My system requires a power which is supplied by a Li-ion battery. However, I need to keep this battery charging at all time so it won't die. Is it possible to connect the battery 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 ?


2 Answers 2


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.

  • \$\begingroup\$ Thanks a lot Jon, this answer is more relevant to my question, however, the problem that I am having is I can't get my source directly from my solar panels (not sure if mentioned i'm using panels) the weather is not that great here so panels are just a way to keep my batteries full, while the batteries are supplying the main power to my system. Like a water tank which is being filled by rain once in a while. I hope you get my point \$\endgroup\$
    – Caspian
    Aug 15, 2013 at 13:38
  • \$\begingroup\$ That use case will still work with the "always plugged in" design, even though it's not "always" plugged in, as long as the sum total of the charging events is higher than the sum total of energy draw! However, your suggested panels seem too weak! There exists micro-energy harvesters and solar chargers that are designed to solve exactly this problem already. If you want to just get it done, try something like the LiPo Rider: seeedstudio.com/depot/lipo-rider-pro-p-992.html (This particular thing may or may not fit your specific need, but it's an example of the use case.) \$\endgroup\$
    – Jon Watte
    Aug 16, 2013 at 15:48
  • \$\begingroup\$ wow, awesome, awesome , and awesome. You got my problem solved. Thanks \$\endgroup\$
    – Caspian
    Aug 16, 2013 at 16:34
  • \$\begingroup\$ Isn't this simultaneous charge/discharge behavior the same thing that laptops do with their batteries? \$\endgroup\$
    – Ian
    Jul 2, 2014 at 23:29
  • \$\begingroup\$ There is no "simultaneous charge and discharge." Depending on whether the externally supplied energy is higher or lower than the draw from the device, the battery will charge or discharge by whatever the difference is. \$\endgroup\$
    – Jon Watte
    Jul 4, 2014 at 0:57

Think about what you are asking

You say: -

I need to keep this Li-Po charging at all time so it won't die

"Charging at all time" MUST mean you have power all the time so why not get rid of the LiPo and feed the circuit (load) from a regular power supply.

If in fact you don't charge all the time, then why not switch your circuit (load) to the alternative power source (via a regulator) when it is available. Then you don't have to worry about the battery charging complications.

  • \$\begingroup\$ Most source switching circuits are actually pretty annoying to get right, if you want them to also not drive the battery down to dead when the cell voltage goes too low. There's also the problem of not back-driving regulators, which may not be particularly happy about that. If you can manage it, a battery in parallel with a charger is a cheaper, simpler, solution. \$\endgroup\$
    – Jon Watte
    Aug 10, 2013 at 0:52
  • \$\begingroup\$ Well as I explained below on Jon's comment, I have a board which draws ~ 1 Amp @ 5V constantly, I am using two small panels in parallel which provides 1 Amps @ 5 Volt(Best case). obviously these panels alternate constantly so I need to have a more reliable source which keeps my main board going, and I am using the panels to "re-fill" my batteries whenever they get some light. \$\endgroup\$
    – Caspian
    Aug 15, 2013 at 13:43
  • \$\begingroup\$ @Caspian - I think you need to revamp your question to precisely indicate what your current setup is and what you see as current difficulties and what you expact from an answer. If solar can't supply enough energy over the longer period then an AC supply is the only option - that's currently what I infer from your comment. \$\endgroup\$
    – Andy aka
    Aug 15, 2013 at 14:06

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