Want to make a load sharing / battery management for my circuit

Hi. Connected to an Arduino Nano I have 3 x 5V PIR sensors, 1 x 5V servo and 2 x white (3.2V) LEDs. When one of the PIRs detects motion the servo moves and the LEDs light up for 20s. Maximum current draw is under 1A.

I have a 9V walled power supply (waiting on a 7.5V supply in the post) and a battery pack 6V supplying power (alternately) via diodes. The wall power preferentially provides power due to the higher voltage.

My question is, how could I make this into a load sharing / battery management system so that the wall power pack charges the batteries when plugged in? I don’t mind on which battery type, so long as they are rechargeable. I would also like to have at least, say 3 Ah.

I have scoured the internet and I can only find chips and circuits with about 3.2 - 4V output. I know I can step up voltage but I’ve heard they play havoc with PIR sensors.

Is there an inexpensive chip/circuit I can implement? Can I combine load sharing circuits somehow? In the latter case I would surely need balance charging so as to prolong the battery life?

Thanks!

3.3Volts is the new 5V… Also, most sensors and LCD/OLED displays are jn the 3.3V range.

Look into 3.2 Volt LiFePO4 cells. I use these in a few projects that I want to be “on” all the time, such as digital clocks. LiFePO4 cells can be float-charged to between 90% and 95% full capacity… a proper voltage-current chip is required for 100% charge, but the cells are cheap, very safe, and I do not mind loosing 5% to 10% for the ease of float charging.

I bring 5V - 6V into a buck DC-DC 94% efficient module. I set the output for 3.40V to use with my 3.3V projects. Do refer to the absolute max Vcc before using a voltage greater than 3.3 however.

You can also use gel-cells and float them up-stream from the DC-DC buck module. Or even NiCad!

Ray

A less sophisticated option would be use a 12v lead-acid battery and a suitable trickle charge circuit. If the charge voltage stays below 13.3v it won't overcharge.

...R

The LM317 datasheet has a scheme for battery backup. Basically you use two regulators, one from the wall power and one from the battery. The regulator from the wall power outputs a slightly higher voltage than the one from the battery so under normal circumstances the battery’s regulator will not output any power.

@mrburnette
I could get a 3V servo and a 3V arduino. Can you recommend a chip/board for float charging the LiFePO4 batteries and enabling the load sharing? I am drowning in the information about the myriad of chips out there. Would the MCP73871 be a good choice? The highest mAH I could find on ebay was 1500, so I would want at least two of them in parallel. How is this possible?
http://www.ebay.com.au/itm/4pcs-ETINESAN-3-2V-1500mah-lifepo4-1450-AA-rechargeable-battery-toys-ect-battery-/252297962244?hash=item3abe216304:g:GnMAAOSwzgRWzRU1

@Robin2
Thanks for the suggestion, but lead acid batts are too heavy for my application.

@Chagrin
In the LM317 datasheet and beyond I couldn't find any information about having the battery backup and load sharing ability. It can be used to step up the voltage and as a battery charger, but did you misunderstand that I wanted both battery charging and load sharing?

Now I'm starting to think that load sharing means having two power supplies running simultaneously. I'm getting mixed up between power management, BMS and all the other terms etc. It would be like a solar controller with the ability to use wall power rather than solar power that I am after. Some of those solar controllers are 5A and about $8 but I'm not sure if wall power would kill it. Not even sure what I should be searching for anymore?!

I understand the weight problem with lead-acid batteries.

NiMh cells are also very safe. Could you make up a pack with a suitable voltage and use an LM317 to provide a trickle charge voltage that can run all the time?

I would be much more cautious about using LiPo cells if they are being charged while unattended. At the very least I would put them in a container where they could do no damage if they burst into flames.

I am not familar with the LiFePO4 cells mentioned by @mrburnette.

...R

When I searched for "power path and battery charger chip" a few options come up. At the following website I selected LiFePO4 and selected power path and battery charger. Boy did I open a can of worms!

Mixed-signal and digital signal processing ICs | Analog Devices(PowerPath,Battery_Charger)#!1806_LiFePO4!1069_Linear|Switching|Shunt!2170_Linear%20PowerPath|Switching%20PowerPath!chem_LiFePO4!vmax!icharge_!bat_bat!path_path

The first of the two options was the LTC4098-3.6 chip (link below). It says "700mA Maximum Load Current from USB Port, 2A Maximum Input Current from Internal Switching Regulator and 1.5A Maximum Charge Current with Thermal Limiting". What would be the maximum output current to the load? When charging, would it be the same as the 700mA input current via the USB. My load needs to get just under 1A. I'm not sure if the "easy" option in the diagram on page 1 is sufficient. Maybe I have to go with one the two diagrams on page 15. Page 22 has additional circuitry for Dual Input Overvoltage Protection. Page 23 has additional circuitry for Dual Polarity Voltage Protection. Not sure if I need these extra components or not.

The second option was the LTC4156 (link below). It says it may draw up to 3A from a high power wall adapter. It seems current-wise this one is good enough as is. My head is starting to hurt looking at all the numbers.

Any thoughts on the use of one of these chips? Remembering that I will draw just under 1A at peak load. It is like that for a second or so when the servo starts moving. The diagrams for these chips have 3.3V battery supply labeled. Am I limited to 1 lonesome LiFePO4 battery or can I put a bunch in parallel?

How about using a LiPo? I saw a single cell 5000 mAh for $10. I read that they can be safely trickled charged but I think they would require regular discharge which would no doubt complicate things if this was to be automatic.

@Robin. As I replied to Chagrin I don't know of a way to do the load sharing with the LM317. As for the LiPo, if I was to use one then I would be happy to put it in one of those flame proof bags from hobbyking or something similar. But alas I think they are only fire resistant so I would need to check that out.

Thoughts please.

The LTC4412 chip is a power path controller and the data sheet (below) has circuit diagrams of charging multiple batteries. I'm not sure if the circuit in Figure 3 on page 10 (link below) can do multiple battery charging. What sort of battery charger would I put here? Figure 6 on page 12 is about multiple battery charging from a single charger. Why does the circuit say "To load or power path controller" when the thing IS a power path controller? I guess I would go straight to load here. This circuit could be a little easier and more flexible than the ones in my previous post (not requiring inductors?). They talk about using Lithium Ion batteries. I assume this includes LiFePO4? I think I'm in over my head here. Could someone more eloquent in the ways of circuitry please help out?

bodkin77:
@mrburnette
I could get a 3V servo and a 3V arduino. Can you recommend a chip/board for float charging the LiFePO4 batteries and enabling the load sharing? I am drowning in the information about the myriad of chips out there. Would the MCP73871 be a good choice? The highest mAH I could find on ebay was 1500, so I would want at least two of them in parallel. How is this possible?

I was using Amazon for reference, the selection is wide and various chargers are available for different physical sizes. LiFePO4 chemistry never produces raw lithium as a byproduct, hence battery chemistry prevents a lithium fire after catastrophic rupture - there is still a possibility with any sealed system to have steam produced ruptures if the cells are shorted; so fuse appropriately.

You may parallel "fully charged" cells. Never attempt to parallel partly charged cells. The charger will bring the full-cycle voltage of any cell to the optimum and then when inserted in parallel, there is only a minuscule inter-cell current for a brief amount of time.

Shop wisely ... cell prices for 4x can range from $10 to $60 for essentially the same cell ... just the suggested "application use" is different!

Ray

Ray, when you were talking about your float charging projects did you mean you plug em in and keep using em like a cell phone, or you leave em plugged in and switch them off, or you totally disconnect them for charging?

bodkin77:
Ray, when you were talking about your float charging projects did you mean you plug em in and keep using em like a cell phone, or you leave em plugged in and switch them off, or you totally disconnect them for charging?

In one project, I use a single LiFePO4 which is permanently in the ESP8266 clock. The 5V cellphone adapter runs through a DC-DC buck converter which is set to 3.40 V which then goes to a switch which then goes to the ESP8266/OLED. The clock is intended to be on 7/24/365.

In another project, 1 LiFePO4 cells is installed in a minibox with a 2xAA cell holder. When using alkaline 1.5V cells, I use 2x and when using the lithium, I use 1 with a shorting-cell. This is a handheld unit, so the cell come out to be charged. As the project is an ASCII test generator, it is used only infrequently, so the external recharge is very simple ... And yet another just-for-fun project that uses a LiFePO4.

Ray

Ideas anyone? ROugh nudge.

bodkin77:
In the LM317 datasheet and beyond I couldn't find any information about having the battery backup and load sharing ability. It can be used to step up the voltage and as a battery charger, but did you misunderstand that I wanted both battery charging and load sharing?

Sorry, it was the LM1117 datasheet; figure 20 on page 16.

@Chagrin I kind of have the inputs under control - it's the charging and outputting of current AT THE SAME TIME (sorry I don't know how to bold or underline) that I'm debating on which path to go down. Maybe what I'm trying to do is "battery charging and power management" or "normal mobile phone power scheme"? I don't seem to be saying the right thing. The figure 20 has an asterisk to change a certain resistor value for battery charging, but not much more to go on.

Some more notes here...

bqSQITCHER - I think this one requires a separate power management chip but the diagrams are clear and the extra components are minimal.

http://forum.arduino.cc/index.php?topic=24261.0 Good info

Someone used the LT4412 for powerpath control and the MAX1555 for Li-ioncharging. There is also a circuit diagram for the LT1512 charger and LTC4412 powerpath controller combined.

@Ray
That ESP8266/OLED project you made, could you describe the switch? Sounds like you are float charging and powering the clock and screen all at the same time?

bodkin77:
@Ray
That ESP8266/OLED project you made, could you describe the switch? Sounds like you are float charging and powering the clock and screen all at the same time?

Yes.

(Note: you can only do this with one cell, or parallel cells, but never with serIes batteries.)

@3.35 - 3.45 Volts, the ESP8266 is well within its Vcc voltage as is the OLED display. The LiFePO4 3.3 Volt cell can be thought of as a big capacitor across the output of the DC-DC module. There is a switch but only used to reset the clock when I change the daylight-savings jumper (only read once in setup().)

Using constant voltage of 3.4'ish Volts, the LiFePO4 cell stays charged to around 90% capacity; which is all I need to get through short power outages which occur far too often here.

The nice part about LiFePO4 is that the chemistry never produces raw lithium. Reread the "Note" I posted above - you cannot apply this trick to series cells. I have one test cell in this configuration for 12 months with no issues - longer term, I have no data.

Ray

So you have no fancy schmangles going on here, you just have the battery in parallel with the DC supply of 3.4'ish Volts? With a fuse I assume. I thought you'd get runaway charging. Is this going to work for the type of load I'm talking about? It sleeps a lot and occasionally pulls 250 mA.

bodkin77:
So you have no fancy schmangles going on here, you just have the battery in parallel with the DC supply of 3.4'ish Volts? With a fuse I assume.

The ESP8266 draws between 40mA and 700+ mA (you need a scope or recording peak meter to see high-end current.)

Yes, I have a 650mA polyfuse inline with the battery.

I thought you'd get runaway charging.

Reference

  1. Fast "forced" charging:
    Because an overvoltage can be applied to the LiFePO4 battery without decomposing the electrolyte, it can be charged by only one step of CC to reach 95%SOC or be charged by CC+CV to get 100%SOC. This is similar to the way lead acid batteries are safely force charged.

  2. Longer cycle life
    In comparison with LiCoO2 battery which has a cycle life of 400 cycles, LiFePO4 battery extends its cycle life up to 2000 cycles.

  3. High temperature performance
    It is detrimental to have a LiCoO2 battery working at elevated temperature, such as 60°C. However, a LiFePO4 battery runs better at elevated temperature, offering 10% more capacity, due to higher lithium ionic conductivity.

Is this going to work for the type of load I'm talking about? It sleeps a lot and occasionally pulls 250 mA.

I cannot guarantee it, but it has worked for me without issue for 12 months.

Lithium Iron Phosphate batteries are among the longest lived batteries ever developed. Test data in the laboratory show up to 2000 charge/discharge cycles. This is due to the extremely robust crystal structure of the iron phosphate, which does not break down under repeated packing and unpacking of the lithium ions during charging and discharging

Ray