diy solar

diy solar

What is the point in charging Lifepo4 all the way to 14.4v when we have about 99.8 SOC at 13.88 v ?

Danny said:
Scotty, What is your BMS cell vpc cuttoff to keep the dilithium crystal cells from melting down. My crystals don't like vpc above 3.65? If the cutoff hits on the high end you will lose your shields!
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Spock could merely estimate the remaining time to shield disruption but Data could divert diLITHIUM crystal core energy to the shields without causing a containment breach.
 
nosys70 said:
you also need to take in account that voltage measured on the charger is not always what your get at the battery terminal
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When charging, your reading the charge voltage not the battery voltage, captain.
 
If you're trying to dial in voltage-based stopping, and your charger doesn't have differential voltage sense wires to read the voltage at the battery, then you are going to be in guessing game mode.

You can still achieve any desired stopping point with trial and error, but it will be current-specific (charge at higher or lower rates later and your offset will be wrong) and you'll be seeing charger values that don't match the charge curve data that you'll see others publishing.

It's definitely worth it to get a charger that has sense lines if you're getting serious about this stuff.
 
Do any of the "residential-grade" MPPT chargers have this? We're using a Victron Smart Solar MPPT, with a BMV 712 with temp sensor/voltage sample that bolts to battery +. We networked them so that the temp and voltage from the BMV is shared with the MPPT. Is the MPPT using the battery sample or its internal voltage sensor as a reference?
 
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orangeminnie said:
Do any of the "residential-grade" MPPT chargers have this? We're using a Victron Smart Solar MPPT, with a BMV 712 with temp sensor/voltage sample that bolts to battery +. We've networked them so that the temp and voltage from the BMV is shared with the MPPT. Is the MPPT using the battery sample or its internal voltage sensor as a reference?
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When networked, the BMV voltage should be being used as the reference.
 
Fortunately, by the time the battery is topping out and entering absorption, the charge current and therefore the wire/connector voltage losses should be small. In our case, I compute the voltage differential to be 0.0078v.
 
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orangeminnie said:
Fortunately, by the time the battery is topping out and entering absorption, the charge current and therefore the wire/connector voltage losses should be small. In our case, I compute the voltage differential to be 0.0078v.
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It's really system-dependent. Fuses, bus bars, wire cross section, wire length, and current of course are the main variables. But low-rate charging to a low CV threshold might also mean the current stays high the whole time (mine does).

I have a drop of somewhere around 40mV per cell under full charging. That's enough to skew things a good bit.

But, you're right: if your circuit is not creating much drop at the decision point, the compensation is not important. I think with larger systems it's almost unavoidable, though, because geometry, bussing, and fusing become major contributors to the resistance and currents get big to try to fill the bank on a reasonable schedule.
 
nebster said:
It's really system-dependent. Fuses, bus bars, wire cross section, wire length, and current of course are the main variables. But low-rate charging to a low CV threshold might also mean the current stays high the whole time (mine does).

I have a drop of somewhere around 40mV per cell under full charging. That's enough to skew things a good bit.

But, you're right: if your circuit is not creating much drop at the decision point, the compensation is not important. I think with larger systems it's almost unavoidable, though, because geometry, bussing, and fusing become major contributors to the resistance and currents get big to try to fill the bank on a reasonable schedule.
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Yeah, some breakers have a slightly shocking voltage drop, also.
 
Dhowman said:
How's this for a different cultural metaphor for this discussion?

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Lol, Marshall amps are great, I bought one as a gift once upon a time. I use a Behringer bass guitar amp but it only goes to 10.

So please tell me that when voltage sampling occurs to determine the actual factual static for real voltage of a battery, there is no charging current present which would skew the reading, correct? ~ Regards, a voltage purist
 
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if you measure battery under charge, you will get the voltage of the charger that is supposed to give a higher voltage of the battery because that's the way you can make current flow (electrician call that "pressure") into the battery.
so to know the real voltage of the battery , you should disconnect charger and put battery under very small charge and check voltage.
that is what some chargers do.
you can also measure what current is flowing into the wires , so when voltage is not able to drive more amps into the battery, it means the battery is charged.
That is why some people charge battery up to 4.2v instead 3.65, because increasing "pressure" let you "push" more amps in the battery.
 
nosys70 said:
if you measure battery under charge, you will get the voltage of the charger that is supposed to give a higher voltage of the battery because that's the way you can make current flow (electrician call that "pressure") into the battery.
so to know the real voltage of the battery , you should disconnect charger and put battery under very small charge and check voltage.
that is what some chargers do.
you can also measure what current is flowing into the wires , so when voltage is not able to drive more amps into the battery, it means the battery is charged.
That is why some people charge battery up to 4.2v instead 3.65, because increasing "pressure" let you "push" more amps in the battery.
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Yes, I know what you mean, the voltage going in must be higher than the battery voltage or current flow for the most part stops due to equalization.

In simple terms, the only way to know the actual State Of Charge of a battery is to take a voltage reading with the battery disconnected from anything that might cause charging or discharging, HOWEVER, with grandpa's old fashioned lead acid batteries for example, a no load battery reading might be perfectly fine but if the battery is placed under load and the battery has weak or undercharged cell(s), the voltage could drop below the threshold of usability. Not sure is Lifepo's and such exhibit the same load/no load characteristics.

Perhaps to ascertain the, "fullness", of a battery that is being actively charged or discharged, perhaps an algorithm is used to compare the current flow into a battery where the battery's electrical profile is known to the algorithm and the State Of Charge is derived from that?

Regards.
 
yes this is due to the fact that lead battery heavily change the internal resistance in regard of SOC.
That is also why you do not need a special charger for Lead battery because you just set the maximum voltage, and let the internal resistance of battery regulate the incoming current.
But this is also why you can measure a correct voltage and get no current.
I think the LFP has a a permanent low internal resistance versus the lead battery that drastically change with the SOC.
this is probably due to the structure of the battery itself.
The LFP as a huge surface compared to a lead battery, so even if LFP has a bad conductivity (that is why the aluminium of the electrode is coated with carbon), it compensate by the huge surface.
 
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nebster said:
If you're trying to dial in voltage-based stopping, and your charger doesn't have differential voltage sense wires to read the voltage at the battery, then you are going to be in guessing game mode.

You can still achieve any desired stopping point with trial and error, but it will be current-specific (charge at higher or lower rates later and your offset will be wrong) and you'll be seeing charger values that don't match the charge curve data that you'll see others publishing.

It's definitely worth it to get a charger that has sense lines if you're getting serious about this stuff.
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I agree. Excellent comment. You seem to understand the problem well that I had. I think the problem gets worse when you have no voltage sense wires and multiple mppt chargers even if you have large wire the chargers read the others voltage. The chargers tend to think the battery voltage is *higher* than actual if you don’t have a dedicated battery sense with no load on it.
 
nosys70 said:
and if you use BMS (common charging) , you do not reall yknow what happens after BMS if you measure just before.
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Right! My charger voltage sense negative is on the BMS B- lead and the positive is on the battery terminal. I have had issues. Great comment.
 
nosys70 said:
if you measure battery under charge, you will get the voltage of the charger that is supposed to give a higher voltage of the battery because that's the way you can make current flow (electrician call that "pressure") into the battery.
so to know the real voltage of the battery , you should disconnect charger and put battery under very small charge and check voltage.
that is what some chargers do.
you can also measure what current is flowing into the wires , so when voltage is not able to drive more amps into the battery, it means the battery is charged.
That is why some people charge battery up to 4.2v instead 3.65, because increasing "pressure" let you "push" more amps in the battery.
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On my bench charger when I top balanced I ran my charger up to 4.2v but my battery terminal cell voltage was still under 3.65 When I was pumping in 20amps. However, when full the voltage needed to be dialed back to 3.65 at 10ma. Now I don’t trust chargers default lifepo4 settings and chargers without voltage sense leads positive and negative. However, if we have voltage sense wires connected to our chargers and we are charging to a pack voltage goal of 13.8v when batteries are empty we sometimes can read charge voltage up at 14.4v at the chargers output because the charger knows not to exceed 13.8 battery terminal voltage at float. I think my Victron MPPTs may work like this with the networked smart battery sense.
 
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orangeminnie said:
Do any of the "residential-grade" MPPT chargers have this? We're using a Victron Smart Solar MPPT, with a BMV 712 with temp sensor/voltage sample that bolts to battery +. We networked them so that the temp and voltage from the BMV is shared with the MPPT. Is the MPPT using the battery sample or its internal voltage sensor as a reference?
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Yes, they do.
 
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