battery interconnector cable

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What is the best gauge cable to connect two 105AH sunxtender batteries that will be next to each other in the same battery box?

Lou Dalessandro

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Re: battery interconnector cable

You really need to supply the load that the batteries will be supporting... If the maximum load is 1 amp, a 3 amp fuse and 16 gauge wire would be fine.... If you need to supply 400 amps for starting diesel motor with glow plugs--16 awg wire is not enough.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: battery interconnector cable

The load to be supported is six 19watt compact fluorescent bulbs.

Lou Dalessandro
BB wrote:
You really need to supply the load that the batteries will be supporting... If the maximum load is 1 amp, a 3 amp fuse and 16 gauge wire would be fine.... If you need to supply 400 amps for starting diesel motor with glow plugs--16 awg wire is not enough.

-Bill
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Re: battery interconnector cable

Here is a good link for wire current carrying capacity:

http://www.interfacebus.com/Copper_Wire_AWG_SIze.html

To figure out how much current you will need, use Power=Voltage * Current

So Voltage = 10 volts (minimum voltage a 12 volt battery will supply)
Power = 6 * 19 watts = 114 watts

current = P/V = 114 watts / 10 volts = 11.4 amps

NEC (US National Electric Code) requires a circuit to run at only 80% of capacity:

Current NEC = Current / 80% = 11.4 amps / 0.8 = 14.25 amps

NEC will allow 15 amps on 14 awg wiring... (note temperature of room, if conduit is used, etc. can affect your choice of wire)

For safety, you will also need to use a fuse/DC Rated Circuit breaker at the battery to prevent fires if there is a wiring short or lamp ballast failure... A 15 amp fuse/breaker should be used here.

Now, the above is a minimum design... Other issues that you should be aware off--if you are using an AC inverter to power the lights--then you would need to account for efficiency... Somewhere between 60-80%--that would change the current requirements from 15 amps to 15/0.6=25 amps rated wiring/fuse... Or ~10 awg wiring...

Next, you need to worry about how long the wires are... If the wire is running a few feet from the battery to an inverter--than this is not a big deal... You can use the wire gauge recommended...

However, if the wire is running 200' from the solar/battery building to the home at 12 vdc (no inverter), then the voltage drop can be pretty significant (and may prevent the lights from working correctly).

Here is an Excel spread sheet that allows you to estimate voltage/power drop for a run of wire (remember, a 200' run is actually 400' of wire)...

http://www.wind-sun.com/smf/index.php?topic=1477.0

Lastly, you also need to address the size of the battery charger too... If the battery charger is rated, for example, 30 amps, then the interconnect wiring will need to be rated for that current too.

At some point, if you will be doing a lot of solar/wiring for your home or cabin, you may want to pickup a copy of the NEC so you have the actual charts and requirements for home wiring.

Does this help or confuse?

-Bill

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: battery interconnector cable

Thanks!

Lou Dalessandro