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Old 02-19-2018, 06:01 PM   #1
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Default Lithium and AGM combo system like Hymer

OK, time for a hairbrained idea for those that have AGM batteries (or wet cells) and face the common issue of getting them to 100% full periodically because they take 6+ hours to complete the last 20% of the charging.

The thread about the new Hymer lithium/AGM that is touted as being a true shared system brought all this up.

Many of us have skeptical of having AGM and lithium in parallel, but maybe it could be made to be a good thing if used correctly.

Fully charged AGMs will be at about 13.1v right off a full charge and lithium will be substantially higher at over 14 volts for a long part of their discharge cycle. Obviously, the lithium will carry the load first due to it's higher voltage, but it will also be charging the AGM as the voltage is high enough to do it fairly well.

Certainly have no details or if it would even work, but just as a starting point consider a normal 2-3 hour driving time with both the AGMs at under 50% and the lithium at low cutoff. In that time the AGMs would get to about 80% and the lithium would get full. If the lithium is sized to be 20% of the AGM capacity, plus whatever power you normally use in about 6 hours, it would finish the charging for you and eliminate having to be on a charge source for long periods. Many of us use solar for that now, but as we all know solar can be spotty for the longer times needed due to locations and weather.

Doing the whole thing this way might be a way to get a lot lithium benefits without the very high cost of getting large capacity, and also retain cold weather capability with high capacity. Do to the normal weight limitations of carrying lots of AGM batteries, it would likely be good up to about 450ah of AGM with 100ah of lithium, although 300ah AGM and 75ah of lithium, or less, would probably be fine for many. For those that stay off grid for long periods without anything but the van alternator and maybe solar, this could take away any worry about if you are getting your AGM batteries to full periodically because of the long topping off time, if I am thinking straight and cost much less than going all lithium. It would also make it easy to remove the lithium battery in very cold storage if chose to.

It is kind of the mirror image of how the Hymer setup is.

Very odd, but kind of an interesting premise for the right applications.
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Old 02-19-2018, 07:34 PM   #2
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Fully charged AGMs will be at about 13.1v right off a full charge and lithium will be substantially higher at over 14 volts for a long part of their discharge cycle. Obviously, the lithium will carry the load first due to it's higher voltage, but it will also be charging the AGM as the voltage is high enough to do it fairly well.
This actually isn't totally obvious to me. The statement that the two batteries are at different voltages can't be literally true. If you have two batteries connected together, you only have ONE circuit, so there can only be one voltage, no?

I am not saying you are wrong, only that it isn't obvious. My feeble EE intuitions fail me when it comes to how such a system would behave. A very interesting area for inquiry and experimentation, though.
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Old 02-19-2018, 08:22 PM   #3
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This actually isn't totally obvious to me. The statement that the two batteries are at different voltages can't be literally true. If you have two batteries connected together, you only have ONE circuit, so there can only be one voltage, no?

I am not saying you are wrong, only that it isn't obvious. My feeble EE intuitions fail me when it comes to how such a system would behave. A very interesting area for inquiry and experimentation, though.
I think, and I stress think, is that as you say, you would wind up with a voltage that is common to both (assuming low resistance connection) and be determined by the acceptance of the AGM and discharge resistance of the lithium, much as it does when you have a charger that is lower in capacity than the acceptance of the battery are absorption voltage. Thus the same as the bulk charge stage with slowly increasing voltage as AGM gets charged. The good is that the lithium should hold high voltage through almost of it's discharge curve, so more or less a constant voltage source through the whole process, but getting pulled down by the load possibly in the beginning. Certainly would need to be tested and evaluated to find out what it really does. I can imagine it being similar to putting one of the lithium jump start batteries on you car with a mostly dead car battery.

Thinking more about it, the setup might also be very good for boondockers that have an Onan, but only run it long enough to get to 80% on their AGMs because they don't want to run the Onan for 10 hours to get a full charge. Many likely never get to full on their batteries. They would run long enough to get the AGMs to 80% and the lithium full and shut it off and let the AGMs fill to 100% off the lithium. If they then chose to, they could do an even shorter run to fill the lithium again. The very same thing would work on boats where nearly always charge to only 80% so the engine is run so much.

Hopefully, we will get a bit of feedback from some others to see if they see any holes in the thought process, as it pretty obscure to visualize all that would be going on.

It probably could be tested on the bench with a small AGM and one of the jump starter lithiums or other small lithium.
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Old 02-19-2018, 08:32 PM   #4
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It is certainly true that with the batteries connected in parallel the voltage across each will be identical and the current division between them will be governed by the apparent resistance they present at their respective terminals. As the batteries receive charge their internal impedance will change in accordance with their charge curve. As the AGM approaches full charge its internal impedance will rise sharply and the bulk of the current will be going to the Li cells. As long as the charging source maintains a voltage sufficiently high to bring the Li cells to full charge everything will reach 100%. At that point the AGM will be overcharging but the current will be quite low, hopefully low enough that it does not damage the AGM. When charging stops and everything settles out the voltage will hold at the higher Li and some current will be flowing into the AGM. If the batteries are separated and allowed to stabilize the voltages will be in accordance with the characteristic of each chemistry.

At least that is the way I see it.

John
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Old 02-19-2018, 08:35 PM   #5
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Reading Boosters second entry I see his logic with regard to charging from the Onan. When you get the Li cells to 100% you shut the Onan off and allow the Li cells to slowly bring the AGM to 100% on its own.
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Old 02-19-2018, 08:44 PM   #6
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It is certainly true that with the batteries connected in parallel the voltage across each will be identical and the current division between them will be governed by the apparent resistance they present at their respective terminals. As the batteries receive charge their internal impedance will change in accordance with their charge curve. As the AGM approaches full charge its internal impedance will rise sharply and the bulk of the current will be going to the Li cells. As long as the charging source maintains a voltage sufficiently high to bring the Li cells to full charge everything will reach 100%. At that point the AGM will be overcharging but the current will be quite low, hopefully low enough that it does not damage the AGM. When charging stops and everything settles out the voltage will hold at the higher Li and some current will be flowing into the AGM. If the batteries are separated and allowed to stabilize the voltages will be in accordance with the characteristic of each chemistry.

At least that is the way I see it.

John
I think the AGM will not overcharge unless it is already full when the charger is connected as the lithium will be far and away quicker to get full. The risk of long term overcharge on the AGMs would be if the lithium still had enough power in it to maintain a higher voltage than about 13.5v after the AGMs were full.

In the real world, I think I would set it up so that there was a disconnect between the AGMs and the lithium, with the charge sources going to the lithium. In everyday use where you would be getting charged enough to live off the lithium, or even mostly, you get the AGM full and shut it off and live off the lithium until the AGMs are needed. This would also shorten the recharge time on the lithiums and give way less charge cycles.
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Old 02-19-2018, 08:56 PM   #7
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Now someone needs to test this hypothesis with an AGM and small Li batteries and a couple of ammeters. It could be done on a small scale using something like a motorcycle AGM and, say, a 45 AH Li battery.
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Old 02-19-2018, 09:50 PM   #8
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Now someone needs to test this hypothesis with an AGM and small Li batteries and a couple of ammeters. It could be done on a small scale using something like a motorcycle AGM and, say, a 45 AH Li battery.
And be sure to include a fire extinguisher and 911 on your speed dial
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Old 02-19-2018, 10:01 PM   #9
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And be sure to include a fire extinguisher and 911 on your speed dial



In reality it would be very close to what you have, just with the relative battery bank sizes swapped!
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Old 02-20-2018, 03:27 AM   #10
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OK-I'm gonna bite. 20ah Lithium batteries are available for $100, less on Alibaba. That would be a good match for an 80ah flooded lead acid and two of them a match for 225ah lead acid.

Assuming I do that what voltage should they be charged at and any other relevant info I need before becoming the guinea pig. Also assuming I don't burn the thing down.

I would put a manual switch between them and maybe a second battery monitor.
Harry

P.S. It charges at 14.4volts. Charge the two batteries separatly, combine them to top off the lead acid, separate to use the lead acid battery.
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