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battery questions... very long post

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jberry

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If I had a 12V 7Ah NiCa battery fully charged how long could I charge or how many times could I charge my phone using a "modified" car charger?(Phone has a 1500mah Li Ion battery) Also im open to suggestions about using a different battery Im trying to go for best bang for the buck but im also considering using a cheap 20$ Li Ion drill battery that is 12V opinions constructive criticism or other ideas are all welcome.
If you had a power circuit made up of multiple batteries in parallel charging electronics like cell phones and one or more of the batteries from the main power circuit are removed after some time, recharged and put back into the circuit, could this be harmful to the electronics being charged?
Can anyone better explain to me wattage, I just need some examples of how much power 1,5,15watts is. I want to use solar panels to charge batteries but im not sure how much wattage I need. In my head the more the merrier but I know that cant be the best idea.
My goal is to make portable solar power source to take with me into the woods and not only charge my cell phone and my friends cell phones at the same time but also act as a fan for me. Ideally I would leave the solar panel outside during the day charging the batteries when im not in the tent, and charge our phones as well as keep myself cool during the night.
Does anyone think that it would be a good idea to try and save battery life with the fans and use a capacitor that would kind of pulse them instead of run them on nonstop?
And finally, does anyone think that this setup with the up to 15 watt solar panel has the power to run an inverter and maybe charge a laptop or just use something that needs to be plugged in?
Thanks for sticking around and reading all of this any and all suggestions are greatly appreciated, Thanks again.
 
I'm going to a stab at a few points.

Power = Volts * Amps. There is usually an efficiency when dealing with power conversion.

mAh is a measure of capacity of a battery. 1000 mA is equal to 1 Amp. A 1500 mAh battery MAY be able to deliver 1.5 AMPs for 1 hour and 0.15 Amps for 10 hours. Batteries have discharge curves and depend on type.

A USB charger ( 5V * 0.5 Amps) = 2.5 Watts. A Fast USB charger (5 V at 1 A) 5W. An iPad charger 10 W.

A cell phone needs much higher peak currents that a charger can provide. That's why you have to use a battery.

Laptop charger is about 100 Watts.

A 15 W solar panel will deliver 15 W when the solar irradience is 100 mW/cm^2 because that's what they are specified at. A nice clear day at AM 1.5 Global spectrum. AM is Air Mass. AM 0 is space. That leaves a LOT of variables.

Inverters have efficiencies in the 80 to 94% range.

Capacitors are like mini batteries, but there purpose is to smooth out ripple and absorb small blips in current consumption.

The woods is dark. Solar doesn't work in the dark.
 
Hi,

I'd like to add a few notes too...

Consider getting a 'crank' flashlight. I never liked these things until i got one. I got a cheap one under 10 dollars and even that works to some degree without having to crank forever. 30 seconds of easy cranking and you have light. With solar, you never know even if you do have an open area like a field to work in. An overcast day probably wont charge your battery enough.

The 12v to cell phone conversion could be done with a linear regulator but you'll waste some energy. If you had a 12v 7.5 AHr battery and you want to charge a 1.5 AHr battery you would simply divide 7.5 by 1.5, which comes out to approximately 5 recharges of the 1.5 AHr battery.
With a buck converter however, going from 12v to 5v means you get to use more energy to charge and this works out to roughly two times the linear charger type regulator, which means you might get as many as 10 charges out of your 12v battery.
These are estimates that give you some ball park idea what to expect. You should always have a backup plan however just in case something goes wrong with the main equipment like the charger. Think in terms of cave exploration and you'll do pretty well...imagine climbing deep into a cave and having your light go out...you need a backup or you're in big trouble.

If you get a solar panel that puts out 1 amp at a high enough voltage it would take about 10 hours to charge a 12v, 7 AHr battery. That's in full bright sunlight however. If you got one oversized like 2 amps that would be better, but again it is very hard to depend on the sun for energy when you need a lot of sunlight to charge your battery.
 
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Thank you guys for your awesome input. Im a heavy construction equipment operator for the army national guard and wanted to make this "box" for when we are in the field moving dirt. I thought it would work well because it seems whatever job we have it usually includes baking in the sun all day. I like the crank Idea im assuming just put it in series with a charger for what ever power source i decide on? So what im getting from this post is I should keep it simple and just charge phones because the amount of power I would get from the batteries is small and the solar panel even under best conditions would take all day to recharge them. Im assuming running small computer fans at night to keep myself cool while I sleep would be too much of a drain, if that is correct what do you guys think about if I were to use one of those handheld fans that run on like 2 AA? I could tap off the incoming power from the solar panel and charge the AA's at the same time. Can I also get an opinion on my power source? should I use different kind and size batteries or maybe something completely different altogether? I dont know if you guys are familiar with Harbor Freight but they have a bunch of smaller solar panels that would be fun for some projects, for instance I plan on using a smaller one they have that is 1.5 watt and add it to my backpack and using it to keep my cell phone topped off. Sorry if Im not making alot of sense or talking in circles I just want to build this without having complete and total failure.
 
Hi again,

The crank light is self contained in that you dont have to hook anything up or do anything else like that. The crank is right on the flashlight so there is nothing to connect. If you wanted to take it apart and use the crank to charge other batteries you'd have to do some experimenting to see how well it would work with other batteries and of course it would have to be connected correctly to get it to work. You might also need a regulator circuit or charger circuit.

If you are in the sun all day you might get some benefit from solar panels. Since it's hard to judge from here i might suggest that you get a smaller lower cost panel and use that as a test to see how well this works out in practice with your typical routine and locations. You can then scale up later. As you said, maybe a small one to charge your cell phone would be a good place to start. You can then do some calculations to see what you would end up needing to charge more batteries for say your fans or other things. More about that in a minute.

You are asking the right questions that's for sure, and one of them about the batteries is also a good one. The lead acid batteries are not as efficient as most other types, and are used mainly to get a large amount of storage for a lower cost. Another two types are NiMH and Li-ion. Li-ion are the best but they require more care in charging. A special charger has to be used to match the battery size and capacity to some degree. NiMH are easier to use but you do have to string them in series to get some usable voltage beacuse they are only 1.4v each. Li-ion have terminal voltage around 4v or a little less. I would suggest that you read up on Li-ion and NiMH a little before you decide. A special kind of NiMH called "low self discharge" are better than the run of the mill NiMH cells.

Now back to the calculations...
You can get by with some simple calculations with batteries because they work mostly on current (amperes or amps). The terminal voltage is also important of course, but you simply match the battery voltage up to the application. If you have a fan that runs on 12v then you need batteries that put out 12v or close to that. The current is what is important for calculating the charging and discharging needs which affects the run time of the equipment.
As i said, it's very simple and only requires multiplication and division. If you charge a battery for 1 hour with 1 amp, you have 1 times 1 equals 1 ampere hour of charge in the battery (we'll talk about battery efficiency in a minute). If you charge a battery with 2 amps for 1 hour, that's 1 times 2 equals 2 ampere hours of charge. Charge for 3 amps for 10 hours and that's simply 3 times 10 or 30 ampere hours. Ampere hours is often written as "Ahr" or "AHr" or similar for short. Thus 30 ampere hours is written 30AHr for example.
Now say we did charge a battery at 3 amps for 10 hours and so we now have 30AHr in that battery, and we want to run a fan that requires 3 amps. The number of hours run time we get is approximately 30 divided by 3 or 10 hours. In other words, we divide the AHr by the current in amps. If we had only charged that battery with 9AHr we would have only gotten 9/3=3 hours run time. So you start to see how simple this is.

Now battery efficiency also comes into play though which makes it a little more complicated, but not too much. The amount we charge the battery is not exactly what we get out of it because there is a charge efficiency associated with each battery type. For example, using a lead acid battery we would have to put in about 1.4 times more charge into it than we would get out of it. Thus, if we have a fan that runs on 1 amp and we want to run it for 10 hours then we have to charge that battery for 14 hours at 1 amp. So you see we charged for 14 hours but only got out 10 hours of run time. In other words, the more exact run time comes from dividing by the current of the device and also divided by the charge efficiency (really called "charge acceptance") of the battery. We charged for 14 hours at 1 amp so that's 14 AHr, then we divide that 14 by 1 (the current of the fan) and divide by 1.4 (the efficiency) and we get 10, which is the run time in hours.

Now there's the battery capacity that also comes in to play. We can buy small batteries or big batteries. The bigger ones can hold more charge and so have higher AHr ratings. A 20 AHr battery can hold twice as much charge as a 10 AHr battery. If we had a fan that needs 1 amp and we wanted to run it for 20 hours, we would need at least a 20 ampere hour battery.

All this can be thought of in terms of a few formulas...
BatteryCapacity=RunTime*DeviceRunCurrent
ChargeTime=BatteryCapacity*ChargeEfficiencyFactor
Charge=ChargeCurrent*Time

Im sure you'll have more questions but all this gets simpler once you calculate a few examples. It's like pouring water into a tank, you get out what you put in for the most part :)


Example 1:
We have a fan that runs on 1 amp and we want to run it for 10 hours. If we wanted to use a lead acid battery what capacity rating would it have to have, and how long would we have to charge it using a charger that puts out 0.5 amps?

First, the capacity of the battery would have to be: 1*10=10 ampere hours, but we may want to oversize a little to make up for battery aging, so we buy a 12 ampere hour battery.
We have a charger that puts out 0.5 amps and the efficiency factor for lead acid is 1.4, we we have to charge for 10*1.4/0.5=28 hours, or to charge the battery up completely we would go longer.

Feel free to ask more about this as it's very hard to include all the details in one post :)
 
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