Multiplus II and LiFePO4 parameters (free talk)

DIY
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1

Hi,

There is too much information, sometimes discrepant, about setting up the right parameters of a Multiplus II 48V with LiFePO4 batteries. So, I set my values as below and I would like to start a free discussion about why it could be good and get your feedback telling us why it could not be good. My setup is:

Inverter tab:
DC input low shut-down : 44.8V (2.80V on cells)
DC input low restart: 47.0V (2.93V on cells)
DC input low pre-alarm: 47.0V (same as low restart)

Charger tab:
Absorption voltage: 56.0V
Float voltage: 52.8V

ESS:
Sustain: 47.0V
Dynamic Cut off: 44.8V (2.8V on cells)
Restart offset: 3.0V (2.9V on cells)
Voltage to restart PV: 46.4V (2.9V on cells)

Would you do the same? If not, why?

2

I set my Quattro to shutdown at 48 volts. On the odd chance that my battery gets that low, I will likely need to use power on the DC side to troubleshoot. My battery has never seen voltage below 50 volts, so 48 volts is pretty low.

I am not in an ESS environment. My equipment is deployed in an RV, so your mileage may vary.

3

My understanding of “shutdown” is that this is the limit to prevent damaging the batteries. This said, you can tailor what you really want by setting a minimum SoC in ESS (that I use). In this case, the minimum SoC is the deepest the system will go unless if the system is off-grid. I “guess” that if in off-grid for some reason (e.g.: grid is down), then the “shutdown” will be the limit.

4

Look:

image
image2834×1126 185 KB

5

To bad the discussion is so short here as I would like to have some more details about this as well.

2.8V is still a bit high. You limit the SOC (unless grid falls) to 10%
But in your situation , let’s say you do not go below 5% which is perfect

Otherwise if you want to go really low, I ‘guess’ a value of 2.7 * 16 = 43.2V is the minimum?

I don’t understand your restart voltage of 3V, it seems a bit high…

And you sustain voltage seems a bit low (compared to the (i thought) default of 50.0)

6

Are you charging / discharging fully every day ?

Are your settings within manufacturers specifications?

If you answer yes to the above, chances are manufacturers cycle life expectations will exceed around 10yrs.

In 10yrs i would guess your cells will be derogating from age more than use.

7

on which battery percent your battery does cut off now ? Mine does on 10-15% and I think it is too high cut off.

8

I have a Quattro and I don’t use ESS. My understanding is that you are not wrong but your statements need to be nuanced a bit more.

  1. Your batteries will fall below your set minimum SOC in ESS if two conditions are met: (a) grid fails and (b) your system happens to be inverting when grid fails. This is not the same thing as being off-grid. Some people who design their systems to be off-grid (wrongly?) use ESS.
  2. Minimum SOC and shutdown voltage are set to any (allowed) figure by the user. If you want, you can set minimum SOC to 100% (the batteries will not discharge but your system will not necessarily shut down; it will continue running on grid) or shutdown voltage at 51 V though this doesn’t make much sense. If there’s a conflict between the 2 and your system is running on batteries, the system will shut down on the limit hit first. Both limits are different from when the system must shut down regardless (eg in case of a misconfiguration). For lipos, this will often (but not always) be at 0% SOC.
  3. Regarding your settings, I find the difference between your float and absorption voltages quite high. It is typically 0.1 - 0.3 V.