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Measuring Voltage with an Inverter
By DrDave, Section Newbies Posted on Thu May 15th, 2008 at 04:11:22 AM MST
Measuring Voltage with an Inverter

I have a small 5 watt PV charging 12v 5 amp/hr batteries. I have a 1200 watt HF inverter connected to several of the batteries in parallel. Using a digital volt meter, I can't seem to get an accurate battery volt reading, because apparently the inverter holds a charge. I have to unplug the inverter, to get correct readings.
I don't have any charge controler as of yet. Is there a simple way to do this?
Can I put a diode between the battery and inverter or something?

Measuring Voltage with an Inverter | 9 comments (9 topical, 0 editorial)

Re: Measuring Voltage with an Inverter (3.00 / 0) (#1)
by commanda (alwynne at unwired dot com dot au) on Thu May 15th, 2008 at 12:24:09 AM MST
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The inverter is a switch-mode device, so it will be switching on and off really really quickly. The battery voltage will be going up and down slightly as the inverter draws current and then doesn't. Your multimeter can't get a reading because it is seeing this noise.

What I would do is; find a capacitor and connect it in parallel with the multimeter leads, at the meter. Pull the leads out of the meter, stuff one pigtail from the capacitor into each socket, then push the lead back in. Best bet somewhere from 100nF to 100uF.

Your mileage may vary.

Better multimeters (Fluke) tend not to suffer this problem.

If that doesn't work, try adding some series resistance in one lead. Assuming it's a digital meter with a 1 Megohm input impedance, anything up to 10K should help the filtering without changing the actual reading by enough to worry about.

Amanda

Re: Measuring Voltage with an Inverter (3.00 / 0) (#2)
by Flux on Thu May 15th, 2008 at 12:42:18 AM MST
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I think you will have trouble running a 1kW inverter from a few 5Ah batteries, the battery current capability will not supply the peak current of the inverter.

Similarly a few 5W panels are not going to run the inverter for many minutes a day.

Diodes will worsen the problem by adding a volt drop in addition to that caused by the limited battery capability. The inverter when connected and not running will store some charge in its input capacitors but the effect will not happen when running.

Try Amanda's tricks to make sure the meter is reading correctly but expect very low volts from those batteries driving a big inverter.

Flux

[ Parent ]

Re: Measuring Voltage with an Inverter (3.00 / 0) (#3)
by wooferhound (tim((NoSpamAt))wooferhound.com) on Thu May 15th, 2008 at 07:54:18 AM MST
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If you put a 120vac appliance on the output of the inverter that pulls 1 amp. Then you will be pulling more than 10 amps from the batteries. If you are pulling 2 amps from the inverter output, then you will be getting over 20 amps from the battery. I don't think your batteries are capable of this high load.

W o o f -={(

Re: Measuring Voltage with an Inverter (3.00 / 0) (#4)
by DrDave on Fri May 16th, 2008 at 09:40:44 AM MST
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I should have posted more details. I use a Becker Digital volt meter.
The inverter(turned off, nothing plugged 110 side) is connected to the battery and the PV Panel. When measuring voltage of the battery, it reads 12.0 v for example. Now I disconnect the battery from everthing and it only reads 11.5 v. Now that the battery is disconnected I measure the voltage on the back of the inverter, it reads 17 v, and this is the same if the PV is hooked to the inverter or unplugged. I am assuming the 17v is produced by the PV and is held in the Inverter. If I power on the inverter with no battery, it draws down this 17 v to about 2 v. Of course nothing is plugged into 120 of the inverter. And I am assuming that when everthing is connected together, that the 12.0 v is false battery reading, because some power is comming from the inverter. Does that make sense? If so how do I know when the battery is fully charged?

Just for grins, with two fully charged 12v (5 or 7 amp/hr not sure) batteries and the 1200 HF inverter connected to a 13w CF bulb ran for 5.5 hrs. I was impressed.
I was wondering if a 400 watt inverter would run longer (ie be more efficent than the 1200w HF). That is my next test.

Re: Measuring Voltage with an Inverter (3.00 / 0) (#5)
by Bruce S (bruce(dot)stahl <at>gmail (dot)(com)) on Fri May 16th, 2008 at 10:23:38 AM MST
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DrDave;
I personally like to use an inverter that is loaded at about 80% of it's constant run level. This way when on it's using as much of the available power to run stuff with as little waste as possible.
I use a small 100 watter to charge all of our mobile phones and cordless drills.

With nothing running the inverters still use power to keep it available, even the remote on units use a little to pop back on when they sence a need for power.

In my playing around with my small set, the power coming from the PVs unless you have a large system, cannot keep up with the demand from the inverter. This is the reason the panel voltages drop so far when the inverter is switched on and connected directly to the back of the inverter.

The readings your getting tell me the battery your using is in needs of a good long charge. when you disconnect it from the PVs it's 11.5, not good , when everything is connected your getting 12Vs, that means the PVs are doing their job. The battery will pull them down to its level. let it do that.
I'd be careful that this battery doesn't get any lower, it may not survive much longer at such a low level.
The low level from the meter readings to me is correct, BUT I would disconnect the inverter and let the panels do their job all weekend and then check them say Sunday. You should see a higher voltage reading by then, otherwise you may have a battery that has a bad cell and should be replaced.

Those 13 watt bulbs will last a long time, :-) if they weren't able to run, once on a warmed up, it still would have lasted 4 hours before dropping out.

Best of luck with that battery!!
Bruce S

[ Parent ]

Re: Measuring Voltage with an Inverter (3.00 / 0) (#6)
by Flux on Fri May 16th, 2008 at 10:31:06 AM MST
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You are getting yourself into a complete muddle.

With the panel charging the battery ( with inverter connected and idling) the 12v you see is that of a battery on charge.

If you disconnect the battery from the charging source the volts will drop, if you load it it will drop even more.

Feeding the inverter from the panel with no battery is not wise and all you are seeing is the panel volts. If you load the inverter the panel has no chance of driving it and the volts will collapse.

When it is all connected together the volts you measure on the battery are battery volts. It will be higher with the panel charging and inverter turned off with inverter loading it the volts will be lower.

You can't measure state of charge either on load or off load. You will only make sense of it with the panel disconnected or shaded and the inverter turned off and it will need several hours to loose the surface charge.

Just don't go feeding panels into inverters with no battery, if you had bigger panels you would likely kill the inverter.

I think those tiny batteries will have a desperately short life driving that big inverter near full load.

It will not be particularly efficient running a 1kW inverter at 13W the no load current may be close to the load current. The size of inverter will be dictated by the load you intend to take. If you don't want more than 13W then a 50W inverter may be better.

The final size of inverter and load you can use will be determined by the battery capability and for those little batteries your inverter is well oversize.

Flux

[ Parent ]

Re: Measuring Voltage with an Inverter (3.00 / 0) (#7)
by Norm (peppysue@suite224.net) on Sat May 17th, 2008 at 08:16:08 AM MST
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All this stuff is good for experience but in my
opinion...you simply don't have enough electrical
source....a 5 watt PV isn't enough for 25 amp/hr.
Instead I would be checking on the input from
the PV to the battery/batteries.
Even with my bad math I figure that a 12 watt
would be 1 amp....so what you have is something
that will input about 400 Ma and that is being
very optimistic....
...I use a pedal generator to charge 700Ma batteries
6 in series 7.2vdc to start fully
charged they will read 8.4vdc I pedal the gen
at a 400 Ma rate (very leisurly) and it takes
about 45 minutes even at that!
So I figure that a 12 volt 5 smp/hr battery
from 12 volt to 13.50 would take at least 4 hrs
IF you were charging at the rated 5watts.

( :>) Norm

Re: Measuring Voltage with an Inverter (3.00 / 0) (#8)
by CmeBREW (smke833f@hotmail.com) on Sat May 17th, 2008 at 08:48:02 AM MST
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Many times you just have to learn as you go. It is a fascinating experience.
So it looks like you have several 12v 7Ah lead acid batts hooked in parallel which equals 49Ah total. It would be wise to have fuses between each battery so that if one battery goes bad and shorts, it won't ruin all the other ones.

YOu probably already know, that if you want your batteries to last for years, you should practice taking out no more than 25 percent of your total AH capacity. Which in your case, 49Ah divided by 4 is about 12.25 AH. (or;around 147 WH)
So, your one 13watt cfl light bulb combined with your inverter losses will be around 20watts total being spent-- which would be about a total of 7.4 hours running the one light bulb at a safe 25 percent AH discharge with all 7 batteries hooked together.

Your voltage reading is somewhat low. You should test the voltage of each batt separately (disconnected) after the are 'rested'(no load/charge) on them for a couple of hours. Full charge should be around 12.7/12.8 if the battery is new. If it is old it will be somewhat less. You may be getting a bad battery.

You may want to buy a (120vac) battery charger and charge them and see what voltage is 'full charge'. (mine shows about 14.2v is full charging at 6 amps///charging at only 2 amps though, it is about 13.2 volts is full charge)
But with your small 5W PV (which is actually more like 3W in full sun) the amps will be so low, It may be difficult to know what is full charge. (my quess is around 12.8)

That inverter probably spends about 4 watts on idle if it has a little cooling fan on all the time.(I actually put a switch on the top of mine that turns the 2watt fan on/off)
I have never needed to turn the little fan on even with running my Tv/and dvd player (total=160Watts or 13Amps) since it never even gets warm. Ive been doing this for about a year. The aluminum is always cold where the power FETS are even after running it for hours like this.
Now, my 600W rated inverter only uses 2W idle instead of 4W!
I only did this because I couldnt stand the noise of the little fan while watching TV and also I NEVER use over 14 amps at any one time.
(BTW, I am not encouraging anyone to do this!! It could burn yours out!)

YOu really need to get more solar panel. -Keep having fun.

Re: Measuring Voltage with an Inverter (3.00 / 0) (#9)
by DrDave on Sun May 18th, 2008 at 10:12:13 PM MST
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Thanks for all the advice. I am definatly learning. I was able to determin the 1200w inverter is drawing 1.6amps at idle and my 400w inverter draws .6 amps at idle. Upon testing with loads, they both step up at the same rate about 1 amp difference regardless of load. I have upgraded my batteries to a used deepcycle hand me down. And as soon as I get a refund from bad ebay solar company, I plan on attempting to purchase more PV power. The 5 watt pannel creates about .35 amps which will charge, but not as fast as I can use the power.

Measuring Voltage with an Inverter | 9 comments (9 topical, 0 editorial)

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