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Relationship between battery C rating and continuous discharge amperage?

nicoloks

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Jul 20, 2022
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Hi All,

I purchased a 120Ah 12v LiFePO4 battery which specified a maximum continuous discharge of 150A as I have a 2000W inverter connected which needs occasional, short term heavy draw.

I've put a 120A fuse on the battery which to my understanding means the maximum discharge C-rate is 1. I've found that even when discharging at around 90A / 0.75C the voltage of the battery immediately drops way down to 12.1~12.3v despite starting with a full charge.

According to the SoC chart published by the manufacturer 12.5v is equal to 20% capacity. The only discharge curve they publish is at 0.1C / 12A which shows from 35% ~ 95% SoC the voltage should only vary between 12.8v~13.1v.

This is probably my misunderstanding, but I would've thought a battery manufacturer advertising a high continuous discharge current would translate as that battery being capability of holding voltage under such loads.

This comes to an issue I had recently where I was off grid camping and when I went to bed around 10pm the battery voltage was 13v. The load at that time (with everyone else asleep, so would not have changed) was around 5A, so with 13v indicating the battery was anywhere between 70% and 85% full thought I had plenty of capacity to last until morning. 5 hours later I woke up to the 10.8v low voltage disconnect alarm of my BMS and confined the battery was in fact sitting at 10.5v.

Would appreciate if anyone could assist with making sense of this for me.
 
What size wire do you have between the battery and the inverter? For a 2000W inverter you should have at least 2/0AWG (70mm²). But with the 120A fuse being used to limit the load, 2AWG (35mm²) would be enough. Any wire smaller than that is going to cause excess voltage drop.

SOC readings on a LiFePO₄ battery are only useful when the battery is unloaded. A voltage drop from a high load doesn't suddenly mean the SOC has dropped that much. Remove the load and the voltage (and SOC) should go back up a bunch.

Do note that a 2000W inverter on a 12V battery can mean up to 200A of current. With your 120A fuse you can only put at most about 1200W of load on the inverter
 
so with 13v indicating the battery was anywhere between 70% and 85%
13.0V on a LiFePO₄ battery is 40% SOC. Keep in mind though that 13.1V is 60% and 12.9V is 30%. So depending on how accurate your reading was the SOC could be in a 30% range.

A shunt based battery monitor is way more accurate than looking at the battery voltage.
 
Hi All,

I purchased a 120Ah 12v LiFePO4 battery which specified a maximum continuous discharge of 150A as I have a 2000W inverter connected which needs occasional, short term heavy draw.

I've put a 120A fuse on the battery which to my understanding means the maximum discharge C-rate is 1. I've found that even when discharging at around 90A / 0.75C the voltage of the battery immediately drops way down to 12.1~12.3v despite starting with a full charge.

According to the SoC chart published by the manufacturer 12.5v is equal to 20% capacity. The only discharge curve they publish is at 0.1C / 12A which shows from 35% ~ 95% SoC the voltage should only vary between 12.8v~13.1v.

This is probably my misunderstanding, but I would've thought a battery manufacturer advertising a high continuous discharge current would translate as that battery being capability of holding voltage under such loads.

This comes to an issue I had recently where I was off grid camping and when I went to bed around 10pm the battery voltage was 13v. The load at that time (with everyone else asleep, so would not have changed) was around 5A, so with 13v indicating the battery was anywhere between 70% and 85% full thought I had plenty of capacity to last until morning. 5 hours later I woke up to the 10.8v low voltage disconnect alarm of my BMS and confined the battery was in fact sitting at 10.5v.

Would appreciate if anyone could assist with making sense of this for me.
Are the Voltage measured right at the battery terminals. You need to verify if you have Vdrops issue on your wiring or not.
What is the Voltage at the battery terminals after full charge without anything connected to the battery?
 
Last edited:
I suggest you also check all your connections for hotspots with FLIR or other IR gun.
 
Thanks everyone for the replies. It's really appreciated!

What size wire do you have between the battery and the inverter? For a 2000W inverter you should have at least 2/0AWG (70mm²). But with the 120A fuse being used to limit the load, 2AWG (35mm²) would be enough. Any wire smaller than that is going to cause excess voltage drop.

SOC readings on a LiFePO₄ battery are only useful when the battery is unloaded. A voltage drop from a high load doesn't suddenly mean the SOC has dropped that much. Remove the load and the voltage (and SOC) should go back up a bunch.

Do note that a 2000W inverter on a 12V battery can mean up to 200A of current. With your 120A fuse you can only put at most about 1200W of load on the inverter

I'm using the cables that came with the inverter which is two runs of 5AWG each for positive and negative. Looked again this morning and realised I actually have a 100A fuse between the battery and inverter. Inverter is rated as 2000W total output, however 1500W per outlet of which I am only using one atm.

13.0V on a LiFePO₄ battery is 40% SOC. Keep in mind though that 13.1V is 60% and 12.9V is 30%. So depending on how accurate your reading was the SOC could be in a 30% range.

A shunt based battery monitor is way more accurate than looking at the battery voltage.

I have a BMPro BC300 shunt which I am using to read voltage right off the battery. 40% SOC at 13v would make way more sense as it was about 5 hours from that reading to the low voltage cutoff alarm waking me up. Wonder why the manufacturer is stating different for their discharge profile at 0.1C load? Our load was around 0.05C at the time, so I would have thought that'd have less impact on voltage readings.

Are the Voltage measured right at the battery terminals. You need to verify if you have Vdrops issue on your wiring or not.
What is the Voltage at the battery terminals after full charge without anything connected to the battery?
I suggest you also check all your connections for hotspots with FLIR or other IR gun.

Really interesting that you guys mention this. Since first posting this thread my BMPro J35B BMS has been playing up a bit. The output overload light is on and staying on regardless of having removed all pegs from the 12v outputs. Overload protection of my BMS is 35A and I run no 12v loads off the BMS to come anywhere close to this. Wondering if the issue of the drained battery while camping was actually a result of something shorting? I've ordered myself a clamp meter so I can try and narrow down actual 12v overload issues or if my BMS is just on the blink.
 
I'm using the cables that came with the inverter which is two runs of 5AWG each for positive and negative.
"5AWG" probably means cheap 16mm² cables. 2 of them would be 32mm² which is about 2AWG. 2AWG is vastly undersized for a 2000W inverter on a 12V battery (though your 100A fuse means no more than 1000W from the inverter which is fine with 2AWG). The wires that come with cheap inverters should be thrown away and replaced with proper wires. And running double wires means you should have two fuses, one on each positive wire. Another reason to get rid of them and use proper, single wires.
 
"5AWG" probably means cheap 16mm² cables. 2 of them would be 32mm² which is about 2AWG. 2AWG is vastly undersized for a 2000W inverter on a 12V battery (though your 100A fuse means no more than 1000W from the inverter which is fine with 2AWG). The wires that come with cheap inverters should be thrown away and replaced with proper wires. And running double wires means you should have two fuses, one on each positive wire. Another reason to get rid of them and use proper, single wires.

I will do, thanks for that advice. Fuse I have is like this one which both cables terminate at, so should be fully covered.

Have been talking to some fellow caravaner's who have suggested I replace my battery as my requirements are probably too high for it. They are about to get 2 x 100Ah Renogy Smart Lithium batteries (with built in heaters) which at current prices are about the same cost as a single 200Ah battery. This guy who suggested it usually does his homework on this kind of thing, so actually pretty tempted to just do it. That is probably for another thread however.
 
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