48v lifepo4 battery voltage query

Fabian

If i set my mppt controller to charge my 48v lifepo4 battery to 54.6v and it stays at that voltage would this be considered as been charged to 100% or would it be seen as been charged to 90%?  seeing that i did not let the battery charge up to 58.4v. Most sites recommend charging to be 3.4v to 3.65v per cell which would be 54.4v to 58.4v. So i just want to know if stopping at 54.6v would be seen as been charged to 100% or 90%. I want to calibrate by battery discharge/charge monitor correctly.

Horsefly

54.6V can get the battery to 100%, but it will require that you stay in absorption for some time at that 54.6V for some time. LiFePO4 will continue to charge after the battery gets to that bulk voltage, and the current will drop off as the battery gets to 100%. You didn't say how big (Ah) your battery is, but if your MPPT has a tail current setting (the charge current at which the MPPT stops absorption and switches to float), you should set it to 0.02C, or 2% of the Ah rating of the battery. At that point you will be at or very near 100% SoC.  

Horsefly

I should add that 54.6V (which is only 3.4125V per cell) is pretty low. You didn't say why you want to charge to such a low voltage. If you could get to 3.45V / cell (55.2V) or even all the way up to 3.5V / cell (56V) it would be more along the lines of what most people do. I would never charge to a voltage above 3.6V / cell (57.6V). As you get closer to the max of 3.65V per cell, you will have one or more cells that will deviate from the others in cell voltage, so that you may have the BMS do a disconnect because a cell gets to the high voltage cutoff setting of the BMS.  I use 3.5V per cell in my setup, but many others like to use 3.45V per cell.

Fabian

Horsefly said:

I should add that 54.6V (which is only 3.4125V per cell) is pretty low. You didn't say why you want to charge to such a low voltage. If you could get to 3.45V / cell (55.2V) or even all the way up to 3.5V / cell (56V) it would be more along the lines of what most people do. I would never charge to a voltage above 3.6V / cell (57.6V). As you get closer to the max of 3.65V per cell, you will have one or more cells that will deviate from the others in cell voltage, so that you may have the BMS do a disconnect because a cell gets to the high voltage cutoff setting of the BMS.  I use 3.5V per cell in my setup, but many others like to use 3.45V per cell.

My battery bank is 200AH. I want to charge only to 90% and discharge to 20% so i can get max cycles out of the battery as the more charges you do to 100% is the less cycles u get from it. Charging to 90% or 95% is what most websites recommend.

So according to the first reply above if the battery just reaches 54.6v and still accepting a large amount of current i am basically at around 90% charge state until the absorption time finish and the battery goes into float and drawing around 2% of its 200ah capacity which would be around 4 amps then that would mean that the battery is at 100% now. 
Am i correct?

Horsefly

You got it right on the 4A tail current meaning that you should be close to 100%.  I know some of the stuff that has been posted saying stay between 90% and 20% SoC, but I think most people will tell you that it isn't quite the way to think.  Most of the people I've seen with lots of experience say that you should focus on keeping the cell voltages out of the danger zone, not keep the SoC away from 100%.  That's why charging to a lower voltage (3.4V or up to 3.55V) is fine. Keeping it at that voltage until the cell doesn't want more charge (meaning it is at 100%) is still fine. Charging all the way to 3.65V or more is bad for the cycle life of the cell.

On the lower end, it's more like what you said. You want to keep any individual cell from getting to 2.5V lower limit. I've done capacity tests on lots of cells, and the an individual cell only has about 7% of it's capacity left once the cell voltage (under discharge) gets to below 3.1V. The discharge curve turns into a cliff pretty quickly after that. So there isn't really any reason to go much lower than that.

In the three LiFePO4 systems I've built (one for a cabin, one for a sailboat, and one for a portable emergency power system) I charge to 3.5V per cell, cut off when the tail current drops to 0.02C, and the inverter is set for LVCO (Low Voltage Cutoff) at 3.1V per cell. 

Sidenote: For LiFePO4 the voltages don't correspond much to SoC, because the vast majority of the charge / discharge curve is almost completely flat. If you really want to know SoC, you'll have to use a shunt. 

Fabian

My battery bank is a Redodo  and I currently have (4) 24v 100Ah batteries string as  48v 200Ah 2P2S connection and the manufacturer told me this: 

If you only charge it to 90% SOC， it is recommended voltage after 30 minutes of resting after charging is completed is: 13.6V-13.7V .
You are really right. The difference in batteries will get bigger and bigger during the use process, so it is recommended to balance them every 6 months for better protection and longer use time.

If you discharge them to 50% and then recharge them, the experience of using the batteries is very poor, it is recommended to discharge to 10%-20%, if 20%, the battery will have better lifespan and performance.

With this info about is this really the case? I thought discharging only to 50 r 40% would give me more cycles? I didn't know that not going down to 20% would affect my performance and lifespan. Is this true because I don't see this online when i do a lot of research.

Also redodo states that their BMS starts balancing at 3.5v so with that case do i have to charge up to 56v each time or should i  do a safe bulk charge of 55.2V and then maybe do a manually charge of each  battery every 6months to the 56v or full charge voltage to get back all batteries internal cells in a balanced state and each batteries  reading the same voltage before i reconnect them back in 2P2S?

Dave Angelini

I guess you need to depend on redodo.  I think I would ask them how their BMS will deal with how you wire these batteries.
I would want to know this as they build a 48V battery, how will (2) 24V BMS in series be as good. Maybe they are fine in series. 

They appear to be telling you use the battery over it's range so the BMS will balance better. Never heard of this company.