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]

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?

The BMV 712 is a great choice and I love that it reads direct battery voltage. A solution for those of us with out the BMV 712 and Bluesolar MPPTs is Victron smart battery sense. This solved voltage reading issues between the battery on my solar charge controllers.

 

[Danny]

Reading correct battery voltage at the charger @nebster and @nosys70 are bringing up a really important issue about voltage measuring accuracy. I am going to summarize my impression of the conversation. 1) Who cares about the voltage set point if our chargers are reading the wrong battery voltages. 2) Some say just use increase wire size, relocate chargers next to batteries, etc. However, just asking your charger to increase the voltage for the current load conditions on the wire with circuit breakers is a more elegant and usually leads to a better long lasting solution. 3) we need chargers with voltage sense input connected directly to the battery. Here is a video with a discussion on the topic of a battery sense wire.

 

[Sgt Raven]

I'm giving you all I can, captain. She's gonna blow for sure...

Oh, laddie. You've got a lot to learn if you want people to think of you as a miracle worker.

 

[Sgt Raven]

frankly i do not think a 0.5V will change something about the battery life.
An LFP can go up to 4.2v (so over 16V). if you feel the battery is heating too much at the end, you can stick to lower voltage to see if the temperature goes cooler.

Are you sure about that? Li-Ion cells like 18650s are charged to 4.2V. LiFePO4 cells are not exactly the same and are charged to 3.65V.

 

[nosys70]

yes , but we just discussed that.
if your charger just measure what it sends, you could send 3.65V and but have only 3.4 at the battery terminals (passing through or not a BMS).
so the correct way to set your charger would be to set a voltage, and measure at the battery what you really get.
De facto LFP can be charged as high as 4.2V while i would see no reason to recommend that, since the increase in capacity is negligible but the wear will be probably certain.
if you charge an LFP to 4.2, it will not stay at 4.2( like a li-ion cell would) , it will go fast down to 3.7.
The lithium battery has this particularity that if you charge at nominal capacity (3.65) you will reach full charge value at about 80%.
So, most charger are pushing the charge a bit longer even if the battery looks charged and they sens the full charge with thermal measure, because the cell will start to heat faster when fully charged.
Another way would be to measure the current absorbed, since the purpose of charging a battery is not to reach a Voltage, but pushing Amps into it. The voltage is only the what allows you to transfer the amps from the charger to the battery.
the use of voltage to control the charge of the battery is just a lazy way to do it because measuring volt is a lot easier/cheaper than measuring current.
So , to make current pass form the charger to the battery, you need to apply a pressure (voltage) that will make flow the current from the charger to the battery. For technical reason you have to limit that voltage to less than 4.2V, or less you could burn the battery chemistry.
The expected behavior, is if the battery is starved, it will eat current fast and create a drop of voltage. When the battery is full , the voltage will rise.
So measuring voltage can reflect the SOC of the battery. (and we have seen that is not the case for a lead battery for example).

 

[PatrickG]

I'm getting a bit frustrated because I'm finding many chargers are not custom programmable and they charge to 14.4 vdc pack voltage and my Calb 180ah charge cutoff at 3.6vpc or 14.4 pack voltage. Rod at Marinehowto.com has this chart https://marinehowto.com/wp-content/uploads/2015/09/28-LiFePO4-On-Boats.jpg and this article https://marinehowto.com/lifepo4-batteries-on-boats/ that states 13.88 is about 100% full. Yes. I top balanced my pack when I built it. To be safe, I just charge to 13.8 and I'm not giving up much SOC and my batteries will likely last longer. Am I missing something?

I'm Fairly new to building a system, but the way I have my Lifepo4 prismatic cells charged is 14Volts. 3.5V each. I'am new to Lifepo4 but not Ignorant to lithium in general to the practice ( worked with lithium ion prior) and agree that it does not seem worth it to me to try to push it up to "the max", because when I initially top balanced and "maxed them out" and completed a capacity test then, recharged at 14v, and cycled again it was roughly .75Ah difference. My cells seem to be happy going to 3.5, this particular set is being used without a BMS ( and watched closely) I wanted to see how much one was really needed. To me if you know what loads you are pulling and if your controller is set up properly it should never overcharge. (I know nothing is perfect) My Inverter is set to shut down before we get to any dangerously low voltage but have rarely got close to draining it unless it was on purpose. I will be the first to admin that I may not know as much as a lot of you when it comes to Lifepo4, and I'm sure some of you could find a million things wrong with my setup. So Far it has been running smoothly and within 0-3mv of each cell.

 

[orangeminnie]

For Li, has anyone tried programming in a lower absorption voltage (e.g.: 13.8V), and then used the equalization function to absorb at a higher voltage (e.g.: 14.4V) at pre-programmed intervals (e.g.: once a week)?

 

[stobbie]

I Only charge to 3.4 / 13.6 volts. I barely loose any capacity. I have cell overvoltage set at 3.5v per cell. And programmad the BMS to read 100% when one of the cells hits 3.4.
So I'm very much at the safe side, my cells will stay healty for a long time, I barely loose capacity..

 

[Sgt Raven]

^^^^^^ this right here. My DC-DC/MPPT controller will switch to acting like a power supply, once the battery is charged up, if there is a load on the system.

 

[stobbie]

YES, same here! My fets will never switch.

 

[orangeminnie]

I Only charge to 3.4 / 13.6 volts. I barely loose any capacity. I have cell overvoltage set at 3.5v per cell. And programmad the BMS to read 100% when one of the cells hits 3.4.
So I'm very much at the safe side, my cells will stay healty for a long time, I barely loose capacity..

Any top balancing, once in a while?

 

[stobbie]

No, I don't do that, cell are within 3mv of each other most of the time. Maybe they will drift a bit if you charge higher, but at only 3.4v they stay close to each other.

 

[Sgt Raven]

Any top balancing, once in a while?

I start my builds with a Parallel Top Balance to 3.65V.
In the forums, most the people having trouble with a new build skipped that step.

I won't repeat it, unless there's a big problem in my pack. It has not strayed far enough to require something like that.