mobile battery for pi and monitor help

The car uses "12V" nominal. But that means that you will see up to 14.8V on the voltage rail during fast-charging. When the car is turned off or not charging the voltage might be as low as 12V, but seldomly lower. (when starting the car, the voltage may drop as low as 8V, but most accessories are normally disconnected at that time).

With a full 11.1V battery you will be at 12.6V, nicely in the 12.0 - 14.8V range that a car-accessory should expect. But when the battery drains, you might want continue to use it down to about 9V. On the other hand, if the monitor keeps on working down to 10.5 or 10.8V then you're in business. But it depends a lot on how the screen is built.

A friend has a display that has a DCDC converter that converts 7V - 15V down to 5V and then only uses that. So his would work just great on a 3S LIPO. (He actually had 5V available but no 12V, so he made a "5V power input" on his screen, bypassing the regulator on the board.).

I'd say: try it. As the backup-plan, buy a DCDC stepdown (buck) converter and a 4S battery.

i was trying to find a step-up to 12v at .5 amp but no luck yet. so maybe it have to be a higher voltage battery and a step down?

update: how about something like this? Pololu 12V Step-Up/Step-Down Voltage Regulator S18V20F12. a step up/down 12v at 2amps. its more amps then i like but i dont know.

for the pi its self i was thinking OKI-78SR-5/1.5-W36-C or Pololu 5V Step-Up/Step-Down Voltage Regulator S7V7F5 not sure which one is better yet

Step down is "easier" to build for the electronics engineers. That makes them cheaper.

The step-up-down are even more complicated. I'd personally just chose a battery that is higher than what you need, and step down. On the other hand, I'd give you good chances that the display will work down to a reasonable cutoff voltage of 10.5V.

If your battery has 7800mAh when you discharge to 3V, you'll be able to use only about 7200mAh if you discharge to 3.5V (per cel => 10.5V in your case). But you'll increase lifespan of your battery dramatically especially if you have a small leakage current in the "voltage too low, shutdown" mode. Then having the 600mAh left will give you much more time to get to a charger than when you have nothing left. And discharging below the 3V is what kills batteries......

ok let see if i understand, its easyer to step down so if i went 11.1v battery to a 14.8 and use a step down that would be better? cuase how i read it is that if i use a step-down 12v its would have a dropoff at 13volt wear a step up/down could use a wider range voltage and has a dropoff at 2.9volts?

there is a mode of regulator called a buck/boost that automatically will switch from one to the other

look at the booster pack for the TI launch pad as an example, very few components involved.

You can also simply chain a boost then a buck one after the other, it will still be close to 80% efficient and there cheap to buy from Ebay

see this as an example.. problem solved  DC 4 5 32V to 1 25 32V Auto Step Up Down Adjustable Converter Voltage Regulator | eBay

regards

peter

wow, so its auto adjustment to voltage needed?

yup, the TI booster pack uses two of a similar chip for its Lipo Booster Pack I reviewed here  http://www.element14.com/community/roadTestReviews/1712

your li-on battery is fine, and you can use a UBEC which can be found in many RC model store to convert 12v to 5v and power your raspberry pi.

Hi,

I have built what is suggested above. See "here". I list of the components and prices are listed.

I use 6x3.8V LI-ION batteries in 2 parallel sets of 3. I think I should have around 16 hours of PI with that configuration.

looks like recovered Laptop batteries, I have maybe a hundred or more from that process, most of them are still good so it is a great source of cheap power

Must be careful of charging and discharging though, if you are going to parallel them either use brand new from the same batch or keep the sets parallel as they come out of the laptop pack, your asking for problems of self discharge, leakage or worse, fire if not, also packs in series need to be monitored for equalization during charging (Unless your charging them all individually), also if your saying your putting 3 in parallel and there from laptop pack where there typically in pairs, it can be bad for the same reasons.

Also try to limit them from full cycle swings as it will age them very quickly

Either way, be very carful