LiFePO4 Resting Voltage / Shunt Installation

AnOldUR

Using a NOCO Genius10 to fully charge my two LiFePO4's separately and disconnected from the T@B prior to installing a shunt. The first battery is done charging and has been resting for a few hours while the second is connected to the NOCO. The chart below shows a 100% charging voltage of 14.6V and a resting of 13.6V. This is what I've experienced in the past. When charging the two LiFePO4's in parallel, after the NOCO was removed and rested the voltage would settle in the 13.4V to 13.6V range.

I'm not worried, but looking for an explanation for the first battery holding at 14.4V after resting for several hours. Google's not helping. Anyone here have an answer?



edited to add shunt to title ...

Sharon_is_SAM

Maybe let it rest for 24 hours and then check voltage?

Marceline

What are you using to measure voltage? Is it completely disconnected from anything or is it back in the T@B?

RTWCTS

Sounds to me like your battery still has a surface charge @AnOldUR (to go back to my lead acid lexicon). In cars, alternators run over13VDC and ultimately charge the battery over 100 percent SOC. Battery chargers can do this too.I was taught to turn the car headlights on for about a minute to remove the surface charge in a car before running a battery test on a freshly charged battery (just driven or charged). Maybe you just need a temporary load on them to ‘settle them down’ to 100 percent. If it is connected to the T@B, parasitic draw could accomplish this overnight.

AnOldUR

Marceline said:

What are you using to measure voltage? Is it completely disconnected from anything or is it back in the T@B?

To get the full charge on the battery, the only thing connected to the lugs was the NOCO Genius10. I haven't reattached anything. I'm probing the lugs with a multimeter.

The surface charge that @RTWCTS posted is new to me. Google told me, "To normalize the condition, switch on electrical loads to remove about 1 percent of the battery's capacity or allow the battery to rest for a few hours."  The voltage is now at 14.27V after over six hours of rest. Regardless, I would think that the LiFePO4's battery management system or the smart charger would prevented surface charge, but it's the only explanation so far.

My plan is to let the second LiFePO4 charge overnight to be sure they are both at 100% before connecting the shunt. It will be interesting to see if it has the same high rested voltage reading in the morning and if the first battery has dropped significantly.

AnOldUR

The second battery had fully charged by 10:00pm so I removed it from the charger. Both batteries are reading in the 14.2V range this morning using a multimeter and disconnected from any loads or charging.

I'm getting ahead of myself, but wondering if setting up the shunt for 100% charge at this voltage would be correct or if they should be drawn down to the 13.6 range first?

RTWCTS

@AnOldUR As I mentioned in my previous post here, the term surface charge is a term from lead acid battery charging chemistry. A more complete explanation is free electrons accumulate on the surface of the electrode plates during charging. This gives a false (high) reading as to the actual charge condition of the battery when tested with a meter (hence the term surface charge). I thought of this when reading about your experience. The procedure I learned in an ASE training school (decades ago) to correct this condition before testing with a meter is what I went further on to briefly explain in my post.
I haven't yet mastered the intricacies of lithium battery chemistry. I have only a few months experience with them so far, so do not truly know if this condition exists in them. I was trying to offer a suggestion based on associative reasoning (a common method of problem solving in my world). 
In any case, based on the chart in your first post and subsequent reported data it appears your batteries may currently have too many electrons sitting in them.  I'll refrain from dipping my toe in to your pool here anymore as I don't think I helped. Best of luck with the shunt. 

AnOldUR

@RTWCTS even if surface charge doesn't pertain to lithium, it was something new to learn about and made me think and do more research. Thank you for that.

I also need to thank @elbolillo. After getting the shunt installed I was having trouble finding out what to adjust the Victron settings to. The Victron information that I found was geared toward lead acid batteries. His post in this thread was very helpful.

The biggest (stupidest) mistake I made was wiring the negative jumper between my two LiFePO4's to the bus bar. It took me way too long to figure out why the amperage going in or out of the bank was half of what my clamp meter read.  

One interesting thing I had never checked is that the old WFCO is only putting about 5 amps into the batteries. Probably has something to do with the higher voltage of the lithium and not charging in bulk mode. My NOCO Genius10 puts in its full 10 amps and when both are connected it charges at around 15 amps. I though there may be some conflict between the two, but apparently not? Wondering if solar will also be additive?

otr_320

Assuming a 12v lifepo4 battery has been fully charged(*) to 14.6v (100% SOC), it can take 36-72 hours to drop to its final 100% SOC resting voltage of 13.4 - 13.6v (exact voltage varies from battery to battery).  36-72 hours assumes zero load on the battery and all charge devices turned off.

There's nothing you can do to speed up this specific process.  Applying a load to drop the voltage quicker also discharges the battery to less than 100% SOC.  Not sure what would be gained by doing this.

(*) Fully charged defined as 14.6v applied to the battery with zero flow of charge current.

AnOldUR

I've been of the opinion that I didn't really need a shunt. That I could estimate close enough with voltage. To test the shunt, I ran the AC (with 2000W inverter) for over an hour. The shunt put me at about a 50% of available power used. Initially the voltage would have made me think I was lower than that, but this morning, after resting the battery overnight, the voltage has stabilized to what the shunt indicated.

For the $130, it's well worth the price even if just for the peace of mind provided.

AnOldUR

Looking for more information.

In the link posted by @elbolillo it says for the shunt, "Charged voltage: 14.4V but note that this number should be about .1 or .2 volts below the charging voltage set in your charging device."

From what I can find, the charging voltage of NOCO Genius10 is 14.2V and the absorption charging voltage set in the Victron app for my LiFePO4's is 14.4V.

So, where would you set the shunts Charged Voltage? Anything below 14.2 seems too low. Will the shunt stop charging early with a low setting?

Horigan

First, I'm surprised that a shunt needs to know charged voltage.  I don't recall my Victron needing that information.  After inputting the capacity it should just keep resetting the fully charged state as amp-hours go into the battery during charging.

Anyway, I would think the charged voltage would be based on the charger that has the highest charging voltage, in your case being 14.4V.  You could also experiment with different values to see what the shunt output says for remaining capacity.  

AnOldUR

Horigan said:

First, I'm surprised that a shunt needs to know charged voltage.

From Victron:
7.2.2. Charged voltage
The battery voltage must be above this voltage level to consider the battery as fully charged. As soon as the battery monitor detects that the voltage of the battery has reached this “charged voltage” parameter and the current has dropped below the “tail current” parameter for a certain amount of time, the battery monitor will set the state of charge to 100%.

Apparently the shunt stops charging and resets to 100% based on the voltage, not the amps replaced. The part that I don't understand is that the voltage drops after resting. Mine has dropped to 13.32V which (by the charts) is below a 100% charge. I would think that stopping charging would be controlled by the smart charging device, not the shunt.

I plugged my NOCO Genius10 back in and it's sending 1.3 amps to the batteries. I'm assuming it will continue to do this until the voltage reaches 14.2V again. This means that when it reset to 100% the first time it was not actually full. I must be doing something wrong.

(Note that all this is done with the battery switch off. No loads or charging other than the NOCO.)

Horigan

Thanks for the info.