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Old 04-16-2021, 09:39 AM   #1
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Essex , CT
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Low voltage battery lithium

hello all we are using our system for the first time and have 200 ah of lithium lifpo4 with a Victron 712 monitor. Our battery reads 80% but voltage is at 12.8 when it is fully charged it was in the 14 range. Iím concerned we wonít have enough power for heat tonight!!!! We think we were able to change with the truck, but this morning the voltage was down to 11.5.
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Old 04-16-2021, 09:46 AM   #2
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hello all we are using our system for the first time and have 200 ah of lithium lifpo4 with a Victron 712 monitor. Our battery reads 80% but voltage is at 12.8 when it is fully charged it was in the 14 range. Iím concerned we wonít have enough power for heat tonight!!!! We think we were able to change with the truck, but this morning the voltage was down to 11.5.
I'm not a lithium battery expert, but if you are charging with your truck, you may want to rethink that. The alternator in a truck puts out the wrong voltage (too high?) and may not be right for your lithium batteries.

Do you have solar? Have you modified the MPPT solar controller to the right settings? (typically 14.6v)
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Old 04-16-2021, 09:56 AM   #3
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In addition to that good advice, these "lithium charging from tow vehicle" search results may help IMO:

https://www.google.com/search?q=lith...=airforums.com

Quite a few recent threads/posts about the different charging required by lithium batteries.

Good luck.
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Old 04-16-2021, 10:58 AM   #4
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AnAirstream, did you measure the voltage while the furnace was running? That can indicate lower than true battery voltage. 200ah at 80% state of charge (SOC) to start with should not produce an indication of 11.5 volts at rest after one night, I think, without doing the math.

Following with interest as I am a few days away from installing 200ah LiFePo with a small DC/DC charger-isolator.
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Old 04-16-2021, 11:09 AM   #5
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Low voltage battery lithium

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You can charge with your batteries with the 7 pin connector. But you will be limited due to the 12 gauge wire that supplies the power. If you want a quick charge from your vehicle you will need to run 02 wire with a DC to DC charger to avoid over heating your alternator.
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Old 04-16-2021, 11:13 AM   #6
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[ATTACH]393322

You can charge with your batteries with the 7 pin connector. But you will be limited due to the 12 gauge wire that supplies the power. If you want a quick charge from your vehicle you will need to run 02 wire with a DC to DC charger to avoid over heating your alternator.


200 AH should be enough to run your Airstream for a couple of days unless you are using a ton of power and leaving your inverter on. Did you upgraded to a lithium converter ?

I can easily run 3 days in my 2020 Classic with 200AH BattleBorn batteries.
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Old 04-17-2021, 10:31 AM   #7
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Pjs hire was correct. Make sure the batteries are not being either charged or drained for a bit before you measure voltage.

The 7 pin is crazy slow and doesn’t have high enough voltage for lithium I just put in a Renogy 40a dc to dc charger. I also have solar as ‘plan A’. But when I drain my 600Ah down using the AC at a test stop or something like that, solar driving for 2 hours isn’t enough to fill them back up for a night of Harvest Host.

And I still carry a 2200w generator for AC while boondocking. A furnace is a real energy hog, well more of a pig if it is really cold. The AC is a hog.
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Old 04-17-2021, 10:50 AM   #8
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Lithium Iron Phosphate Battery Charging

Quote:
Originally Posted by AnAirstream View Post
hello all we are using our system for the first time and have 200 ah of lithium lifpo4 with a Victron 712 monitor. Our battery reads 80% but voltage is at 12.8 when it is fully charged it was in the 14 range. Iím concerned we wonít have enough power for heat tonight!!!! We think we were able to change with the truck, but this morning the voltage was down to 11.5.
To whom it may concern.

LiFePo4 batteries are totally different from conventional lead-acid batteries as they require special attention to charging and discharging parmeters.

Here is a good web site to gain visibility to these differences:
https://www.powerstream.com/LLLF.htm...ging%20voltage.

Voltages shown on this webpage are for single cells and not typical Lithium-ion battery. You will need to multiple each by 4 in order to arrive a overall battery voltages since traditional batteries are constructed with four cell-banks in series. i.e. Max charge voltage listed as 3.65V per cell equals 14.60V for the overall battery voltage. Nominal operating voltage of 3.2V per cell equates out to 12.8v for the overall battery.

The best of luck to you. Cheers!
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Old 04-17-2021, 11:07 AM   #9
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Hi

Since these are Lithium batteries, do *not* get tangled up in charts and voltages that are intended for lead acid batteries (like the one shown above). The voltages and care issues for each are *very* different. You can *not* use lead acid "state of charge" voltages with a lithium.

The same thing applies to your BMV 712. They come from the factory set up for lead acid. You need to change multiple settings to get them to work right with Lithium. You also need to get the battery fully charged (and have the 712 recognize that) before they can do anything useful. Based on what you are reading, I would guess your 712 is either programmed wrong or wired wrong.

While lead acid's have a "don't go below this" damage point. A properly done Lithium does not. They have a BMS in them that will cut things off before damage occurs. Indeed that's no fun for your furnace issue, but it is a difference. The "half full" voltage on a LiFePO4 is in the vicinity of 13V. You are correct in being a bit worried.

If the batteries are outside the trailer *and* it's "furnace weather", there is another issue to consider. They will not charge below about 32F. The BMS will stop this happening. You can discharge them, but their capacity (as with any battery) is less when they are cold.

If you have a basic converter charger, it likely will put out 30 or 40 A into the batteries at typical charge voltages. That may not be what the spec sheet leads you to believe. Your 200H pack will take 5 to 7 hours to go from zero to full. While it is charging, the voltage will slowly rise.

Charging from the truck (if you have a DC/DC converter likely will put about 7A into the batteries. That assumes the engine is at > 1,000 RPM. Figure about 30 hours to take the batteries from empty to full that way.

The BMV *will* show you the charge current going into the battery (if it's wired correctly ....). It is much better to use current in and out of the battery to diagnose this sort of thing instead of using voltages.

Now for (as always) some math:

If your furnace pulls an average of 3 to 4 A while running *and* your fridge pulls another 1A or so *and* you have other misc stuff going on at about 1/2A average .... That's a drain of 4.5 to 5.5A ( your 712 will give you the real numbers ....). If you call that 5A, then you have 40 hours of "run time" on your batteries. Not quite two days .... If it's cold, you might come up 12 hours short of those numbers.

Fun !!!

Bob
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