Power Bank Shootout – Brunton Resync vs Voltaic V39
Assume the sun is not shining on you. You cannot afford to wait-out the bad weather in the tent. You must go on. The solar kit is in the backpack and the solar panel is large enough to keep you supplied. Now the sun is gone for days at a time. The power bank will save you and keep the GPS and camera running. We have established that the battery should hold over 30 Wh to be worthwhile. The Voltaic V39 and the Brunton Resync are in that range.
On a hiking trip, we care about weight above all else. For a power bank, it is the energy in Watt hours per kilogram that counts. The highest specific energy content (290 Wh/kg) is in primary lithium cells. These are not rechargeable but are still an alternative worth considering and these cells would score 10 out of 10. The Voltaic scores a 5 with a little under half the energy density. The Brunton gets a 4.
Charging Efficiency
This is a very sorry story. Both batteries are marketed for solar applications and have a 12 V input for the panel. None advertises maximum power point tracking (MPP tracking), the gold standard for solar power conversion. When asked about MPP Voltaic replied that the limited gains are not worth the effort. This is probably true, when charging circuit and solar panel are reasonably matched. Sunload speaks about gains of 20% for their MPP battery.
Matters are worse. The PowerFilm F15/600 under full sun is within 80% of MPP between 8 V and 16 V. Under 50% sun this changes to 9 V – 15 V and 6 V – 11 V in the shade (5%) . An input circuit that keeps the voltage between 9 V and 11 V would achieve most of the benefits of MPP tracking and be simple to implement. Instead, Brunton and Voltaic have opted to set the voltage at 4 V – 6 V, thus wasting most of the energy the panel could supply. The batteries score 3 and 4 out of 10 for efficiency.
Message to Brunton and Voltaic
The poor design of the input circuits in the batteries drives up the cost and weight of other system components. The solar panel must be twice as big, costing an extra 75 – 100 Euro and adding a 150 – 200 g weight penalty. Cost and weight is almost the same as the power banks from Brunton and Voltaic. If the objective is a fashion accessory with an environmental friendly solar touch, raise the price and do nothing. If you want to deliver serious gear for the outdoors, go back to the drawing board.
Ruggedness
The Resync has rubberized corners and looks like it will survive a drop on a rock. This is the only but critical feature missing from the V39. Both have means to cover the USB ports but leave the 12 V input open. Both have a button to activate battery and status display that is not protected in a meaningful way when the batteries are in a backpack. The battery may therefore discharge while it is stuffed in the bag. Overall the mechanical design of the housing is well thought out. The batteries may designed as fashion accessories after all. The Resync gets a 9 because of the exposed 12 V port and switch. The V39 gets a 5 because it will be dropped when the solar panel is set up on a rock.
Connectors and Interface
Voltaic has two USB ports on the V39 as outputs and a 12 V input that will accept many commonly used plugs. The Resync has a cable attached to the device, includes an adapter in a small compartment at the back and can thus charge from a standard USB port and deliver output to mini or micro USB without additional accessories. The 12 V input connector of the Resync is less commonly used than the 5,5 mm * 2.1 mm on the V39.
Both can charge an iPad and work with all USB devices that were tested. They also include a cigarette lighter adapter (CLA) that can be used in the barrel that comes with the solar panel. I recommend eliminating the CLA and barrel. They add unnecessary weight and bulk.
Both batteries have well designed connectors and interfaces. The cable solution of the Resync has slightly more value than the additional USB port on the V39. 7 for the V39 and 8 for the Resync.
Street Price
The Resync has come to market only recently and prices may yet drop. Currently it costs 50% more than the V39. Availability in Europe is limited. Voltaic seems to have chosen to not market the V39 separately in Europe. The pricing of the V39 is in line with the pricing of Lithium rechargeable at 4 Euro/Wh. Scores: 7 of 10 for Voltaic and 4 of 10 for Brunton.
Conclusion
In the end, this is a disappointing draw with 5.0 for the V39 and 4.9 for the Resync. You are paying for the extra rubber on the Resync with money and weight. No surprise here. The disappointment is with the poor design of the electronics in both devices. Form does seem more important than function to the designers.
The V39 did exhibit strange behavior when it was first charged. It did heat up close to the 2.1 A output and make a clicking sound. Voltaic did offer to take it back, but time was too short before the trip. It never had the issue since. That is another minus for he electronics and a plus for customer service and did not count towards the final result.
I have used the V39 on the last trip without issues and will try the Resync next time. Both are ok to use.
Wish List
Sunload makes a nice little charger with battery and does MPP tracking too. If only they would make something in between the M5 and M60. We would have the perfect solution. The former is too small to be useful beyond a one day buffer for a phone. The latter is too heavy. PowerFilm Solar could also quadruple the size of the USB+AA solar panel and make it viable as a one stop solution. Goal Zero also has stuff. It seems underpowered (Guide 10 Adventure Kit) or too expensive and heavy for the power it will yield. Brunton and Voltaic could also improve their input electronics to better match their specified input voltage of 12 V. Until then we will waste half the energy and carry too much weight.
Roland von Bremen 