280 4s actual watt hours on inverter?

[jbird526]

Would it be reasonable as a rule of thumb to calculate the watt hours of a LiFePO4 280ah 4s like the following? You could get cells that a bit lower but just thinking about some default calculations to add to a spreadsheet.

Utilize only .80 of the battery as to not drain all the way down and .85 efficiency with use of an inverter. I understand that with lithium you can run all the way down but still seems that a floor of 20% should be reasonable limit before the lithium curve drops off.

Realistic watt hours?
280ah x 12 v = 3360w x .80 = 2688w x .85 = 2284.8w

 

[MisterSandals]

Utilize only .80 of the battery as to not drain all the way down

Exactly how would you determine that you use .80 of the battery? Thats the rub because the voltage curve is mighty flat on an LiFePO4 battery (50%+ of the power is pretty much at the same voltage).

 

[jbird526]

Exactly how would you determine that you use .80 of the battery? Thats the rub because the voltage curve is mighty flat on an LiFePO4 battery (50%+ of the power is pretty much at the same voltage).

Would @2600-2700 watts from a fully charged pack be considered 80%?

Guess I'm not really asking about monitoring the value per se, unless using something like the Victron Smart shunt to tell you the percentage of charge, but what you can realistically expect to get out of a 280ah pack.

 

[MisterSandals]

Would @2600-2700 watts from a fully charged pack be considered 80%?

Guess I'm not really asking about monitoring the value per se, unless using something like the Victron Smart shunt to tell you the percentage of charge, but what you can realistically expect to get out of a 280ah pack.

Yes, while i didn't do the math, simply using what is calculated to be 80% does not tell you anything about its real capacity.

From what I've read on this forum, nearly everybody has been getting the rated AH from the 280's (correct me if i am wrong, anybody!). Some folks kill their batteries trying to feel good about their purchase. YMMV

 

[Steve_S]

You should download this voltage chart. It will help clarify how LFP voltages run.

General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V

I've looked at several places for a simple, easy to read at a glance "chart document" for LFP. I could not find anything comprehensive. I discovered an XLS Worksheet in my stored file and took parts of that to create this chart, I did not...

diysolarforum.com

 

[John Frum]

Would it be reasonable as a rule of thumb to calculate the watt hours of a LiFePO4 280ah 4s like the following? You could get cells that a bit lower but just thinking about some default calculations to add to a spreadsheet.

Utilize only .80 of the battery as to not drain all the way down and .85 efficiency with use of an inverter. I understand that with lithium you can run all the way down but still seems that a floor of 20% should be reasonable limit before the lithium curve drops off.

Realistic watt hours?
280ah x 12 v = 3360w x .80 = 2688w x .85 = 2284.8w

3.2 volts * 280 amp hours * 4 cells = 3584 watt hours
3584 watt hours * .8 bandwidth management factor = 2867.2 watt hours
2867.2 watt hours * .9 @snoobler dishonesty factor = 2580.48 watt hours

@snoobler advises that we should accept 10% less than advertised capacity.

 

[jbird526]

3.2 volts * 280 amp hours * 4 cells = 3584 watt hours
3584 watt hours * .8 bandwidth management factor = 2867.2 watt hours
2867.2 watt hours * .9 @snoobler dishonesty factor = 2580.48 watt hours

@snoobler advises that we should accept 10% less than advertised capacity.

Those all look like reasonable calculations. My post was to learn from others and collect opinions on what a baseline watt hours might really look like. Think if I factored in 2500w instead of advertised 3600w into my decision making that would help calculate how large a battery bank to build. This is of course assuming all appliances running on AC and not any DC appliances that would not have the transfer loss. Would rather err on the low side.

 

[John Frum]

Those all look like reasonable calculations. My post was to learn from others and collect opinions on what a baseline watt hours might really look like. Think if I factored in 2500w instead of advertised 3600w into my decision making that would help calculate how large a battery bank to build. This is of course assuming all appliances running on AC and not any DC appliances that would not have the transfer loss. Would rather err on the low side.

For an inverter 15% conversion overhead is typical.

ac watts / .85 = dc watts

 

[rajseth]

Would it be reasonable as a rule of thumb to calculate the watt hours of a LiFePO4 280ah 4s like the following? You could get cells that a bit lower but just thinking about some default calculations to add to a spreadsheet.

Utilize only .80 of the battery as to not drain all the way down and .85 efficiency with use of an inverter. I understand that with lithium you can run all the way down but still seems that a floor of 20% should be reasonable limit before the lithium curve drops off.

Realistic watt hours?
280ah x 12 v = 3360w x .80 = 2688w x .85 = 2284.8w

I calculate it per cell as
Energy total 280 * 3.2 = 896 VAh * 82% (inverter)= 735Wh output
Rate @ 1C = 735Watts. ( ) factor it by the DOD that makes you happy.

4S (12.8V) = 3kWh, if you need 3kW rate (go to 24+ Volt system) unless you size everything for 300 Amps $$$
8S (25.6V) = 6kWh, if you need 6kW rate (go to 48 Volt system) unless you size everything for 300 Amps $$$

 

[jbird526]

I calculate it per cell as
Energy total 280 * 3.2 = 896 VAh * 82% (inverter)= 735Wh output
Rate @ 1C = 735Watts. ( ) factor it by the DOD that makes you happy.

4S (12.8V) = 3kWh, if you need 3kW rate (go to 24+ Volt system) unless you size everything for 300 Amps $$$
8S (25.6V) = 6kWh, if you need 6kW rate (go to 48 Volt system) unless you size everything for 300 Amps $$$

I didn't even want to get into the 12v, 24, 48v conversation yet... Thought my original post may cause enough flames cast my way.