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Building a multi-output, Multi-voltage charging system from 3.7v 18650 cells

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scornflake

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All - firstly, thanks in advance for reading.

I'd like to embark on building a charging system for mobile devices of many kinds from reclaimed 3.7v (nominal) 18650 cells from old laptop batteries. I'd configure the battery pack in 3S, but understanding the voltage of these cells fluctuates between 3v (fully discharged) and 4.2v (fully charged) it occurs to me I might need some sort of variable input on a voltage regulator to deal with the varying voltage of these cells in a 3S configuration (9v-12.6v depending on the state of charge of the pack).

I've seen variable input and output step up (for example: https://www.amazon.com/DROK-Adjusta...8&qid=1424789189&sr=8-1&keywords=drok+step+up) and step down (https://www.amazon.com/DROK-Adjusta...tabilizers/dp/B00BYTEHQO/ref=cm_cd_ql_qh_dp_t) regulators , but it's unclear if the input voltage on these can be variable (see above) or if you have to specifically select your input voltage as it changes from 12.6v (fully charged) to 9v (fully discharged).

1) Is anyone aware of a voltage converter that'll deal with the fluctuating input voltage (again, from 9-12.6v it seems to me) on the fly?
2) suggestions on limiting the whole system to stop discharging once 9v (3.0v per cell in series) is reached?
3) any other suggestions on this project?

Thanks
 
The real issue is not where you get your input voltage, it's how you control it.
LiPo batteries don't like being over-charged or over-discharged.
I've never used the following part, but have used its single-cell equivalent, which is dead easy. MCP73213
Just supply it with 12V max input and you're running.
 
My expected devices which will be charged by this system would be those which do have LiPo batteries, but would also have internal charge limiters, i.e. iPad, iPhone, Bose external bluetooth speaker, etc., so it's really just a matter of supplying (output) a reliable 5v, 12v, etc from a source battery that will fluctuate based on the status of its charge from 9v to 12.6v.

That said, thanks for the tip on the above part, but it seems that's only available in 1S and 2S variants. Nothing that'll handle 3S seemed to be available, and since I'm not charging other non-charge-controlled items (bare LiPo batteries, for instance) I'm not sure it's something I need.

Good stuff though, keep the ideas coming everyone.

Thanks again.
 
To recharge salvaged 18650s out of battery packs, each cell should be charged individually or in parallel with other cells of the same voltage and charge state if possible.
Apply 4.1000 V from any preset-fixed voltage regulated source and all will be fine and safe.
 
Thanks for the tip. Anyone with any insight on auto-regulating the voltage from a 3s pack as it fluctuates between 9v and 12.6 for the purposes of also outputting a variable regulated voltage?
 
Yes, the input voltage can vary while the output remains steady. Since you have more voltage in than you want out you need the step down (buck) regulator.
 
RonV- if I always wanted only 5v, yes, agreed... But if I'd like 5v in some cases, but 12v in others, I guess I need variable buck/boost option for when a 3s supply battery is anything but fully charged, which is, admittedly what KISS recommended (thanks KISS). Now, any way to get that without dropping $109?

I could buy an (admittedly inferior) production system for $100... The allure was building it on the cheap. :)

That said, maybe I just need to plan on 4s supply battery and always buck it down to 5v, 12v, whatever... It'd be 12v fully depleted(4 x 3.0v).

Thoughts?
 
scornflake:

On the "back burner" I want to make a DC UPS with multiple outputs. Odd vales like 17 V for a repeater. Now it's 5V and for a cell phone.
The DC UPS has to hang out unobtrusively in the living room behind a piece of furniture.

The rest of my network is on a UPS and the DSL modem is powered by POE. I do have two UPS's in the house: #1 is the cordless handsets, ans machine and lights for my elderly mom. #2. Is most of my network stuff. They are 50 W UPS;s which use very little power.

For me, I want a specific form factor.

What I have been thinking about is "some way" to avoid plugging in the wrong stuff OR some way of the "stuff" setting the voltage, current and power limits. Reverse polarity seems harder. Think "big connector with some smarts.
 
If you can't find it on Ebay they don't make it. :rolleyes:

**broken link removed**

To build one would probably cost you 3 or 5 times as much, at least. And, they aren't real easy to home brew. :(
 
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