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Old 11-12-2018, 02:32 PM   #1
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Default Parallel lead acid and lithium circa 2018

Are we going to start seeing parallel lead acid & lfp battery setups now with the drop-in lithium batteries?

There's already a commercial option for this: https://www.bos-ag.com/products/lith...ension-battery

The 110Ah Trojan Trillium LFP battery - https://www.trojanbattery.com/pdf/da...m_12.8-110.pdf - combined with 440Ah AGM would fall into the suggested 4 to 1 range of the bos-ag.com setup.

I'm not thinking of any special setup here, just a typical paralleling of batteries with the full realization that the lithium battery will be float charging the AGM most of the time. No buttons to push, no switches to toggle, no user management of the system, no decisions to be made etc.

We often try for perfect but convenience, economy and extended run-time, not perfect, would be the goal here.
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Old 11-12-2018, 03:37 PM   #2
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The BOS-Ag setup looks like a great concept for extending battery capacity. Put the BMS intelligence into the cells themselves, and make the whole thing plug-and-play.

Nice.
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Old 11-12-2018, 04:08 PM   #3
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Originally Posted by markopolo View Post
Are we going to start seeing parallel lead acid & lfp battery setups now with the drop-in lithium batteries?

There's already a commercial option for this: https://www.bos-ag.com/products/lith...ension-battery

The 110Ah Trojan Trillium LFP battery - https://www.trojanbattery.com/pdf/da...m_12.8-110.pdf - combined with 440Ah AGM would fall into the suggested 4 to 1 range of the bos-ag.com setup.

I'm not thinking of any special setup here, just a typical paralleling of batteries with the full realization that the lithium battery will be float charging the AGM most of the time. No buttons to push, no switches to toggle, no user management of the system, no decisions to be made etc.

We often try for perfect but convenience, economy and extended run-time, not perfect, would be the goal here.

There is probably a pretty high probability that is the system that Hymer was raving about a while ago for the European market. This is what precipitated the question of building similar, which Harry did, but with more charging control.


I think the questions that were asked then would still apply, though, even if we have bit more data now about the system.


It would be interesting to see how the currents flow on charging and discharging, especially out and in of the lithium, as it seems like it would be hard to have the AGM take priority in charging. One would think the lithium would fill first at a 14.3v or absorption voltage that the AGM would want, unless the lithium batteries self limit current going in. It says the lithium would discharge first, which would be expected, and they list a max amps but don't say if it is internally limited to that level. The float they say the lithium gives the AGM might be on the high end of the range, but probably not horrible, but they might also be limiting that internally.


Most of the AGM system chargers have absorption times in the 4 hour range, so the lithium would see that voltage and time essentially every charge cycle. The 4 hours is also not really enough to totally till most AGM discharges so if that was increased by using an amp based charger that time would greatly increase for the lithium. This was why Harry used the 12v to 12v charger in his setup, so the AGM would get fully charged off the lithiums. The other thing the 12v to 12v did was limit the charge current to the AGMs form the lithium, as when I tested that with small batteries the current from the lithiums went way too high ass soon as the charger was shut off if the AGM wasn't completely full or at least over 85%. That my be why this new systems lists max output so low and may limit it to that. If they limit that current, they would eliminate the largest part of the need for the 12v to 12v charger, I think.



At the 4 to 1 ratio, it probably would work best for someone with 300 watts of solar or more and moderate, maybe 50ah, daily use. In that case the AGM would rarely get discharged much at all and the solar would like the low internal resistance of the lithium battery. This would, I think, greatly increase the life of the AGM bank, and you would still have it there for reserve in cloudy weather.


Once power use gets to the point of bigger discharges of the AGM, I think it gets more difficult to predict gains and losses in life for both the AGM and lithium. It will be interesting to see if anyone puts a data logger on one of the Hymers when they get out, and if they are this system, to get some real world information.


The simplicity of the system would be a huge plus, and over the normal time an RV lasts any weaknesses may not be big enough to make a difference. We do know that ARV and Roadtrek both tired full lithium systems with drop ins and quickly went away from them, but don't really know what the issues were for them, if any, but those could have been different control systems.
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Old 11-12-2018, 05:47 PM   #4
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This statement is in the "Product Features" and explains things more like would be expected. Less about floating the AGM and more towards daily cycling and topping off the AGM is necessary.


This is much more in line with what would be expected and what started Harry on the path to his system. To do this as stated, I think they must be limiting the discharge current in the lithiums or they would melt things into a big AGM bank that was discharged a ways. If so, this makes the system much more appealing as it takes care of the relatively expensive addition of the 12v to 12v charger and associated relays.


Quote:
Longer battery lifetime The lead acid battery is not strained by deep charging cycles, as the lithium battery is used for running daily cycles. During its long hours at full state of charge the lead acid battery is floated. Compared to pure lead acid systems both effects positively impact the lead acid battery lifetime. By preventing harmful sulphation, the lifetime can be increased to more than 10 years. This lowers the operating cost of energy storage significantly and has a positive impact on the environment. The lithium battery offers enough charging cycles for more than 10 years of daily use. The lithium battery furthermore recharges the lead acid battery in case that it didn't get fully charged during the day, a case which frequently happens in solar applications.
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Old 11-12-2018, 05:56 PM   #5
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And by keeping them in parallel voltage remains the same.

The 440Ah AGM + 110 LFP combo is likely best suited for dual alternator setups. It would be hard on single stock alternator if current couldn't be controlled.
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Old 11-12-2018, 06:09 PM   #6
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And by keeping them in parallel voltage remains the same.

The 440Ah AGM + 110 LFP combo is likely best suited for dual alternator setups. It would be hard on single stock alternator if current couldn't be controlled.

Yeah, the voltage staying the same on both is probably the biggest weakness in it. If the AGMs are full after a charge, and so is the lithium, the AGM will see too high voltage for while. If the AGM is not very full after a charge, the lithium might not get it full before the voltage drops to far. If the charge runs long enough to fill a discharged AGM bank, the lithium will fill quickly and then see higher than desired voltage for some amount of time.



The big question is if the downsides are worth worrying about, and if the current limiting, hopefully both in and out of the lithium would balance a lot of it out, or not.



Definitely intriguing, and we know from Haryy's experience that it does work with the 12v to 12v charger to attain his goal of shortening generator run time. If it will do the same without the 12v to 12v it would be really nice.
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Old 11-12-2018, 06:34 PM   #7
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Voltages seem to match up OK.

14.4V charge voltage seems to be OK for both types.
12.0V seems to be as low as you'd want to ever go on both types.
13.6V OCV for LFP is a higher than typical float for AGM but likely acceptable. 13.6V is the typical Normal mode for PD chargers for example. That 13.6V would trend down.

If I had any need for more battery than what I have now I'd be really tempted to try it.
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Old 11-12-2018, 06:56 PM   #8
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Voltages seem to match up OK.

14.4V charge voltage seems to be OK for both types.
12.0V seems to be as low as you'd want to ever go on both types.
13.6V OCV for LFP is a higher than typical float for AGM but likely acceptable. 13.6V is the typical Normal mode for PD chargers for example. That 13.6V would trend down.

If I had any need for more battery than what I have now I'd be really tempted to try it.

Agree, the basic voltages are well within reason. The only issues would in transition type times like the examples I listed earlier. How much issue that would be is anyone's guess, as those time are quite transient.


I am not sure what would be the biggest benefit at this point, if you already have the AGMs so not buying them. The plain capacity increase would be higher cost than more AGM batteries, the weight less, charge times shortened as long as the lithiums covered your normal daily use, AGM life increased if the lithiums cover daily use, the lithiums probably couldn't sit out in below -4*F.



My guess would be that the shortened generator times, be it engine or Onan, would be the biggest benefit, but only a guess. Shore power charging times normally are easier to have plenty of time to full charge AGMs, but generator and driving/idling time shortening would be a big plus for a lot of people
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Old 11-12-2018, 08:53 PM   #9
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Just as a note on whatever the possible non optimum things may be with the hybrid system. It is likely that they are probably less of an issue than a less than stellar battery charger in the system. As other discussions have talked about on the forum, most do not do a very good job of charging the batteries so whatever else happens with the hybrid setup might make those issues better or worse depending on what charger is used and use patterns.


Also along the same lines is that very many folks with non engine generator systems that use a the van alternator through a separator can easily be putting full charge voltage on full batteries during an entire day's drive, so up to maybe 12 hours, which is very hard on AGM batteries.


Sometimes it is easy to not think of where in big picture different weaknesses appear, and how the rank of consequences order is in the real world.
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Old 11-12-2018, 11:26 PM   #10
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I can have a parallel system just by flipping the switch to both with a 1 to 1 ratio, not 1 to 4.

The RV is in the shop due to a deer strike but let me know what you want to know. It will get new lead acid batteries in January.

Current read outs with AGM at 50% and lithium at 100% may be a place to start. I'll let you know. It isn't getting out of the shop anytime soon.

The system works very well with all the switches. Power is no longer a problem. I do not have a compressor fridge.
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