Help on building a "portable" battery bank

Purple_1

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Aug 10, 2016
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I am trying to build a battery bank that can charge up to five mobile devices at once, but I have absolutely no idea how i'm supposed to do this. My current plan is to get 20 18650 batteries with a capacity of 2500 mAh each, and hook them together in parallel, and then to somehow get a 20w solar panel to charge those batteries when idle. Then I would get five 5v usb ports and attach them to the batteries using some voltage regulator. My question is how can I go about this without running 74 v (my math is basic and probably wrong don't judge me) through a phone and killing all electronics I plug into it. Also, is it possible for this to not waste much if any electricity in the process of charging? I know much of this is probably wrong, and that is why I ask for help.


Info:
Solar panel - 20w,12v,1.6A(I think)
20 x 2500mAh 18650 battery cells
5 x 5v 1A usb ports
Some electronic boards that I can purchase on ebay

Thanks!
 

punkncat

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Apr 3, 2018
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Well, you have me curious. Why those specific batteries?

A thought that I have for ease would be to get a deep cycle marine (12V) battery. Make up a simple top where you can splice to the connectors with a "high" amperage car "cig lighter" charger into a hub. Connect each mobile device you want to charge off the hub with w/e format wire/plug you need. When the battery gets low you can charge from your car, a battery charger, or solar solutions. The battery will even come with a handle.
 

Purple_1

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Aug 10, 2016
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I chose those batteries with my basic knowledge of how the alternative, lead acid batteries worked. I used to use a car jump starter with a USB port to charge these devices, and it had a lead acid battery in it. It lasted for four weeks. I got another, and it lasted a year of light usage. To my knowledge lithium ion batteries have a longer lifespan than the lead acid ones.

What is the capacity of these deep cycle marine batteries? considering the high price of lithium ion batteries, about how much would I save? Also, they look like lead acid. Are they? and HOLY SHIT the amperage of these things is incredibly high. Would that mean a faster discharge capability?
 

punkncat

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Apr 3, 2018
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Deep cycle batteries are designed to be run down and recharged, unlike "car" type batteries. As with any battery, you can take it's amp hour rating and calculate your load for expected charge times for the devices.

Consider taking a look at sealed lead acid batteries, without something going wrong, they don't leak. Just heavy. There are a variety of cases and carriers you can find for these, mostly as related to it's use for a boat (trolling motor).
 
If you don't have engineering experience with lithium-ion batteries and their chargers, I would strongly discourage you from trying to build your own. Incorrectly charged, they can catch fire or explode. Incorrectly discharged, you can kill the cells after a single use.

http://batteryuniversity.com/learn/article/building_a_lithium_ion_pack

Instead, try to find a pre-made battery pack where an engineer, who knows the ins and outs of lithium-ion batteries and their charging quirks, has designed a safe and functional charger to go along with the pack.

Or if weight is not a concern, go with the aforementioned deep cycle lead-acid battery.
 

punkncat

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Apr 3, 2018
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Something to keep in mind for this build. Put in a disconnect switch, or make it easy to undo your terminals. Leaving a charger "on" even without a device being actively charged will significantly reduce your standby time.
 

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