Small solar panel + TP4056

[doobedoobedo]

I'm working on a project which I want to be solar + a single 18650 powered. Power is only used occasionally, but I want to keep the battery topped by solar.

I have a 250mA 6V open circuit solar panel which I hooked up directly to a TP4056 board only to discover that this would only charge to about 3.6V after lowering the charge current I can get the battery to about 3.8V.

So I decided I needed to do some more reading . Now I understand that the voltage from the solar panel drops rapidly when too much current is drawn I was wondering about how to solve this.

I found this article http://www.eetimes.com/document.asp?doc_id=1273171 which suggests using the bq24071, but they're quite expensive and not particularly readily available. Can anyone suggest an alternative chip or approach?

My first thought is to use a fet in series with R_prog on the TP4056 which could be tuned to increase the resistance as the voltage from the panel falls. Has anyone tried this? Or have any other ideas?

[fourtytwo42]

I think your problem is your solar cell is a bit small for the application, it sounds like it's IOC is 250mA well that might mean its only reasonable to expect to get about 150mA out of it at some reasonable voltage so that should be your starting point for your charge current setting. Unfortunately a simple linear charger will simply clamp the cell input to the battery voltage because the current cannot rise to the setpoint, the cell on the other hand cannot generate any more voltage as its current capacity is exceeded. Try to get a spec for your pv panel or similar with mppt graphs and you will see the problem. Mppt controllers are expensive, they are possible to DIY but take quite a lot of work.

[doobedoobedo]

I changed the R_prog for a 10k which drops the charge current down to 130mA I could easily drop it some more. I think I need to do some more calculations on how much power it's actually going to use.

The battery will power an arduino and a couple of LEDs for about 5 minutes (50mA) and a servo for about 10 seconds at around 1 amp draw each week, so even with no charging the battery should last a while. I'm probably over-thinking it as by my reckoning that's about about 200mAh/year, call it 300mAh from the battery after boosting to 5V and the battery would still last 4 years without a charge. So anything it gets is a bonus. I'll just put a higher value resistor on it so it charges over the summer.

[David Hess]

Use a shunt regulator which can dissipate the maximum power available from the solar panel, just a couple watts, and it will have no trouble charging the battery.

[doobedoobedo]

I hadn't heard of that. Looks like I have some more reading to do .

The max output of the panel is a massive 1.25W

[aries1470]

@doobedoobedo

Thanks for your post. I had been looking at a similar scenario in the past, of which a PowerPath IC would be bet suited.
I ended up though with a simple design that included the MCP73831 in SOT-23-5, 100mA charge, and a MIC5252.
Here are the links on digikey
Unfortunately, I didn't end up making the board due to other things getting in my way.

So what would happen is, since the solar panel will get loaded, it will drop under 6V, so you are within its operating voltage. Now, the panel I had is a 9V version, so it made sense to use the LDO, in your case, you can omit it.

Now, in regards to this post:

I changed the R_prog for a 10k which drops the charge current down to 130mA I could easily drop it some more. I think I need to do some more calculations on how much power it's actually going to use.

The battery will power an arduino and a couple of LEDs for about 5 minutes (50mA) and a servo for about 10 seconds at around 1 amp draw each week, so even with no charging the battery should last a while. I'm probably over-thinking it as by my reckoning that's about about 200mAh/year, call it 300mAh from the battery after boosting to 5V and the battery would still last 4 years without a charge. So anything it gets is a bonus. I'll just put a higher value resistor on it so it charges over the summer.

You are making some fundamentally flawed assumptions, sorry mate.

You will ALWAYS be consuming electricity, unless you have a physical switch to turn things off. Second, you forgot the self discharge of the battery. Li-Ion batteries hate that! Please read up on the maintenance of Li-Ion batt's. Keeping the battery topped up constantly at 4P2V for a long period of time will degrade it quickly, same as keeping it non charged. For long term storage, it needs to be held at around 3P6V.

As for using a higher value resistor, you will then be better off using a powerpath chip, leaving it at 200 mA - 500 mA. That way, you don't need to do a thing, as you can connect the bost circuit to the output, and if there is not enough current on the input, it will se it from the battery.

I found these chips suited best for a simple circuit that I was aiming for, and think they will be for you too:
LTC4099 You can use i2c to set it up and monitor it too. Great for use with Arduino, and yes, you can use it as a normal charger too.
It needs a front end though for the voltage, and a great solution would be this one:
LTC3129 or the LTC3129-1 for a fixed voltage.

There are also solutions from T.I. and a few otehr vendors.

The question is, did you buy a ready made module, or are you able to make your own PCB's?

[doobedoobedo]

ok the whole circuit as it stands is:

solar panel -> tp4056 with battery protection -> b6286y based chinese boost board (via an N-mosfet on the low side, 15k resistor from G to S, normally off push switch between S and D. Pushing the switch boots the arduino which holds the gate high for a set time) -> arduino, servo, RC522 RFID board.

Off current is limited by battery voltage and the 15K GS resistor as that's the only path to ground. The 'on' LED on the arduino does glow very faintly when it's off, but it can't be drawing any more than 300uA, I can't measure it with the equipment I have. I don't have Dave's uCurrent, which is what I would need, my multi-meter on the 2000uA range reads zero, but the led goes out when I put it in circuit...

@aries1470
I'm using an old chinese 18650 which has been sat unused at 3.9V for about 3 years. Li-Ion cells have practically no self-discharge. The charge will deplete over the winter and with the low power available from the solar panel I very much doubt if it'll get back up to 4.2V for long, if at all (read my first post). I was looking for a simple, cheap, very low power mpp type thing (IC or circuit), which it seems doesn't exist.

It's been interesting to read up on the suggestions, but it'll probably stay as it is. It's for key-fob entry to my back gate, it's not as if I won't have a work-around if I need to take the battery out and charge it periodically, or even replace it if it fails due to being at 4.2V too long.

[doobedoobedo]

I know this is an old topic, but I've found a more or less drop in replacement for the TP4056, the Consonance CN3063 http://www.consonance-elec.com/pdf/datasheet/DSE-CN3063.pdf. I'll need to change the charge current resistor, cut a trace on the board and add a bodge wire (pin8 goes to the battery +ve not the input). It does exactly what I want.