Solar panel for lithium

v4000ds

Member
Hello. I need to make a solar charger for lithium. 3 cells, each with its own charger. Then they should be connected in series and through the stabilizer I should get a stable 5 volts.
The solar cell provides little current. The charger should continue to work until the current drops to 10 mA.
I will put the supercapacitor in parallel with the solar panel.
It will probably be useless if the charge chip is capable of automatically detecting the charge current.
But so far I do not know if she is capable of delivering the greatest current herself without squandering the source. Therefore, I will program it for the lowest current (50 mA, 10 mA).
The microcircuit must support low current.
Does the microcircuit support low current?

rjenkinsgb

Well-Known Member
The microcircuit must support low current.
The 73812 appears to be programmable down to 10mA or even lower, I'd guess a setting resistor somewhere around 47K?
(From a quick look I did not see a formula for that).

However, the IC also automatically switches on and off, depending on the input voltage being adequate to charge the connected cell, so you could set the limit somewhat higher, eg. 50 - 100mA or whatever the maximum from the solar panel is, and allow the supercap to act as a reservoir to provide current each time it reaches adequate voltage if the solar panel is not capable of supporting continuous charge.

Edit - typo corrected.

Last edited:

v4000ds

Member
What is 43812? Cheap 73812?

rjenkinsgb

Well-Known Member
What is 43812? Cheap 73812?
Sorry!
A typo for 73812... I must have seen the 4 at the start of the suffix..

v4000ds

Member
How to program it to 10mA? In a datasheet, a circuit with a resistor gives more current than without a resistor. MCP73811 without resistor 450 mA and MCP73812 with resistor 500 mA

rjenkinsgb

Well-Known Member
They are different devices with different operating modes. The input on the 11 is a logic select pin.

As I said, try 47K and see what current that gives, then adjust by trial and error; but setting a current less that the solar panels can give is just wasting energy.

v4000ds

Member
47K serial connection or between 5 and 2 pins?

rjenkinsgb

Well-Known Member
47K serial connection or between 5 and 2 pins?
Yes.

Looking through the data sheet again, I've just found the resistor <> current info.

To convert to an equation, 1000 / resistor in K Ohms = mA.
So, 47K will give around 20mA, 100K would set 10mA

v4000ds

Member
Sorry,
for MCP73811 85 mA minimum, for MCP73812 50 mA.
minimum external components for MCP73812 ceramic capacitors and 1 resistor
From datasheet:
The constant voltage regulation is fixed at 4.20 V, with a tight regulation tolerance of 1%. For the MCP73811, the constant current value is selected at 85 mA (low power USB port) or 450 mA (high powe USB port) with a digital input sugnal on the PROG input.
MCP73812 need resistor, MCP73811 85/450 mA getting information from USB port.
Resistors for MCP73812, 50 mA-20k, 100mA-10k, 500mA-2k .
Stutdown Vdd<=Vbat-100mV
On resistance 400 mO !
Junction temperature 150 degrees Celsius.

The MCP73811 suspends charge if the die temperature exceeds 150 degrees Celsius. Charging will resume when the die temperature has cooled by approximately 10 degrees. The thermal shutdown is secondary safety feature in the event that there is a failure within a thermal regulation circuity.

Feature?!

If the PCB layout used at heatsink, adding may vias in the heatsink pad can help conduct more heat to the backplane of the PCB, thus reducing the maximum junction temperature
Reducing the maximum junction temperature!

400 mOhms, 150 degrees junction temperature (75 degrees if charge current is 500mA), PCB layout must be used at heatsink. Charging will resume when the die temperature has cooled by approximately 10 degrees.
Without external thermistors.

Cheap, but worst quality!