Bests & Worsts Reviews from Amazon

When I first received this product (Opteka BP-SC4000 Ultra Thin Solar Powered High Capacity Backup Battery and Charger), I was surprised by its small size and light weight. That makes me suspicious about its claim of '4000mAh' battery capacity. However, after thoroughly testing the unit, I'm convinced that the capacity rating is accurate. Here's how I conducted my test: First, I make sure that the internal battery is fully charged from external USB adapter. Next, using the provided USB cable and plugs, I connected a 50W 5-ohm load resistor to the USB output. The load current I measured is 0.98A, and the output voltage is regulated at 4.85V. This gives an output power of 4.75W. I then timed how long it took for the converter to shut off. The last status indicator bar went out shortly after 158 minutes. How does this runtime relate to the battery capacity? The 5V output has to be boost-converted from the internal 3.7V battery. Assuming a typical conversion efficiency of 85%, the internal battery has to supply at least 1.51A (3.7V*1.51A*0.85 = 4.75W). A 4000mAh battery should then last for 4000mAh/1510mA = 2.65 hours, or 159 minutes. So my test above proved that the battery capacity rating of '4000mAh' is right-on. However, in the process of this test, I also discovered a confusing behavior about the battery status bars... There are four status bars for the battery indicator, with 4 bars being full and zero being empty. Naturally, people will assume that each status bar represent 25% of the capacity left. This is NOT the case. The status indicator is actually a non-linear representation of the battery capacity. Here is what I figured after repeating my experiment (all percentage numbers are approximate): 4 bars = 87 - 100% full 3 bars = 73 - 86% left 2 bars = 58 - 72% left 1 bar = 1 - 57% left 0 bar = empty For example, when the status indicator drops from 2 bars to 1 bar, people will assume that there's less than 25% of energy left. But in fact the battery is still more than half full. So people will under-estimate the capacity of this battery pack during discharge. On the other hand, after I recharge the internal battery from empty at 1A for 1.5 hour, the status shows 3 bars, even though the battery cannot be more than 40% charged. So in this case people will over-estimate the fullness of the battery, only to be disappointed later since it does not provide 75% runtime as expected. As a solar charger, the Opteka BP-SC4000 suffers from the same problem as other similar products: it takes days to recharge using solar power. The maximum charging current is only 250mA under optimal condition, so to recharge a 4000mAh battery it will take 16 hours of peak sunlight. In real life this means at least 3 sunny days. Previously I evaluated another far more expensive product, the Soladec Hybrid All-in-One Portable Solar Power Charger (SD4000LP). Both claim the same 4000mAh battery capacity. But this Opteka unit offers several advantages: - It can supply at least 1A of output current continuously, while the Soladec maxed out at just 800mA. - It has an on/off switch to conserve power while not in use. The Soladec drains 3mA from its battery all the time. - It comes with an USB cable plus 10 adapters for charging various devices. The Soladec only comes with an USB-to-microUSB cable. - It's cost is just 1/3 that of the Soladec Bottom Line: The Opteka BP-SC4000 performs exactly as advertised, which (sadly) is a rarity nowadays. The only drawback is its non-linear battery status indicator, which can cause confusion for the user. Overall, this is still a great product, especially considering its low price. [Update on Dec 3, 2012] During my summer vacation trip, the SC4000 proved to be a life-saver. On many occasions, I have used it to rescue other battery-powered appliances such as camera, cell phone, GPS and media player. For that I'm satisfied with this product. [Update on Dec 7, 2012] I just noticed there is another similar product sold here at Amazon: EZOPower Universal USB Solar External Rechargeable Backup Battery - 3000mAh It looks identical to the SC4000 except for battery capacity. So I suspect some customers may have received this lower capacity unit instead of the SC4000. Use caution when you shop.