USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultracap

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pedrorojas
 
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USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultracap

Post by pedrorojas »

I want to create a system backup for my Arduino project and this device have everything I need, the problem is that I don't know if the MCP73871 can take a short-circuit in the Vbat pin. Can somebody help me with this doubt? Is important that I gonna use a solar panel as the power source.

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zener
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by zener »

Are you saying you want to use this to charge a ultracap instead of a battery?

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pedrorojas
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by pedrorojas »

Exactly, the ultracap is a short-circuit when it hasn't charge.

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zener
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by zener »

Have you seen an app note suggesting that IC for that application? It may be a bit too smart for that.

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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by adafruit_support_bill »

I'm pretty sure that the charger chip would interpret a short as a battery fault.

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pedrorojas
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by pedrorojas »

Ok and, in this condition, is it going to shut down or cut off the output current?

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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by adafruit_support_bill »

Yes. As a safety measure. It is dangerous to charge a LiPo cell that has been damaged or over-discharged. And the charger has no way to distinguish between a LiPo and a capacitor.

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zener
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by zener »

What is it you like about this IC for charging the supercap? Usually a constant voltage is used (regulator) with some current limiting for the start up condition. A series resistor is a simple way to handle that. A supercap is a bit "simpler" than a battery. The main thing you need to avoid is overvoltage.

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pedrorojas
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by pedrorojas »

Thank you both, and do you know how can I limit the current without a resistor? this take, minimum, the have of the power and I need to make it with very low power consumption.

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zener
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by zener »

What is your power source and what is the specific capacitor? It depends on all the specifics. Keep in mind the resistor would only waste a lot during the initial charge, then would dissipate much less. There are lots of ways you could do it but it is hard to recommend without knowing the details. Some super caps such as Panasonic "Gold" caps have much higher ESR so they will not present such a big load as other types. I designed a supercap backup circuit once and it required a lot of calculations and testing to get what we needed. I had a customer once who had a whole forest of caps on a board. About $400 worth per board. I don't know how he charged it though.

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pedrorojas
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by pedrorojas »

The power source is a solar panel (7 W and 5 V output) the load is an arduino with several sensors (I think that the all load can consume a peak of 300 mA) the supercap is a Maxwell K2 series BCAP2000, 2000F, ESR = 0.35 mΩ, 2.7 of rated voltage and I'm gonna use two in series. I'm thinking about using a boost converter for using almost all the power of this bank of supercaps. But a important objective of the project is make it very very efficient in the power consumption issue.

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zener
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by zener »

Those details help. But it is still a "Devil in the details" design. That is a big cap.

For reference, here is a part that is built for this job:

http://cds.linear.com/docs/en/datasheet/4425f.pdf

One important thing this chip does is keep the voltage balanced between the caps, so one doesn't get over charged. It also takes care of the max current, which is selectable.

The design I worked with before used a booster for the exact same reason you state. But in the end it didn't add much life. The reason is, as the voltage sags, the booster draws even more current, making the voltage sag faster, making it want even more current. You do get some extra life but not as much as you think. And you have the overhead of the booster quiescent current. I would say you might get more return by trying to reduce your load as much as possible.

So you have a balancing act between the output of the panel, the needs of the cap and the needs of your circuit. But it is a good one to play with and you will learn a lot.

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pedrorojas
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by pedrorojas »

I got it, thanks. I choose the LTC4425 but I have a doubt, in the datasheet said that I just have to connect the load to the supercap bank, and they gonna limit the current to the supercap but I don't know if the load can be keep running while the supercap charges.
This works in that way?

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zener
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by zener »

The load will just see the voltage that is on the cap at that time. The voltage will change very slowly. Whether it goes up or down will depend on how much current is going into it and how much current is coming out of it. Think of it like a big tank of water. You can add water at one place and you can take water out at another place. You can do both at the same time. My main concern with that part is the small package. Super tiny. Hopefully they sell a development board for it. I am curious how they handle the current limit. They might PWM it but I am guessing they just create a variable resistance and the heat that goes along with it.

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pedrorojas
 
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Re: USB / DC / SOLAR LITHIUM ION/POLYMER CHARGER with ultrac

Post by pedrorojas »

Ok I got it. The problem is to make a backup but I'm thinking in put a Schottky diode in the input line and another one after the supercaps. Another thing is, I have doubt about boost the solar panel. I read that is a normally practice but I wanna use the LTC3128 for the supercaps and make the diode's arrangement but the "load" will be another boost converter and after that it's going to be the real load. Is a good idea boost twice the solar panel? Do you have a better idea?

Think that I'm complicating the design but I need to let it a 100% functional without mistakes.
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