Cobbled together USB charger

Re: Cobbled together USB charger

peakbagger wrote: »

In my collection of solar left overs is a 30 Watt 12 volt panel (along with some 48 watts Siemens panels). While at a recent trade show, I got a free 12 volt USB adaptor for a cigarette lighter plug. I am curious, could I simply wire the USB charger with a fuse to the panel leads and have a cheap charger?

Maybe. Typical "car" voltages are 9-16V and most USB chargers are designed to handle that range of voltages. 12V panels typically put out over 18 volts so you risk damaging it. However, if your USB charger can survive that voltage without damage/overheating and you add a capacitor to ride through peak draw it might work.

Re: Cobbled together USB charger

zoneblue wrote: »

Take a gander at the input voltage spec. With the proliferation of SUV type vehicles, many if not most usb car gadgets now are rated for both 12 and 24v input. In which case you are pretty much guaranteed a workable solution. In day light hours anyhow!

99.99% of vehicles on the road (including SUV type vehicles) use 12 volts for accessory power. Aircraft, large trucks (i.e tractor-trailers) and some military vehicles use 24 volts (or even odder voltages) and the 42 volt standard is appearing in very few vehicles. And in vehicles like EV's or hybrids the traction battery operates at voltages from 200 to 500 volts. But even in these latter vehicles, the cigarette lighter port is 12 volts.

Unfortunately the input voltage rating is often not given in the device's data sheet. In such cases 9-16 volts is a reasonable assumption, although it is a little risky because of potential heating at the upper limits of voltage.

Re: Cobbled together USB charger

I know that most recent Radio Shack 5V usb adapters have ratings of 12-24v if you check their specs. Sometimes you may see it on the web site, or some may only be on the packaging. I've used a variety of them with my 20 watt powerfilm folding panel with no problem attached to just the cigarette lighter adapter. Since the panel only puts out a maximum of 19-20v, no problem.

Also check the input current rating - for solar you'll probably want one that can accept up to 1A input or more, rather than the older style that only accepts 0.5A input. Usually the 1A input (at 12v+) adapters provide at least 1A output or more (at 5V) as well.

Most of the usb 5V adapters these days are of the switching regulator type, and not the 3-pin linear regulator, which would require a pretty massive heatsink regulating 18v down to 5v. The switching types are more efficient, and obviously smaller as in the flush-mount types that can supply up to 2A of output current sometimes. Some of the more quality ones may also include internal overcurrent and overtemp protection.

I haven't tested the dropout-voltage of the RS usb adapters - that is how low the supply voltage can get before they stop regulating, and the 5V output starts to drop too. I don't drop my agm's below 12v anyway, so no problem - but something to keep an eye on if your supply battery drops below 12v and now your device is no longer charging.

Remember to have the panels fully illuminated before attaching the device to the adapter! A slow introduction of current from shadows or standing in front of the panels while hooking things up may fool the device into going into a protection mode, or negotiating a slow default current rate when it is capable of much more.

Quality varies, so remember that you may be hooking up 2 cents worth of parts and engineering to several hundred dollars of phone / device. You pay your money and take your chances.

Re: Cobbled together USB charger

This brings up an excellent point when it comes to they typical camping / folding panel with built in usb 5v port:

At what voltage is the manufacturer basing/marketing their panel power at? Use PIE (P-watts over I-current times E-voltage) formula.

This is just like my 20 watt powerfilm with RS usb adapter setup. If we do the math, then at the unregulated solar output we have 20w/18v = 1.1A available. But now, let's do it at 5V output (assuming a lossless regulator) 20w / 5V = 4A available for the 5v output! BUT, my little usb adapter is not capable of that, and is limited to 1A maximum due to the choice of internal regulator. At the end of the day, my 20W panel is acting like a 5W panel (5W / 5V = 1A) because of the regulator's limitation.

So just do the math at the appropriate voltage levels, and be aware that you may be artificially limited by the hardware itself. I've had some friends go absolutely nuts when trying to learn solar and doing the P/I*E formula and coming out all wrong in their estimations due to this. They may also forget to do the math at the 12v level, and find that hanging multiple usb adapters off a panel simply cannot be done because they did not stick to wattage (P) as the common denominator and the panel was severely underpowered.

For example: If I were to hang four of those 1A usb 5V adapters off a panel, how big of a panel do I need at minimum to accomplish this?

Total current drawn - 4adapters x 1A = 4A needed.
Voltage? 5 volts

4A * 5V = 20 watts.

So I would need at minimum at 20 watt rated panel to support 4 simultaneous 1A usb power adapters. Due to losses, the panel should be even larger at say 30 watts or so.