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High load when fridge kicks in


Lindsay
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Hi, I have a 24V Mecer (Axpert) inverter which I've only recently hooked up to my fridge.

Before I did this I ran the fridge on mains and used a power meter to determine that when running it used around 160W and about 1.8kWh a day.

However, now on the inverter, when the fridge kicks in I've noticed loads of over 2100W, calculated based on almost 80 amps being measured on the shunt to my battery bank.

I haven't seen such a high number on the load reported by the inverter itself, only 600W and a bit over 1000VA at the same time with the battery voltage reported by the inverter dropping below 24.4V (and then bouncing back to 26V+ straight after). 

I measure/log my shunt and inverter values every 15 seconds, but these are the highest loads I've seen so far.

 

I only have 16mm2 cable to my battery bank and originally had a 60 amp fuse (since when I originally set up my system 2 years ago I never used more than 40 amps) and a "C characteristic" 63 amps DC circuit breaker. I've since switched the fuse to 100 amp but have left the circuit breaker as based on its technical specs it should be ok surging to even double its rating for around 10 seconds. I had been a bit concerned that if it did trip that the inverter might be damaged since these Axpert type units apparently can get damaged if they are connected to solar panels with the battery disconnected.

So I'm just wondering, is such a high load normal when a fridge kicks on? And is the battery voltage read by the inverter actually correct? Or is it likely that the there are losses along the 1 meter+ length of cable to my battery when the fridge kicks in the inverter is under-reporting the real value at the battery bank? Should I be concerned about my cable/fuse/circuit breaker choices due to these surges?

 

Thanks in advance!

 

 

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1 hour ago, Lindsay said:

when the fridge kicks in I've noticed loads of over 2100W, calculated based on almost 80 amps being measured on the shunt to my battery bank.

That sounds a little high, but certainly not impossible. I assume that you only get one reading of 80 A, and after that it settles down to under 10 A?

1 hour ago, Lindsay said:

And is the battery voltage read by the inverter actually correct?

Axperts are generally pretty good, though occasionally I've had to calibrate them. One of my pair had to be calibrated, for example.

1 hour ago, Lindsay said:

I only have 16mm2 cable to my battery bank

That's only good for about 60-75 A continuous (depending on the temperature rating of the insulation), so your 60 A or 63 A breaker was appropriate. It should be able to handle a fridge start-up with ease, though, as it lasts well under 1 second. Cable can safely be overloaded by up to 100% for a minute or more. The breaker/fuse should allow this as well, without nuisance tripping/blowing. You should not use a 100 A breaker or fuse on 16 mm² cable; size the protection for the cable, not the peak load.

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39 minutes ago, Coulomb said:

That sounds a little high, but certainly not impossible. I assume that you only get one reading of 80 A, and after that it settles down to under 10 A?

Thanks for the responses Coulomb. Yes, it doesn't do it for very long as I don't seem to pick it up every time the fridge compressor starts up (as mentioned I only log every 15 seconds). I had the fridge off for half an hour last night and when I turned it on the voltage reported by the inverter went down from 26.2V to just over 25V for about a second. So seems like it's just a brief heavy load. After this it settles to under 10 amps, until it turns off again.

My watts calculation based off the shunt might be slightly high as I use the (voltage from the inverter) x (amps across shunt) and could end up reading the inverter a few seconds before the shunt. But I wouldn't expect it to be out by more than 10% and I'd expect at some point to see 2000W+ reported directly from the inverter, unless it doesn't pick up on loads that are briefly that high. Not really a big concern though.

42 minutes ago, Coulomb said:

Axperts are generally pretty good, though occasionally I've had to calibrate them. One of my pair had to be calibrated, for example.

I thought maybe when there's a brief heavy load though it might report lower at the inverter than the voltage at the batteries (for about 1 second). I haven't really had a chance to look at the voltage reported by the BMS when the fridge kicks on, but this is what I'll check next.

47 minutes ago, Coulomb said:

That's only good for about 60-75 A continuous (depending on the temperature rating of the insulation), so your 60 A or 63 A breaker was appropriate. It should be able to handle a fridge start-up with ease, though, as it lasts well under 1 second. Cable can safely be overloaded by up to 100% for a minute or more. The breaker/fuse should allow this as well, without nuisance tripping/blowing. You should not use a 100 A breaker or fuse on 16 mm² cable; size the protection for the cable, not the peak load.

Thanks, that makes me feel more relieved about my setup, I was worried about those peak loads blowing the 60 amp fuse. I've actually changed the fuse holder, even though my old fuse holder with moulded in cable was supposedly rated for 60 amps its cable was getting quite warm even when charging at 20 amps and seemed to be a bit thinner than 16mm2. So I bought a fuse holder at Outdoor Warehouse that takes those strip/flat fuses that NationalLuna and LittelFuse etc make, but I couldn't find a 60 amp fuse available locally so went 100 amp. Guess I will have to order from Joburg, or just move up to 25 or 35mm2 cable :)

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Well since I swapped out the fuse holder that was getting warm the battery voltage reported by the inverter doesn't seem to be going quite as low when the fridge kicks in, so maybe that made a difference.

9 hours ago, Coulomb said:

It's very important that it's DC rated. Flat doesn't sound like it's DC rated.

This is probably me just getting the terminology wrong. I've seen these fuse types names strip/flat/ane/anl/mega/midi etc, so I'm not sure what the correct name is for these with DC rating. I know that the National Luna fuse holder with 2x 100 amp fuses included that I bought was for "dual-battery systems" according to the packaging and most of their fuses are stamped 32V. So yes I believe it is for DC, however I will look at replacing it with this 60 amp one from Mantech since you said that the 60 amp fuses will be ok overloaded 100% above the rating : https://mantech.co.za/ProductInfo.aspx?Item=14M3521

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2 hours ago, Lindsay said:

This is probably me just getting the terminology wrong.

It's possibly me not understanding 24 V systems. My understanding is that 24 V is barely the start of where DC arcs become a problem, so that these "automotive" style fuses may actually be acceptable.

Personally, I would not feel safe connecting a battery capable of thousands of amps to a fuse like that. I'd want to see a ceramic or sand-filled fuse that is capable of quenching a sizeable arc. But that's me. I would at least check the fuse's maximum current rating; I think it should be able to clear at least 2000 A. If you were using a large lithium based battery ongoing, it would likely have to be able to clear even more current than that.

2 hours ago, Lindsay said:

you said that the 60 amp fuses will be ok overloaded 100% above the rating

I emphasise that this is for short periods of time only, of the order of one minute, depending on the fuse characteristics. Some fuses are better at carrying short term overloads than others.

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8 minutes ago, Coulomb said:

Personally, I would not feel safe connecting a battery capable of thousands of amps to a fuse like that. I'd want to see a ceramic or sand-filled fuse that is capable of quenching a sizeable arc. But that's me. I would at least check the fuse's maximum current rating; I think it should be able to clear at least 2000 A. If you were using a large lithium based battery ongoing, it would likely have to be able to clear even more current than that.

I know my 63 amps circuit breaker has a breaking capacity of 6000 amps. I mainly went with the inline strip fuse as they look like they bolt down nicely, with very good contact to my lugs. Is there a type of ceramic or sand-filled fuse you recommend? I always thought the ceramic tube type fuses might not make a good connection in their holder (although I do use these on my solar panel strings).

15 minutes ago, Coulomb said:

I emphasise that this is for short periods of time only, of the order of one minute, depending on the fuse characteristics. Some fuses are better at carrying short term overloads than others.

Yes, I understand, I should have clarified and said that I understand it's fine for at least a couple of seconds, I don't expect to overload it for any longer than a second or two ;)

 

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9 minutes ago, Lindsay said:

I mainly went with the inline strip fuse as they look like they bolt down nicely, with very good contact to my lugs. Is there a type of ceramic or sand-filled fuse you recommend?

That's a good point; especially when people don't use soft starting of their inverters, and use the fuse as an isolator, the splat when connecting can leave pit marks that could lead to high resistance.

I've not had any trouble with mine, but I rarely disconnect the fuses, and I have pre-charge built in to my system. I use the fuses that are I think 18 mm or 22 mm diameter. They and their fuse holders are not cheap.

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2 hours ago, Coulomb said:

I've not had any trouble with mine, but I rarely disconnect the fuses, and I have pre-charge built in to my system. I use the fuses that are I think 18 mm or 22 mm diameter. They and their fuse holders are not cheap.

Thanks Coulomb, I'll take a look.

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