Solar Fridge Battery Calculation with LiFePO4

[klassen55]

Hi everybody. I apologize for the extremely-beginner question here, but I have received conflicting information from my local solar dealers about a solar fridge for my family's off-grid camp and was hoping for an extra opinion.
The solar fridge lists its power draw 2 ways for a 24V system:
524Wh / 24 hrs
24Ah / 24 hrs

I'm looking to use 2 x LiFePO4 batteries, each of which is 12V 100Ah, although the battery capacity is listed at 1280W. For calculation purposes lets just round it down to 1200W.
Hooking them up in series to achieve the necessary 24V would continue to give me the same 100Ah but I should now have 2400W, correct? (24V x 100Ah). One dealer told me I would have 2400W and another told me only the voltage increases but nothing else, giving me 24V but still with 1200W. Clearly this is a huge fundamental discrepancy and I'm shocked that professionals would differ on this information. Anyway, it has me second guessing things so just looking for clarity.

With 2400W, I should have 2400 / 524 = 4.5 days runtime (assuming zero solar charge).
With 1200W, I have half that.

Using the Ah for calculation instead of Wh, I have 100 / 24 = 4.2 days runtime (assuming zero solar charge). Makes no sense to me that the dealer's calculations could show 4.2 days runtime using one unit, but 2.3 days using another.

Any help here would be much appreciated!

 

[rickst29]

'Watts' is an instantaneous measure of power, volts * amps. The battery capacity of each battery is 1280 Watt-hours. (100 Amps * 12.8 Volts = 1280, and the batteries are run down to zero State-of-Charge after one hour.)

Hooking in Series is the same Amp-Hours, they still die after one hour of being "drawn upon" at 100A. But you have doubled the total capacity in Watt-Hours, because the Voltage is approximately doubled. You are consuming double the power.

I suggest that you ignore the Watt-hours, even though Ah directly converts to Wh (by multiplying the Voltage). They already gave you the 24Ah per day "average power usage", at 24 volts. So they already did that math for you, and translated into Ah for a 24 Volt battery. You are building your "24 Volt" battery by using two 12V volt batteries in series. The Amp-Hours is unchanged, but the Voltage increased. You still have 100Ah. That's 4.2 days, if you run them from 100% State of Charge down to Totally Dead.

(Don't do that - throw out the food and drink the beer warm when the batteries still have about 15% left. Save the batteries for a sunny day, instead of killing them. )

 

[klassen55]

'Watts' is an instantaneous measure of power, volts * amps. The battery capacity of each battery is 1280 Watt-hours. (100 Amps * 12.8 Volts = 1280, and the batteries are run down to zero State-of-Charge after one hour.)

Hooking in Series is the same Amp-Hours, they still die after one hour of being "drawn upon" at 100A. But you have doubled the total capacity in Watt-Hours, because the Voltage is approximately doubled. You are consuming double the power.

I suggest that you ignore the Watt-hours, even though Ah directly converts to Wh (by multiplying the Voltage). They already gave you the 24Ah per day "average power usage", at 24 volts. So they already did that math for you, and translated into Ah for a 24 Volt battery. You are building your "24 Volt" battery by using two 12V volt batteries in series. The Amp-Hours is unchanged, but the Voltage increased. You still have 100Ah. That's 4.2 days, if you run them from 100% State of Charge down to Totally Dead.

(Don't do that - throw out the food and drink the beer warm when the batteries still have about 15% left. Save the batteries for a sunny day, instead of killing them. )

Amazing, thanks for the detailed reply! This is exactly what I was hoping for! No plans to run them down to zero, but was rather simplifying things as much as possible, haha. It's a pretty rare event we have no sun for that many days up here in Ontario, so I think this system should make life much more comfortable!

Thanks again!