12v LED lighting for cabin

[bruceb58]

Don't bother using a starting battery. Once it gets discharged a few times, it will bring down your other one. Batteries in parallel should be similar age and type.

 

[JASinIL2006]

Ok, thanks Bruce.

 

[MTboatguy]

If you are going to set up a two battery bank, they need to be about the same age, same capacity and same size, that way they will last longer, mixing batteries is detrimental on both of them, because they state sending juice back and forth between each other and tend to draw each other down to a point they won't take or keep a charge any longer.

On my motorhome, I replaced the group 27 12 volt batteries(2) with 4 deep cycle golf cart batteries and they work quite well and are a long term storage solution. Most houses in our area that convert to solar use 6 volt type batteries and they hold out for years.

 

[achris]

Batteries in parallel also need to be isolated from each other when not in use. If not, they set up a current loop and drain each other. (as alluded to by MT).

Chris........

 

[bruceb58]

Batteries in parallel also need to be isolated from each other when not in use. If not, they set up a current loop and drain each other.

This is not true. Large solar and RV systems have multiple batteries in a parallel arrangement. There are no current loops that all of a sudden develop from batteries in parallel.

The problem with 2 batteries in parallel is that one battery may start developing a bad cell and the good battery will only charge up to the level of the bad battery. This causes the good battery to not get fully charged. If they are just left in parallel, they will sit at the same voltage and no current will flow between the two unless one has a faster parasitic discharge current.

 

[achris]

This is not true. Large solar and RV systems have multiple batteries in a parallel arrangement. There are no current loops that all of a sudden develop from batteries in parallel..

If they are attached to a charging system then they are considered 'in use'.

2 perfectly good batteries of the same type and size, connected in parallel, with nothing else connected WILL discharge each other. All batteries have they own internal resistance, and it varies slightly battery to battery. When you setup 2 voltage sources with 2 resistances you have a circuit in which current will flow. That current may be small, but over time it will flatten both batteries.

But yes, putting a low level charging system on does negate that.

Chris.

 

[bruceb58]

No...2 batteries will NOT discharge each other if left in parallel other than the normal parasitic discharge. I used to work with the GM EV1 program that used lead acid batteries and that was never an issue.

If one battery has a higher level of charge that the other, there will be a flow of current from the higher potential(voltage) battery to the other until both are at the same potential. At that point, either battery can have a different internal resistance but since the potential is the same, no current will flow. The battery with the higher parasitic discharge will cause current to flow from the lower parasitic discharge battery to the higher parasitic discharge battery...that is true. Both batteries would indeed flatten but both would anyway over time if they were connected in parallel or not.

 

[achris]

Sorry bruceb58 but in this case you are incorrect.

As well as being a certified marine mechanic, I have been an electronics tech for almost 40 years. I started as a computer hardware engineer. So as well as the theory, I have practical examples of boats bought in to me with near new flat batteries, with the only cause being the battery switch left on. I would charge the batteries (or replace them if they had been flattened too far down), and then check for any current flowing into the electrical system. Usually no current. The only reason the batteries were flat was the switch left on 'BOTH'. Explaining that to the customer and suggesting they flick it to one or the other at the end of the day stopped the problems.

Chris. ...

 

[bruceb58]

Electrical engineer for 35 years including work on electric vehicles. Easy test... connect 2 batteries in parallel. Allow voltage to equilize then replace the parallel connection with a multimeter in current measuring mode. You will see ZERO current.

Have left my 2 AGM batteries in the both position accidentally for a month. Batteries were still nearly fully charged after that month.

There is no theory you can possibly show that would cause current to flow between the batteries when in parallel...I will wait for you to explain that theory.

Perfect example are Tesla batteries. A Tesla battery pack consists of multiple 203V modules that are then all paralleled together. Teslas can sit for over a month and lose less than 15% of their total charge. This is mainly due to the car still having to run basic electronics. BTW, I am friends with a Tesla engineer.

 

[MTboatguy]

Oh boy, Oh boy!

The heat is on!

:lol:

:facepalm:

 

[bruceb58]

So...I went and did my test. Hooked up two batteries with my meter in current measuring mode to make a parallel connection. My meter's resolution is down to 10mA so anything less than 5mA would measure as 0. 2 fully charged identical batteries that are around 4 years old.

 

[JASinIL2006]

Is it safe to assume that two batteries would be wired in parallel with cable that matches the heaviest gauge cable used elsewhere in the system?

 

[bruceb58]

Is it safe to assume that two batteries would be wired in parallel with cable that matches the heaviest gauge cable used elsewhere in the system?

Yes. In theory you could go smaller since, assuming equal batteries, only half the current is flowing through it but there is no point in doing that.

 

[MTboatguy]

My wire between the batteries are the same as regular starter/battery cables, from there I run 10 gauge to the fuse block and then wire the lights up with 12 or 14 gauge wire, the LED's do not generate the heat like the incandescent type lights, the resistance is quite a bit less with the LED's and they have a high tolerance for voltage..

That said, it is best to always use the electrical formula to figure out what size wires you should use over various distances. That is the way we did it at my buddies RV shop when I was working with him a couple of years ago, all of the RV's had formula's figured out for each size RV and our wiring runs. Also, anytime I am attaching connections on my wires, I solder them on the larger connections, which some say is overkill, but I have never had any problems in my systems.

 

[bruceb58]

My wire between the batteries are the same as regular starter/battery cables, from there I run 10 gauge to the fuse block and then wire the lights up with 12 or 14 gauge wire,

I agree....that's what I would do too. Get a big fuse box with lots of fuses and do homeruns to your light circuits. When you want to add another circuit later, it's easy. The bigger the wire used, less likely it is to get damaged. You don't really need to worry much about voltage drop with LED lights..they draw so little current.

 

[JASinIL2006]

Thanks, guys. I have 8 gauge to run from the battery bank to the fuse block. I was thinking of using the same for connecting the batteries in parallel. I thought of getting heavier auto-sized batteries to bonnect the batteries, but 8 AWG over a short run should be ok up to 30 amps, and I can't see ever pulling anywhere near that much current.

 

[bruceb58]

If you haven't bought batteries yet, seriously think about 2 6V Trojan golf cart batteries in series or go with what you have until they die and switch it up to the 6V in series after that. That's what most RVers do.

 

[JASinIL2006]

Another question: winter storage. We close our cabin for six months or so... winter temps regularly hit sub-zero (F) and can stay well below freezing for extended periods of time. Can I leave the batteries hooked up to the solar panel/charge controller over winter or am I better off making sure they are fully charged and install a disconnect switch?

 

[JASinIL2006]

If you haven't bought batteries yet, seriously think about 2 6V Trojan golf cart batteries in series or go with what you have until they die and switch it up to the 6V in series after that. That's what most RVers do.

Yikes, those things are pricy! They have a version with thicker plates just for renewable energy applications, and they are even more expensive. I think the Trojan batteries may have to be part of version 2.0 of my solar project...

 

[bruceb58]

Another question: winter storage. We close our cabin for six months or so... winter temps regularly hit sub-zero (F) and can stay well below freezing for extended periods of time. Can I leave the batteries hooked up to the solar panel/charge controller over winter or am I better off making sure they are fully charged and install a disconnect switch?

Will the solar panels be free of snow? A fully charged lead acid battery can go to -92?F before freezing.

http://www.trojanbattery.com/pdf/WP_...orage_0512.pdf

Yikes, those things are pricy! They have a version with thicker plates just for renewable energy applications, and they are even more expensive. I think the Trojan batteries may have to be part of version 2.0 of my solar project...

An alternative are the golf cart batteries they sell at Sams Club.