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Is my math off for LiFePo4?

MartyByrde

Off-Grid Innovator
Joined
May 16, 2022
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Location
USA
Hello,

I have four 16s 280Ah LiFePo4 batteries paralleled for a total of 1120Ah. I’m not sure if my math is incorrect or if I have some settings wrong.

I am running a 3500w load. It’s my understanding that the four batteries would last for approximately 15 hours.

48v * 1120ah = 53,760 Wh
53,760 Wh / 3500w load = 15.36 hours

At 4pm today the batteries read 52.9V. By 11pm today the batteries reached 48V reading on the inverter and shut off. I checked all the BMS’s and confirmed they’re at 48V (48.4-48.6v). Over 7 hours I got 24,500 watts and some PV was still coming in until 6pm. I have 7kw of panels.

It seems like the batteries are draining rather fast. Is this correct or should I be getting longer battery power?
 
Your math is not really off but if your starting voltage was 52.9V you batteries where not fully charged to begin with.
Typically you should be charging to 54V and the batteries will taper of the charging when they get full.
There is no way a 54KWh bank is going to get fully charged in one day by 7Kw of panels. Best case scenario is probably around 35KWh of charging.
Your figure of 24.5KWh sounds like a typical day for this time of the year unless the sky is perfectly clear and your panels have a good angle.
Also I would not drain the batteries down to 48V. Your going to shorten their life. It's best if you cutoff at 51V but I know that many people will go down to 48V to get a bit more out of the battery even if it shortens the life.
 
What are you running for load that’s constant 3.5kw for hours on end?
 
Your math is not really off but if your starting voltage was 52.9V you batteries where not fully charged to begin with.
Typically you should be charging to 54V and the batteries will taper of the charging when they get full.
I’ve seen it reach 54V around mid day then drop down into the afternoon. Is this normal?

There is no way a 54KWh bank is going to get fully charged in one day by 7Kw of panels. Best case scenario is probably around 35KWh of charging.
Your figure of 24.5KWh sounds like a typical day for this time of the year unless the sky is perfectly clear and your panels have a good angle.
Yes, between 35-38KWh per day. What size system would you recommend for the 54KWh batteries to be filled daily?

Also I would not drain the batteries down to 48V. Your going to shorten their life. It's best if you cutoff at 51V but I know that many people will go down to 48V to get a bit more out of the battery even if it shortens the life.
 
That matches my back of the napkin math.

Did all four start out near fully charged?
Was each 16s balanced, all cells same state of charge?

How did you determine load was 3500W?
Just added up computers pull
 
I have a 4p16s which is the same, it sees a 1000w constant load. And on average it use around 19% each night. I started out very low target charge but found it was not sufficient. I now charge to 54.4 or 3.4v and they are doing very well in terms of balance. So at more than 3 times my load I would really charge them higher.
 
Last edited:
Just added up computers pull

Measured?
W vs. VA?

Check the battery current draw, which would account for inefficiencies including how inverter deals with poor PF of computer supplies.
(I would expect diode/capacitor front end to require higher wattage rated inverter, but not to burn extra power dealing with out of phase current as inductive load might.)

Inverters have no-load consumption which can be significant when lightly loaded, but yours has a pretty good load.
Is there an inverter efficiency spec? Does it have a curve giving efficiency at that load, rather than "peak" or a single number?
 
I’ve seen it reach 54V around mid day then drop down into the afternoon. Is this normal?
Yes and No. It should get to 54V and then stay there pulling a lot of power for awhile. I have 26KWh and it will hit 54.4V and stay there drawing 3-4KW for about an hour and then taper off over the next 30 minutes until it is drawing nothing. Then the batteries will settle over the next hour or two to 53.4V
Yes, between 35-38KWh per day. What size system would you recommend for the 54KWh batteries to be filled daily?
12KW of panels would be good amount to have but the question is what will your Inverter handle and how much current can it deliver for charging.
 
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