LifePo4 Voltage at .5C - .6C Discharge

[otrfun]

Our two GC2's (with 100% charge) drop below 12v while under a 100a - 125a load. When they reach 50% (at 100a - 125a), voltage drops into the low 11's while under-load.

What kind of under-load voltage can I expect with two 100ah LifePo4's (paralleled) with a 100a - 125a load? At 100% charge? At 10-20% charge?

[Itinerant1]

BFL13 wrote:
T

Just curious--can you get 12v to the rig via 7-pin with your set-up if the trailer batts are 'dead in the water' for any reason? Will that light up the Magnum?

-With the trailer batts ok, do you mind paralleling the truck battery with the LFPs via 7-pin? I worry about mixing battery types, but I am told here that is not an issue really. What do you do? Thanks.

I have been thinking about this option but not from the 7 pin. The 2016 Ford diesel truck with dual batteries is always there. Front compartment is always accessible so the nose of the truck could be put near. I was thinking something like from that video from mortons on the move that was posted about the comparison of agm to bb, where he showed the cables from engine compartment to rear of truck camper batteries. Maybe big quick disconnects to something permanent mounted on either end in the engine bay of truck & 5th wheel compartment, for emergency purposes.

Or something like this?

[BFL13]

Some RVs have a big warning sign by the stove to never use it for heating the rig (fumes) Our 5er did. Owner's manuals have that too. And with no 12v, no range hood exhaust fan either.

[pianotuna]

Itinerant1,

Thanks for the excellent report. I would have used my stove top as I don't have an alternative heater. 1 failure in over 1000 days is impressive, to say the least.

Is there a reason you don't go with an electric start genny (with its own battery)?

[BFL13]

Thanks for the long report! (I added your original post to my question while you were posting that)

So the inverter/ charger had lights at that point, so it did have 12v then, but not known where it stood during the "no 12v" time? It was off as shown by having the OEM settings afterwards?

Just curious--can you get 12v to the rig via 7-pin with your set-up if the trailer batts are 'dead in the water' for any reason? Will that light up the Magnum?

-With the trailer batts ok, do you mind paralleling the truck battery with the LFPs via 7-pin? I worry about mixing battery types, but I am told here that is not an issue really. What do you do? Thanks.

[Itinerant1]

BFL13 wrote:
We always are off grid except at home, so redundancy is good. Don't want to have to bail and go home early.

Itenerant1, how did the sequence work when you had your BMS shut-down when one of your cells "went rogue"? In particular, how did you get your Magnum to light up so you could use its charger with the House batts shut down?

This all happened while I was away working in a different state and wife was with the 5th wheel and going on 1,017 days of continuous boondocking and never an issue.

Cells didn't go rouge they shut off charging acceptance due to low cell temp (bms doing exactly what it was designed to do). The batteries were still able to power the rig "but" without charging time is against you. Once the cells warmed up above 32f solar started to charge but this was in the afternoon when it started charging. Generator ran out of gas and she couldn't get it going again. Next night before me coming home she called me and said the system shutdown "no 12v power". She had no idea why. This happened in Nov 2019 while in Kansas with nighttime lows between 7-11f, daytimes just above freezing (cold front dropped down).

Temporary solution... she winterized the trailer (we carry rv antifreeze with us during cold months), pulled out the big buddy heater.

Cause of the system shutting down was one of the senseboards on the cells stopped communicating with the cpu, it did exactly what it's supposed to do...shutdown (one of many safety features, if it doesn't see 20 cells it shuts down). Why did the senseboard die, I don't know why...age, electronic failure or just sheet happens?

When I returned I picked up a new eu2200i generator on the way back. I wanted to upgrade the eu2000i anyways this just sped that up.

First thing I did was put a multimeter on the pack and it was showing 13.28v so it was ok and had a charge. I talked to my installer and he explained how to bypass the cpu (bms) "but" doing this the 20 cells were unprotected.


Now for the next part of the problem, the batteries were not being charged from the Magnum inverter/ charger or solar. After talking to the installer and trying different things...nothing, no charging as if the system was dead but I could see the lights on the equipment showing power? In the meantime plugged in a car battery charger (10a) to the generator so not to kill the batteries while thinking about my options.

After thinking about this overnight, the next morning I went and looked at the Me-Arc setup menu for the system and everything was not as it was set up originally, it cleared itself back to factory settings. After 15 minutes of resetting all of the setup menu from before these issues started the system was back on line charging properly from both the inverter/ charger and solar. I was told that Magnum equipment will remember settings if this were to happen...well it didn't.

This was a inconvenience for sure, more for the wife than me because she was with the trailer and I was in a hotel room that night.

Leason learned from this experience.
Keep generator full of gas. 😉
How to bypass cpu.
Don't go off the word of others, check everything including settings.
Carry extra senseboards now.
Don't put off insulating compartment, tapped into furnace heat duct.
Everything thing is written down now with pics if I'm away and the wife has to do it. Good thing I wrote the settings down that were programmed originally because 3 years later I wouldn't of remembered them all. Hint to all write your settings down, you never know.;)

I like to think I was prepared but this showed the wink links that needed attention to. 😞

Something else between "dropin" and a more diy system is the control over exterior components verses the sealed box. It won't be the end of the world just some inconvenience, you hope anyways.

[3_tons]

“Don't class A with generators installed have an inverter/ charger with some sort of remote displays that can have auto start when voltages drop to x amount or is this a BLF13 black hole all the stars went into misalignment, sky is falling and it's time to assume the fetal position.”

After several threads of twisted gobbledygook I see it more as a ongoing misinformation crusade to promote ‘off-topic’ SiO2’s - after all with all these make believe nagging maladies associated with LFP’s, who in their right mind would ever consider one - lol!!

Soooo much interest (subjective disparaging) from someone who’s an ardent campaigner pushing SiO2’s - lol 🙂 !!

3 tons

[BFL13]

Itenerant1, how did the sequence work when you had your BMS shut-down when one of your cells "went rogue"? In particular, how did you get your Magnum to light up so you could use its charger with the House batts shut down?

EDIT, read this again and not sure it applies. Might be he only lost his BMS but still had 12v "unprotected" until he got that fixed. Was "dead in the water" too for a time there? so might apply?.

My question is about the inverter/charger needing to have 12v before you can use the charger at all with a low voltage BMS shut-down.

-RE: Lithium Battery Question by Itinerant1 :

"I don't know how the dropin batteries work as far as disconnecting that's why I asked. On my system there are 2 solenoids, an over voltage disconnect and under voltage disconnect which will stop the charging or discharging but keep the batteries powered.

For instance at the tail end of last year I had a sense board go out on one of my strings which shut the entire system down (dead in the water)

one of the safety features of my system is it has to see all 20 cells or it will shut down protecting the batteries. I had to do a 5 minute rewire to bypass the selenoids but doing this I was using my batteries without any protection at all.

After 3.5 years and felt fairly confident that if I lowered my charging to 13.9v and kept 13.6 float I should be ok. I got busy doing other things for 3 months and by the time a got new sense boards installed and rewired the solenoids back for protection I set the generator like usual to charge at 100a and set absorb voltage to 14.2v. During the absorb I had 1 cell that would go to 3.8v (the balancer wasn't able to burn off enough) which would disconnect charging but still kept the batteries powered (it would do this with solar also). I had to lower my charging to 14.0v absorb which was keeping that one cell from hitting 3.8v and letting the balancers start doing their thing of charging the other cells and catch up. It took about 2 weeks of doing this in steps of 14.0v, 14.1v, 14.2v after all the cell got within 40mv of each other like they always have been I lowered it back to 14.1v and all is good. I should also note it took that long for the SOC capacity to reset itself because it could never get the voltage per cell high enough for a reset."

[pianotuna]

To use a phrase I started here, many years ago. "capacity is king". Design sufficient worst case capacity and add 10 to 20%. That may lead to lack of angst on camping trips.

Itinerant1 has done so, and any thinking person would do the same.

BFL13 approaches solar and storage from the minimalist point of view and has great fun doing so!

[pianotuna]

StirCrazy wrote:


when they wredoing the capacity tests on LFP batteries they found the amps went up in relation to the volatage going down, so that would suggest the capacity remains constant going by watt-hours.

Steve

Then you need to do more research. I suggest you watch the two links I placed on separate threads. They are quite scholarly, and in my own opinion, definitive.

[Itinerant1]

Don't class A with generators installed have an inverter/ charger with some sort of remote displays that can have auto start when voltages drop to x amount or is this a BLF13 black hole all the stars went into misalignment, sky is falling and it's time to assume the fetal position.

I believe but could be wrong that any of the A, B, C rigs with lfp already installed today have displays with gen charging or alternator charging already set to go. Whether it's functional is a different story.

I'll bet if your scenario happens and the person was so totally oblivious to the "what if" it probably won't happen again and if it does.... hmmmm, is it the system or the user? Which can happen with any battery chemistry.

Proper planning prevents piss poor performance

[BFL13]

StirCrazy wrote:

BFL13 wrote:
After BMS cut-off you can charge the batt and eventually it will come back on. Until it comes back on you have no 12v in the RV from that batt.

If you have an inverter/charger, which needs 12v to even be on so you can use its charger, how are you going to use the inverter/charger? It is dead.

You can use your converter as long as you have a gen to run it with. The converter can also then supply 12v to the RV while you wait for the battery to come back to life.

If your installed gen needs that batt to start it, your gen can't be used to run the converter. A portable gen would start.

Solar could be used unless this happens at night.

So you need a way to deal with it in your RV. Too late if it happens and you never thought of that.

this isnt a problem spicific to LFP, you run any battery dead and you need a way to charge it. I think part of building a battery no matter what the type is insuring you have enough capacity to do what you do. if not then you have to ensure you have a way to charge it.

Steve

Not the same thing IMO. If you run a FLA down to 10.5v it is "empty" but it can still run the lights (but dimly). When you run the LFP down to BMS cut off, you can't run the lights at all.

Anyway, yes, "be prepared". I am thinking mostly of a newbie Class A owner with installed gen that uses the House battery, inverter/charger, and LFPs. He somehow has low voltage BMS cut-off. Oops! Now what?

He needs to get at the 12v engine batts. He can start the engine and get alternator power if that will start his gen and light up the inverter/charger. (would it? I don't know) A trailer could use the 7-pin and start the towing vehicle if that supplies enough to start the gen and inverter/charger.

[StirCrazy]

pianotuna wrote:
Hi BFL13,

As a battery discharges the voltage starts to drop.

By the time it reaches 50% of the amp-hours, voltage is some what lower than it was at 75% state of charge.

When the battery reaches 10% state of charge voltage is again lower.

Since watts are the product of amps X volts, and the voltage is lower--there is less energy in watt-hours remaining, even though the amp-hours are at 50% of full.

I started a thread on Battery Monitoring basics that shows some graphs to help bend your mind around this.

I know you hate watts--but watt-hours are a better measurement of energy than amp-hours.

when they wredoing the capacity tests on LFP batteries they found the amps went up in relation to the volatage going down, so that would suggest the capacity remains constant going by watt-hours.

Steve

[StirCrazy]

BFL13 wrote:
After BMS cut-off you can charge the batt and eventually it will come back on. Until it comes back on you have no 12v in the RV from that batt.

If you have an inverter/charger, which needs 12v to even be on so you can use its charger, how are you going to use the inverter/charger? It is dead.

You can use your converter as long as you have a gen to run it with. The converter can also then supply 12v to the RV while you wait for the battery to come back to life.

If your installed gen needs that batt to start it, your gen can't be used to run the converter. A portable gen would start.

Solar could be used unless this happens at night.

So you need a way to deal with it in your RV. Too late if it happens and you never thought of that.

this isnt a problem spicific to LFP, you run any battery dead and you need a way to charge it. I think part of building a battery no matter what the type is insuring you have enough capacity to do what you do. if not then you have to ensure you have a way to charge it.

Steve

[pianotuna]

Hi BFL13,

As a battery discharges the voltage starts to drop.

By the time it reaches 50% of the amp-hours, voltage is some what lower than it was at 75% state of charge.

When the battery reaches 10% state of charge voltage is again lower.

Since watts are the product of amps X volts, and the voltage is lower--there is less energy in watt-hours remaining, even though the amp-hours are at 50% of full.

I started a thread on Battery Monitoring basics that shows some graphs to help bend your mind around this.

I know you hate watts--but watt-hours are a better measurement of energy than amp-hours.

[BFL13]

"With a battery the first half of the Ah drawn from the battery are at a higher voltage than the second half, so yes at 50% Ah you have used more than 50% of the energy in the battery."


The total AH is 100% of the capacity between full and empty
The total energy is 100% of the capacity between full and empty

When you use up half the "capacity" it doesn't matter what you are measuring, AH or energy, you are at half the capacity, 50% with 50% remaining of whatever units you want to use for "capacity"

2 + 2 = 4 where 4 is the total capacity
3 + 1 = 4 where 4 is the total capacity

2 is half way to 4, 3 is not half way to 4.

So they have two different kinds of "capacity" and they are not speaking English. Meanwhile, they are trying to make their monitor read "half way down" when they have half the energy left,

BUT they want that to be half the AH too where they rate the battery in AH. They have two sets of books! 🙂

EDIT--They talk about a "reserve capacity". Perhaps that is from making 50% of the energy pretend that is 50% of the "pretend AH", which is less than half the "real AH" when you get to "empty" on energy , you still have some "real AH" left to go? (but no energy) Makes no sense.

So are they using real AH at the 20 hr rate, or "pretend AH" ?

If I run a 5 amp load on a 100AH FLA it takes 20 hrs to be "empty"
If I run a 5 amp load on a "100AH" LFP how long will it take to be "empty"?