A Ravpower USB iSmart battery doesn’t like low loads

I have one of these Ravpower iSmart USB batteries, and it works a treat when used as the manufacturer intended  – to power a mobile phone or an iPod (4th gen touch in my case). No complaints whatsoever.

I constructed a remote GPS module with MAX232 RS232 chip, and all this wants to run off 5V  – the MAX232 is specced at 4.5V to 5.5V, the GPS is probably more tolerant. So the obvious thing to do is to cut off a USB cable, use the USB A plug and wire the power to my device from this. No need for a regulator, job done, and indeed the GPS fires up. Dandy. No need for 5V regulators, no need to mess about with undervolt cutoff, 5V power straight out of the box, what’s not to like?

An intelligently managed battery

The USB battery gives me a USB chargeable device and integrated power management, you can’t overcharge these or run them flat, and as someone who has just trashed a LiPo battery by leaving it connected overnight and flattening it, I appreciate that thought. Until I find out that

iSmart is too darn smart

and decides my device isn’t drawing enough power and pulls the plug after a couple of minutes. Damn. My GPS draws a hefty 50-60mA, depending on whether the unbelievably bright LED the Chinese makers decided to fit is on or not.

a good 60mA current draw is not enough for iSmart, it seems

a good 60mA current draw is not enough for iSmart, it seems

so we aren’t talking about some micropower application. There is much discussion and complexity associated with providing a smart-ish but dumb (in not having a microcontroller) USB charging socket, which involves connecting resistors to the data lines of the output socket, but there’s not so much on what do I have to look like to draw power from a smart battery. Maxim have quite a bit more on what a charger looks for, and I tried their trick of putting a couple of 27k (must be >24k) resistors to ground, with no success.

So it’s probably back to the simplicity of four NiMH AAs and a 500mA fuse. At 1800mAh I’m good for 30 hours runtime. Which stinks for a general purpose GPS but is good enough for me.

ISmart is well suited for a Raspberry Pi

The new Raspberry Pis (Raspberry Pi 2 Model B+ v1.1 in my case) with the new switching regulators drop power significantly when you do sudo halt to power down. This works a treat with the Ravpower which pulls the plug entirely and depowers everything, includig the static LED, all you need to do to restart would be to bump the power supply – a NC relay in the 5V line pulsed for half a second would restart it and only draw relay power for half a second. I’ve only tested this by pulling out the USB connector and plugging it back in, but earlier models of the Pi at least don’t have anything on the data pins of the power socket, these are floating. There are no obvious switches in the Ravpower USB sockets to reset the outputs.

The Pi2 ModelB+ draws 230mA and up, which is enough to keep the iSmart outputs active, it seems.

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