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Old 11-14-2021, 02:32 PM   #1
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12V inverter and LiFePo4 batteries?

I am in the process of upgrading to Lithium batteries (a pair of 200AH LiFePo4 batteries) in my 20 year old Safari Trek, well almost 20, its 20th birthday is next week. in doing so I am changing out a fair amount of wiring, and after running the numbers, vs the wiring advice from the inverter manufacturer (Xantrex), I have to wonder if it is time that we re-evaluate wire size recommendations for inverters when they are connected to Lithium batteries vs traditional lead acid.



The inverter in question is a Xantrex Prowatt SW 2000 pure sine wave inverter (installed 5+ years ago), which has a max amp draw of 205 amps for up to 5 minutes in surge mode, and typical max running draw of 175 amps at rated load. Traditional wisdom says that such an inverter must be connected using 4/0 cable and be no more than 8-10 feet from the battery. I question if this is still true in the era of LiFePo4 batteries, with their slightly higher voltage, much flatter discharge curve staying at over 13.0VDC through the upper 80% of the discharge curve, and much lower amount of voltage sag under load due to lower internal resistance. Note that the 3% voltage drop charts only requires 2/0 cable at 20 wire feet from the battery to the load.


What are your thoughts on the subject?
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Old 11-14-2021, 04:02 PM   #2
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IMO the battery bank is immaterial to the capabilities of the inverter. The inverter built-in voltage and amperage safety controls will prevent exceeding design specifications.
The battery cable size to the inverter is adequate for it's capabilities.
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Old 11-14-2021, 04:38 PM   #3
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Ray my point is that LiFePo4 batteries may not need as oversized of cables as the standard advice has been forever for inverters due to the lower amount of voltage sag under load and slightly higher operating voltages thereby lower the chance of the inverter seeing an undervolt condition.
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Old 11-15-2021, 08:23 AM   #4
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Wire size selection

There are ampacity tables. These table show what current is safe (will not get hot). They do not consider distance, only wire size.

Larger size is needed for longer distances to keep voltage up at the end of a run. If your inverter does not shut down prematurely, then it is good to go.

Often an inverter/charger also charges the battery through the same cable. Battery charging may slow down during high current charging phase. Battery charging is more sensitive to voltage loss than most appliances.
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Old 11-15-2021, 09:36 AM   #5
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Quote:
Originally Posted by Isaac-1 View Post
Ray my point is that LiFePo4 batteries may not need as oversized of cables as the standard advice has been forever for inverters due to the lower amount of voltage sag under load and slightly higher operating voltages thereby lower the chance of the inverter seeing an undervolt condition.

If you want to spend the money to switch to smaller cables, go ahead. It is not necessary, just your desire.
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Old 11-15-2021, 06:18 PM   #6
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I am still using mostly larger cables, though as part of the refit the location of the main battery bank is moving to a location inside the coach so as to not be exposed to low temperatures. As to charging being more sensitive to voltage loss, this is probably another lead acid thing, with lead acid batteries it is generally accepted that you need a minimum of 1.0VDC before current flows (ie 13.8VDC float voltage for a 12.8VDC battery bank, and 14.2-14.4VDC to bulk charge a depleted 12.0VDC lead acid battery), with LiFePo4, it only takes about 0.1VDC of voltage difference over the current battery voltage to get effective (high amp) charging due to their much lower internal resistance.


The point being that all those lead acid rules are in question when it comes to Lithium batteries, of course ampacity and voltage drop will remain the same for cable, I just question the importance of staying below the 3% voltage drop curve when dealing with a battery like LifePo4 with its very flat discharge curve, higher voltage, etc. On a lead acid battery we see a swing between 12.8VDC down to 12.0VDC for 100%-50% discharge, by comparison we only see a drop of 13.4-12.9VDC on LiFePo4 between 99% down to 20% state of charge, and we see only a fraction of the voltage drop under load we see on lead acid, which negate a low of the voltage drop seen at an inverter under load, perhaps making the line losses less important.
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Old 11-15-2021, 09:13 PM   #7
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I imagine it will function with smaller wires, but the wires are likely to get warmer and you will have some power loss.

If you are putting new wires why not spend the extra 50 -100 dollars and get the 4/0 wires and then you don't have extra power loss or any concerns about wires over heating.
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