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Battery Dilemma

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Bought one of the cheap things off eBay, from China. Backup plan will be PP3s with spares in my pocket... or about 100 23As.
 
I found this very simplistic battery life calculator (**broken link removed**) and pumped some numbers into it...

If I can assume that my circuit will draw about 100mA (just a standard default value because most effect programmes flash so it's not constant - at 9v the min. is 0.06A and max. was measured at 0.15A) so it should be indicative of what I'll see, assuming (not worrying about battery type voltage for now) is this a set of fair expectations...

23A battery with 33mAh would last around 0.23 hours.
PP3 battery with 565mAh would last around 3.95 hours.
AA battery with 2000mAh would last around 14 hours.

Obviously, my circuit wouldn't run with 1 x AA, but what I'm thinking of now is combining 3 x AA or 4 x AA into pack and using a boost converter module to provide 12v to the circuit (or 9v, depending on what I can live with)... how would I then change my figures in these basic expectation calculations?

Is it linear, like, if I had 1 x AA with 2000mAh and that would last around 14 hours, if I had 4 x AA would that be 8000mAh - so 56 hours, but I'd then need to cater for the change in voltage - the AA provides 1.5v and I need 12v, so is the calculation 12 / 1.5 = 8 - so the lifetime expectation would be 56 / 8 = 7 hours? Or - simplistically - 4 x AAs at 12v would provide 2 times the lifetime than a single PP3 running at 9v and 30 times the lifetime of a single 23A running at 12v?

Or... and I do expect people to say this... am I way off base?

P.S. - experiments with a single 23A battery seem to indicate a fairly short lifespan (as everyone said) with the reds looking fine after more than 20 minues of running, but the blues starting to look much less bright than they were at the start.

P.P.S. - no, if I have 4 x AAs, then the mAH doesn't increase, just the voltage, from 1.5v to 6v. So the lifetime would still be 14 hours, but I'd only be running at 6v, so would my simple calculation become 14 hours / 2 (12 / 6) = 7 hours. Same answer - of 7 hours - but a rather different way of getting there.
 
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With batteries, capacity (usually in mAh, miliamp-hours) decreases as current draw increases, so say you had a 100% efficient boost converter, which could convert any voltage to any other voltage, with the same power out as in - 4xAA's would last more than double the life of 2xAA's, because 4xAA's have double the voltage, therefore, for a given load current, provide half the current :)

As you rightly pointed out though, the greater the difference between input voltage and output voltage in a boost converter, generally the lower the efficiency. So 3V to 12v wouldn't be as efficient as 5V to 12V - but in todays standards, at your power requirements, it can still be really quite high.

Here's how I tihnk of it. Your rated capacties are 'ideal', as in, 2000mAh is pretty much the maxcimum you can expect from an alkeline battery from fresh to 1.1V (when the voltage is too low for most apps, and they are considered 'dead'). Draw 100mA from thos battery, and maths says it sohuld last 2000/100 = 20 hours. - this isn't the case, because at 100mA draw the capacity will drop. If we bring voltage into it (series batteries, same capacity different voltage) it just becomes easier to think interms of power, watts.

An alkeline AA battery starts at ~1.5V, dead at 1.1. Its discharge curve isn't 'flat' so over the discharge its 'average' voltage is probably more like 1.4 worst case. Now we have an average voltage for each battery, multiply by the capacitity to get 'mWh' miliwatt-hours. For 3 x AA's, 2 x 1.4V = 4.2V average voltage. Thats 8400mW hours, 8.4W-hours. With 4 x AA's being 11.2w-hours.

Again 'assuming' worst case efficiency for a boost converter is 75%, to boost from our battery voltage to 9V, and with a peak current draw of 100mA on the 9v load, thats 0.9W. 0.9W / 75% = 1.2W. So theoretically, with 3 AA's, 8.4/1.2 = 7 hours. With 4 AA's, 11.2/1.2 = 9.3 hours.

You made a mistake regarding using 4 x AA's instead of one :) Indeed, the capaciity doesn't change, each cell is in series, so each cell has the same current flowing through it. But... that is assuming you're drawing constant current - which, with a boost converter, you're not. In a linear circuit, the battery voltage sags (so the circuit most likely draws less current) until its value is too low to keep the circuit running, then the batteries are considered 'dead' regardless of how much juice they have left. In switched mode supplies, such as boost converters, power is converted, with power lost because nothing is 100% efficient. If you have 9V out, with a load of 100mA (0.9W) then the input current is determined by the input voltage. More cells = higher voltage = less current draw. The less current draw, the longer the battery life :)

That said, it should be noted that boost converters (or buck converters for step down) aren't always more efficient than linear supplies. They are more expensive, take up more space, can add 'noise' to the power lines (horrible for RF and analogue circuits) and are more trouble than they are worth if you require a 5V supply from 6V. But in your case, I think it's a great way to get 9-12V at a 'medium' current draw (0.1A - 0.4A) from a battery pack of 3-4AA cells.

I'm sorry if I've explained this badly, I tend to just rant.

If you have access to some components (as in, no need to order especially), I would be happy to post a schem of a simple boost converter with a 100-150uH inductor. Might be useful to play with to see how input current changes with input voltage given a fixed output voltage/load.
 
A 9V alkaline battery starts at 9V then quickly drops to about 7.2V, then drops slower until your circuit won't work anymore. A 9V lithium battery is much better as shown on my graph.

One single 2000mAh AA cell can be boosted up to 9V but then its current is more than 6 times higher so it will have much less capacity of a little 9V battery. Its voltage drops the same as the 9V alkaline battery but faster.
 

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I have three tactics in-play right now. The first is that I am searching for a 8 x AA battery holder (as recommended at the very beginning of the thread) with mounting holes - that will give me the 12v to start - if I find it, I will order it and handle the weight of 8 x AAs in my hat. The second is that I have a number of battery holders that take either 3 x AAs or 4 x AAs and I could couple these with a cheap boost converter module (a recommended later on) which I have already ordered (but might take some time) and use either 4.5v or 6v to create either my required 9v or 12v and a higher longevity. The final - backup - tactic is to install 2 or 3 of the 23A battery holders in the hat (you can see there is one bolted in already) and connect them in parallel, thereby using up my supply of 23A batteries through the day, carrying spares as necessary.

I think, in order of preference, they are actually 1, 2, 3.

The only final thing I have started thinking of is the following...

I already have two of the 3 x AA battery holders that provide 4.5v (tested with multimeter) - they are obviously connecting 3 x 1.5v AAs in series to get the 4.5v. I suppose that I could consider connecting two of these holders together in series to end up with 6 x AA 1.5v batteries providing 9v, and it would last for some time, couldn't I? By that, I mean the rough 2,000mAh provided by an AA.

P.S. - my two 3 x AA battery holders already have mounting holes. :)
 
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two 3 cell AA holders in series is just as good as a 6 x AA holder IMO :) Its just an extra wire between the two holders.

Although, remember, the 9V from the AA's will drop. You won't get 9V through-out the battery life, so check how your circuit does at lower voltages, down to 8V maybe. I know your PIC runs on 5V provided by the regulator, but what about these LED modules? If you have a link to the spec, they may require a minimum voltage of 9V, or have an inbuilt boost power supply of their own (becoming more common now days, boosting a 3-12V input for LED strings in series). Considering you've ran them off 9-12V I'm guessing its two-three LED's in series, with a current limiting resistor. Blue/white LED's tend to have the highest forward voltage requirement 3.2-3.6V and so, simply won't conduct/light up with less than 3V across them. But it all depends on the config :)
 
The totality of what's being powered is either 45 or 135 LEDs. The more I think about it, the more I'm impressed with the little 23A battery and what it can do.

This is what it looks like, all outside of the hat, but ready to go back in when I have suitable batteries installed and I've added some more heatshrink around my crappy solder joints. In this picture it is obviously being powered by a single 12v 23A.

**broken link removed**

Getting 6 or 8 x AAs into the top hat is what I think is going to be the main challenge.

The lower the voltage, the more the LED strips dim. So I can't afford them to go much lower than 9v or they'll look like they're not turned on.
 
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Here is the first **broken link removed** that popped up in ebay, and it has mounting holes.

An alternate would be 3 LiCo (li-ion) protected AA's, at 3*3.7V (11.1V). Special charger and precaution required for charging though.
 
That would work, yes... it wouldn't have come up for me as I'm in the UK and this is ebay.com - however, it might be worth a punt to see if it gets here on time. If it doesn't, I'll progress with linking others together (I reckon) and keep it for the future. Thanks.
 
Hobby stores sell many different sizes of 11.1V Li-Po batteries and chargers for them. They are small and light weight.
 
I've just Googled these and seen quite a few... the mAh seems relatively low for reasonably priced ones, but I guess the beauty of them is that they're rechargeable. About 800mAh for below £20, but then there look to be quite a few amazing ones, right up to 8,000mAh for over £150! Interesting, but I think it would distract me to look at these further right now. I've just ordered some PP3 single connectors so I can run these 3 or 4 x AA blocks in series to get 9v or 12v. I still have my cheapo boost converter on order, but I have no faith that it will arrive in a reasonable time.

Ones I've been looking at appear big too, 135mm long. Scanning for smaller ones, just for interest...
 
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Good old Wikipedia says "The major risk factor is the volatility. When punctured, Li-po batteries react quickly by smoking and causing large fires."

I'm not sure I should put these into a hat... sure, I might easily be called risk averse and over-cautious, but the non-risk averse folk have probably never had their head set on fire. Neither have I, but I'm also unlikely to. I think having these batteries in a hat where drinking large quantities of alcohol is involved is probably not overly smart for me.
 
I have drinked, drank and drunk large quantities of alcohol at parties like everybody else and nobody has stabbed me (I am not in USA).
When I go to the park to fly my Li-Po powered RC airplanes I have a pocketful of Li-Po batteries in my pocket beside the family jewels and I have never had a problem with being stabbed there.

A little alkaline battery will burn you if it is stabbed.
 
W-e-ell, it was more a tongue-in-cheek reason for not looking into these in great detail. I think I'll stick with AAs and see how they work out - either 6 x AA at 9v, 8 x AA at 12v or some combination with a boost converter.
 
W-e-ell, it was more a tongue-in-cheek reason for not looking into these in great detail. I think I'll stick with AAs and see how they work out - either 6 x AA at 9v, 8 x AA at 12v or some combination with a boost converter.
Six AA alkaline battery cells produce 9V only when they are brand new!
Then you will see the LEDs dimming as the battery voltage runs down.

Eight AA alkaline battery cells produce 12V when new to make the LEDs bright but soon drop the voltage to only 9.6V which is a noticeable dimming. Maybe your LED strips do not light with only 9.6V (the blue LEDs).
 
These are only problems... not solutions ;)

It's all about experimenting to see what's possible and acceptable. If we find that AAs aren't the way to go I'll revert to 23As and just replace them every 30 minutes... or PP3. I've tried both of those already, so let's just see if loads of AAs offer something better.
 
Noticed a pack of 4 x AA Energizer Ultimate Lithium for £6.99 in Maplin today. They were £7.99 at Staples. However, Maplin also have a buy one for £6.99, buy two for £9.99 offer on until Christmas Eve. Not sure there's much better deals out there.
 
They are much more expensive nere in Canada and nobody has an offer for them.
 
If its the sort of device/application, which is likely to be used often, and is always used by you, then sure, specific batteries such as LiPo battery packs are ideal. Lightweight, high power density, predictable discharge curves, and many dedicated IC's available for charging. They can be charged up at any time, regardless of their state as most charing IC's monitor things, and prevents over discharge.

Downsides are, expensive, requires a dedicated charging circuit (inside the box). AA alkaline can be brought anywhere, have virtually no self-discharge, they are convenient as a standard. If you plan on using this LED thing once in every so often, or you're making it for someone else to use (who's not always technically minded but intuitively knows how to plug a few AA's in a box) then alkalines would be better in my opinion. Of course, a lithium in 'AA size' would do just as well, also being the standard AA size. They'll last longer, more expensive, lighter, but aren't rechargable.

Rechargables pay for themselves if you're running this regularly, but NiMH have 1.2V per cell, so 8 would give you only 9.6V.
 
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