DC to DC Converter

Hi,

I use one of these:

http://www.ebay.com/sch/i.html?_sacat=0&_nkw=lm2596&rt=nc&LH_BIN=1

I currently use a 30cm cable made from normal wiring cables. These are MUCH thicker than what you normally find in micro USB cables. I'm currently using a micro USB plug that I bought at farnell for this. But next time I'm going to plug in on the GPIO connector and just provide 5V unfused.

I've adjusted the module to provide 5.01V unloaded. Now under load it provides 4.99V. Then I have a 0.05V drop in my cable, so I end up with 4.94V on my 'pi. Next the fuse drops 0.15V so the rest of the 'pi only gets 4.79V.

If you only power your 'pi, the LM2596 will need only 1V of "dropout". So 6V at the input will be enough. If you're powering other stuff as well, at 2 or 3A, the LM2596 will require up to two volts of headroom, or at least 7V of input.

So... powering the 'pi from two lithium cells should be possible. Make sure you cut off by the time the voltage drops to 6V. Three cells gives you more margin, but probably slightly more losses.

4 NiMH cells is cutting it too close. Use 6. 12V lead acid should work too.

Above some people are saying things about ventilation of the lead-acid batteries. IMHO, if you treat them well, they won't vent. That's the thing about sealed lead-acid batteries: they don't vent. The trick is that a catalist in the battery lets the oxigen and hydrogen recombine producing heat and not an explosion. The "treat them well" means you don't charge anywhere NEAR the maximum conversion capacity of the catalyst. The "treat them well" also means you don't apply enough power to significantly heat up the battery when the catalist is recombining the oxigen and hydrogen to produce water and heat.

So: Sealed lead acid batteries are nice. Stick to the charging rules and they have a long lifetime and do not pose an explosion danger.

The day before you asked this question we started designing our "pipower" module. This module takes 5V input (e.g. from a LM2596 module as linked) and allows software to decide when to power the 'pi. For example the pi might "ask": "Wake me up in an hour". So the 'pi might wake up, find there is not much too do and go to sleep again (i.e. powerdown).

Configuration 1:

You get 5V from somewhere. The module powers the 'pi whenever it feels like it. In the case of the LM2596 module, that will cost you the 7.5mA "no load current" as a battery drain, limiting the "low duty cycle" stamina of the battery powered pi system.

Configuration 2:

You power the module separately. The pipower module switches the 12V to the LM2596 module. A simple non-switching regulator is used to power the pipower module. With a bit of luck the linear regulator providing power to our AVR processor on the pipower module will use less than the 7.5mA that the LM2596 uses in the no-load situation.

Your post has made me realize we haven't designed a battery power monitor yet (i.e. measure the battery, cut off the 'pi when it gets too low), nor a "power up now" button. Those will be included.

Also the software will be able to "watchdog" functions. We can powercycle the 'pi if the raspberry doesn't contact us within an agreed-upon timespan.

Do you have any other requirements? If the current design is not able to do what YOU want, it is not good enough. I want to make it "good enough". :-)

Andrew if you buy a rpi_serial board and one of the expansion boards, I'll throw in a beta "pipower" board for the field-test. :-)

Hi,

I use one of these:

http://www.ebay.com/sch/i.html?_sacat=0&_nkw=lm2596&rt=nc&LH_BIN=1

I currently use a 30cm cable made from normal wiring cables. These are MUCH thicker than what you normally find in micro USB cables. I'm currently using a micro USB plug that I bought at farnell for this. But next time I'm going to plug in on the GPIO connector and just provide 5V unfused.

I've adjusted the module to provide 5.01V unloaded. Now under load it provides 4.99V. Then I have a 0.05V drop in my cable, so I end up with 4.94V on my 'pi. Next the fuse drops 0.15V so the rest of the 'pi only gets 4.79V.

If you only power your 'pi, the LM2596 will need only 1V of "dropout". So 6V at the input will be enough. If you're powering other stuff as well, at 2 or 3A, the LM2596 will require up to two volts of headroom, or at least 7V of input.

So... powering the 'pi from two lithium cells should be possible. Make sure you cut off by the time the voltage drops to 6V. Three cells gives you more margin, but probably slightly more losses.

4 NiMH cells is cutting it too close. Use 6. 12V lead acid should work too.

Above some people are saying things about ventilation of the lead-acid batteries. IMHO, if you treat them well, they won't vent. That's the thing about sealed lead-acid batteries: they don't vent. The trick is that a catalist in the battery lets the oxigen and hydrogen recombine producing heat and not an explosion. The "treat them well" means you don't charge anywhere NEAR the maximum conversion capacity of the catalyst. The "treat them well" also means you don't apply enough power to significantly heat up the battery when the catalist is recombining the oxigen and hydrogen to produce water and heat.

So: Sealed lead acid batteries are nice. Stick to the charging rules and they have a long lifetime and do not pose an explosion danger.

The day before you asked this question we started designing our "pipower" module. This module takes 5V input (e.g. from a LM2596 module as linked) and allows software to decide when to power the 'pi. For example the pi might "ask": "Wake me up in an hour". So the 'pi might wake up, find there is not much too do and go to sleep again (i.e. powerdown).

Configuration 1:

You get 5V from somewhere. The module powers the 'pi whenever it feels like it. In the case of the LM2596 module, that will cost you the 7.5mA "no load current" as a battery drain, limiting the "low duty cycle" stamina of the battery powered pi system.

Configuration 2:

You power the module separately. The pipower module switches the 12V to the LM2596 module. A simple non-switching regulator is used to power the pipower module. With a bit of luck the linear regulator providing power to our AVR processor on the pipower module will use less than the 7.5mA that the LM2596 uses in the no-load situation.

Your post has made me realize we haven't designed a battery power monitor yet (i.e. measure the battery, cut off the 'pi when it gets too low), nor a "power up now" button. Those will be included.

Also the software will be able to "watchdog" functions. We can powercycle the 'pi if the raspberry doesn't contact us within an agreed-upon timespan.

Do you have any other requirements? If the current design is not able to do what YOU want, it is not good enough. I want to make it "good enough". :-)

Andrew if you buy a rpi_serial board and one of the expansion boards, I'll throw in a beta "pipower" board for the field-test. :-)