user34854: I have a system that uses an Arduino Uno board as a "brain". All the elements in the system are powered by a single LiPo battery (3cell, 11.1V, 1800mAh). I made a circuit so the arduino can check the battery voltage and decide when it needs to recharge. I also have a circuit that controls the battery charge sequence and interrupts the power when the battery is charged, sending a signal to the arduino interrupting it's power saving mode. The arduino enters the power saving mode when the charging sequence is initiated. The charging circuit uses 12V and 450mA which I plan to supply with a 12V 6W solar panel.
My question is: can I charge the battery while the arduino is still on, but on power savig mode?
If not, how can I make this system self charging while keeping the arduino continuously on?

user31191: If you need to charge it and rum at the same time, I would recommend getting a Poweboost module from Adafruit. They can handle both, and they also have a low battery indicator. 

user27050: Application Note AN1149 by Microchip Inc:

It is NOT encouraged to attach the system load directly to Li-Ion
  batteries when using a stand-alone Li-Ion battery charge management
  controller with automatic termination feature. 


Why not?

1. The charge may never end. 
Most Li-Ion battery chargers are based on
  Constant Current and Constant Voltage (CC-CV) modes. The termination
  is based on the ratio of charge current and preset constant current
  (Fast Charge). If the system draws current from the battery, the
  charge current will never meet the termination value. This causes the
  non-termination of the charge management circuit.
2.  The total system current is limited by the charge
  current.
It is because the charger will deliver total
  system and battery charging current through the
  output pin. This solution may be feasible for
  some applications that run on constant current,
  but it is not recommended.


So what's the alternative?
A load-sharing switch with a P-MOSFET:


Q1 is OFF When Gate is High
  and No Current Flows from the Battery Cell to the
  System Load



Q1 is ON When the Gate is
  Low and Current Flows from the Battery Cell to
  the System Load.

The App Note contains some really good practival advice on component selection, I highly recommend it.

user4143: 
can I charge the battery while the arduino is still on, but on power savig mode?

You can draw power from a Li-Ion battery whilst charging it, but it isn't recommended. This is because the load on the battery affects the ability to detect the different phases of charging, and determine the end-of-charge condition.
Instead you should provide a bypass facility so that if external power is applied the external power is routed to the Arduino to power it, but when the power is removed the battery provides the power to the Arduino.
The circuit I usually use is:

simulate this circuit – Schematic created using CircuitLab
When 5V is applied the battery charges up. Q1 is held off. The 3.3V regulator is then powered through D1. When the 5V is removed Q1 is turned on by R1 pulling the gate low and the battery powers the 3.3V regulator.

34854: <p>I have a system that uses an Arduino Uno board as a "brain". All the elements in the system are powered by a single LiPo battery (3cell, 11.1V, 1800mAh). I made a circuit so the arduino can check the battery voltage and decide when it needs to recharge. I also have a circuit that controls the battery charge sequence and interrupts the power when the battery is charged, sending a signal to the arduino interrupting it's power saving mode. The arduino enters the power saving mode when the charging sequence is initiated. The charging circuit uses 12V and 450mA which I plan to supply with a 12V 6W solar panel.</p>

<p>My question is: can I charge the battery while the arduino is still on, but on power savig mode?</p>

<p>If not, how can I make this system self charging while keeping the arduino continuously on?</p>
 31191: <p>If you need to charge it and rum at the same time, I would recommend getting a Poweboost module from Adafruit. They can handle both, and they also have a low battery indicator. </p>
 27050: <p><a href="http://ww1.microchip.com/downloads/en/appnotes/01149c.pdf" rel="nofollow noreferrer">Application Note AN1149</a> by Microchip Inc:</p>

<blockquote>
  <p><strong>It is NOT encouraged to attach the system load directly to Li-Ion
  batteries when using a stand-alone Li-Ion battery charge management
  controller with automatic termination feature.</strong> </p>
</blockquote>

<p><a href="https://i.stack.imgur.com/EEVtp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/EEVtp.png" alt="enter image description here"></a></p>

<h2>Why not?</h2>

<blockquote>
  <p><strong>1. The charge may never end.</strong> </p>
  
  <p>Most Li-Ion battery chargers are based on
  Constant Current and Constant Voltage (CC-CV) modes. The termination
  is based on the ratio of charge current and preset constant current
  (Fast Charge). If the system draws current from the battery, the
  charge current will never meet the termination value. This causes the
  non-termination of the charge management circuit.</p>
  
  <p><strong>2.  The total system current is limited by the charge
  current.</strong></p>
  
  <p>It is because the charger will deliver total
  system and battery charging current through the
  output pin. This solution may be feasible for
  some applications that run on constant current,
  but it is not recommended.</p>
</blockquote>

<hr>

<h2>So what's the alternative?</h2>

<p>A load-sharing switch with a P-MOSFET:</p>

<p><a href="https://i.stack.imgur.com/wv3FD.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wv3FD.png" alt="Plugged in"></a></p>

<blockquote>
  <p>Q1 is OFF When Gate is High
  and No Current Flows from the Battery Cell to the
  System Load</p>
</blockquote>

<p><a href="https://i.stack.imgur.com/WAkg1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WAkg1.png" alt="Battery-powered"></a></p>

<blockquote>
  <p>Q1 is ON When the Gate is
  Low and Current Flows from the Battery Cell to
  the System Load.</p>
</blockquote>

<p>The App Note contains some really good practival advice on component selection, I highly recommend it.</p>
 4143: <blockquote>
  <p>can I charge the battery while the arduino is still on, but on power savig mode?</p>
</blockquote>

<p>You can draw power from a Li-Ion battery whilst charging it, but it isn't recommended. This is because the load on the battery affects the ability to detect the different phases of charging, and determine the end-of-charge condition.</p>

<p>Instead you should provide a <em>bypass</em> facility so that if external power is applied the external power is routed to the Arduino to power it, but when the power is removed the battery provides the power to the Arduino.</p>

<p>The circuit I usually use is:</p>



<p><img src="https://i.stack.imgur.com/hA5Yg.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fhA5Yg.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="noreferrer">CircuitLab</a></sup></p>

<p>When 5V is applied the battery charges up. Q1 is held off. The 3.3V regulator is then powered through D1. When the 5V is removed Q1 is turned on by R1 pulling the gate low and the battery powers the 3.3V regulator.</p>
