Optimising charge rate and LiFePO4 life

[Csimmer]

I have 7kw solar, 2x MS5048, 500 amp smart Daly, 2 amp balancer, 2P16S 280Ah LiFePO4.
I am trying to keep my batteries between 20-90% SOC for longevity but I want to get to 80-90% as quickly as possible.
Setting up the charging voltage at 56.8v and letting the BMS stop charging at say 53.8v or less caused my inverters to go overvolt on the DC side because of the spike which is generated when the BMS abruptly cuts the charging.
Am I wrong doing it this way, is there a better way?
I have ordered some ultracapacitors to wire them in parallel with the BMS to soak up the charge more gently when the BMS cuts the charging. I will also place a 60v TVS diode across the charger. What do people think?
Circuit simulation

 

[Ampster]

What do people think?

I just set the current higher on my inverter/charger so I could use a lower absorb voltage. That got me to where I wanted and the current tapered when it hit the absorb voltage befor it shut down charging. Maybe that is not an option with yours.

 

[the_colorist]

I haven't messed with the MPP series but when your BMS stops the charging, are you saying it disconnects the batteries from all loads and charging sources? Or just the charging source? I would assume it's the former if you're using the internal MPPT to charge with.

 

[Csimmer]

I just set the current higher on my inverter/charger so I could use a lower absorb voltage. That got me to where I wanted and the current tapered when it hit the absorb voltage befor it shut down charging. Maybe that is not an option with yours.

All I can set are the bulk, float voltages, bulk time and max charging current. I don't want to limit my charging current too much.

 

[Csimmer]

I haven't messed with the MPP series but when your BMS stops the charging, are you saying it disconnects the batteries from all loads and charging sources? Or just the charging source? I would assume it's the former if you're using the internal MPPT to charge with.

The BMS just disconnects the charging, the batteries can still supply power to the inverter

 

[Pappion]

True spikes are very short duration. Maybe some ferrite cores on the wiring will help.

 

[the_colorist]

The BMS just disconnects the charging, the batteries can still supply power to the inverter

Are you using a Lithium CC/CV charging profile? Are you charging with the MS5048?

 

[Ampster]

I don't want to limit my charging current too much.

I understand. I thought you set the voltage higher than your target to get more current to flow. That is what happens with my power supplies and that is how I get them to put out more current. It is not an issue with my inverter charger because I can set the current higher and set voltage to my target so when it hits constant voltage stage the current begins to taper and then it is not the shock when it cuts out. I get to target voltage faster but still get a nice taper of current before it shuts off. Can you set a termination current in absorb stage?

 

[Csimmer]

Are you using a Lithium CC/CV charging profile? Are you charging with the MS5048?

 

[Csimmer]

I am charging with the ms5048, there is no lifepo4 profile.

 

[Csimmer]

I understand. I thought you set the voltage higher than your target to get more current to flow. That is what happens with my power supplies and that is how I get them to put out more current. It is not an issue with my inverter charger because I can set the current higher and set voltage to my target so when it hits constant voltage stage the current begins to taper and then it is not the shock when it cuts out. I get to target voltage faster but still get a nice taper of current before it shuts off. Can you set a termination current in absorb stage?

No I can't, all I can set is my max charge current and my charging voltage. Yes I want to have my charging voltage at a reasonable value so I can get the amps into the batteries but I want to stop charging at a lower battery voltage. I know with more expensive chargers anything is possible but I have these units and want to use them.

 

[Csimmer]

I understand. I thought you set the voltage higher than your target to get more current to flow. That is what happens with my power supplies and that is how I get them to put out more current. It is not an issue with my inverter charger because I can set the current higher and set voltage to my target so when it hits constant voltage stage the current begins to taper and then it is not the shock when it cuts out. I get to target voltage faster but still get a nice taper of current before it shuts off. Can you set a termination current in absorb stage?

I have tried using various combinations of bulk and float voltages but I found the float always kicks in way too early and even if it is set a a high enough voltage the amps get throttled the float stage progresses.

 

[the_colorist]

I am charging with the ms5048, there is no lifepo4 profile.

Just curious then, how is the BMS disconnecting the charging without disconnecting from the inverter as well?

 

[Ampster]

Yes I want to have my charging voltage at a reasonable value so I can get the amps into the batteries but I want to stop charging at a lower battery voltage.

That is the part I was not understanding but your explanation below makes more sense if you options are that limited. There is no magic in a LiFePO4 profile, it is just CC/CV. Float is just a lower current form of CV. It is unfortunate your equipment is not that flexible.

I have tried using various combinations of bulk and float voltages but I found the float always kicks in way too early and even if it is set a a high enough voltage the amps get throttled the float stage progresses.

 

[Csimmer]

Just curious then, how is the BMS disconnecting the charging without disconnecting from the inverter as well?

The BMS can cut the power going into the batteries but still allow power out of the batteries. The BMS is like two switched diodes.

 

[Dhopewell]

I am charging with the ms5048, there is no lifepo4 profile.

I just saw this. Is there an AGM profile? For my setup CC current as high as possible and CV at 14.1 x 4 worked well. I did that with new 280ah cells and was over 95% charged at the end of CC. I have the option to set 13.6 as absorption so have done did that since anything higher is unnecessary. Batteries are nearly full as they exit CC, they just finish charging a little more slowly. I could skip absorption entirely if my inverter let me.

In any event the highest current and lowest absorption voltage profile you have is probably fine as long as absorption doesn’t exceed 14.4x 4. Any higher CV than that and it would be wise to limit the time there if you can. It’s amazing how much energy these cells absorb even at 3.4v as long as you have (a little) more time and don’t need the last few percent capacity.

 

[Csimmer]

I just saw this. Is there an AGM profile? For my setup CC current as high as possible and CV at 14.1 x 4 worked well. I did that with new 280ah cells and was over 95% charged at the end of CC. I have the option to set 13.6 as absorption so have done did that since anything higher is unnecessary. Batteries are nearly full as they exit CC, they just finish charging a little more slowly. I could skip absorption entirely if my inverter let me.

In any event the highest current and lowest absorption voltage profile you have is probably fine as long as absorption doesn’t exceed 14.4x 4. Any higher CV than that and it would be wise to limit the time there if you can. It’s amazing how much energy these cells absorb even at 3.4v as long as you have (a little) more time and don’t need the last few percent capacity.

Ideally I want to stop charging at between 80-90%. I still have the problem at 14.1v x 4 that my BMS goes over volt towards the end unless it goes to float and then it doesn't charge fast enough. I want to charge at say 14.2v x 4 at maximum amps but stop charging when the battery voltage gets to say 13.4v x 4 (or thereabouts I will determine that by trial and error). Using my BMS to stop charging trips my inverter because of the induced spike that's why I am thinking of ways to soften the bms switching on the system with super capacitors and/or TVS diode.

 

[toms]

I am trying to keep my batteries between 20-90% SOC for longevity but I want to get to 80-90% as quickly as possible.

Charging your batteries “as quickly as possible” isn’t compatible with longevity.

Ideally you want to charge your batteries as slowly as possible.

Charge rate has a greater effect on lifespan than time spent at 90% charged.

 

[Csimmer]

Charging your batteries “as quickly as possible” isn’t compatible with longevity.

Ideally you want to charge your batteries as slowly as possible.

Charge rate has a greater effect on lifespan than time spent at 90% charged.

When I say as quickly as possible, that is not over any recommended charging rates, it is as quickly as possible within allowable charge rates. The most I'd be charging at the most it would be 0.25C but would usually be closer to 0.15C.
Can you point to some references in regards to charge rate having a greater effect on longevity than SOC?