lithium ion
My system requires a power which is supplied by a Li-Po. however, I need to keep this Li-Po charging at all time so it won't die. Is it possible to connect the Li-Po to my system and also have it connected to the MAX1555 which is charging it? (think of it as using a cell phone while it's always plugged in to the wall, but I am not sure if they use a more complicated charging circuit or not)
if no, could you give me a simple recommendation ?
(the numbers are given for LiCoO2 chemistry, for LiFePO4 they should be lower) Short answer: yes, this is OK. I charge lithium cells with a bench supply set to 4.2 V and whatever C/10 figure is for the cell I'm charging. If you are not in a hurry, slow charging is better, even for cells claimed to be capable of withstanding higher charging currents.
However (quoting you):
charging at a constant voltage (say 4.2V) so long as the maximum current is limited to a reasonable value for the cell
means you will have constant current charger till your cell is at ~95%. Up to this point the voltage across the battery will be less than 4.2V if you measure it. Only when your charger starts outputting 4.2V it will become constant voltage.
What you are about to build is CC/CV charger and this is the right thing to do. "Constant voltage only" charger will be set to 4.2 V with no current limiting and it will charge the lithium cell very slowly. You can check it youself, just construct var.voltage circuit and measure the current into (discharged) cell at 3.5, 3.7, 4.0, 4.2, and 4.5 V. Cheap Chinese chargers are constructed like that, they restrict the voltage to 4.2 V so the cell won't ignite after being charged but the consumer would have to wait longer. I once bought a portable emergency charger which could be emptied in a couple of hours then took 3 days to recharge its internal cells.
There are other precautions to observe while charging lithium, you can learn them from any modern charge controller IC datasheet (my favorite is Linear Tech., their literature is very high quality). If you don't implement these precautions in your design never leave it unattended while charging, otherwise it may ruin your morning one day.
I don't know whether this is a 100% perfect solution as per my requirement or not but seems to be working good and very much similar to the solution I was looking for.
Solution is Load sharing: charging the battery and have the main circuit run normally
We can use any charge controller ic I am considering here is MCP73831 because of low cost and good features and 5-Pin SOT-23 package, just simple to use.
Will have to test this for charging cycles per day because my requirement is to have low number of charging cycles(as charging/discharging continuously is not preferred and my circuit will be at remote position and will be powered ON 24x7).
For time being this solution looks good to me.
References: Thanks Microchip for good explanation on Load sharing concept ww1.microchip.com/downloads/en/AppNotes/01149c.pdf
and a good blog from
blog.zakkemble.co.uk/a-lithium-battery-charger-with-load-sharing/
as I have lower reputation so cannot share more than two links hence links posted as a string(sorry...)
Please correct me if I am wrong.
Best Answer
What you're describing is an "always-plugged-in" device with a "backup" battery. In addition, the "backup" battery may be able to provide significantly more current than the built-in power source, for short cycles, and when the load is removed, the battery is charged back up.
LiPo batteries don't like staying at top voltage (4.2V rated, typically) "trickle charging," because this will metalize the lithium, which will kill the battery.
However, it is safe to "float" a lithium polymer cell at a lower voltage -- typically somewhere between 3.9V and 4.05V, depending on the manufacturer and cell specifics. Thus, it is totally safe to design a cell "charge/float" circuit that provides a float voltage that won't go above the safe float voltage. This circuit will basically provide no current into the battery when there is no load. When there is load applied, the power will come from the battery and from the "float" voltage in relation to their respective source impedances (internal resistances.)
If the use is for something like an "always ready flashlight" that can also be on while plugged in, yes, you CAN build one of those with a LiPo (or a LeFePO4, which is a little safer, but has lower voltage per cell) by floating the cell at the appropriate voltage. You have to design the source impedance of the charger/floater to have the appropriate characteristics for the case of a heavy load on the output, versus charging a discharged cell, versus there being no load and a "full" cell.