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Is it normal for output to say when input is close to LVC?

LMF5000

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I have a small homemade portable generator setup comprising a 12V 36Ah deep-cycle lead acid battery (VMF DC36-12) and a 500W pure sine wave inverter bought from AliExpress (the brand says Carutu). I use it for backup power during power cuts.

I have a hall-effect coulometer for measuring voltage across the battery terminals and current flow, and several kill-a-watt style inline power meters for measuring AC power and voltage at the output.

Today I was powering a 3D printer with it, and when the battery was getting low, around 10.5V, the inverter was no longer able to hold 230V output, it sagged as low as 192V. The current on the DC side at that point was 33 Amps (almost 400W). When the bed heater cuts out (several times a minute, to maintain a constant temperature with bang-bang control), power falls to 120W, DC current falls to 12A, battery voltage rises to 11.5V and inverter output rises to 230V.

Is this normal behaviour for an inverter? Originally I thought it would maintain the output voltage constant over the whole range of permissible input voltages (10.5-15V). But it seems on mine there's some degree of dependence on input voltage.
 
10.5-11.0V is the typical cut-off for 12V inverters. A 33A load on a 36Ah lead acid battery is massive. You may only get 20 minutes of run time.

The issue here is you have a grossly undersized battery for a 350W load. Your battery will not last long being subjected to high current deep discharge.

Also note that "deep discharge" just means it is designed for lower current, deeper cycles than automotive starter batteries. This does NOT mean that you should discharge it to less than 50% capacity remaining.

36Ah batteries are typically rated for 20 hours, i.e., to get 36Ah of capacity, you can only consume 36Ah/20h = 1.8A.
 
Your inverter is not regulated output, they are not easy to find for small inverter. As DC input Voltage goes down, the DC current will go up to try to maintain AC output Voltage. Most inverter will not maintain AC output when batteries are low.
Here is example of Inverter with regulated output.
 
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10.5-11.0V is the typical cut-off for 12V inverters. A 33A load on a 36Ah lead acid battery is massive. You may only get 20 minutes of run time.

The issue here is you have a grossly undersized battery for a 350W load. Your battery will not last long being subjected to high current deep discharge.

Also note that "deep discharge" just means it is designed for lower current, deeper cycles than automotive starter batteries. This does NOT mean that you should discharge it to less than 50% capacity remaining.

36Ah batteries are typically rated for 20 hours, i.e., to get 36Ah of capacity, you can only consume 36Ah/20h = 1.8A.
When you say it won't last long do you mean in terms of runtime (Peukert effect) or lifetime (cycles/years)? What's a typical load (in C) for a deep cycle battery?

I selected this battery after making a spreadsheet of specs of all the models available locally. The main constraint was weight because this assembly needed to be portable, so my upper limit was about 10kg for the battery. Unfortunately LiFePO4 is very hard to get in my country (no shops stock it and no logistics companies will ship them) and the only local option is Victron (by special order) which are 8x the cost of lead. The VMF came out the winner in terms of cost/Ah and close to the top in terms of Ah/kg so I chose this one. Typical load usually doesn't exceed 15A@12V to run a desktop computer and two monitors for up to an hour, but I wanted to run the printer to see what would happen hehe.

It's not a constant 33A load, the heated bed is a 250W heater that cycles on and off every 20 seconds or so to keep a constant temperature. The other loads are the hotend heater which is 30W, a large stand fan of 60W to exhaust the fumes, and electronics/lights/motors that consumed the rest. I noticed something was amiss because the fan would noticeably slow down every time the heat bed cycled on.

So, to circle back, is it normal for the inverter's output to sag when the input gets on the low side? I thought inverters were meant to keep a constant output until they hit LVC?
 
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Both. The more aggressively you use a battery, the less runtime you get, and the less life you get.

As I indicated in the quoted text, 1.8A for that battery. If you use more, you will have reduced capacity. Does your battery have a reserve capacity or "RC" number?

I can't comment on what's normal for your inverter; however, it is common for undesirable outcomes when one runs a system at or beyond the limits.
 
Both. The more aggressively you use a battery, the less runtime you get, and the less life you get.

As I indicated in the quoted text, 1.8A for that battery. If you use more, you will have reduced capacity. Does your battery have a reserve capacity or "RC" number?

I can't comment on what's normal for your inverter; however, it is common for undesirable outcomes when one runs a system at or beyond the limits.

I've attached the datasheet of my battery. No published RC number, but that's easily estimated from the table in the datasheet. If RC is minutes at 25 Amps then reading off the constant-current table in the 10.5V row I get somewhere between 30 minutes and 1 hour.

I get that C20 is determined at the 20-hour rate, but that would need a battery 20 times bigger for my application ;). I meant what's a reasonable design limit for the battery discharge rate? And which limits am I exceeding? The max current per the datasheet is 330A (although for 5 seconds only - lol!)
 

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