LifePo4 Voltage at .5C - .6C Discharge

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."

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!

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.

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

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.

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

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

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.

"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"?

Navigation guidance instrumentation drifts (and such) on Commercial Aircraft, but for some odd reason it hasn’t seemed to keep many folks from flying - I would submit, it must have something to do with confidence in technical engineering - but just a guess...

3 tons