# How to Choose a Battery

I would like to start off by saying I have little actual experience in electronics, so bare with me, and for all I know I could be way in over my head. Anyway...

I was thinking of making a rechargeable portable battery charger that is able to keep a smartphone charged for roughly 3 to 5 times the normal complete discharge time of the battery (so I would supposedly be able to make the battery on my smartphone last three to five times longer). I have a Galaxy S4 that I wanted to model the project after. The battery is rated at 2600 mAh, so I had figured I would buy a Li- ion (I assumed their wide use in electronics made them good for this kind of thing [specifically the Lithium Cobalt because of their large capacity]) 8Ah battery or higher, or two 4Ah batteries or higher. Doing a quick search on Mouser and Newark, I saw the prices for batteries rated at that Amp Hour level are expensive ($200 for a 12V 6Ah lead acid battery). My questions are: Am I missing something here? Am I looking at the wrong batteries? The cost to buy a "long term" charger for a smartphone seems to be between$30 and $100, half the cost of the batteries I found. Am I approaching this the wrong way? If I am looking for the wrong thing, what should I buy? Or is it hopelessly expensive to make a portable battery charger, and I should just buy one? • Not sure where you are looking at but a quick search on Amazon and I found 12v 6Ah lead acid batteries for$30 or less. – Michael Choi Apr 3 '14 at 6:40

For the average DIY guy I guess you have these technologies to tinker with:

• Lithium (mix of different sorts)
• Nickel Metal Hydride

By specifying it as "portable" you probably don't want the charger to be heavy. Then you can remove Lead acid from your list. The NiMH batteries have quite high energy density nowadays but the charging and maintaining of the cells are a bit more complicated than for Lithium. We assume Lithium for this case.

The nominal cell voltage for a lithium cell can be around 3.6V per cell (depends on make and model). If you're going to charge directly to the USB port you would need 5V input to the phone. So you would be best off using two cells at 7.2V nominal. This would then have to be converted with preferably a DC-DC to 5V.

The Galaxy S4 has a 3.8V cell with 9.88Wh. To double the battery capacity you would need an equal amount of Watthours (cell voltage x Amperehours) in your backup battery. Calculating for 10% loss in your DC-DC you would need a 7.2V cell pack of at least 1.35Ah.

And then you still have the task of doing a charger for your backup battery.

• Sounds good, but I'm confused. Why do you use Watt Hours and not Amp Hours to find the how much charge you need? – Translucent Dragon Apr 3 '14 at 23:08
• Because Watt Hours are a measure of energy, whereas Amp Hours as a measurement only makes sense if you also know the integral of voltage over time. – Jon Watte Apr 4 '14 at 4:16

Don't bother trying to make your own. There are several models of external chargers with higher capacities than you found. For example, the Anker® Astro Pro2 is rated at 20Ah, is designed to charge the Galaxy S4 among several other models, and is available from Amazon for $100 (marked down from a$300 list price). It can also be used for laptops and tablets. I doubt if you can build your own with that capacity for $100. Or is it hopelessly expensive to make a portable battery charger, and I should just buy one? If there's a commercial product already on sale that does the job: mass-production, economies of scale and purchasing power will almost always trump the hobbyists savings on salaries, marketing, HR and other corporate overheads. The motivation for making things yourself is often fun not finance. A good source of cheap LiPo batteries is RC hobby mail order sites. Either get a 3.7V cell with a step-up DC/DC converter, or a 7.4V or higher battery pack with a step-down DC/DC converter. Then get an RC hobby LiPo charger to actually charge the pack. A good way to build this would be to use an off-the-shelf 5V drop-in 7805 replacement switching DC/DC converter, such as the Murata OKI-78SR. Power it from a 3S (11.1V nominal) battery pack. Wire it to the "+" and "-" parts of a USB socket, and short the "D+" and "D-" pins together in that socket to indicate to the device that it's a "dump charger" that can provide 1A of current. Finally: LiPo batteries are common, because they are high capacity, charge quickly, and are light-weight. However, if you physically damage them, or puncture the protection, or over-charge them, THEY WILL CATCH FIRE. This can burn down houses, and has done so. Also, if you over-discharge (much below 3.2V per cell) the battery pack will no longer take a charge, and recovery is very dangerous (see the "burn your house down" part.) The LiPo chargers will also balance the cells in the pack, because they will not charge equally well over time. There is a special little connector on RC battery packs for this purpose. The 78SR is$5, a USB socket is $1, a high-capacity 11.1V battery pack will be about$30, and a charger (which you can leave at home) might be \$16 or so.

Or you can just buy a complete package; there are many to choose from, and that's a lot easier. Again, see the "burn your house down" part :-)

• Great point! Do not overheat, overcharge, over-discharge, puncture, or distort the shape of the lithium batteries. Very dangerous. Balance charging also important for anything over 1S, which adds a layer of complexity- that's why I recommended the 1S battery with step up converter that controls the charging and prevents over discharge. – Filek Apr 5 '14 at 0:12

Or is it hopelessly expensive to make a portable battery charger, and I should just buy one?

I built my own for practically free, and it is capable of charging my ipad from 0 to 100%, and still has power left to charge my iphone a several times! It's measured capacity is about 16Ah at 4.2 volts. It should charge my iphone at least 9 times, but I haven't had it long enough to test it out.

I used 18650 Li-ion cells that I rescued from laptop batteries that were discarded. I ripped apart the discarded batteries and tested the individual cells, and on some of the batteries, there were only a couple of dead cells making the battery inoperable, but the other cells were still in good health.

Then using a DC-DC 3.7-5V buck/booster like this one:

http://www.dx.com/p/fx-608-pcba-diy-1-2-lcd-dual-usb-output-5v-boost-pcb-module-w-led-for-mobile-power-green-221064#.Uz3JPvldWSo

This allows you to both charge the backup battery and to use it to charge your phone etc.

You could use these cells:

http://www.dx.com/p/genuine-panasonic-18650-2900mah-rechargeable-battery-grey-pair-121450#.Uz3KavldWSo

They are a little pricey, but have excellent capacity, and just wire as many in parallel as you need for what ever capacity that you want.

• How are you able to/what do you use to charge the portable battery from the DC-DC Booster (like do you charge it from the USB or micro-USB or what)? Do you hook the wires from the booster up to the battery terminals (is it that simple)? Does this provide a protection circuit for the Li-ion Batteries (I think it says it does but I didn't understand the whole description, plus there doesn't seem to be an online manual for it)? – Translucent Dragon Apr 4 '14 at 3:42
• Yes, it is that easy. – Filek Apr 4 '14 at 4:23
• I should clarify: I didn't use that exact unit, but a slight different one off of that site, I just didn't have time to search for the one I used. Yes, you just hook up the black wire to the neg of the battery and the red to the pos of the battery. Yes, that unit provides overcharge and over discharge protection. To charge the battery you use the micro USB, then to charge your phone you use the regular USB ports. 1 USB port can provide 1 amp, the other can provide up to 2.1 amps for tablets etc. So I soldered several batteries together in parallel and then connected up the boost/buck unit. – Filek Apr 4 '14 at 4:26
• It says it may not work on some Samsung models. Do you know if this Booster works on a Galaxy S4? Or do you know of a different booster that, for sure, work for an S4? – Translucent Dragon Apr 4 '14 at 23:19
• I believe that the unit I put the link to won't charge some Samsung phones with the cables provided. I have a feeling that if you used the able that came with your S4 it would work. This one should work also:dx.com/p/…. Also, consider joining the website in these links and then you can ask them directly if it will charge the S4. They have a forum and likely someone has bought one and can tell you if it works. – Filek Apr 5 '14 at 0:32