Can I plug in a lamp to my inverter to test the state of my batteries?

[kathyg]

You're running in 24V mode? Batteries in series? One battery may be good, the other bad. The bad battery will pull the good one down. Charge the 'good' (12.4VDC) battery with a conventional battery charger, let it rest 6-12 hours and check it again,

Yes 24 Volt system. batteries are joined in Series. So if one battery is good, how will I know without putting it into use? I have come to feel that voltage meters don't tell me anything and so I test the battery with a load. I have been charging the batteries using the Inverter/grid. This morning before the load test: one showed 13.5 and the other showed 13.2.

 

[kathyg]

Mechanical timer?
But not good to fully discharge battery. Better to look up what voltage is 50% SoC at your discharge rate, plot a few readings over time to predict when it will get there, stop before fully discharge. 50% SoC, 30% SoC, or whatever you've determined you would like to use.

The inverter is programmed to stop at 50% discharge (I think). I should stop the test before 50%? Anyway- I guess I am asking if one battery ended up at 11.6 v after the test and one ended at 12.4, then neither seem to have stopped at what is supposed to be 50%- which I am told is 12.3 volts. Remember my goal here is to know whether to use or not use these batteries. I do have a third battery which I could use with the good one.

The Wattage on the DC input side of the inverter will be higher than the Wattage on the AC output side of the inverter due to conversion and system loss, so the inverter may consume 120W on the input side of the inverter to produce the usable output of 100W on the AC output side of the inverter. You also do not want to discharge the LA batteries below 50% SOC.

I think you are saying what i was told is that my inverter runs at 26 watts

 

[svetz]

Results of the test of the power in two marine batteries:

1. Charged the two 12 volt batteries to show FLOAT mode on the AIMS inverter - using the grid
2. Plugged in two 60 watt light bulbs that registered a load of 108 watts on my Kill A Watt meter.
3. The batteries went dead after 2 hours and 44 minutes.
4. Multiply 2.75 hours by 108 watts= 295 total watts available in the battery (should be watt-hours)

Two 12V batteries in series for 24V power, 108 watts load for 2 hours 44 minutes, the inverter is set to turn off at 50% SoC.

2.75 h x 108W = 295 Wh

Now I have been told to conclude that these batteries are no good because the provided only 295 watts of power in such a short time on a small load.

Since the inverter was set to cutoff at 50%, the batteries should have 295 Wh / 50% = ~600 Wh.
To know if the batteries are good we'd need to know the number of amp hours the battery was rated for.
600 Wh / 24V = 25 Ah.

Two 12 volt batteries should give me 12 volt X 105 amp-hours X 2 batteries = 2520 watt-hours X 50% useable watts = 1260 watt-hours.

My first guess would be improper voltage settings on the Aims charger settings. Let them sit off float for 15 min and see if the voltage of each is 13V.

What's the voltage of each battery after the test concludes (take the voltage while failing and then again 15 min afterwards)? At 50% it should be about 12V:

If the voltages are not same then one may have a dead cell. You can probably get them tested for free at a local autoparts store. Worth it if they're still under warranty.

Batteries are also rated at the 20 hour rate, that is you won't see the 105 Ah out of them unless they are discharged slowly over 20 hours (this is known as Peukert's law). Discharging them faster means less total power. 105 Ah / 20 hours = 5.25 amps, at 24V that's 126W, so your two-light bulb test should be fine.

Here's an example chart, but check the datasheets on your batteries if you can find them:


Finally, lead acid batteries have less power availabe when they're cold. See the Battery FAQ, for more on batteries...
Hope that helps!

 

[Supervstech]

I agree, one battery should not be so much lower than the other.
Try the desulfator setting, but watch it, because it will send 32V at the bank, and can BOIL the good battery.
Likely boil the bad one also.
Verify if the batteries are FLA that all cells have full distilled water above the plates.

 

[Hedges]

Anyway- I guess I am asking if one battery ended up at 11.6 v after the test and one ended at 12.4, then neither seem to have stopped at what is supposed to be 50%- which I am told is 12.3 volts. Remember my goal here is to know whether to use or not use these batteries. I do have a third battery which I could use with the good one.

I would charge the two batteries independently. If they settle to different (or too low) voltages, then I would perform an equalizing charge. If they still settle too low, maybe 3rd battery will work.

I have some AGM automotive batteries which were getting weak, and equalization brought up their rest voltage. Normally not recommended for AGM but I have instructions from one vendor, and with a bad battery what is there to lose?

If two batteries in series are diverging in discharge, consider determining what pack voltage corresponds to 50% (or other target) for the weaker one and adjust low battery cutout accordingly.

Voltage alone is a poor indicator for a battery, since IR drop results in lower voltage under load. A more sophisticated system which counts watt-hours as well would be better.

 

[kathyg]

I agree, one battery should not be so much lower than the other.
Try the desulfator setting, but watch it, because it will send 32V at the bank, and can BOIL the good battery.
Likely boil the bad one also.
Verify if the batteries are FLA that all cells have full distilled water above the plates.

Thanks. I have a small cheapr disulfator I could use. I would not use the one on the inverter.

 

[kathyg]

Two 12V batteries in series for 24V power, 108 watts load for 2 hours 44 minutes, the inverter is set to turn off at 50% SoC.

2.75 h x 108W = 295 Wh


Since the inverter was set to cutoff at 50%, the batteries should have 295 Wh / 50% = ~600 Wh.
To know if the batteries are good we'd need to know the number of amp hours the battery was rated for.
600 Wh / 24V = 25 Ah.


My first guess would be improper voltage settings on the Aims charger settings. Let them sit off float for 15 min and see if the voltage of each is 13V.

What's the voltage of each battery after the test concludes (take the voltage while failing and then again 15 min afterwards)? At 50% it should be about 12V:

If the voltages are not same then one may have a dead cell. You can probably get them tested for free at a local autoparts store. Worth it if they're still under warranty.

Batteries are also rated at the 20 hour rate, that is you won't see the 105 Ah out of them unless they are discharged slowly over 20 hours (this is known as Peukert's law). Discharging them faster means less total power. 105 Ah / 20 hours = 5.25 amps, at 24V that's 126W, so your two-light bulb test should be fine.

Here's an example chart, but check the datasheets on your batteries if you can find them:
View attachment 55819

Finally, lead acid batteries have less power availabe when they're cold. See the Battery FAQ, for more on batteries...
Hope that helps!

Thanks for your thoughtful reply. I can see I am starting to realize that this gets much more technical than I was expecting. I had the batteries tested by two profesionals and they said they were good. They used a voltage meter. I guess I will run the disulfator on the low one. Or just use a third used battery which is showing 12.7 volts. The two that were tested with the load this morning are now showing 12.1 and 12.5 after resting two hours ( two house ago they're were 11.6 and 12.4 ). I think my attention for this problem has reached its limit. It was a fabulous learning experience but it consumed a chunk of my life for three days. Now I have to connect the 4 new marine batteries to the system and test them so I am ready for a storm. What will get to me is if these new batteries don't give me decent amp hours/watt hours. I'll be back here after that test. Thanks very much to everyone who gave me advice.

 

[svetz]

I had the batteries tested by two profesionals and they said they were good. They used a voltage meter. A voltmeter won't cut it. It'll tell you the current state of charge, not battery health. Let me rephrase... take it to a well-equipped parts store in a big town where there are people that know what they're doing. Probably should call them and explain your issue and ask if they have a battery tester that can measure the internal resistance. The battery test doesn't take long, but it does takes specialized equipment that runs a frequency test through the battery that can detect the internal resistance. It's very quick, but a great test for batteries. My fault for assuming that would be standard equipment in an autoparts store that sells batteries just because mine has one.

 

[Bud Martin]

If you are reading 108W on the AC side, the input power on the Inverter side will be higher than 108W, you can easily verify that using the DC Volt meter and the clamp on DC Amp meter, you can also look at the spec of the AIMS inverter. You have to put more in than what you get out due to inverter conversion loss + system loss.

 

[Hedges]

Now I have to connect the 4 new marine batteries to the system and test them so I am ready for a storm. What will get to me is if these new batteries don't give me decent amp hours/watt hours. I'll be back here after that test.

"Marine" batteries won't deliver as many deep cycles as other batteries meant for that application. Data sheets should have specs on cycle life. Even within a particular technology (e.g. AGM), there is a range of life and of prices.

 

[svetz]

A voltmeter won't cut it.... measure the internal resistance.

As batteries age, the internal resistance of the cells starts to get more and more off the original spec. From Ohm's law V=IR, so the charger applies the V, the battery the R.

Lets use a water analogy. Think of a battery as series of empty glasses. When you're recharging it you're putting water on the top of the cells.
Because the cells have different resistances (think of these as the diameter of holes allowing water to pass through), different amounts of water flow into the cells. So, when it's done charging one glass will be full and the others not so full. When it drains, as soon as the first glass empties the battery can no longer produce power regardless of the water level in the other cells.

On a new battery they're all very close to the same so all the glasses are full. This is one of the dangers of putting an old and new battery together in series. The old battery can prematurely age the new battery. More on that in Incrementally adding DC batteries

The pro battery testers do a number of tests besides the internal resistance. For example, plates can warp and short out making the battery useless.

 

[kathyg]

A voltmeter won't cut it. It'll tell you the current state of charge, not battery health. Let me rephrase... take it to a well-equipped parts store in a big town where there are people that know what they're doing. Probably should call them and explain your issue and ask if they have a battery tester that can measure the internal resistance. The battery test doesn't take long, but it does takes specialized equipment that runs a frequency test through the battery that can detect the internal resistance. It's very quick, but a great test for batteries. My fault for assuming that would be standard equipment in an autoparts store that sells batteries just because mine has one.

I had six batteries on my system. My installer told me they were all bad and I took them all back to Advance Auto - It takes me 4 hours to get there since I live on an island off the coast of PR. They gave me three new ones under the warranty and I bought the forth one. That leaves me with three old batteries to decide whether to keep. I asked them to test the new ones before I took them. He attached a handheld device to each one - it took a 2-3 minutes for each batteries and showed me the display which said "Good". Unfortunately they did not test the old ones. Thanks for the explanation.

 

[Hedges]

Are these batteries for an occasional power failure? Or do you cycle them day in and day out?
Short term, marine batteries may be the lowest up-front cost. Cycled nightly over a decade, other lead-acid types or lithium is likely to cost much less.

My preference is to have extra PV capacity and get by with as little battery as possible, due to cost and lifespan.

 

[kathyg]

If you are reading 108W on the AC side, the input power on the Inverter side will be higher than 108W, you can easily verify that using the DC Volt meter and the clamp on DC Amp meter, you can also look at the spec of the AIMS inverter. You have to put more in than what you get out due to inverter conversion loss + system loss.

I have been communicating with AIMS tech support consistently. They told me , "With no loads the inverter draws 26.5 Watts". I figured that into my calculations. Thanks.

 

[Supervstech]

If you are reading 108W on the AC side, the input power on the Inverter side will be higher than 108W, you can easily verify that using the DC Volt meter and the clamp on DC Amp meter, you can also look at the spec of the AIMS inverter. You have to put more in than what you get out due to inverter conversion loss + system loss.

This has been tested by average joe youtuber… the aims inverter consistently drew the same current. Standby was the same 56W ( on a 6000W inverter) at zero watts draw, 100Watt draw, and 1500W draw… the standby current was unchanged no matter what the load on the inverter.

 

[Bud Martin]

I have been communicating with AIMS tech support consistently. They told me , "With no loads the inverter draws 26.5 Watts". I figured that into my calculations. Thanks.

That is no load power draw, it is not conversion lost, all of those affect the total lost when convert DC to AC.

6.5. Efficiency of Inverters | EME 812: Utility Solar Power and Concentration

CEC Inverter Test Protocol

Inverter efficiency is the ratio of the usable AC output power to the sum of the DC input power and any AC input power. Typical grid-tied inverter efficiencies exceed 95% under most operating conditions Efficiency changes as a function of AC output power, DC voltage, and sometimes inverter temper...

pvpmc.sandia.gov

If you look at the spec you will see 'Nominal efficiency:' rating. It is less efficient when load is not heavy.

2500 WATT LOW FREQUENCY PURE SINE INVERTER CHARGER 12 VDC to 120 VAC

The AIMS 2500 watt low frequency inverter charger is perfect for most off grid systems, semi-trucks, 5th wheels, toy haulers or any remote locations needing power. Providing 2500 watts of continuous, clean and reliable pure sine wave power, ideal for sensitive electronics and household...

www.aimscorp.net

 

[kathyg]

That is no load power draw, it is not conversion lost, all of those affect the total lost when convert DC to AC.

6.5. Efficiency of Inverters | EME 812: Utility Solar Power and Concentration

CEC Inverter Test Protocol

Inverter efficiency is the ratio of the usable AC output power to the sum of the DC input power and any AC input power. Typical grid-tied inverter efficiencies exceed 95% under most operating conditions Efficiency changes as a function of AC output power, DC voltage, and sometimes inverter temper...

pvpmc.sandia.gov

If you look at the spec you will see 'Nominal efficiency:' rating. It is less efficient when load is not heavy.

2500 WATT LOW FREQUENCY PURE SINE INVERTER CHARGER 12 VDC to 120 VAC

The AIMS 2500 watt low frequency inverter charger is perfect for most off grid systems, semi-trucks, 5th wheels, toy haulers or any remote locations needing power. Providing 2500 watts of continuous, clean and reliable pure sine wave power, ideal for sensitive electronics and household...

www.aimscorp.net

thank you. I do not understand much of this. Are you saying I have to factor in a SIGNIFICANT amount of energy to that loss? If two batteries ran 2 light bulbs for only 2 hrs and 44 minutes - I think there is something wrong.- probably with the batteries.