Critique proposed solar battery upgrade

Itinerant1 wrote:

OldSmokey wrote:

1000 watts and 12v is a bad idea, better to be in 24 or 48 territory.

Other than wire size which could accounted for why would 12v be bad after 1000 watts?

basically it's all about power. Watts in this case

eg: you have a 100 bulb at 12 V then your amps is 100/12=8.3A

now for 1000 Watts you get 1000 / 12 = 83.3A
that's a lot of amps to handle and basic economics and common sense come into play here

several issues come to mind.. they are:

1. An MPPT charge controller capable of handling 80 to 100A is expensive
typically in the $900 to $1200 dollar range.
2. You will need high current cabling and terminations.
3. Batteries oh yes, there are limts here...

Batteries have min and max charge rates, the ideal is c/10. ( min )
and c/4 ( max ) that is a good range to avoid heating,overgassing and lifespan

The venerable T105RE is a 225Ah battery for example...
and would be very happy with 22.5A charge (225Ah / 10 = C/10 rate)
that same battery would not be happy at 83 amps ( C/2.7 ) too much amperage.
generally the higher charge you apply over repeated cycles, the faster you
will destroy your battery

so you might go down the wrong path here and decide to parallel enough 12V batteries
to satisfy the c/4 max.. but there is a simpler and cost effective way.

you guessed it... go UP in voltage. now if we double the voltage then we will halve the current
so 1000 watts into a 24V battery is now 1000/24=41.6A and you would select a 40 amp controller..
40amp controllers are in the $200 to $300 range .. 1/3 the cost. also you
can reduce cabling requirements, for 83A you would have required 3awg and now you only need
8awg.

another issue here is the amount of amperage that can be drawn from a battery..
I see no end of posts here on the RV forums of issues of inverters and low voltages.
the main reason of course is too big of a power draw and too low voltage, with 12V any loss
from cables or terminations amounts to a large percentage and thats the problem.

for example a 1000 W inverter at max load will draw approx 1100 to 1200 Watts due to inefficiencies
and at 12 volts this is 1200 / 12 = 100Amps ! if we were to do this to a T105 then the voltage drop
would be the ( internal resistance of the battery ) * 100Amps
a T105RE is 0.001 Ohms per cell so thats 0.001 * 6 ( for 12 volts ) so the Vdrop = 0.006 * 100 = 0.6 Volts
your 12 v battery is now only 11.3 volts ! add to this contact resistance and it just gets worse.
your inverter will soon trip out on a low voltage alarm.

if we had 1000 W inverter on 24 Volts you would get: 1200/24=50Amps and Vdrop is still 0.6V

I could write a whole paper on the negative aspects of 12 Volts in systems like this, but
instead I will post a tutorial soon covering recommended solar and battery practices
that should give you guys a good foundation on which to base your systems on.