SparkFun Forums

[ChriX]

I'm looking to build a GPS speed meter as small as possible, but am having trouble deciding on a power source. I've got a tiny 2x8 character HD44780 LCD, the EM406 GPS on the way, and will be using a 16 pin PIC of some kind.

Does anyone have any ideas for a suitable power source (battery) for these components, in keeping with the current scale - the GPS is 30mm square, and the LCD is 30x40mm. I don't want something that is going to dwarf all the current components as that means I may as well have gone with a bigger LCD.

[rod_vdb]

What is your power requirement ie how many mA do all the components draw? What is peak power what is quiecient? will the power draw be large current pulses or will it be constant very low draw?
Do you want rechargable or primary? How long do you want it to last?

Chosing batteries is almost always left till last but it should not be!

[ChriX]

From the GPS module datasheet it says 70mA, doesn't say at what voltage, I guess 5.5V as thats the middle of the supply range. The LCD datasheet states 3mA max at 5V. Not sure about the PIC as I suppose it depends on what it's doing at the time. Obviously it would be a better idea to wait until all the components arrive and actually measure the current, but this gives some idea.

I wanted rechargable, and ~4hrs would be a minimum run time.

[ohararp]

lcd with a backlight can have some major power consumption. I know that a Crystal fontz GLCD 128x64 can suck nearly 400 ma for the backlight alone.

If you run li-ion you will have to step voltage up to 5V from the batteries 3.6V.

[ChriX]

I'm not using a backlight for that very reason.

[SOI_Sentinel]

For you, I'd suggest placing a momentary on button somewhere recessed a bit so you could light it up to read the display if you had to. No extra logic, and a push every so often shouldn't hurt run times too badly.

Now, rough numbers

70mA GPS
3mA display
30mA PIC (Overestimating here. I've measured a PIC18 running at 40MHz with a few LEDs and I was getting 20mA)

So... let's just call it 130mA with some minor losses for other components.

I'm going to assume you're looking at a single LION battery for the moment.

Toss in a buck/boost regulator, an example is Microchip's MCP1252, 60% efficiency (low end, improves as voltage drops actually). My estimate is actually a bit high for this part, but this is an example and I know this data sheet fairly well. So... 4.2V fairly new LION vs 5V 130mA load. Math for 60% efficiency results in a draw of 257mA from the LION. at 3V (dead), we're seeing 85% efficiency and 254mA. Oddly linear looking.

Anyway, for 4 hours of run time, you'd need about a 1000maH battery set here. I'd recommend at least 1200, especially if you go with LION. They don't like to be drained dry at all. Other power systems may give you better efficiency. At this rate, 3 NIMH AAA's may be an option. Maha PowerEx line has 900maH batteries, for instance. They are a bit low for your needs, but a different power system may be more efficient. Three 2500maH AA's would provide over 8 hours of run time. Not too shabby. There's also enough extra in there to permit some backlight action, if your backlight is somewhat voltage tolerant and could be connected directly to the batteries and bypass the digital power system.

[phalanx]

Maha PowerEx line...

Excellent batteries. I have a few sets of the AAA and several sets of AA Mahas. They take a lot of abuse and can deliver very good run times on a lot of equipment. Maha also makes great battery chargers too.

-Bill

[ohararp]

SOI_Sentinel

I am looking at doing something very similar here as well. I did have question for you concerning how to prevent draining a li-ion beyond 3V. Is there a way to cut power off? The current sparkfun data logger could have some issues if you were to use li-ion batttery since the power led is always on? Any insight would be appreciated, especially since I want to take my "geek" toys outside now that the weather is finally getting nice!

[SOI_Sentinel]

Yes, there is! Pretty spiffy too. This particluar part is from TI, samples available.

http://focus.ti.com/docs/prod/folders/p ... 952-1.html

I don't think you'll have issues with the 3A max current.

It also provides basic over-voltage protection, too.

TI also has parts for multi-cell packs. Looks like you'd need a multi-cell pack to toss on a battery fuel gauge IC.

[ohararp]

Not sure is you'd know this or not, but appears there are a ton of li-ion batteries for cell phones out there for really cheap (on ebay at least). No one can be sure but is it common for these batteries to include these protection circuits into the battery and the hard packaging?

I am thinking of using the batttery that is commonly found in boost mobile phones (i415). Another question I have is that often these batttery packs have 3-4 connections. The i415 has 4 to be exact. Obvisouly there is + and - (3-4V and GND) and there are two other terminals that are not marked. Any insight would be appreciated since schematics or data sheets for these batteries are impossible to find. On other reading it seems that these terminals may be used for discharging or possibly for sensing the battery's remaining charge?

[SOI_Sentinel]

Possibly. You really might have to open one. If you are careful not to puncture it, this is not dangerous. The case is not part of the battery proper if it's one of those "part of the case" batteries. If it's fully internal to the cell phone, then it is the entire battery, do not attempt to open it.

The individual connections could be links to internal cells, connections to current shunts, or a thermistor connection. I don't know at this time.

[ohararp]

I was able to "hack" this battery and it basically was a li-polymer cell with a protection circuit pcb. It looks like you can get a very similar device at this site:

Looks like Li-ion batteries are prone to catching on fire if not charge properly (MAX1551 - Sparkfun - Thanks) and not discharged properly (< 3V). This link should offer a good protection circuit http://www.all-battery.com/index.asp?Pa ... ProdID=549.

[ohararp]

Neat trick picked up I picked up from "Mr. Analog" at the mircrochip seminar:

http://www.microchip.com/stellent/id...cName=en024786

or

http://www.national.com/ds/LM/LM3478.pdf#page=17

SEPIC regulator allows you to keep 3.3V to your devices even when input can vary from 4.2V-3V (battery voltage decreases with usage). Nice.

[nova]

Those Buck-Boost SEPIC regulators look really nice - Maxim/Dallas also makes some.
Also, if you are looking for small LiIon batteries, Sparkfun sells 1000Mah and 860Mah, both of which are very small. There is a new 100Mah that is about the size of a quarter, but doesn't last for long enough.
Sparkfun also carries 2700 Mah AA NiMH cells that would last for about twice what you want. They work really well, and also look cool as an added plus.