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Batteries Solar Panels And Inverter - Will This Work ?


sampvt
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I have just installed a 250w solar panel with 2 x 120ah batteries connected in parallel but the pos from one battery and the neg from the other with the regulator supplied voltage across the poles of the first in line battery cable. My 1500/3000w pure sine wave inverter is fed from the later battery situated nearer the axle. Naturally my mates have been looking and one of them showed me his set up. ....

 

He has a 110 ah battery which isn't new and a 75ah which is also pretty used and they are straight connected in parallel with each other. His regulator charge leads are connected to the 110ah battery in the front bay. All the batteries and his inverter are connected with 30 amp cables with the feed for his 5000w modified sine wave inverter and it works. He scoffs at the same type battery idea as they are both 12v and his philosophy is right in as much as he will loose a little on the usage because one battery discharges into the other but he has run this system for ages.

 

Is this an accident waiting to happen or will he get away with it but loose some input and performance. I was there last week when his wife used the hair dryer and his cables stayed pretty cool. Am I in overkill mode or is he in suicide mode.

Im back to motorhoming with a scooter on the back again.

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I do not think I would want to be in his van when his wife is using a hairdryer.

 

At minimum reasonable power for a hair dryer, you are talking about 1kw, that equates to over 100 amps from the batteries to the inverter, 30amp cable is definitely too thin,, or do you mean 30mm2 ?

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How do you get 100 amps draw from the battery. That seems a heck of a lot of amps to me. A 1000w hair dryer draws around 8. 5 amps from the inverter so why would the inverter need to draw 100 amps from the battery. even at an efficiency rating of 80% the draw would constitute just over 10 to 11 amps. Please correct me if I am wrong, but 100 amps seems very large a draw. His cables were defo only 30 amp wires that you buy from Halfords and they were not hot or even warm.

Im back to motorhoming with a scooter on the back again.

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For 1000w, current = 1000/12=83amps.

At 80% efficiency this would be 104amps on the 12v side.

 

On the 240v side, the current would be 4 amps

 

Brian

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As Brian says that's why inverters are connected with cables the size of starter motor cables because of the high amperage .

 

 

Is any of these circuits fused as I would of thought a fuse protecting 30A cable would of blown buy now . If no fuse protection sounds like a fire waiting to happen.

 

Dave

Jeep Commander 3. 0 V6 CRD

Isuzu D- Max Utah Auto

Elddis Crusader Storm 2000 Kgs, Unipart Royal Atlas Mover .

 

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I have sent all this info to him and he promises to change the cables before he goes out again. He now realises that the cables are too small and if a fire happens, he could ruin other vans. I am going to help him as its really easy, all he needs to do is move the inverter closer to the battery and use thicker cables. The cables from the inverter to the appliance can be via an extension cable so no big deal there.

 

I think he has got away with it for so long because his wife only uses the dryer for a short time at a go. any longer would compromise the cables, he has been lucky.

Im back to motorhoming with a scooter on the back again.

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I remember the same argument with split charging that once the relay makes between the two batteries the current will sore as it equalised between the two batteries. Being worried I used a 25A in line fuse and it never ruptured. The problem is with two things in parallel is at 12 volt the volt drop is huge and to use two batteries the cables from both batteries need to be same length and size or the batteries will not load share. Narrow boats with bow thrusters and rear engine have had real problems with this. Normally a local battery is put at the bow but on a 60 foot boat getting it to charge is a real problem.

 

We must look at worse case and you have to think what current if local battery becomes disconnected so cables between batteries must be able to carry total load so looking at 16mm for around the 100A although the 110 AH battery will give some where like 250A very quickly the battery will become discharged so no real time for cables to over heat at 4 meters with 16mm and 100A the volt drop will be 1. 12 volts with internal volt drop in the battery this will be near the point where the inverter will switch off with under voltage.

 

Should really use 25mm cable and same length from both batteries but for 1500W inverter really need around 500AH of battery not 220AH we with a 3kW inverter with 6kW peak used twin 35mm cables to each terminal i. e. 4 cables to battery bank 3 x 160AH and the cables did not over heat inverter switched off first. We really needed to double up on batteries but boat sold first.

 

Consider a starter motor with 250A draw is often supplied with 16mm cable because it is for such a short time. However both 16mm and 25mm cable is used for meter tails in the house so sold by the meter in most electrical whole sale outlets tri-rated so easy enough to buy both types so why use 16mm even if it would do may as well use 25mm. I do know some people do use 16mm because that's the limit of their crimp pliers to fit lugs but unlikely you have either size so solder or in the vice with ball bearing which was my trick before I had 120mm crimp pliers.

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Hi,

​You wonder about getting a 100 amps from a battery ?--- years ago ( must have been the late 1950's or early 60's ) it was possible to buy a welding set ( sort of ) you connected it to a spare car battery and used it as a normal welder ( not for long i'll bet ! ) another way --- drop a spanner across the battery terminals --- and ( if small ) watch it melt ! -- NOT RECOMMENDED !!!!!!

david!

Skoda Scout 4x4 pulling a coachman Amara 520/4 at 93%---- when full!

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I would be more concerned about just how much they might give out.

The batteries could well have been designed to withstand yielding cold cranking currents anything from 400 to 750 Amps so potential be "okay" in parallel up to 1000Amps, and be "capable" of even more. They can give the required current!

The 30 Amp wire being no more than a fuse, or incandesent heater!

 

This would not apply if they were super "deep discharge" traction batteries, but as theses cost twice to three times convention starter batteries I feel that is unlikely

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I have just come back and rewired the whole thing for him using 350amp cables, similar to jumper lead cables. Ive also taken his 75 ah battery out of the loop and left snap on battery connectors to install a 110ah battery to match the one in his battery box when he buys one. All done and dusted and he will sleep better now, so will I.

Im back to motorhoming with a scooter on the back again.

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Don't forget the fuse. Lead-acid batteries can supply massive amounts of energy if shorted, waaaaaay more than your 350A cable is rated for. My 1,600W inverter has a peak current output of 3,000W, that's 250A. My cabling is rated for 345A so I've fused at 250A. There should be no reason such a fuse would rupture in normal usage but in a fault condition it would rupture before the cabling was put at risk.

 

His 5,000W inverter could in theory draw 417A (5,000W / 12V = 417A) so your cabling is 'the weakest link'. I'd fuse at 350A and accept that you'll not be able to draw maximum inverter output. Or you could increase the cable size but, as you've spent time fitting 350A cable, this might not be the preferred option!

 

There is a preferred method for wiring up two batteries in parallel. I'm not going to repeat it because it is well documented here.


p. s. 417A through 30A cable. That would be a sight to behold. .. ;)

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p. p. s. Littlefuse do a range of in-line fuses in this current range ('Mega' fuses), further info here: http://www. littelfuse. com/products/fuses/automotive-aftermarket-products/fuse-holders/mega-holder-boxed. aspx or you could do as i've done and fit a DC distribution box inside, further info here: http://www. 12voltplanet. co. uk/mega-midi-fuse-power-distribution-box. html

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post-63455-0-10745300-1392750746_thumb.jpg

Edited by tictag
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