Direct charge, grid tie or battery bank?

What is the best use of your pedaling time on a pedal generator? There are 3 typical avenues you could direct your watts:

  1. Direct charge: Directly use the watts coming out of your pedal generator, once converted to useful 12 volt current, and charge your devices using car chargers.
  2. Grid Tie: Use a grid tie inverter to send all your pedaling effort into the nearest wall outlet.
  3. Battery bank: Charge a bank of lead acid batteries.

There are advantages and disadvantages to each of these.

Method Pros Cons
Direct charging Efficient – 90%+ charging efficiency. You will need to round up enough gadgets to create the appropriate resistance for a good workout.
Grid Tie Directly offsets your utility bill with watts generated. Grid tie inverters are about 80% efficient in practice.  Put in 100Wh from pedaling, only about 80Wh will be put back into the grid.
Battery bank For those living off grid, augmenting their battery bank may be a necessity. Lead acid battery charge efficiency is only about 85% – so if you put in 100Wh, you only get out 85Wh.  Couple this with the efficiency losses of a charge controller and an inverter to get it back out as AC, and you might get 50% of what you put in.

I advocate for direct charging, and I’ll explain why.  Going out on a limb here, but I’ll assume everyone uses a mobile device or two, and maybe a tablet. Assuming you charge these things regularly, let’s look at the scenario of charging these using the methods above.

Direct charging:

Pedal generator -> Charge controller (90%+ efficient) -> 12v car charger  -> device(s)

Grid Tie:

Pedal generator -> Grid tie inverter (80% efficient) -> 110VAC wall charger -> device(s)

Battery Bank:

Pedal generator -> Charge controller (90% efficient) -> Battery bank (85% efficient) -> DC to AC inverter (90% efficient) -> 110VAC wall charger -> Devices(s)

As I discovered in the previous blog post, wall chargers have losses up to 30%+, whereas 2 out of the 3 car chargers tested had losses of just over 1%. Using either grid tie or a battery bank may result in 50% or more of your efforts wasted on conversion losses.  I do want to point out that the charge controller used in my pedal charger design is 90%+ efficient, so you will lose up to 10% in that conversion, resulting in a max of about 12% loss if you include the car charger losses – but far better than the other two alternatives!

Wall Charger Verses 12v Car Charger Efficiency

Have you ever wondered which kind of charger is more efficient?  Are wall chargers we use to charge our devices just as efficient as 12v car chargers?  In this blog post, you’ll find out!

I grabbed 3 quality wall chargers and 3 quality car chargers with the intent to see how many watts each consumed, and how many they actually put into an Apple iPad Mini.  To find these measurements, I used a wall AC meter, a DC power meter, a USB dongle meter and an Anker 1 ft Powerline cable.  Here are the results.

Charger Watts In Watts Out Percent Loss
Apple 5 watt 6.6 watts 4.6 watts 30.3%
Apple 12 watt 11.6 watts 9.2 watts 20.7%
RAVPower iSmart 11.0 watts 9.1 watts 17.3%
PowerGen 20w car 8.9 watts 8.8 watts 1.1%
Anker 42w car 10.7 watts 9.9 watts 7.5%
Anker 24w car 8.2 watts 8.1 watts 1.2%

Wow!  I didn’t expect to see such a difference in efficiency between the wall chargers and the car chargers.  After a little digging, it makes sense – the wall chargers need to convert AC (alternating current) into DC (direct current), whereas the car chargers are just changing 12 volt direct current into 5 volt direct current that is the standard for USB.  Even the worst car charger was 10% more efficient than the best wall charger – crazy, right?

I suspected AC wall chargers were less efficient from my experience using my pedal generator.  I could see the wattage used by the wall chargers put more of a load than the DC car chargers, but it was great to quantify the real difference.  I had no idea how much more inefficient wall chargers were!  I will definitely be using car chargers whenever possible for the most efficient charging of my devices.  On the other hand, if I need more of a challenge on my pedal generator, I can always substitute in the AC chargers!

If you’re living off grid, you may find this information very helpful in deciding how you should charge your mobile devices.  Directly charging with solar or a pedal generator would be the most efficient way to go, rather than dumping all power to a battery bank, then converting to AC and charging with a wall wart (thoughts of all the efficiency losses in this cycle is making my head spin!).