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Approximate Battery Life

Usangira

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Dec 4, 2020
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Hi

I have a second hand 48V 400Ah battery bank I bought from China just a month ago. The battery is 24/7 on load hooked to solar panels and an inverter which is drawing around 500watts as the sun goes down around 5pm in a tropical hot Dar es Salaam.
By the time sun goes down, according to the charge controller the battery bank is 52.8V ( I usually assume the battery bank is fully charged by then as my consumption is hardly 12kwh/day Against a 5KW solar panels capacity.

So, I don't have measuring instruments in place, but I just want to deduce a fairly reasonable remaining capacity number. Does 52.8V of a battery which is on such a small load can indicate the battery capacity is only 70% of its original capacity ? And that means this is the remaining capacity of the battery. Mind you the 52.8V is on load voltage as have mentioned.

Thanks, Innocent
 
This is LFP
52.8V is 3.3 volts per cell.
Your battery is not fully charged.
Do you have a shunt based battery montior?
If not I suggest you get one.
Also what is your charging profile.
You need a charge voltage >3.45 volts and <3.65 volts per cell to get your battery full.
This is assuming your battery is typical LFP.
 
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Ok....do you mean even if the storage capacity has degraded to less than 70%. The battery bank will charge to ?
 
Your question will require a bit more information so that folks can answer it. I suggest you post as much of the technical specifications of the system including in particular the solar charge controller model and specification's

A photo of the system is often helpful too. Picture tells a thousand words etc etc

Shunt based battery monitors are relatively inexpensive and very useful. Definitely worth adding to your system. The junktek battery monitor is popular in NZ

 
In addition, you say there's a 24/7 load of aprox 500w but you don't say the size of your PV array. Power will go to the load first and its only the 'left over power' that goes to charging. And if the panels aren't making 500w (some clouds go by for example) then the load will lean on the battery discharging it. To get a clear view into your system it would help to
1) Track the kwh(s) coming in from the PV
2) Track the kwh(s) going to your inverter or coming out of your inverter.
There are cheap meters to you can buy to get/see this info.

Note because of your comment above. You're batteries will charge to the same voltage, and operate in the same voltage range regardless of capacity. A new battery vs an old battery of the same type/chemistry both charge to the same voltage. If a battery has lost significant capacity you'll see the voltage drop from hi to low faster with the same load than a 100% battery.
 
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To add a new question to the thread : does anyone know approximately how fast LFP experiences calendar aging or have data on it? 4000-7000 80 percent cycles is obviously a 15-25 year lifespan on solar loads. But that's assuming no calendar aging, how much will that cut it down.
 
Nope not saying anything about capacity degradation.

16 * 3.45 volts = 55.2 volts
16 * 3.55 volts = 56.8 volts
16 * 3.65 volts = 58.4 volts

Suggest you charge to the second option.
For me to achieve 56.8volts as voltage at resting. What should be my highest charging voltage to be set at the charge controller ?
I tried to do 56.8v as bulk same float voltage. The moment the sun goes down and scc stops charging. The resting voltage becomes 52.8volts
 
To add a new question to the thread : does anyone know approximately how fast LFP experiences calendar aging or have data on it? 4000-7000 80 percent cycles is obviously a 15-25 year lifespan on solar loads. But that's assuming no calendar aging, how much will that cut it down.

This is off topic, but it depends a lot on the temperature. 4000/6000 cycles and whatever are based on 25C operating temperature. If it's 45C, you can half that. Conversely, keeping them at cold temperatures reduces calendar aging.
 
For me to achieve 56.8volts as voltage at resting. What should be my highest charging voltage to be set at the charge controller ?
I tried to do 56.8v as bulk same float voltage. The moment the sun goes down and scc stops charging. The resting voltage becomes 52.8volts

How big is your solar array?
 
For me to achieve 56.8volts as voltage at resting. What should be my highest charging voltage to be set at the charge controller ?
That is the charging voltage.
I tried to do 56.8v as bulk same float voltage.
Float should not be that high.
Are you entering float phase before the sunset?
The moment the sun goes down and scc stops charging.
If your charge source is a solar charge controller that is expected.
The resting voltage becomes 52.8volts
That is pretty low for a resting voltage.
Is it really the resting voltage or is your system under load?
How many amp hours are you putting into the battery during the charge cycle?
Do you have a shunt based battery monitor?
 
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For me to achieve 56.8volts as voltage at resting. What should be my highest charging voltage to be set at the charge controller ?
I tried to do 56.8v as bulk same float voltage. The moment the sun goes down and scc stops charging. The resting voltage becomes 52.8volts
Since you say the inverter is on 24/7 and has a 500watt load at sun down, then the pack voltage dropping to 52.8 seems reasonable. That’s just the beginning of the flat part of the curve. It started out fully charged as the sun went down. You can probably drop the float to 54.4 and still go into sundown with a fully charged pack.
 
Since you say the inverter is on 24/7 and has a 500watt load at sun down, then the pack voltage dropping to 52.8 seems reasonable. That’s just the beginning of the flat part of the curve. It started out fully charged as the sun went down. You can probably drop the float to 54.4 and still go into sundown with a fully charged pack.
No....I have already said that I tried 58.6 and yet I didn't achieve full charge by sundown
 
No....I have already said that I tried 58.6 and yet I didn't achieve full charge by sundown
Why do you say you didn't achieve full charge? Did the battery pack reach 58.6? Did the amps going in eventually drop way down (even with sun up) (and no load)? Did it transition to float?

If so, it's fully charged.

If the amps stayed high (somewhat higher than the inverter load) and it did not transition to float then you are right, it is not fully charged. You need more amps per day. So, you need more panels and/or a bigger charge controller. (Not a higher charge voltage.)

Just because it drops back down to 52.8 with a 500 watt load does not mean it isn't fully charged. It could be. We don't have enough information from you to be sure.
 
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Why do you say you didn't achieve full charge? Did the battery pack reach 58.6? Did the amps going in eventually drop way down (even with sun up) (and no load)? Did it transition to float?

If so, it's fully charged.

If the amps stayed high (somewhat higher than the inverter load) and it did not transition to float then you are right, it is not fully charged. You need more amps per day. So, you need more panels and/or a bigger charge controller. (Not a higher charge voltage.)

Just because it drops back down to 52.8 with a 500 watt load does not mean it isn't fully charged. It could be. We don't have enough information from you to be sure.
I just tested it yesterday.
I changed bulk voltage to 59.6 same as float. It never went into float. I achieved 53v resting voltage and I got the highest generation ever 19.8kwh which I believe most of it just went into the battery bank as no body was at home. I ended getting more than 7 hours of air-conditioning.
 
I just tested it yesterday.
I changed bulk voltage to 59.6 same as float. It never went into float. I achieved 53v resting voltage and I got the highest generation ever 19.8kwh which I believe most of it just went into the battery bank as no body was at home. I ended getting more than 7 hours of air-conditioning.

My opinion: your solar array is too small. 5kW of panels (just found that in your first post) with a 19kWh battery with a 12kWh/day consumption. 19.8kWh production - 12kWh use = 7.8kWh reserve on your best day ever.
 
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