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Topic: Power Arduino with 4x batteries in series or 2x2 in parallel/series (Read 3722 times) previous topic - next topic

redfuse

Hi all,

I want to power my Arduino Uno with four Nokia BL-5C 3.7V 1020 mAh Li-ion batteries. What would be best in terms of endurance: four batteries in series (with average voltage of 14.8V and 1020 mah), or 2x2 in parallel/series (with average voltage of 7.4V and 2040 mah). I guess what I'm basically trying to find out is whether there is a difference in efficiency in the Uno's step-down regulator, going from 14.8V down to 5V or from 7.4V to 5V. And perhaps there is a difference for the batteries discharging cycle.
Any insights appreciated!

Arrch

four batteries in parallel (with average voltage of 14.8V and 1020 mah)


I think you mean in series here.

As far as your question, dropping 14.8V across the regulator is going to dissipate a lot heat, which equates to wasted energy.

redfuse

Yes, you're right, that was meant to be series.
So 2x2 parallel/series seems best then. Thanks.

retrolefty

#3
Aug 30, 2012, 05:43 pm Last Edit: Aug 30, 2012, 05:48 pm by retrolefty Reason: 1
I would also recommend using Li cells in series Vs parallel/series configurations. The reason is that unless you have the means to closely match the individual cells you can have problems with charging them and insuring that they maintain a good balance between them as they are charged and discharged. Even series cells can be a problem maintaining good balance without using a proper charger that can detect each individual cells terminal voltage while charging (so called 'balance chargers') to ensure that there is no individual cell being overcharged or undercharged Vs the other cells. Obtaining a commerical Li battery pack that already has your desired output voltage and MAH capacity is always preferred over building your own, and of course always use the proper charger that is recommended or compatible with a specific Li battery pack. Li battery are currently the best choice for best energy density (MAH Vs weight and volume) of any other popular battery choice, but they do come with more specific charging and discharging safety requirements.

As far as:

Quote
I guess what I'm basically trying to find out is whether there is a difference in efficiency in the Uno's step-down regulator, going from 14.8V down to 5V or from 7.4V to 5V.


The on-board +5vdc regulator is just a simple linear regulator so the higher the input voltage the higher the efficiency losses will be. So the lower input voltage you can run within the +7- to +12vdc rating, the less losses you will have, which is very important when powering a project from batteries as that directly effects the run time one gets from a given and fixed maH battery capacity.

Lefty

redfuse

So putting Li-ion in series is safer than in parallel? I do have a lithium charger on which I can select to charge 1, 2, or 3 cells (http://www.jperkinsdistribution.co.uk/detail.php?JPNO=5509989&activepage=1&Navmain=Batteries/Chargers&subcatname=Chargers%20and%20Balancers).
I understand that a pre-assembled pack is better than building your own, but I have some difficult dimension restrictions and found the batteries that I have perfectly sized. Or would you know a good resource for finding (cheap) batteries in certain dimensions? There are plenty of battery sellers, but more often than not they don't show the battery size, they just say it fits in this or that device.

drjiohnsmith

DO NOT CONNECT LiION BATTERIES IN PARALLEL.

there is a small, but significant possibility that one will self discharge into the other,
    boiling one of the batteries and if your luck catching fire,
       if your unlucky,
         spewing molten Lithium electrolyte all over you.

most cells have electronics in them to protect against this, but not all,
    and not all protection systems work as well as they should,

series connect is ok,
    use a dcdc converter to get longer life.

retrolefty

Check with this site, they seem to give good data on their offerings (note there are two pages of selections), just click on each to read the data:

http://www.all-battery.com/li-polyerbatterypacks.aspx

Lefty

redfuse

Thanks a lot for the advice. I have put two li-ion's in series, will also look for a better battery pack.

Lefty, I had another question specifically for you - because I'm using lithium I want to be very careful with over-discharging, so I want to have an auto-shutdown mechanism when the battery level gets too low. I think a relay would work well for that. I noticed that you wrote about this in an older thread: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1232478475. Unfortunately the ebay-link does not exist anymore. Would you know of any other seller that sells a simple and small (single coil) latching relay? I can only seem to find relays that are meant for switching main ac currents, which are quite bulky. Thanks!

retrolefty


Thanks a lot for the advice. I have put two li-ion's in series, will also look for a better battery pack.

Lefty, I had another question specifically for you - because I'm using lithium I want to be very careful with over-discharging, so I want to have an auto-shutdown mechanism when the battery level gets too low. I think a relay would work well for that. I noticed that you wrote about this in an older thread: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1232478475. Unfortunately the ebay-link does not exist anymore. Would you know of any other seller that sells a simple and small (single coil) latching relay? I can only seem to find relays that are meant for switching main ac currents, which are quite bulky. Thanks!


I just did a search on e-bay for '5 volt latching relay' and got this:

http://www.ebay.com/sch/i.html?_nkw=5+volt+latching+relay&_sacat=92074&_odkw=latching+relay&_osacat=92074

I suspect any of these would work. I bought a quantity 10 listing when I got mine as I thought they would be quire useful in the future.

Here is a link to my schematic for manual on / auto off operation.

http://img25.imageshack.us/img25/563/08miq7.jpg

Lefty

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