Super simple solar LiPo-charger

Thread Starter

Engineerer

Joined May 22, 2013
18
Hi

I have a 2000 mAh 3,7 V (nominal) mobile phone battery that I would like to charge super simply. Mostly because I don't have space for circuit board.

I ordered 5,5 V, 110 mA solar panel. I need some kind of regulation because LiPo cell voltage should not go over 4,2 V

My idea was to use AP2125-4.2 fixed LDO. It outputs 4,2 V. Max output current is 300 mA, so I don't need over current protection between solar and LDO.

Will this LDO drain my battery if solar panel won't produce voltage? meaning output voltage is higher than its input.

Datasheet: http://diodes.com/datasheets/AP2125.pdf
 

Thread Starter

Engineerer

Joined May 22, 2013
18
Are you sure I need special profile since my charge current is only around 100 mA or 0,5 C?
 

Papabravo

Joined Feb 24, 2006
12,405
You absolutely do need this unless you're fond of performing the stupid exploding battery trick. Anything with Lithium in the name is nothing to mess with. Be safe or win a Darwin Award.
 

Thread Starter

Engineerer

Joined May 22, 2013
18
Have you actually ever worked with Lithium-Polymer battery? Why would lithium battery explode if it ain't overcharged? I'm not sure what I should read from batteryuniversity. I read http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries and it gave me no new info.

Hmm.. Maybe that AP2125 won't work as I wanted. I think it shuts down because it can't raise the output voltage to 4,2 V. I have manually charged Lithium-batteries with laboratory power supply with current limit, but AP2125 can't work the same way.

Maybe I'll try to find approriate IC that can lower its output voltage to prevent over current and stop at ~4,2 V. Maybe specific Lithium-ion charger IC. It would have to be SMD simple (no other connections) because space is tight.
 

NorthGuy

Joined Jun 28, 2014
611
The simplest form of charger is a comparator which connects the battery when its voltage is lower that your threshold and disconnects it when it gets higher than this.
 

ian field

Joined Oct 27, 2012
6,543
You absolutely do need this unless you're fond of performing the stupid exploding battery trick. Anything with Lithium in the name is nothing to mess with. Be safe or win a Darwin Award.
Its worth checking the E-cigarette suppliers for LiPo guard flame resistant charging bags whether you use a proper manufactured charger or a home brew one.

My first reaction would be to use the actual mobile phone as the charge controller, my usual salvage source recently gave up 4 mobiles in one go - all had batteries.

For months now, I've been using a pretty simple TL431 based shunt regulator for my E-cig 18350 with no problems (yet).

The prototype was the bare TL431 and a current limiting resistor selected to pass the max 100mA the TL431 can handle when no battery is connected.

Normally the battery charges within 24h and is swapped over to use every morning - although once or twice I've forgotten and a battery has been left charging for just under 48h.

It probably could be a problem if left on charge for long periods, but that situation hasn't come up yet.
 

Thread Starter

Engineerer

Joined May 22, 2013
18
Otherwise this circuit seems to be perfect, but my charging current should be bigger than 50 mA.

I chose MCP73832-2DCI/OT. It is integrated LiPo-charger circuit with programmable charging current up to 500 mA

https://www.sparkfun.com/datasheets/Prototyping/Batteries/MCP73831T.pdf

It seems that solar panel gives it maximum power when it is loaded so that its voltage is around 0,76 x open-circuit voltage. Current depends on how much light the panel gets.


This picture is for ~37 VOC solar panel. As you can see the largest amount of power is drawn when voltage is around 0,76 x VOC. In the picture's case 37 V * 0,76 = 28,1 V. MPPT chargers track maximum-power-point by changing load.

I think I'm going to set my charging current to 100 mA. (My panel is rated for 110 mA). In optimum lighting conditions I should get pretty much everything my panel can offer.

When light intensity falls my charger continues to draw around 100 mA until solar panel voltage falls under UVLO (Undervoltage lockout). Which is around 3,5 V in my charger. When UVLO is reached charger stops charging and disconnects load from solar panel. When load is disconnected solar panel voltage rises again and goes out from UVLO. Then charger should load panel until UVLO is reched again.

UVLO at 3,5 V is about 0,63 * VOC so I should stay pretty close to MPP (Around 85% of maximum power) before charger starts switching.

This is how I think it works.
 
Last edited:

Metalmann

Joined Dec 8, 2012
692
Its worth checking the E-cigarette suppliers for LiPo guard flame resistant charging bags whether you use a proper manufactured charger or a home brew one.

My first reaction would be to use the actual mobile phone as the charge controller, my usual salvage source recently gave up 4 mobiles in one go - all had batteries.

For months now, I've been using a pretty simple TL431 based shunt regulator for my E-cig 18350 with no problems (yet).

The prototype was the bare TL431 and a current limiting resistor selected to pass the max 100mA the TL431 can handle when no battery is connected.

Normally the battery charges within 24h and is swapped over to use every morning - although once or twice I've forgotten and a battery has been left charging for just under 48h.

It probably could be a problem if left on charge for long periods, but that situation hasn't come up yet.


We've been "Vaping" for a few years now, and we leave the batteries plugged in all the time.
The chargers run 24/7, for at least 3-4 years.

The only problem I've seen with those batteries, is forgetting to wipe all liquid off before screwing them in. That will cause the contact corrosion.

When the tips get corroded, I use my Foredom tool with a tiny steel wire brush, to polish the contacts right back up to new condition.
 

Thread Starter

Engineerer

Joined May 22, 2013
18
I already ordered MCP73831. It is complicated but cheap chip. And you only have to put one external resistor to set charge current.
 

alfacliff

Joined Dec 13, 2013
2,449
kit probably wont help, but I was looking for a solar charger for an Icom 2at some years ago, and happened to find that a small 12 volt solar panel from radio shack had the same ratings as the wall wart that came with the radio, 12 volts at 25 ma. problem solved, the charge circuit was already in the battery pack on those radios.
 

MrAl

Joined Jun 17, 2014
6,489
Hi,

I dont know if i would trust that AP-4.2 chip because it's voltage could actually go over 4.2v by an amount that would be considered too high for an Li-anything cell.

I might be OK with an adjustable version though, where i could set the output voltage with two resistors and then set the voltage for a bit less than 4.2v, more like 4.15v. Maybe the 4.15v version would work better.

The charge current might not be that great though. Maybe around 70ma to 80ma nothing near 300ma.

How the regulator behaves when the output is clamped to say 3.0v and the input is current limited remains to be seen. A guess would be that the regulator drops maybe 0.1v so the input will be clamped to around 3.1v and so current will be reduced due to the curve of the solar array. If you can order a chip you can test it. I dont think it will go into short circuit current limit but that's still 50ma.
 

Thread Starter

Engineerer

Joined May 22, 2013
18
I'm not using AP-4.2. I will use MCP73831 lithium charger circuit. 4,2 V version output 4.232 V at max (tolerance). Actually my phone's (Ascend p6) battery is 3,8 V nominal. Huawei's software shows over 4,3 V when fully charged.

My charging current is not going to be anywhere near 300 mA. I bought my panel rated 5,5 V 110 mA, but I'm afraid the current ain't gonna reach 100 mA.

I measured current of 30 mA with multimeter with sunlight coming trough window (short-circuit current)...
 

ian field

Joined Oct 27, 2012
6,543
We've been "Vaping" for a few years now, and we leave the batteries plugged in all the time.
The chargers run 24/7, for at least 3-4 years.

The only problem I've seen with those batteries, is forgetting to wipe all liquid off before screwing them in. That will cause the contact corrosion.

When the tips get corroded, I use my Foredom tool with a tiny steel wire brush, to polish the contacts right back up to new condition.
There are some real cheapies turning up on the market with no charge termination cut-off chip - every few weeks there's a house fire on the news.

The ones that look like a cigarette and have disposable cartomiser were never intended to be topped up with fluid, if you're not careful it can run through and contaminate the sensor capsule containing the pressure sensor and charge control.

At best it will become unreliable when you draw on it - at worst it can screw up the charge control with the potential for venting with flaming gas!

I've not had any of these problems with the clearomiser that was designed to be refilled in the first place.

Even with (unauthorised) refilling; the disposable cartomisers work out pretty nearly as expensive as smoking - its well worth investing in a clearomiser/tank and battery system.

The 510 thread is the most common, and a 510 clearomiser (that I know of) will fit an ego battery - but not an ego clearo on a 510 battery.
 

ian field

Joined Oct 27, 2012
6,543
Give post #8 a read again if you want simple.
Another option is to strip the charge control capsule from an e-cig, but it'll take more than 24h to charge a typical phone battery.

It might be possible to reduce the current limiting resistor (in the adaptor between the e-cig & USB wall-wart) but you have to monitor the capsule temperature for a while before declaring job done.

The breakdown voltage on the switching semiconductor probably isn't over generous, so a shunt regulator to ensure the solar output doesn't exceed 5V is advisable.

A while back I won a Nikkon S220 with no charger, I was able to charge the battery with an e-cig setup - but it took a while.
 
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