LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[ebaugh]

Originally Posted by boat_alexandra

You can use differential inputs, and one one leg the battery voltage, the other leg is connected to the reference divider which is lots of resistors in series across the battery. These two inputs are then very close so then you can use programmable gain 10x or 20x or so to gain more bits.

Another way which is not as good is to take lots of samples, say 1k per second for each channel or more, and average them to essentially gain a few bits.

Last resort is something like an external 24bit adc

For balancing, depending on circuitry there may not be measuring needed.

The small Arduino's don't have differential inputs? Thats the problem with all the microprocessors I looked at. All analog inputs were referenced to the same ground.

Plus, you must "load" each cell with whatever voltage divider network you use exactly the same. Otherwise, it will unbalance the pack...or make balancing a requirement to reverse the effect.

Other than cost, which I grant is not insignificant, what is the disadvantage of this method?

[T1 Terry]

It always amazes me how people can take a really simple system and try to make it extremely complicated and less dependable with a far greater cost involved. A Junsi cell logger with logging capabilities is roughly $28, one without logging capabilities is around $14, one connected to a simple timer circuit available off the shelf to cut a relay for a set period to look after HVC and one with all alarm parameters set with the alarm port connected to a smoke alarm, a double redundancy safety back up for around $100, why try to reinvent the wheel?

T1 Terry

[goboatingnow]

Their are several ways to address this single sided issue , but all need a few components. You can use OP-amps to give you differential capacity etc

Furthermore you can adjust the gain to provide the correct dynamic range , if you think about it , you are not measuring 0-16 or 24 or 332v , you are measuring between about 2.8v to 4.0. So the dynamic range is a little over a volt , adjust yiu input gain according

The voltage dividers for the ADC have no effect on the balancing by the way

Bit you can't use the ardunio out of the box easily


Dave

[goboatingnow]

Originally Posted by T1 Terry

It always amazes me how people can take a really simple system and try to make it extremely complicated and less dependable with a far greater cost involved. A Junsi cell logger with logging capabilities is roughly $28, one without logging capabilities is around $14, one connected to a simple timer circuit available off the shelf to cut a relay for a set period to look after HVC and one with all alarm parameters set with the alarm port connected to a smoke alarm, a double redundancy safety back up for around $100, why try to reinvent the wheel?

T1 Terry

Because we want a better wheel

[T1 Terry]

:lol: what ever happened to the KISS principle, you will end up building something only the builder fully understands, so a failure then needs to be returned to the builder for repair. My approach is the system owner is also the builder, keep it as simple as possible, the owner builds it so the owner knows where the bits are, then a simple flow chart allows the owner builder to also be the one who diagnoses the problem and effects the repair, something every traveller to unknown areas needs to be able to do.
There are multiple ways to skin a cat, I guess each builder/owner has to decide which system suits them best.

T1 Terry

[roetter]

Originally Posted by T1 Terry

:lol: what ever happened to the KISS principle, you will end up building something only the builder fully understands, so a failure then needs to be returned to the builder for repair. My approach is the system owner is also the builder, keep it as simple as possible, the owner builds it so the owner knows where the bits are, then a simple flow chart allows the owner builder to also be the one who diagnoses the problem and effects the repair, something every traveller to unknown areas needs to be able to do.
There are multiple ways to skin a cat, I guess each builder/owner has to decide which system suits them best.

T1 Terry

For me the Arduino is KISS.

I can easily program in C++ or C, which I have done for years.It is a few years back, but it is like riding a bike.

I have just tested the alarm functions with the Arduino and they work great.

The voltage divider appears to be pretty even. All voltages are within 20mV of the CellLog. The cell drain of the voltage divider circuit is way less than self discharge. I use 10kOhm resistors. That would be a 0.00033A load.


Advantages of the Arduino:
- I now have full control over hysteresis and/or timer delay for HVC or LVC. It is as easy as changing some definitions at the beginning of the source code, hooking the board up to the USB and pressing the upload button. All is up to date. Anybody can do that. All tools are freely downloadable form the Internet.

#define HVC 3.6
#define LVC 2.8
#define HIGHHYST 0.2
#define LOWHYST 0.2

- I have a larger LCD display - easier to read

- I can start thinking about other functions.
I got some amp-sensors (less tha $10 each) in the 12 to 75A range that hook up directly to the Arduino. I plan to hook them into the two fridges I have on board. Now I can not only log the duty cycle (these fridges are RPM controlled, so duty does not tell much about consumption) but measure the Ahs used. Now I can easily check how much savings I get from placing a little 12V computer fan under the cabinet to get the hot air out.

The sky is the limit of what can be done with this.
-There are GSM module so you can have it send a SMS or email in case of alarm to your smart phone.
-You can watch the bilge pumps and get alarms for that.
-You can hook up motion sensors to let you when when someone is on the boat.
-There are GPS modules that you can use to send the location via SMS or email.
-How about using the accelerometers to log sea state?

The Arduino board is $29. The LCD display is $20. The 4-channel relay board is $3.95. Some cables, some connector maybe another $25. It is possible to do the cell voltage alarms for less than $100, excluding the big solenoids.

Going crazy? This is fun.

[SVI]

Good idea. Can it be implemented as a PC app? More user-friendly.

[roetter]

Originally Posted by SVI

Good idea. Can it be implemented as a PC app? More user-friendly.

Interesting idea, but:

I am not sure how you would like to see that as a PC app? Maybe you can explain.

Some thoughts from my side:
- A PC has much higher power demands than the Arduino. The board uses about 25mA at 5V. This processor is used in all kinds of low power devices, including the Junsi CellLog. So you would not like to have the PC running all the time.
- I am planning to put a SD card on the board to record the voltages with time stamps. The card can be read by any PC and analyzed by Excel.

- The settings for low and high voltages, etc. could be set using a simple interface on the LCD display.

What are you planning to do on the PC?

[SVI]

Originally Posted by roetter

- A PC has much higher power demands than the Arduino.

What are you planning to do on the PC?

Power demand depends on the PC model
It is not viable to have Arduino just for battery monitoring. SD is not convenient.

Better to make a PC app, as PC is used for navigation, entertainment etc. Look at BEP CZone app (for tank monitoring), they have the same model for electricity. May you just use couple of them?

[goboatingnow]

Originally Posted by SVI

Power demand depends on the PC model
It is not viable to have Arduino just for battery monitoring. SD is not convenient.

Better to make a PC app, as PC is used for navigation, entertainment etc. Look at BEP CZone app (for tank monitoring), they have the same model for electricity. May you just use couple of them?

Sorry , this doesnt make sense, an Ardunio a 2 line LCD , etc is cheap as buttons, Czone is very expensive, !!!!

PC apps are useless on board as its not possible to leave the PC running all the time. If you are talking about using teh SD card, thats only for logging. All you want day to day is the ardunio implementing the LVC and HVC protection and warning of any out of balance situation,

equally with cheap ardunio add-ons, you could so Ethernet or Wifi reporting, or even just serial port or USB.

Its a nice solution.

dave

[tsmwebb]

I think a raspberry pi could run Open Cpn and do other general "pc" stuff and also do battery monitor stuff for not much cash or amps. But I'm pretty sure it would be harder to implement the hardware / software for the monitor side with the pi than with a controller. Tom.

[goboatingnow]

Originally Posted by tsmwebb

I think a raspberry pi could run Open Cpn and do other general "pc" stuff and also do battery monitor stuff for not much cash or amps. But I'm pretty sure it would be harder to implement the hardware / software for the monitor side with the pi than with a controller. Tom.

Micros are cheap , have loads of them , see ubiquitous ( persuasive) computing for reference. You run one one thing properly per processor. Then you mesh them together.

Dave

[daddle]

#define parameters ??? That's so 1980's.

Using a programmed device is certainly the way to go rather than complex linear circuits ... Until the software bugs start creeping in.

[goboatingnow]

Originally Posted by daddle

#define parameters ??? That's so 1980's.

Using a programmed device is certainly the way to go rather than complex linear circuits ... Until the software bugs start creeping in.

Because ardunio code is easy to reload, there's no point building in a UI to accept parameters. And given its written in C++ what you suggest ? ( don't say const !!!, that's so noughties!)

Dave

[roetter]

Originally Posted by goboatingnow

Because ardunio code is easy to reload, there's no point building in a UI to accept parameters. And given its written in C++ what you suggest ? ( don't say const !!!, that's so noughties!)

Dave

No constants in my code, but a few defines. A bit rusty on the C++ stuff. Have a class for Battery and for Alarm so far. The code is running well and less than two hundred lines including comments and empties.

#define HIGHALARMPORT 3
#define LOWALARMPORT 4
#define HIGHALARMON HIGH
#define HIGHALARMOFF LOW
#define LOWALARMON LOW
#define LOWALARMOFF HIGH
#define HVC 3.6
#define LVC 2.8
#define HIGHHYST 0.2
#define LOWHYST 0.2
#define TRUE 1
#define FALSE 0

Purely for educational purpose I am playing with some other means of reading the cell voltages.
I am playing with differential op-amps using dangerous Internet half-knowledge. I use an LM348 as differential amplifier so see if I get more precise results in creating voltages to measure, to compare to just the resistor bridges. However, the first cell reads off. So instead of getting 3.22 for each cell, I get 3.72 for the first cell. All other cells create 3.22 as a difference amplification value.

Strangely though, reading from ground to the second cell gives 6.44 which is correct. Maybe this is because this is not a rail-to-rail op-amp?

Anybody?

I am waiting on some fancier items that I saw today in a circuit on Ruggedcicuits.com. They have a circuit to create individual cell voltage to ground using op-amps and reference voltages. Circuit #5 I ordered the parts to have some more fun.