In this article, David answers a question about how to correctly connect a portable solar panel to an existing battery management system.
Hi David,
I recently read the article in June’s Caravan World (and online here) regarding extra solar panel connections. I have two 170W panels on the roof of my van thus totalling 28A connected to a BMPRO BatteryPlus35-II-HA allowing a total of 35A input.
I have purchased an iTechworld 100W solar blanket therefor an additional 8.3A. I have an Anderson plug connect to the additional solar connection terminals on the BMPRO. I have the solar blanket connected directly without the using the MPPT regulator thinking that the extra solar connectors on the BatteryPlus does the same thing. Is this the case? If not, how do I connect the extra solar?
Cheers,
Mal Flew
Hi Mal,
Thanks for your question, as this highlights a common scenario faced by many RV owners looking to boost their off-grid power capacity: how to correctly connect a portable solar blanket to an existing BMPRO BatteryPlus35-II-HA system.
Effectively there are two options.
The first — as you rightly point out — is to use the secondary solar inputs. BMPRO recommends that any solar installations exceeding 350W panels, including any portable panels which are likely to be installed, should use both solar input connection points to ensure adequate electrical connection at the solar input.
Using the two fixed 170W solar panels on the roof of the van and a 100W portable panel will provide a maximum of 450W in ideal conditions (more likely is 70–80 per cent of this 450W — so, around 340W). The best way to do this is to parallel the two outputs on the BMPRO unit and then parallel wire to the panels.
The second option is to run your portable panels through a small MPPT regulator and go directly to the battery. This would require an additional dedicated BMPRO shunt designed to specifically monitor the battery current direct to the battery and feed this data back into the system.
The BMPRO BatteryPlus35-II-HA includes a built-in MPPT solar regulator capable of accepting a total solar input of up to 35A, so adding a solar blanket that contributes 8.3A is perfectly within the system’s capacity.
A very important warning: The solar panel negative output (0V) is not a common ground and cannot be grounded if connected to the BatteryPlus35-II. Grounding the negative output can result in damage to the BatteryPlus35-II and batteries. Many caravans ‘earth’ their 12V system to the caravan chassis, however that should only ever be for a load not a source such as solar. Both positive and negative must be wired directly from the Anderson connector to the BatteryPlus35 without grounding on the negative.
Another key to successful integration is connecting the blanket directly to the BMPRO’s unregulated solar input, not directly to a battery or with a separate external solar regulator. This is because the BatteryPlus35 already has a high-quality MPPT regulator onboard. Feeding it pre-regulated solar power can confuse the system, reduce efficiency and potentially limit charging performance.
There’s no need for a manual selector or switching between the two — as long as both are unregulated inputs, the BMPRO will take care of the rest.
Using an Anderson-style connector is a great choice. These are reliable, robust and well-suited for caravan environments where vibration, dust, and weather are common.
When wiring the system, ensure:
- Correct cable sizing: Use at least 6mm² twin-core automotive cable (or larger, depending on run length — most setups are 8 B&S) to minimise voltage drop and protection.
- Quality terminations: Avoid flimsy or temporary connections. All joins should be crimped and, ideally, heat-shrunk or sealed against corrosion.
- Secure mounting: A loose connection increases electrical resistance, which not only reduces charging efficiency but can generate heat and lead to longer-term issues.
While solar blankets typically aren’t as efficient per watt as fixed rooftop panels, they do have a major advantage: portability.
With a solar blanket, you can position it wherever the sun is shining best. This is especially useful in shaded campsites or during early morning and late afternoon when the angle of the sun is less optimal for roof panels. You can even move the blanket throughout the day to track the sun and maximise exposure — something roof panels simply can’t do.
Additionally, when parked under trees or awnings, a solar blanket allows you to make use of that clear patch of sunlight a few metres away, keeping your battery topped up while enjoying a cooler campsite.
Your approach is spot-on. Connect the solar blanket via an Anderson plug directly to the BMPRO’s additional solar input terminals, without using a separate MPPT controller. It’s a straightforward upgrade that offers flexibility, redundancy, and peace of mind when travelling off-grid.
Safe travels and sunny skies,
12V Guru
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12V Guru: Adding portable solar panel to an existing BMS