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Pi Battery Trouble

Posted: Tue Feb 12, 2013 11:51 pm
by bobcool09
So I followed these directions and have made a battery. Unfortunately, now is where I run into the problem.
The battery is discharging at 16v, but only .35A! After my 5v linear regulator, it is 5v and still .35A of course.
So, what I am wondering, is how could I harvest that wasted energy.
If it matters, I'm charging the batteries not with the charger from the guide, but this one. Also, it's set to only .7A charging because the power supply I found is only 800mA, 12v.

Thank you!

Also, is there any way I could have a switch with 3 positions: charging, charging with pi on, discharging, and everything off?

I'm attempting to make my pi fully portable in the shell of an old laptop, and the gentleman ceteras has helped me with the screen

Re: Pi Battery Trouble

Posted: Wed Feb 13, 2013 3:21 am
by rpdom
bobcool09 wrote:So, what I am wondering, is how could I harvest that wasted energy.
Stick a mug of water on top of the regulator and make a cup of tea? :lol:

Seriously though, ditch the linear reg - those things are really inefficient - and get a switch-mode regulator. Search for UBEC and 5V on ebay and you should find loads that will convert from 5.5-26V to 5V with easily enough current capacity to run a Pi and a few peripherals, and over 80% efficiency (rather than the 30% or so you are getting on your linear on at those voltages). Input at 16V should be less than 0.15A and your batter pack will last a lot longer :)

Re: Pi Battery Trouble

Posted: Wed Feb 13, 2013 11:54 pm
by bobcool09
http://www.hobbyking.com/hobbyking/stor ... 3A_5v.html

Is that the sort of thing you are talking about?

Re: Pi Battery Trouble

Posted: Thu Feb 14, 2013 12:00 am
by mahjongg
thats a possibility all right.

Re: Pi Battery Trouble

Posted: Thu Feb 14, 2013 12:18 am
by bobcool09
And I measured the temperature of the regulator 10 seconds after I started it, and the temperature of it was 207° F (97.2° C). If I would have left it I might have been able to make a cup of tea on it! :D

Re: Pi Battery Trouble

Posted: Thu Feb 14, 2013 8:26 am
by rpdom
bobcool09 wrote:http://www.hobbyking.com/hobbyking/stor ... 3A_5v.html

Is that the sort of thing you are talking about?
Yes, exactly that :-)

Re: Pi Battery Trouble

Posted: Thu Feb 14, 2013 7:36 pm
by bobcool09
I solved the small current issue by moving to a better terminal block and tightening it down better, II am now getting 1.5A.
I am going to power a screen with this. The description says it needs a 12v 4a input, so I have been looking for a Ubec that is 12v and at least 4a. I found ones that can handle up to 4.5a, but output only 3a. Will I have to have 2 In parallel?

Re: Pi Battery Trouble

Posted: Wed Feb 20, 2013 1:28 am
by kaos
If the screen takes 48W (4A at 12V), you won't be able to power it from a battery that delivers max. 24W (1.5A at 16V), no matter what kind of regulator you are using. You will either have to change to a bigger battery pack or find a less power hungry screen. What kind of screen is that anyway? 48W is rather excessive for any kind of screen I could imagine running on battery power.

A few additional points to keep in mind:
- Don't connect regulators in parallel; no two will have precisely the same output level, which means that only one of them will end up driving the load, with the other possibly even (depending on the design of the regulator in question) just providing an additional load.
- Switch mode regulators capable of handling >4A certainly exist. I don't know what is out there of ready-made modules, but LM2678 to name one, is a switch mode controller capable of handling 5A. Note that this kind of controller requires external components to make a complete regulator circuit (see datasheet).
- For relatively low voltage drops, like 16V to 12V, linear regulators may be worth looking at again; 16V to 12V would mean 75% power efficiency; bad by switch-mode standards but not outrageous.
- On the other hand, with high currents, each percent of lost efficiency means considerable power waste in absolute terms.
- Remember that all power "lost" will wind up as waste heat, meaning that inefficient and/or high power circuits will require a heatsink, in extreme cases a quite large one. Allowing silicon components to reach tea-making temperatures is not recommended ;) .

--
Best regards, Kári.