Solar powered Arduino charging issue

I have stand alone Arduino with a GPS that is powered by a 3.7V li-po battery. To save the battery I put both the Arduino and the GPS to sleep which works very well. To keep things going I have a solar cell to recharge the battery. I am using a Max1555 1 cell li-po charger which I have seen a bunch of other people in this forum use for a similiar purpose. When I populate ONLY the Max1555 charger and decoupling caps on the PCB the charging works great. In this configuration I am measuring about 17mah which I am very happy with. To clarify I am measuring in series on the solar cell going to the charger. However, as soon as I add a load to the battery the charge current drops dramatically. I am seeing the charge rate drop to about 10mah which is not really enough for my application. My components in sleep mode draw about 200uA which should be low enough to have limited impact on charging. Has anyone else had problems using a Max1555 to charge a battery while it is still under load? I read the datasheet and did not find anything that says it can or cant charge a battery while under load.

Thanks in advance for the advice!

I'm not a pro with these, but I have several solar panels. None that small. They are a pretty much a constant current device.

Have you checked the input voltage to the 1555 from the solar panel just charging and under load?

What are the ratings on the solar panel? These might help: Maximum Power (Pmax) Voltage at Pmax (Vmp) Current at Pmax (Imp) Short-circuit current (Isc) Open-circuit voltage (Voc)

I don't know how to check for all of those specs and none are given by the manufacture. The solar cell is designed to be 5V and I have measured 6V in good sun with no load. A close approximation to what I have is this unit from solarbotics When I measure the input side of the max1555 with no load I see 4.2V with about 17mah which seems correct since this is what it is trying to charging to. When I add a small load (about 200uA) I see the solar input voltage rise to 4.9V and the current is about 10mah. When I put a heavier load on the battery (about 35mah) the input voltage is about 4.6V and the current from the solar cell remains about 10mah. i am not sure exactly what the results mean but they seem odd to me.

You might find this to be a good read about charging via solar. The solar charger she sells works great. John

Sounds like the regulator is not functioning as you expect. The info you gave me helps.

The solar panels are constant current. If the panel produces 17ma at 5v, then it should produce about 18ma short circuit, and about that current at every voltage in between.

But as you can calculate, the power it produces depends on the output voltage of the panel. At 5v output, that would be 85mw. At 1 volt output, that would be 17mw.

Since the panel output voltage is increasing when you add a load, I would think the challenge is in the battery charger, not the panel.

@bHogan: Good article on MPPT.

agreed, that is a good article and thanks for posting it!

I am not using a regulator since the GPS and Arduino work well at the li-po voltage. I did more testing and have some observations though. When I put a fully charged battery on the charger I read 5.5V on the solar input line. When the battery requires charging it appears to draws on the solar as hard as it can and brings the voltage down to 4.2V on the input side. As the battery gets close to being charged it ramps down the charge and the solar voltage works its way from 4.2V up to the 5.5V when the battery is charged and it stops. When I connect a discharged battery and put it under load (even small loads) I see the solar input voltage go up from 4.2 to 4.6-4.9V which suggests that the charger is not sensing that the battery needs to be charged. It seems to think that the battery is mostly charged and it is starting to taper off the charge. It is not clear to me why a load on the battery would impact the voltage the charger see's but somehow it seems to.

I am not using a regulator

The battery charger is a regulator of a sort. I should have use battery charger instead.

In that case it may be some type of low voltage or overheat problem. The datasheet does mention on overheat, it will taper off the charge rate current. On low voltage (3.5v) it will shut off until the input goes above 3.9v.

How does that charger do if you power it with a 5v-6v wall wart?

Edit: My apology. I was interrupted by work.

If the voltage from the solar panel drops below its Vmp (voltage at maximum power), then you are probably using more power than the solar panel is capable of producing. But I can’t see the “big picture” from here.

I just gave a 6V wall wart a try. The input voltage was steady at 6.24V with or without a load on the battery and the charge rate was about 80Mah with and without a load on the battery. So by all accounts the charger worked properly with a wall wart.

fbriggs4: I just gave a 6V wall wart a try. The input voltage was steady at 6.24V with or without a load on the battery and the charge rate was about 80Mah with and without a load on the battery. So by all accounts the charger worked properly with a wall wart.

Then in the now almost famous words of Tim Taylor, "MORE POWER!"

Solution: Bigger solar panel.

unfortunately in my case size and weight are critical so more power is not an option. Even 1 gram more would be problematic so I think I need to find an alternative charger.

May I suggest a lower current, switching-type charger. You might take a look at that other charger posted above by bHogan.