PWM Inverter

stmoloud

My 600 watt PSW inverter has PWM internally included. But now I believe I have discovered why it was on special. Reading the specs it says that maximum PV power (W) is at 400 W @ max current of 20 A. 

My limited understanding is that this inverter will restrict me to a maximum of 4 x 100 watt panels as at that size the cumulative parallel current will be near the max.

Question is can I use another PWM fed from another separate 400 watt array into the same battery array as that of the inverter?  And so on.

Would the fuse between the battery positive and the inverter need to be upgraded?

Thanks

BB.

Yes, you can parallel 2 or more chargers (or inverter-chargers) just fine. Each controller should have the proper charging voltage and float voltages programmed in... And each controller will make up its own mind based on what is sees at the battery bank.

Normally, you have a common bus point on the battery (where all battery current/voltage will be picked up).

At that point the negative wires go out to each major load (star connected wiring). The positive leads also go to each major load (or power source) with the wiring sized to carry the necessary current safely--And there is usually a fuse or circuit breaker near the battery + to protect against short circuits from causing a wiring fire.

I don't know about other counrty's wiring standards in detail... But in North America using AWG (American Wire Gauge) numbers and the NEC (national electric code)--I always suggest that the DC wiring be "upsized" to minimize voltage drop (less losses) and to minize heating of the wiring... Our NEC code uses a 0.8 / 1.25x deratring factor (plus many more if you read the code) for circuits that have continuous current flow... Also our fuses and breakers are design to not open below 80% of rated current, and will open (minutes/hours) >100% of rated current

For example, we would use a 20 AWG cable to carry 20 amp current in the NEC.

However for DC wiring, the derating would mean:

• 20 amps * 0.8 NEC derate = 16 amps max continuous suggested for branch circuit
• 20 amps * 1.25 NEC derate =  25 Amps minimum rated breaker+wiring for "20 amp continuous loads"

For example, we rarely run our house AC wiring at rated power (electric heaters/hair dryers/etc. can be an exception).

For off grid power systems, when charging the battery bank, the battery can draw "rated/designed" current for 4-5 hours (or more)... Much "harder" on wiring than normal AC house wiring sees.

-Bill

stmoloud

Yes rules will probably be similar here. Just good practice. All my wires are 25 Amp, best to have a little extra. Good to know I can parallel charge controllers. So just to clarify. All the outputs from the chargers go to a negative or positive bus bar from which is connected to the relevant terminals of the battery bank. The inverter and the DC feed is then connected to the same terminals. There is a 20 Amp fuse on the positive cable going to the inverter and there is also a 20 Amp on the positive cable going  to the vehicle's DC feed. Does the negative need to be earthed or can it just be left to float?   

BB.

The technical answer is that if you "ground reference" your DC battery bus (tie to vehicle frame, cold water pipe, etc.), then the return (negative bus usually) never gets above zero volts, so there is no reason to breaker/fuse the grounded return leads.

I think your inverter-charger will need a larger fuse/breaker (breakers can be very nice--Easy to turn on/off the branch circuit to work/debug system). A 600 Watt 12 volt inverter should be something like:

• 600 Watts * 1/0.85 AC inverter eff * 1/10.5 volts battery cutoff = 67 Amps max continuous draw (may surge to 2x 600 Watts)
• 67 Amps * 1.25 derating = 84 amp minimum branch circuit/breaker/fuse suggested rating

With 12 volt (and other low voltage DC bus voltages), it is usually better to run a dedicated +/- wire from the main battery bus to each load. Rather than running from Bus to Load A to Load B to Load C (daisy chain).

For example, you have two battery charger systems, each should home run back to the battery bus/+fuse/+breaker. Charge controllers generate a lot of electrical noise (PWM --- Pulse Width Modulation, basically a series of On/Off pulses) which can confuse other charge controllers if they share the same daisy chain circuit (or cause LED lighting to "strobe" in they share the same daisy chain).

If you have multiple batteries, here is a good write up on how to wire the bank:

http://www.smartgauge.co.uk/batt_con.html

And, the technical discussion on how to protect a "floating power system"... Technically, a floating power system will not short circuit if one of the wires is grounded--But if there is a second short, then you can have high current flowing through either of the power leads.

In the US, we have breakers that are connected together--If one breaker detects a high fault current, it shuts down and also turns off its mating breaker... For a well designed power system, that is how it should be done. Does anyone do it--Not very often.

For smaller power systems, even if they technically have floating outputs, just one fuse or breaker is normally used.

-Bill

BB.

One of our posters here, 2ManyToyz has a nice website that has lots of do it yourself solar power system pictures, and how his systems evolved over time (about 1/2 way down on the right):

http://2manytoyz.com/

-Bill

stmoloud

Thanks Bill great links, I will bookmark both of them