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Large Portable DC Bank
#1
I am still short on cells, and time, at the moment. I have about 70 so far, and have two projects planned. For one, I want to build a high capacity portable charger. My daughter is involved in club volleyball and it is not uncommon to go 12 hours not being near a USB charger. I am looking into making a high capacity portable box, probably coming in around 80k mah. Just cause I can.

Check my thinking... I can wire this up, all positives, and all negatives, providing a bank wide voltage of 4.x. Then I want to locate some of those USB charger chips, around 10x and wire those up. Does anyone see a problem using this method? From what I can tell of those cheap single cell 18650 pocket chargers, they must step up the voltage of the battery to 5v at the cost of capacity? Then find a method to charge the bank as well. Are there any flaws to my thinking here?
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#2
You should be fine using one of those 5v units you mentioned. Then just parallel a ton of cells together. You'll be wise to put a separate voltage meter to monitor low end voltage so you don't drain below 3v.
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#3
(06-22-2017, 04:55 PM)Toochi84 Wrote: You should be fine using one of those 5v units you mentioned. Then just parallel a ton of cells together. You'll be wise to put a separate voltage meter to monitor low end voltage so you don't drain below 3v.

Good point, thanks! Those little chips probably contain a meter anyway, but, in either case, do you have any you recommend? Or would I be ok to just find any good inexpensive meter with good reviews?
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#4
You can get them for $1-$2each on eBay. Just search "mini digital voltmeter" make sure you find the one that has the adjuster pot on the back. The volt range should be 2.5-30 I believe

Example:
There are other colors around, just browse.

http://www.ebay.com/itm/New-ONE-Mini-DC2...SwDrNZR1ru
Cell progress: 2016/10/17:                                       Business Owner                                       Projects:
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600cells@2.1-2.6a                                                                                                                8Kw Powerwall  
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#5
Just remember that the more cells in parallel, the less mA each one will get. If your charger puts out 1A max, then with 10 cells each cell will get a max of 100mA. If you have 20 cells, each will only get 50mA. That equates to a longer charge time.
Also note it is not good practice to parallel the chargers as they are not in sync with each other. Meaning, with two chargers reading 1 cell, 1 charger may see 4.121V, the other charger may see 4.128V. This may not seem like a lot, but if the 2nd charger got the cells to 4.2V and shut off, the 1st charger will continue to pump power into the pack and essentially over charge the cells.

Or, another way to look at it:
Each charger charges at 1A. You put 5 chargers on the pack connected in parallel. You figure based on those numbers it will take an hr to charge (assuming you know the mAh of the pack). Well, as the pack is charged, the chargers will start shutting down at different times. Your charge time is not increased as they don't read the same voltage and as each charger shuts down, there's less amps going into the pack.

Just something to think about.
What pocket chargers were you referring to, btw. Are they TP4056 modules we've been using?
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#6
(06-22-2017, 05:01 PM)Toochi84 Wrote: You can get them for $1-$2each on eBay. Just search "mini digital voltmeter" make sure you find the one that has the adjuster pot on the back. The volt range should be 2.5-30 I believe

Example:
There are other colors around, just browse.

http://www.ebay.com/itm/New-ONE-Mini-DC2...SwDrNZR1ru

Thanks! This I assume just wires up between the batteries and the USB port/chips?

(06-22-2017, 05:56 PM)Korishan Wrote: Just remember that the more cells in parallel, the less mA each one will get. If your charger puts out 1A max, then with 10 cells each cell will get a max of 100mA. If you have 20 cells, each will only get 50mA. That equates to a longer charge time.
Also note it is not good practice to parallel the chargers as they are not in sync with each other. Meaning, with two chargers reading 1 cell, 1 charger may see 4.121V, the other charger may see 4.128V. This may not seem like a lot, but if the 2nd charger got the cells to 4.2V and shut off, the 1st charger will continue to pump power into the pack and essentially over charge the cells.

Or, another way to look at it:
Each charger charges at 1A. You put 5 chargers on the pack connected in parallel. You figure based on those numbers it will take an hr to charge (assuming you know the mAh of the pack). Well, as the pack is charged, the chargers will start shutting down at different times. Your charge time is not increased as they don't read the same voltage and as each charger shuts down, there's less amps going into the pack.

Just something to think about.
What pocket chargers were you referring to, btw. Are they TP4056 modules we've been using?

I was going to eventually ask about parallel charging, so you are a step ahead thanks. I won't need to charge the bank quickly, so 1A is fine by me. I will have a minimum of a week to charge the bank, probably more.

If I wire up 10x USB ports, will they all be able to draw the max current they are designed for? I don't see why not until the bank voltage drops to the cutoff.

Yes, I have browsed those TP4056 but haven't looked in detail yet. Here is a link to one similar to what I talking about, figure one mini USB charging input, and 10 or so USB-A female for output. Haven't gotten far enough in my thinking to determine if I want to go from 1A to 2A on any of them.

https://www.walmart.com/ip/ALLPOWERS-540.../165562997
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#7
Maybe you can use a buck/boost converter that has a fault( cut off): http://www.ebay.ca/itm/LTC3780-DC-5V-32V...210e6c111b
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#8
(06-22-2017, 08:14 PM)Jeremy Choy Wrote: Maybe you can use a buck/boost converter that has a fault( cut off): http://www.ebay.ca/itm/LTC3780-DC-5V-32V...tem210e6c1
 Interesting though, I will look into it. Thanks
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#9
If your activity is outdoor most of the time.... another alternative is to use a small foldable 70-80W Solar Panel USB charger... is light and portable.
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#10
(06-22-2017, 04:26 PM)ridiculously_necessary Wrote: I am still short on cells, and time, at the moment. I have about 70 so far, and have two projects planned. For one, I want to build a high capacity portable charger. My daughter is involved in club volleyball and it is not uncommon to go 12 hours not being near a USB charger. I am looking into making a high capacity portable box, probably coming in around 80k mah. Just cause I can.

Check my thinking... I can wire this up, all positives, and all negatives, providing a bank wide voltage of 4.x. Then I want to locate some of those USB charger chips, around 10x and wire those up. Does anyone see a problem using this method? From what I can tell of those cheap single cell 18650 pocket chargers, they must step up the voltage of the battery to 5v at the cost of capacity? Then find a method to charge the bank as well. Are there any flaws to my thinking here?

Or....

Just get a bunch of these off of ebay.  I work with kids doing robotics.  I got 5 of these and I filled them with 2500 mah cells.  So I guess that I am at 50k mah...

They are easy to charge from any USB port.  And in a crunch I can just swap out flat cells for freshly recharged cells, giving and effective recharge time of 0 minutes.

http://www.ebay.com/itm/4-Slot-18650-Bat...2749.l2649

 

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