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Old 08-06-2019, 10:56 AM   #1
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Did I Do This Right?

Hey there, I just got little solar "generator" and ran a test to confirm the watt hours. But I'm totally confused by the result. Can you guys tell me, did I do this right?

I just wanted to do a super basic test, so I plugged in 2 lamps with incandescent bulbs totaling 100 watts (a 60-watt bulb in one, a 40 watt in the other). The generator is rated for 288 watt hours, so I figured it should power the bulbs for 2.88 hours... right? 288 / 100? That's 2 hours and 53 minutes, not taking any conversion/inversion losses into account.

The reason I'm so confused is that the generator exceeded that significantly. I was expecting less output, but I ran the test twice and both times got considerably more. So much more that I at first thought I'd forgotten to disconnect the solar panels, but no, this box was not getting any input, and was only putting OUT power. It ran the lights for well over 3 hours and I stopped with 3.5% still available.

So... did I miscalculate? Or is this thing simply a BEAST?
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Old 08-06-2019, 11:44 AM   #2
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Originally Posted by firebuild View Post
I just wanted to do a super basic test, so I plugged in 2 lamps with incandescent bulbs totaling 100 watts (a 60-watt bulb in one, a 40 watt in the other). The generator is rated for 288 watt hours, so I figured it should power the bulbs for 2.88 hours... right? 288 / 100? That's 2 hours and 53 minutes, not taking any conversion/inversion losses into account. It ran the lights for well over 3 hours and I stopped with 3.5% still available.

So... did I miscalculate? Or is this thing simply a BEAST?
Rated watt-hours may be based on a maximum load that you weren't anywhere near approaching. So, yes, I believe that could be a factor. Resistance can make a difference here as well. Orrrrrr..... you could have gotten a much higher-capacity setup that was packaged in the wrong box...

Keep in mind, I am not an electrician by trade, I just remember a few things from my father, who was. ;)
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Old 08-06-2019, 11:50 AM   #3
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A 60W bulb only uses .06KWH
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Old 08-06-2019, 12:10 PM   #4
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Rated watt-hours may be based on a maximum load that you weren't anywhere near approaching. So, yes, I believe that could be a factor. Resistance can make a difference here as well. Orrrrrr..... you could have gotten a much higher-capacity setup that was packaged in the wrong box...

Keep in mind, I am not an electrician by trade, I just remember a few things from my father, who was. ;)
Ah, OK, if it's based on maximum load that could explain it.

I just bought a second one that looks identical but has a significantly higher watt hour rating, and it almost seems to line up with what I found, so I wonder if perhaps you're "wrong box" theory is correct... These devices are theoretically from two different manufacturers but their appearance is the same, so I'm thinking they were made in the same factory, and this first one is mislabeled.
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Old 08-06-2019, 12:12 PM   #5
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A 60W bulb only uses .06KWH
Right... I'm talking WH not KWH.
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Old 08-06-2019, 12:28 PM   #6
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Pretty much what Cheese_Wagon said.

A battery's actual amp-hour performance depends on the load you're powering. Less current per time, the greater the amp-hours available. Or, in other words, the faster you discharge a battery, the less you're going to get out of it in terms of total energy output. So the discharge rate they based their amp-hour rating on is most likely significantly higher than that presented by your test load-o'-lights.

I doubt (though don't know for sure) that their advertised rating is the maximum 'safe' discharge rate for the battery - unless their marketing department was asleep at the wheel - so you'd likely see the reverse behavior (lower AH performance than advertised) pulling a heavy load.

Now that you've satisfied your curiosity, I'd suggest you don't continue to see how long you can run something before the battery gives up the ghost. Each time you do you're shortening your battery's lifespan and reducing its total capacity.
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Old 08-06-2019, 01:09 PM   #7
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Well the bare minimum to do a CC load discharge test is a DMM and an ammeter, usually combined.

Ah counters are more convenient, but less accurate.

Actual load testers that automatically keep Amps constant as voltage drops are about $50-60 through the usual Ali channels, maybe Amazon marketplace and eBay.

They can be paralleled if you need to pull a higher load.

0.05C aka 20-hour rate is standard for lead batteries in the US, but for the various lithium chemistries, Peukert coefficient is close enough to 1.0 that a 3-5 hour test would be fine.

I spelled out more details here: https://www.expeditionportal.com/for...?posts/2633829
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Old 08-06-2019, 02:14 PM   #8
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Well the bare minimum to do a CC load discharge test is a DMM and an ammeter, usually combined.

Ah counters are more convenient, but less accurate.

Actual load testers that automatically keep Amps constant as voltage drops are about $50-60 through the usual Ali channels, maybe Amazon marketplace and eBay.

They can be paralleled if you need to pull a higher load.

0.05C aka 20-hour rate is standard for lead batteries in the US, but for the various lithium chemistries, Peukert coefficient is close enough to 1.0 that a 3-5 hour test would be fine.

I spelled out more details here: https://www.expeditionportal.com/for...?posts/2633829
Thanks so much for the info, but I really wasn't trying to get that fancy. I don't actually understand much of what you just said.

I am taking baby steps in my solar understanding, and for that reason have been focusing on watt hours, which seems like such simple concept. But now I'm completely confused again:

Quote:
Originally Posted by TheHubbardBus View Post

A battery's actual amp-hour performance depends on the load you're powering. Less current per time, the greater the amp-hours available. Or, in other words, the faster you discharge a battery, the less you're going to get out of it in terms of total energy output. So the discharge rate they based their amp-hour rating on is most likely significantly higher than that presented by your test load-o'-lights.

I doubt (though don't know for sure) that their advertised rating is the maximum 'safe' discharge rate for the battery - unless their marketing department was asleep at the wheel - so you'd likely see the reverse behavior (lower AH performance than advertised) pulling a heavy load.
OK, I was aware that amp hours would vary but I thought watt hours was kind of like a dollar: whether it's comprised of 10 dimes, 4 quarters, or one paper bill, it's still a dollar. I thought the whole point of watt hours was that, as a unit of measurement, it allows you to compare batteries of different voltages and to calculate your power needs across a wide range of devices.

I did not, in fact, find worse performance with a larger load, or better performance with a smaller load. Part of why I decided to test with lightbulbs (I don't have any kind of meter) was that I was surprised at what was happening when I ran a larger load, and I thought my math might be suspect, which is why I figured 100 watts would make it super easy to spot a bald spot in the logic. It's very discouraging to discover that the tiny little corner I thought I understood, I don't understand at all.

I've come across things in my research on this which would explain why the rated watt hours would not be accurate, but it seemed like it would be consistently inaccurate, not load dependent, and that performance would be worse, not better, than stated. I give up. Well, not really, but the more people explain to me, the more abbreviations and terms come out, the more confused I get. I thought it would be better to just get started than to wait until I know what Peukert coefficient is. Now, I don't know.
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Old 08-06-2019, 02:33 PM   #9
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Amp hours are a much better measure of battery capacity. Watt hours are needed when converting from one voltage to another.

The intentional obscurity and outright scamming of this market segment,

and let's not sanction their fraudulent use of "generator" - portable powerpak is better,

makes it difficult to determine just how bad their value truly is.

With lead, a high Peukert value means a bank hold many more Ah if discharging at a lower C-rate.

A 12V 100Ah bank discharged at 5A exactly, from 100% SoC down to 10.5V is the 20-hour rate, as I said is the industry standard.

Discharging at 20A rate 0.2C will give a completely different capacity number. That's Peukert in a nutshell.

Your mistake with the bulbs is assuming the nominal watts rating is the actual power consumed. Not the case, need to measure with an ammeter, and be able to vary the consumption in order to keep the amps rate as constant as possible.
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Old 08-06-2019, 05:12 PM   #10
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I disagree with the statement above.
The easiest way to estimate use is to calculate everything in watt hours and amps. Here's the best formula I use. 100ah at 100amps at 12v is 100amps x 12v 1200watt hours. It will produce 100amps for 20 hours according to spec, but who's using 100amps. You're using 100 watt appliance for 10 hours or 1000 watt hours.

Your 60w bulb may be 60w equivalent too. Most don't draw anywhere near the power stated. Try using a battery monitor. Pop this bad boy over the the chord and then you can see what's really happening.
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