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Old 05-30-2017, 11:56 PM   #1
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Default Losing 15% of charging power?

This came up in relation to a bunch of converting off amp hours to watt hours, to amps to watts that was going on when messing with solar output sun position calculator stuff. I don't really know if it makes any sense, or not, as it is something that I have never heard mentioned.

Batteries don't store amp hours, even though they rate them that way, as that is not an energy term. They store watt hours or kilowatt hours, which adds the voltage term so it is energy. The question is what happens when we use amp hours for both charging and discharging?

The example that comes to mind is if you have a 10 amp load at 12.3 volts for an hour, you would use 123 watt hours of energy from the battery. If you then go on charge and replace those 123 watt hours at 14.4 volts, it would only take 8.5 amps for an hour. So all of a sudden, we have a much bigger actual energy loss than you get from the charge efficiency, and also bigger than the recommended overcharge amount to get to full state of charge. Your monitor won't know the difference and think they are different amounts of energy because the amp hours are different and think you haven't replaced the full amount of energy, but have you, or not.

So the question is if the extra energy from the charge voltage gets used, or wasted? My guess is wasted, as you already have to add amp hours for charge efficiency and for required overcharge.

So, if you add this to lost charging energy with the others, it starts to get to be a big % of your generated energy, especially for solar.

Charge efficiency modifier is listed as much as 20% for wet cells and 8-9% for AGM. Based on amp hours, not watts.

Overcharge is listed as 15-20% for wet cells and 3-10% for AGM. Also based on amp hours, not watts.

Now we add another 15% that seems to disappear due to the higher than battery voltage when charging, if going by amp hours, not watts.

That would mean that you could lose as much as 1/2 your charging energy with wet cells, and 1/3 with AGM to get you batteries full. In other words you would need double the energy out of you solar, that you used from your batteries for wet cells and 50% more with AGM.

This does kind of match with what we have seen with the solar on our van, especially in bad conditions with limited output. We find that for the same amount of solar generated, or batteries stay noticeably more full if we use the power generated by the solar as it is generated, rather than putting in the batteries and then using it later. Things like device charging are a big one for that or running fans to dry towels. We also always see in the range of 10-15% overcharge by amp hours to get to the .5%C that Lifeline says is full charge. This is on top of the already in charge efficiency in the meter.

Has anyone ever heard of this, or noticed it in their systems.
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Old 06-01-2017, 08:32 PM   #2
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A pretty interesting tech paper here:

http://www.otherpower.com/images/sci...Efficiency.pdf

The guy measured in and out actual amp hours at various states of charge, and the results certainly aren't pretty at the high end of the SOC.

He doesn't mentioned total watts going in and out, so we would have to assume that any extra energy from the source being at higher voltage charging than discharging is also lost. He didn't mention if the charger was holding voltage at absorption for the very low charge rate he used, so it is possible he could have been close to battery voltage at the less than near full SOC tests.

It would certainly show that you lose a lot of precious solar output at the high end of SOC, and would explain why you can get get "death spiral" type SOC loss for a while doing multiple recharges on solar only. With efficiencies in the 50% range, the amp hours you put in will only cover 1/2 of the amp hours you took out. When the SOC gets low enough for the efficiency to be higher, the solar may be able to stop spiral down, but it will just sit there and not recover to full.
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Old 06-03-2017, 01:16 AM   #3
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I wonder if the charge efficiency mentioned in your first post already takes into account the higher voltage needed to overcome the battery's internal resistance.

I had to check the date on that study and found that it is 20 years old. The info is good and it probably help lead to today's modern solar controllers. I have a old solar charge controller of that vintage that I have since upgraded. It was a pulse type on then off, on, off etc.

One interesting thing on the old controller brochure was that even back then (20 years ago) there was an option for a low voltage disconnect that could automatically start a generator.
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Old 06-03-2017, 01:42 AM   #4
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Quote:
Originally Posted by markopolo View Post
I wonder if the charge efficiency mentioned in your first post already takes into account the higher voltage needed to overcome the battery's internal resistance.

I had to check the date on that study and found that it is 20 years old. The info is good and it probably help lead to today's modern solar controllers. I have a old solar charge controller of that vintage that I have since upgraded. It was a pulse type on then off, on, off etc.

One interesting thing on the old controller brochure was that even back then (20 years ago) there was an option for a low voltage disconnect that could automatically start a generator.
I have most of those questions in to Blue Sky solar now. The tech is forwarding it to the designer, as he didn't have any idea, but it got him thinking.

Reading the Magnum remote manual about charge efficiency, it explained that it takes charge efficiency into account by reducing the number of amp hours the SOC % and the in/out amp hours. That would imply that there is even less ah actually getting into the battery than what the solar output is at the higher charge voltage. The extremely low total charge/discharge efficiency the guy saw in the testing would seem to confirm there must be another adder, as charge efficiency is usually said to be not as high as he got. And then you have the recommended overcharge ah to add in.

I have no idea what is really right, and especially with AGM batteries.

I do like the test method he used, though, it wasn't a way that I had even considered. By running at a fixed discharge rate until a voltage cutoff, he gets a pretty consistent baseline to start from. He than adds a fixed amount of ah (I would have to set the charge efficiency to 100% to do that), and then discharges at the same rate to see how much he gets back. Quite accurate, I would think, and something we would be able to do at home pretty easily. I do think I would chose a cutoff that was higher than than fully dead, though, to save the batteries.
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