wire size for solar panels

[davidmh]

I am going to put a 20W portable panel on the boat so that it keeps the 85 AH house battery charged up when I am away from the boat. Often when I come back from a sail the engine battery is fully charged but the house battery is not fully charged. The boat is moored on a mid river pontoon facing SE so I hope to fix it facing due south and tipped up a bit. The cable run from the panel to the battery including the controller will be between 5 and 10 meters depending on how I can get the cable from the outside to inside. Question is what would be minimum cable size to avoid unacceptable voltage drop. At some point I need to have a plug and socket, the socket would remain live so the contacts need to be coverable and must only connect one way round, what sort of plug and socket would the panel suggest. My budget is small.

Regards
David MH

 

[Croftie]

Not an expert but 20W is very small so I doubt you would need larger than 2.5mm² and with that size if panel you will get virtually nothing in the winter months. No doubt someone will give other opinions/options

 

[lw395]

20W might give you about 60Ah a month this time of year, maybe half that in January.
That's a useful top up for a single 85Ah battery.
Plenty for keeping it float charged, but how many Ah do you use on a typical weekend?

Voltage drop between panel and controller is not really critical, losing half a volt won't change things much unless the cable overheats, but losing volts between the controller and the battery affects charging.
Use an online calculator to find the drop.
2.5sqmm will probably be fine from the panels, if the run to the battery is long you might benefit from thicker there.

 

[PaulRainbow]

I am going to put a 20W portable panel on the boat so that it keeps the 85 AH house battery charged up when I am away from the boat. Often when I come back from a sail the engine battery is fully charged but the house battery is not fully charged. The boat is moored on a mid river pontoon facing SE so I hope to fix it facing due south and tipped up a bit. The cable run from the panel to the battery including the controller will be between 5 and 10 meters depending on how I can get the cable from the outside to inside. Question is what would be minimum cable size to avoid unacceptable voltage drop. At some point I need to have a plug and socket, the socket would remain live so the contacts need to be coverable and must only connect one way round, what sort of plug and socket would the panel suggest. My budget is small.

Regards
David MH

With a 5m to 10m run 1.5mm cable would be OK, at 10m the voltage drop at 1.5a is about 0.2v. Keep the controller as close to the batteries as possible and use at least 2.5mm here.

You will need to fuse the cable from the controller, close to the battery. Plug and sockets will pretty much all only go one way, lots to choose from if you Goggle deck sockets.

 

[ithet]

I found a 10w panel without controller fine for keeping a 115Ah battery topped up over winter.

 

[macd]

I found a 10w panel without controller fine for keeping a 115Ah battery topped up over winter.

I've no doubt that's so, but much will depend on the self-discharge rate of the batteries in question. I presume yours are lead/calcium, which discharge only slowly. Lead/antimony batteries might not be so happy with whatever fraction of 10W your panels deliver in a British winter.

 

[VicS]

I am going to put a 20W portable panel on the boat so that it keeps the 85 AH house battery charged up when I am away from the boat. Often when I come back from a sail the engine battery is fully charged but the house battery is not fully charged. The boat is moored on a mid river pontoon facing SE so I hope to fix it facing due south and tipped up a bit. The cable run from the panel to the battery including the controller will be between 5 and 10 meters depending on how I can get the cable from the outside to inside. Question is what would be minimum cable size to avoid unacceptable voltage drop. At some point I need to have a plug and socket, the socket would remain live so the contacts need to be coverable and must only connect one way round, what sort of plug and socket would the panel suggest. My budget is small.

Regards
David MH

What Prainbow says regarding cable X section for a 10m cable run. You could reduce it proportionally for a shorter run if necessary but 1.5mm² is a reasonable size from a physical robustness point of view.

Consider the Bulgin Buccaneer plugs and deck sockets

 

[William_H]

For a 20w panel not only is the current small but volt drop is not a great concern. Consider that the panel has an internal resistance of around 20 ohms so even 1 ohm of cable resistance will not matter much. So the primary concern re cable size is the simple robustness of the cable in the marine environment so don't skimp on size for that reason. You do need a fuse near the battery. ol'will

 

[gregcope]

One way to make wiring easier and benefit from large cables is to wire the controller in to the positive and negative bus bars. You will benefit from thick cables from there to the batteries. Also those bus bars tend to be near where you want to mount the controller. You may need to connect the positive to your main master switch (the battery connection) as this will power down the positive bus bar.

 

[doug748]

I found a 10w panel without controller fine for keeping a 115Ah battery topped up over winter.

I was surprised to find the same so I looked (very roughly) at the figures. Only 15 Ah per month charge keeps you well ahead of the game.
With low self discharge at low winter temperatures and more charge than you might expect, it all works out very well provided you have little use for 12v over your winter.

 

[Red Panda]

I've got a 20w panel permanently mounted on the hatch garage, it charges both 100Ah house and 65Ah engine start batteries via a cheap controller from Photonic Universe.
I used 4mm wire, which is probably overkill for panel-controller but kept things simple.

Both batteries are always at 100% when I come on board. Such a small panel would take some time to replenish the batteries if they were significantly discharged, but has no trouble dealing with self-discharge and light domestic use (LED lights, stereo etc) from when I spend some time on board. I'm delighted with how well this arrangement works.


Agreeing with what gregcope says above: my house battery charging +ve connection goes to the unswitched positive section of the fusebox, the starter battery +ve charging lead has an inline blade fuse and then connects to the #2/engine start pole of the 1-2-B switch. There's already great big thick leads going to the batteries, makes sense to use them!