Off-grid solar system

[Cinn]

I have a small off-grid system consisting of: 6 panel charging a battery bank of 8 12v 88 Ahr deep cycle leisure batteries. These are configured in series/parallel to supply 24vdc to an 5000w pure sine inverter. The problem I have is that when using bank to supply 1kw oil filled radiator or AC unit pulling 850watts, the positive battery terminal into the inverter gets so hot that on 2 occasions it has melted the terminal. Each time I have increased the cable sizes and am now using 10mm (the same as for the bank connections) on the assumption that less resistance would cure the issue. Unfortunately, it has not. On top of this the inverter stops drawing for low voltage when bank is at 27v, when the manual says it will not cut until 21v. Everything works okay for less power hungry devices e.g. TV, Hi-fi, kettle. The inverter is called 'Power Jack' and was sourced from China. My thoughts are that it is the inverter not controlling the current draw correctly. Does anybody have any thoughts?

 

[Octopus]

Ask the supplier for advice?

They sold it to you.

 

[Cinn]

Thanks. It was supplied via ebay from china. The manual might just as well be in chinese as well. I bought it about 5 yrs ago. I am just watching a youtube video reviewing Power Jack and it seems to confirm my theory. But I thought I'd just ask people with more electrical knowhow than me in case there was something I was missing. As far as I can fathom if it provides 230v (like mains) and cabling bank to inverter started as MP4, then 6mm now 10mm with no improvement in overheating of terminal and connecting cable it must be the inverter?

 

[SparkyChick]

At 240v, 1kw is roughly 4.2A, ignoring losses in the inverter, 240/24 = 10. Input current will be in a ratio that is reversed... so Output Voltage/Input Voltage = Input Current/Output Current. So roughly 42A. This is a substantial current and even low contact resistances can result in high power dissipation. P=(IxI)xR. If there is a contact resistance of 0.5 ohms, P=(IxI)xR = (42 x 42) x 0.5 = 882w. Nearly the same as the 1kw heater. Even a contact resistance of 0.05 ohms will yield losses of around 82w, which is significantly more than most soldering irons and will result in the heating of the terminal.

You should clean all the surfaces of the contacts very thoroughly, ensure they are as flat as possible and if possible, use two cable terminations... one on each side of the terminal to maximise the contact area and hopefully reduce the contact resistance. You can also use thick washers to spread the load created by the screws to ensure there is even pressure over the entirety of the cable terminations.

 

[HandySparks]

I would say that 10mm² cable is barely big enough for a 24V inverter running at 1kW. As SparkyChick says, the connections for these sorts of currents need to be well made to avoid problems. If the battery terminal is being damaged, you might bring all the battery outputs into one separate commoning terminal or bussbar and then on to the inverter (applies to pos and neg supplies). That keeps the current at each battery terminal to just that for the one battery.

When the inverter is dropping out on low input voltage, what is the voltage at the inverter input, as opposed to the battery? Quite likely that voltage drop in the undersized cables is causing the inverter to drop out.

 

[johnduffell]

Agree with the above, if you are making 40+ amp connections that's into mains electric shower territory so the terminals should be at least that substantial even if the insulation didn't have to be.
10mm cable sounds plenty, if it were pvc insulated singles, but it would still be hot to touch. Is the insulation being damaged by the heat along the cable? Or just the terminal itself melting the metal?
Are you sure the heat is coming your your termination, or could there be a bad termination inside the unit that is conducting back to the input terminal.
I don't suppose you have access to a thermal camera, but a normal photo might help with suggestions

 

[Cinn]

Thank you all for your replies. I can see what you are all saying makes sense. The connecting cable is single rubber insulation and it does get hot but not as hot as the terminal. I think John may be on the right track re: unit conducting back into terminal. I have just found a very thorough youtube review

relating to aforementioned inverter. I suspect from this that it is actually a pile of pants. Possibly not 5000w, poorly ventilated, substandard parts, incorrectly rated efficiency, and cutting out due to overheating. I installed the whole grid system myself 5yrs ago and, as far as I could see, the configuration should work fine with the correct components. The reviewer tested large loads and his inverter (even after he had repaired and modified some components) could not do what it was rated to do, cutting out even quicker than mine but, presumably not then feeding back heat. It would appear to be both useless and dangerous, and I must now weigh up as to whether I can afford to replace it with something more reputable.

 

[SparkyChick]

Looking at that, if your inverter has similar sized input terminals then they aren't going to be giving you a great contact area, especially if you're sticking a crimp terminal on that post and just using the washers they provided.

I would be inclined to use some sizeable lugs and, if you can, clamp it to the screw at the very bottom so a large portion of the lug is in contact with the brass plate at the bottom of the post. Then stick a washer on to spread the load a little and tighten it down with the nut.

If the two sets of terminals are paralleled internally, you could also run a copper busbar across the two, clamping it down at the bottom of the post to the main body of the terminal (to maximise contact area) and then use bigger terminals to connect to the bus bar.

As for it being safe... things can go wrong at any time and whilst items from quality manufacturers may have better warranty periods and mean time before failure figures, they are only based on statistics. They don't all fail bang on 30,000 hours. Some will fail much sooner and some will run for much longer. Providing you don't do anything crazy, like leave it unattended with a heavy load on it, if the input is adequately protected to prevent shorting out the batteries, it will probably work fine for ages. Who knows.

But that's how I'd set about trying to resolve the issue of the overheating input terminals.

 

[Cinn]

Thanks.

 

[HandySparks]

Looks like I misunderstood which terminal was overheating.

 

[Cinn]

Thanks handyspark. But apologies as I also have made an error. The cable now connecting all the batteries as a bank and then from neg and pos terminals of bank to inverter poles is actually 25mm^2 not 10 as previously stated. I can supply picture of the connection set up if that would help.

 

[Squid]

Damn, where is all that Chinese spam when you need it.........

 

[DennisG]

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All of these brands will easily adapt to the battery you need if you want to select off grid options.

1. WINAICO
2. Canadian Solar
3. Csun China Sunergy
4. Hanwha SolarOne
5. LG Solar Panels
6. QCells
7. SolarWorld
8. Silfab
9. Solaria
10. SunPreme
11. Talesun

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