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Old 11-14-2018, 03:49 PM   #21
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Booster - I was looking at this site: https://www.solacity.com/how-to-keep...tteries-happy/ - for the discharge curve and SOC chart. That info is approx 2/3 down the page.
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Old 11-14-2018, 04:11 PM   #22
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Thanks Marko, just what I was looking for.


Discharge curve: Seems to show that the AGM will not start to discharge until the lithium is at about 15% (12.7v) so that would be their priority discharging of the lithium







Charge curve: The charge curve seems to show that the lithium will not charge much past 10% until the AGM is nearing the bulk to absorption transition point. The voltage of AGM charging seems to go from 13.6 to 14.3v pretty quickly, so they probably would be approaching 70% full. This would pretty much fullfill their priority charging for the AGM, I think.








If all this is right, you would not need any fancy controls to do what the literature claims for the system.


This is very similar to what we had guessed in the initial and later discussions on a hybrid system that culuminated with Harry building his. Our biggest issues were the limiting of the charge and discharge amps from the lithium, which in the system likely done with the BMS, and the fact that while the lithium will finish off the the AGMs, sort of, the charge voltage is going to be too low to do a good job except at the very beginning. After that it is basically charging at float voltage, which won't take care of sulfate fixing and maintain capacity. Better than leaving it at 70%, but not as good as a full charge would be.
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Old 11-14-2018, 04:37 PM   #23
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Charge curve: The charge curve seems to show that the lithium will not charge much past 10% until the AGM is nearing the bulk to absorption transition point. The voltage of AGM charging seems to go from 13.6 to 14.3v pretty quickly, so they probably would be approaching 70% full. This would pretty much fullfill their priority charging for the AGM, I think.
Why isn't the priority to charge the lithium first, as that gives you access to the most power? Then charge your reserve AGM or LA batteries.
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Old 11-14-2018, 04:44 PM   #24
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Why isn't the priority to charge the lithium first, as that gives you access to the most power? Then charge your reserve AGM or LA batteries.

Yes, if you are going to charge just one first like they indicated they do, and called it an advantage to charge the one first. As Marko and I both indicated earlier, it would make more sense, as you say, to charge the lithium first if you had the choice.


Since then, and looking at the charge and discharge curves it looks like they probably don't do anything special at all and charge both at the same time. Physics takes care of all their "priority charging and discharging" which makes the AGM charge mostly first and the lithium to discharge mostly first. What they did, most likely, is just make that fact appear to be intentional and an advantage by claiming it was something revolutionary.
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Old 11-14-2018, 06:23 PM   #25
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Booster's observation looks to be spot on.

My understanding:

When charging, you bring the system voltage up by supplying current at a voltage that overcomes the internal resistance of the batteries. When charging the combo AGM/LFP bank from a deeply discharged state like 12.0V OCV the AGM has to accept the charge current to increase in voltage and therefor also cause the system voltage rise to where the LFP battery begins to charge. The competition for charge current between the two probably begins at around 13.6V.

I'm not sure exactly how the current will be shared from there. The LFP would want to rapidly charge but it will probably be restrained because the system voltage also has to rise and the rate of that rise is tempered by the AGM acceptance rate. When the AGM begins to taper the LFP will happily and rapidly absorb the surplus current.
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Old 11-18-2018, 11:01 AM   #26
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Now I'm thinking an A, B, Both, Make before break, switch should be in the system. The main reason would be to disconnect the LFP battery to prevent below freezing charging. Heating pads are always an option of course but they could fail without notice.
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Old 11-18-2018, 11:52 PM   #27
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As soon as I get my system back I'll run some tests. Up to now I haven't paused at the both setting but switched through it. Pretty easy to let it sit there, run the sat TV and see what happens. I'll do the same thing while charging.

I expect a switchable system with separate chargers to be better but we will see how the other one works with nothing but battery physics controlling things.
Harry
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Old 11-19-2018, 12:00 AM   #28
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As soon as I get my system back I'll run some tests. Up to now I haven't paused at the both setting but switched through it. Pretty easy to let it sit there, run the sat TV and see what happens. I'll do the same thing while charging.

I expect a switchable system with separate chargers to be better but we will see how the other one works with nothing but battery physics controlling things.
Harry

The one unknown that might bite you is that a big part of the reason to have the B to B charger is to limit current from the lithiums to a low AGM bank. You might wind up with way high current and blow fuses or melt something. The Hymer system seems to indicate they limit current out of the lithium battery to prevent that issue, but we don't know that for sure at this point.
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Old 11-19-2018, 04:31 AM   #29
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The system in our Roadtrek has the normal 2 - 6V flooded golf cart batteries in series (not sure what capacity, maybe 220A/H) and 320A/H of LFP. At first I had them separated with separate chargers but the lead acid charger failed (and the replacement failed, both because they overheated in the compartment RT puts them in in my year 210V) so I've had them connected in parallel for several months now. They are charged from the alternator or the inverter/charger (we have 400W of portable solar too, but the portable panels have not seen much use and I don't really want to put panels on the roof of this coach).

They operate pretty much as you all have posted, I've verified that the LFP discharges first, but I haven't discharged the LFP to the point where the lead acid has to discharge substantially. I have Thornwave battery monitors on both batteries feeding a Rpi with Domoticz on it to collect the data, I'll post the results when I do discharge them, maybe sometime this winter.
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Old 11-22-2018, 12:02 AM   #30
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The one unknown that might bite you is that a big part of the reason to have the B to B charger is to limit current from the lithiums to a low AGM bank. You might wind up with way high current and blow fuses or melt something. The Hymer system seems to indicate they limit current out of the lithium battery to prevent that issue, but we don't know that for sure at this point.
This has been a great thread and I'm now seriously considering adding a LFP battery to my existing AGM/solar/alternator setup. On the point mentioned above; if the hybrid battery is directly connected, I don't see how you would get a very high current from the lithiums to the low AGM bank. They would both go low together, right?

What situation would allow the AGM to get low while the lithium remains full of energy? Connecting them together let's the lithiums do most of the cycling by draining them first).

I'm thinking I might have an isolation cutout relay on the lithium battery for the case where the battery temp goes to 0C (or lower), but otherwise it would just be connected to the existing system in parallel. A basic BMS on the lithium battery would protect it from overcharge and overdischarge. Any major 'gotcha's' with this proposed setup?
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