Hi all,
A few years ago I had an electrician set up a Multiplus II and 4x12V 120Ah batteries in a 24V configuration for a “tiny home on wheels”. I usually use a caravan park mains supply and I have a small 2000W generator if needed.
There is not a lot of drain on the batteries: lights are DC and fridge and freezer are AC running through the inverter. After 24 hours of running fridge and freezer, gas-fired califont for hot water, charging a laptop (all on AC) and lights on DC, the batteries were at 25.3V, irregular monitoring showed current draw of 2.1 - 3.6A. Time period was 6pm to 6pm.
I am now ready to add solar and would like to stay with Victron products. Where do I start?
Thanks in advance,
Stephen
PS. I have VictronConnect running through a dongle for monitoring.
Have a look here, try the sizing calculator. You need an MPPT controller, I recommend one of the Smart ones to communicate with VictronConnect on your phone. This will charge the batteries.
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Thanks for this, it’s a new world for me…
In another thread, it was mentioned that the solar system should deliver 20-25% of battery capacity so I figure I need 1200-1500W of solar.
Using the sizing calculator, I can get this with 3 x 175W 12V panels and the MPPT 75/15. I only need the solar during the summer months - I leave the country (New Zealand) for winter. Does that look right?
The 175W panels are sold out here so the sizing calculator tells me that 4 x 115W panels (2 in series, 2 strings) with the same MPPT will do the job.
Is it really that simple?
What you need is more to do with how much you use daily rather than the size of battery bank, and on boats, caravans and RVs the roof area. My boat is 700Ah of batteries at 12V and I have 700W of panels and typically use 100-150Ah (at 12V) daily and the 700W mostly covers this during spring and autumn in the UK and during summer I have excess solar.
Thanks for this.
Roof space is not a problem, the “RV” is a semi-trailer so I have (outside measurements) 11m x 2.5m of flat roof. A relative luxury.
Do you recommend a way of determining daily use? And, technically, what happens if the solar provides more power than the batteries need? Does the MPPT just let it “evaporate”? How closely do the daily needs and solar input need to match?
A solar panel has a current & power curve vs voltage as below.
When power is not required, the MPPT controller forces the panel voltage higher towards Voc where the current and power fall to zero.
To know how much you use, look back at previous trips and see how low the SOC falls between generator runs if you are off grid. Otherwise do a power audit, look at how long you run each piece of equipment and its current rating and add them up.
e.g Microwave, uses 1600W input, at 24V this is 67A, allow 10% eficiency loss in the inverter = 73A. Runs for 15 minutes daily, so 0.25hr, 73 x 0.25 = 18Ah. Lights, 50W of LEDs, = 2A at 24V, on for 4 hours so 2 x 4 = 8Ah. The real eater of power is a fridge. This could use 2A (50W) on 24hrs/day, but actually only running 50% of the time when the thermostat needs it to, so 12 hours run time. 2 x 12 = 24Ah. Same for a freezer. Ad it all up, water pumps, TV, laptop use etc.
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I did a power audit and it came to 70Ah per day on 24V so about 1700W per day. Yes, the fridge and freezer are about 60% of that. Pleasantly surprised that this corresponds to a little under 3 amps, about the mid-point of my irregular current monitoring over a 24 hour period.
I presume I now use the sizing calculator to find a combination of panels and controller that give me 1700W during Spring and Autumn, more in Summer can “disappear” and I won’t be using anything during Winter, i.e. the solar will just be keeping the batteries topped-up.
Added later: the 24 hour monitoring I did had the batteries at 25.3V which I think corresponds to an SOC of about 90%. Is the purpose of the solar panels to cover all load so that the batteries do not significantly discharge, i.e. the batteries remain at near-full SOC?
That is about my use so it seems to be a reasonable result. Use a solar predictor for your location with flat mounted panels to get the required production.
