All About Circuits Forum LiFePO4 Simple Charger circuit
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#1
04-18-2011, 05:00 PM
 dweeb4 Junior Member Join Date: Apr 2011 Posts: 12
LiFePO4 Simple Charger circuit

My first post here & it will become clear fairly quickly that I'm no E'ee but I can put together a circuit from a schematic & understand basic electronics.

Now, I want to do a simple LiFePo4 battery charger for the 26650 cells (2300mAh) . I know a bit about these batteries. I know that the ideal charging algorithm is a constant current charge until a set point (3.65V) & then constant voltage (3.65V) until the current reduces to 1/10 of the Battery C (2300mAh). I have looked at a LM317 + TL431 circuit designed for Lithium battery charging & does CC & CV charging. But the LM317 overheats & goes into thermal limitation. I didn't want to have to use heatsinks as there is not much room in the box that this charger is going into & I'm also concerned about the heating effect on other circuits in the box. I

So I don't want to use a switching circuit.

Anyway, I just read that if the charging current is kept to < 0.18C then when the battery reaches 3.6V it will be 99% full. Can anybody confirm this?
#2
04-18-2011, 06:14 PM
 dweeb4 Junior Member Join Date: Apr 2011 Posts: 12

Here's the CC/CV battery charger circuit but if using a highish current, it overheats the LM317. If using a low current I'm not sure how long it will take to charge the 2300mAh battery as it works by using a reducing current once in constant voltage mode!

So I wanted to use the simpler low current trickle charger with automatic cut-off. I can set this to < 0.18*C which is 0.18*2300 = 414mA current. About 300mA would be fine & would charge a completed depleted battery in 8 hrs or overnight which is my operational target
Attached Images
 CC- CV battery charger.jpg (95.4 KB, 216 views) battery charger with cut-off.JPG (31.1 KB, 195 views)
#3
04-18-2011, 06:27 PM
 dweeb4 Junior Member Join Date: Apr 2011 Posts: 12

Can someone explain to me what the "minimum load condition" is - as mentioned in the passage below from the LM317 datasheet? I suspect I'm having this problem as I don't get the calculated voltage when I use the LM317 as a simple voltage regulator as per the datasheet. I'm using 2K for the top R (R1) & 4K7 for the bottom R (R2). I calculate 4.19V & read 6.6V with a 2K load on the output.

Quote:
 The LM117/217/317 provides an internal reference voltage of 1.25 V between the output and adjustments terminals. This is used to set a constant current flow across an external resistor divider (see Figure 3), giving an output voltage VO of: VO = VREF (1 + R2/R1) + IADJ R2 The device was designed to minimize the term IADJ (100 ľA max) and to maintain it very constant with line and load changes. Usually, the error term IADJ × R2 can be neglected. To obtain the previous requirement, all the regulator quiescent current is returned to the output terminal, imposing a minimum load current condition. If the load is insufficient, the output voltage will rise. Since the LM117/217317 is a floating regulator and "sees" only the input-to-
#4
04-18-2011, 06:57 PM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

The "minimum load current" for an LM317 is 10mA. It is provided by the 120 ohm resistor from the output to the ADJ terminal and has 1.25V across it so its current is 10.4mA.
If there is not enough current and the input voltage is high then the output voltage will rise.
#5
04-18-2011, 07:12 PM
 dweeb4 Junior Member Join Date: Apr 2011 Posts: 12

OK, thanks for that. The 120 ohm you talk about I don't see - is this on the datasheet? But I also see 240 ohm as the top R in the voltage divider - this would give only 5mA - I'm confused again.

Also in the CC/CV circuit the top R is 6K8 but it uses 1K between OUT & ADJ - are these paralleled - still will be below the minimum laod current? Does the TL431 provide the current to the ADJ pin?
#6
04-18-2011, 07:29 PM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

240 ohms is used for the more expensive LM117 that is shown in almost every schematic in the datasheet.

The circuit you found using a TL431 is wrong. The other circuit is correct and has a 120 ohm resistor from the output of the LM317 to its ADJ terminal.
#7
04-18-2011, 07:38 PM
 dweeb4 Junior Member Join Date: Apr 2011 Posts: 12

Quote:
 Originally Posted by Audioguru 240 ohms is used for the more expensive LM117 that is shown in almost every schematic in the datasheet. The circuit you found using a TL431 is wrong.
Aha X 2, thanks
Quote:
 The other circuit is correct and has a 120 ohm resistor from the output of the LM317 to its ADJ terminal.
OK, so this explains my issues - I should have checked here first I'll go back to putting together a circuit that I can understand & operates as predicted.
#8
04-18-2011, 07:54 PM
 dweeb4 Junior Member Join Date: Apr 2011 Posts: 12

Can I ask some more questions if I'm not too much of a pest at this stage?
In the second circuit the charge reduces to a trickle charge when battery is full - is this trickle determined by the 0R56 - Oops I calc this as 2.2A? What am I missing?

Edit: I guess I don't want to use this circuit anyway as it has the same reducing current output & makes it difficult to calculate how long it would take to charge a battery,

Last edited by dweeb4; 04-18-2011 at 08:33 PM.
#9
04-18-2011, 08:16 PM
 dweeb4 Junior Member Join Date: Apr 2011 Posts: 12

Here's my final circuit that seems to fulfill all my criteria:
Current can be set to < 0.18C & is constant so it makes charging time easy to calc.
Cut-off voltage is defined by zener.

It looks simple & easy to understand - I hope I'm not missing anything?

Back to my original question - battery charging @ a constant current < 0.18C will result in 99% full battery charge when the voltage reaches 3.65V. Anybody got experience with this?
Attached Images
 battery charger with cutoff.jpg (20.9 KB, 168 views)
#10
04-18-2011, 08:27 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,038

The maximum charge current is set by R1. You can roughly calculate the value for R1 by:
R1 ~= 0.63v/Desired_Current
So, if you wanted 500mA max current, R1 ~= 0.63/0.5A = 1.26 Ohms.

I built something similar to the 2nd schematic to use to recharge a 3.6v NiCD battery at a slow rate. See the attached.
Attached Images
 140mA 3.6v NiCD and NiMH Charger.PNG (71.2 KB, 115 views)
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 Tags charger, circuit, lifepo4, simple

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