutting down and re-starting, Very normal

Now.. If it's an MSW inverter the APC may not like it... Just so you know. (I stress MAY.. this means that it might be ok with it) you never really know till you try it.

Of course, adding a 2nd "inverter" battery. and isolating it,,, That might solve your problem too.
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Old 04-05-2010, 04:57 AM   #9
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Blue74:

I used the modified sine wave inverters to power my computers, even a microwave oven in the old coach. The computers ran fine, the microwave made some noise it did not make while on shore power but cooked food well, we did not notice the power loss they are supposed to have on MSW inverters.

There are a few things that will overheat and fail when you power them with MSW, most notably some Dewalt (Black and Decker) power tool battery chargers, but they will power most things well. Lots pf people use MSW inverters every day while dry camping (for example, Tioga and George has had at least 4 computers over the last 7 years and not had a problem using any of them on MSW inverters).

Having said that, I did install a true sine wave inverter in the new coach, and with the price of these Xantrex inverters I see no reason not to use true sine wave in new installations.

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