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Battery math


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I have a bunch of batteries packs from work, and I'm trying to put together the best configuration for my pineapple case.

For arguments sake, I have only enough room for 24 batteries (each a little fatter than a AA). Hypothetically, each battery is 3v - 1100 mAh.

I could do 12 banks of 2, 12-(6v @ 1100) = 6v @ 13200 mAh

I could do 8 banks of 3, 8-(9v @ 1100) = 9v @ 8800 mAh

I could do 6 banks 0f 4, 6-(12v @ 1100) = 12v @ 6600 mAh

What is the best configuration and why.

Does it even matter the configuration because 24 batteries are 24 batteries of power no matter how it is configured?

Is my math even right for this?

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In my opinion you are better off with;

8 banks of 3, 8-(9v @ 1100) = 9v @ 8800 mAh

This is similar to the Pineapple Juice battery pack; should get 8-10 hours approximately.

You could try: 12 banks of 2, 12-(6v @ 1100) = 6v @ 13200 mAh, for extended running time, but if the output drops below 5v the pineapple might become unstable and power-down.

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i would go with either the 9v or the 12v configuration, when i tested with a 5v configuration it would become unstable in a shorter period of time. i am running mine with this DC-DC converter http://www.amazon.com/gp/product/B009HPB1OI/ref=oh_details_o08_s00_i02?ie=UTF8&psc=1 i have it set to output 9v because some of my batteries I was messing with at full charge actually put out almost 13v and I have been getting some real good uptimes.

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This isn't an easy question to answer as the best choice will depend on a number of factors.

The big question is does the pineapple have a voltage regulator built in. If not then you will want to aim the Volts at the those it requires (or you risk frying some of the components).

If it does have a voltage regulator built in, or you are piping the batteries through one, then you will want to find out the specs and that voltage regulator and make sure that you don't overload it.

Another consideration is the type of cells you are using and what their discharge curve is. As a general rule rechargeable batteries tend to hold their voltage for longer while discharging, but drop off quickly towards the end. Alkaline's voltage tends to show a steady drop but last a bit longer that rechargeables, they also tend start with a higher voltage as well (e.g. AA's will have 1.5v compared to a rechargeable 1.2).

Assuming a suitable voltage regulator and your hypothetical figures I would recommend trying 12 banks of 2 if the cells are rechargeable and 8 banks of 3 if using Alkaline. But the best way to figure it out might be to just try each combination and see which one lasts the longest.

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There's a crucial variable missing here! How much current does the Pineapple draw?

I don't have numbers, but in general, at higher voltages, it will draw less current. So a battery pack with fewer Ah but higher voltage could actually last longer than one with more Ah but lower voltage.

There should be a particular voltage range within which the Pineapple's regulator is most efficient. You need to determinate what that is (e.g., from spec sheets) before it is possible to answer the question of optimal battery pack configuration.

Trial-and-error is an option too, but the thread title does specifically ask about doing it with math.

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  • 2 weeks later...

LABTEST Mark V

Standard settings, both radios on and normal traffic.

load may vary on config.

test voltage and draw

5v - 0,6A - 3W

7,2v - 0,4A - 2,88W

9v - 0,32A - 2,88W

11,4v - 0,26A - 2,96W

12v - 0,24A - 2,88W most efficient rating, since a battery will usually have deeper discharge at this voltage.

13v - 0,23A - 2,99W, not recommended at thislevel it begins slowly drawing more current. stressing the regulator

this shows the efficiency area of the input.

example and theoretic battery calculations under these conditions.

12v x 8400mah - 100wh/2,88w giving 35hours

Remember LVC circuits on lithium polymer and lithium iron phosphate cells so you dont brake em.

protected lithium ion 18650 cells is recommended if you dont have lvc, cheap and you can find them everywhere.can also be bulk charged.

now you can calculate batteries with more accuracy

Edited by SirHaXalot
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  • 2 weeks later...

I could do 12 banks of 2, 12-(6v @ 1100) = 6v @ 13200 mAh

I could do 8 banks of 3, 8-(9v @ 1100) = 9v @ 8800 mAh

I could do 6 banks 0f 4, 6-(12v @ 1100) = 12v @ 6600 mAh

I've come to find out that it doesn't make much of a difference.

6v @ 13200 mAh = 79.19 watts

9v @ 8.8 mAh = 79.2 watts

12v @ 6600 mAh = 79.19 watts

If I run the pineapple at 6v, it needs to draw twice the amount of milliamps to function than if I ran it at 12v.

So in general, when buying a battery pack, given a choice between a 12v/13200 mAh battery pack and a 6v/13200 mAh battery pack, the 12v delivers more power than the 6v. However, the 12v/13200mAh battery pack would have double the amount of batteries than the 6v/13200mAh.

Given that I can only fit 24 batteries, for the most part it doesn't matter how I configure them because the pineapple will use the same amount of watts in either configuration. It will just compensate by pulling more or less milliamps.

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Hi Sir Haxalot

thanx for sharing that numbers. I'm going to run also some test with different antennas to see if that differs the power drain. I mena with two 14dbi panel, giving the device more capability to detect more signals thus making it to to pobably work at a full load the processor. This are only asumptions.

I bought the markV and the external battery from the shop. the antenas from ebay. and also Im planning to try to expand the autonomy with a small solar panel.

I bought this small panel to start testing the autonomy. but has not yet arrived.

http://www.ebay.com/itm/160791345174?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I'm on the rigth path with the solar panel, can I use a smaller one?

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[12v - 0,24A - 2,88W most efficient rating, since a battery will usually have deeper discharge at this voltage.

http://www.ebay.com/itm/160791345174?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I'm on the rigth path with the solar panel, can I use a smaller one?

Normal draw is 240ma without doing anything crazy. Your solar panel produces 570ma max(full sun and at the right southern angle). That's 330ma charging current during the day.

I'm going to guess that on a partly cloudy day, it should keep power in the evening and crash early morning and restart at dawn.

Also it lists that the max volts is 17+ volts, may be a problem.

Normal charging volts for a 12v battery is ~14.5 volts, may be a problem.

Full charge of a 12 volt battery is 13.5 volts, I have not had a problem here.

Pushing above 13.5 volts may smoke the pineapple.

I think that there is going to be a lot of volt meter testing before hooking it up.

Edited by yabasoya
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