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Ken Rackowski avatar image
Ken Rackowski asked

MPPT 150/85 wiring to multiple battery strings

Helping out a friend put together an off-grid solar system and looking to see if the components she has purchased will work together ok, and if my concept of the wiring is correct.

System components:
(12) Slightly used Hyundai HiS-S365KI
(2) Victor MPPT 150/85-Tr Chargers

(1) Quattro 24v/5000va/120amp/120vac Inverter
(8) Rolls S6 L16-HC 6 volt batteries

This is my first solar experience and concerned about lack of detailed information about the system sizing and configuration. My initial questions are about the MPPT battery wiring and solar panel configuration. I have included a wiring diagram that I pulled off this site and modified.

1. Since we have 8 - 6 volt batteries and 2 - MPPT chargers, figure that we will have 2 - 24 volt strings. Do I just the connect the 2 strings in parallel and connect the MPPT outputs together?

2. After looking over way too many articles on how to wire the PV panels (Parallel, Series # Panels) I honestly have no idea what the optimum configuration for this application is. Based on the equipment we have and the fact the panels will have direct exposure with no shade, what is the best configuration for 2 PV strings.

Thanks

Ken R

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MPPT Controllers
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4 Answers
Ken Rackowski avatar image
Ken Rackowski answered ·

Sorry, this should be under the Questions and answers space.

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Mike Dorsett avatar image
Mike Dorsett answered ·

1) Since the modules are ~47V Voc, you can wire max 3 modules in series. This is most efficient from volt drop in the pv cables point of view.

2) The 85A charger at 24V can handle ~2kW of modules. so each charger can run 2 parallel strings of 3 series modules. The two chargers are independently connected to the battery busbar.

3) the 8 batteries are connected in 2 strings of 4 to give 24V. See documentation on parallel wiring of battery strings - total cable lengths in each string wiring must be equal.

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Kevin Windrem avatar image Kevin Windrem commented ·
I'm not sure you can get 3 panels in series and avoid going over the 150 volt max for the MPPT.


Compute the Voc at MINIMUM temperature since voltage goes up as temperature goes down.
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Ken Rackowski avatar image
Ken Rackowski answered ·

Hi Mike,

Thanks for the reply. Still have lots of design details to work out but at least the system concept is ok. A few things I am still looking at include:

How to build a grounding grid (this is a small island with a thin sandy top cover)

Best way to power the 120v 16amp RO pump that runs to fill a 200 gallon fresh water tank. The inverter will carry the load, but might be better to just run the generator when pump is need. This would also charge the battery if run during no solar time.

Is there a distance limit on the wire between the MPPT's and the batteries, or can we just up-size the wire as the distance increases. Not sure if the NEC tables apply to the 24vdc circuit.


Ken

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Sarah avatar image
Sarah answered ·

Hi @KenR i can’t answer the grounding grid question but with regard to your other questions.

1. The water maker uses up to 2kw of power so provided there is sufficient solar you can run the water maker and charge the batteries at the same time.

2. For the solar controllers keep them as close to the batteries as possible. The PV arrays operate at a much higher voltage which allows for longer cable runs.

As an added point I would recommend a Cerbo and smart shunt to monitor the battery status and provide charge limiting (the total charging current solar + generator is likely to exceed the rated battery charge current). Also I would consider using the Lynx bus bars as there is the potential for high currents and you want to ensure all of your cable connections are appropriately fused.

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