Does my solar electric system design look OK?

Hi bc buck,

I assume that you are considering the Morningstar  Tristar MPPT 45 Charge Controller - is this correct?

You would not really need a Combiner if you have only one string PVs.   Sometimes,  running PVs in parallel can work a bit better for RVs,  if they are really mobile,  as PV Shading would be less of an impact,  it the RV is parked where there is some shading.

If you did run these  three PVs in parallel,  you would need a Combiner,  AND  a PWM CC would probably serve you better.

If you were to run the three PVs in series,  into a PWM CC,  then,  the CC would run hot,  and you would be wasting production from two of the PVs.

FWIW,   Vic

Hi bc buck,

Thanks for the info.

Morningstar in its divine wisdom has chosen to have two CCs with very similar names  --  the venerable TriStar TS 45 (A PWM CC),  and the TriStar TS 45 MPPT (an MPPT CC).

Am familiar with each of them,  to some extent.

The way that PWM CCs work is to act like an electronic switch that connects the PV input voltage to the battery.  And,  when a regulated voltage is needed (Absorb,  Float and EQ),  this connection is turned off and on at a rate that equates to the average of the needed regulated voltage (as long as there is enough PV input power to maintain this needed voltage).

So,  the is no voltage to current conversion process.   The maximum amount of current available for charging the batteries or for loads is the available current from the PV module/s.   If one wires three PVs in series,  the current available remains the same as a single PV.   Would guess that much of the excess power from the added series PVs  would probably just be used in heating the PVs,  so probably the CC would not dissipate much of this excess power,  as heat.

You really have no choice,  but to wire the existing PVs in parallel,  if you already have the TS 45 PWM CC.   You will lose more from  cable voltage drop  compared to a series string and an MPPT CC,  but you will be far ahead,  as you will have three times the current to use,  less a bit of loss in cable..   If you do not already own the TS PWM CC,  then,  perhaps an MPPT CC would be in order,  but a 30 A MPPT CC would cost several hundred dollars more than the TS PWM.

You would probably want to increase the diameter of the PV cable from the PVs to the CC,  and add a Combiner box,  but  you will produce  considerably more power with parallel PV connections.

Irrespective of which CC you might use,  please make certain that you have the Battery Temperature Sensor  (BTS/RST)  for it,  as it will help the CC adjust charge voltages for the measured temperature of your batteries.

FWIW,   Vic

Hi bc buck,

Yes,   with three 12 V PVs in series the power from two of them would be not used at all -- wasted,  if using a PWM CC.

If needed,  use # 4 AWG cable,  and make the transition from #4 cable,  to the largest cable that the TS 45 will accept,  at the circuit breaker which is needed at the input for the PVs on the TS 45.   If the input breaker will not accept #4 AWG,  then you could still make a transition from #4 to the maximum size cable that is accommodated by the breaker.

The cost of the cable is very small,  compared to the cost of an MPPT CC.  IMO.

More later,    Vic