lipo monitor for 2 cell battery

[tony_g]

as the title says im after some information for monitoring both cells in my 2s lipo.

occasionally after i have been running my rc crawler i check my individual cell voltages with my dmm to see how the cells have discharged after running and a few times i have found that one cell has significantly disharged way more than the other so i want to avoid this.

the esc lipo cut off of 3v per cell is too low for my liking so i would like to make my own with user set voltage warnings and to be able to monitor the individual cells and give an audible warning if they end up differing too much.

the only things im really unsure of and need advice/help is about connecting the two individual cells to check them.

i know each cell is up to 4.2v when fully charged and i would like to use the balance plug but am not sure if i can use the taps for both cells to go to 2 individual ADC channels to check the cells or if this will count as over voltage to the picaxe being that the 2 cells are in series


hopefully you get my ideas of my concerns through my ramblings lol


tony

 

[premelec]

Assuming the 2 cells are in series you can use a voltage dividers and FVR2048 internal ref on an 08M2 - running on a 5 volt LDO regulator from the 2 cells.
Then do the math in the 08M2 to determine the individual cell voltages. I don't know how your 'balance' plug is set up... You need to prevent overcharge as well as undervoltage. The 08M2 can control a relay which disconnects the cells when one falls below low voltage. [and disconnects the 08M2 as well]. Obviously you could use a buzzer instead of relay disconnect if you can risk overdischarge... A small - I hope - problem is that the voltage divider resistors stay connected across the cells drawing a few tenths of a milliampere unless you arrange to disconnect them also.

 

[g6ejd]

LiPo balance plugs give a series connected view of the cells.
1-Gnd
2-Cell1 terminal (fully charged you will measure 4.2 volts)
3-Cell2 terminal (fully charged you will measure 8.4 volts)
4-Cell3 terminal (fully charged you will measure 12.6 volts)
n-Celln terminal (fully charged you will measure cell-count x 4.2volts)

A fully charged (100%) cell LiPo is 4.20volts
A fully discharged (20%) cell LiPo is 3.60volts

Your ESC cut-off voltage is designed to keep the +5volts running as long as possible, so not really an alarm device.

Seen these, so many variants...: http://www.ebay.co.uk/itm/LIPO-LOW-VOLTAGE-ALARM-2S-3S-MONITOR-CHEAP-TESTER-QUADCOPTER-HELICOPTER-PLANE-/111336543938?pt=UK_ToysGames_RadioControlled_JN&hash=item19ec2cd6c2

 

[techElder]

Hobby King sells these to check up to 8 cell lipos for around $8 US.

 

[erco]

Hobby King sells these to check up to 8 cell lipos for around $8 US.

Agreed, no need to reinvent the wheel if you can use an off the shelf solution.

$1.69 Alarm: http://www.ebay.com/itm/NEW-7-4V-11-1V-Lipo-Battery-low-voltage-Alarm-Indicator-TR-/181297737877?pt=LH_DefaultDomain_0&hash=item2a362ff095

$1.99 voltmeter: http://www.ebay.com/itm/RC-1S-6S-Lipo-Battery-Voltage-Indicator-Checker-Tester-DR-/301267298552?pt=LH_DefaultDomain_0&hash=item4624ee28f8

 

[g6ejd]

Even though there are already products that achieve the design aim, it's a good project for a PIC and will exercise quite a few disciplines of hardware and software design.

 

[tony_g]

thanks for the replies guys, i now their are already cheap enough lipo monitors out their but seeing as i have all the smd bits and pieces then why not lol.

besides i want a lipo monitor that will give me custom warnings based on what i want to keep an eye on after my esc cutoff let me down.

g6ejd
"Your ESC cut-off voltage is designed to keep the +5volts running as long as possible, so not really an alarm device".

yes im aware the speed controllers cut off is not an alarm, but its supposed to cut off above the minimum voltage when set to lipo mode and a couple of times its let one cell drop below 3v because it monitors overall pack voltage which doesnt keep the cells in the safe range if one discharges more than the other so i want to be able to give an audible warning not only when my overall pack voltage gets to the critical point but also be notified if one cell gets too far out from the other or too close to cutoff volts



tony

 

[g6ejd]

Yes, well as you know ESC cut-off voltages are typically 3v/cell and ordinarily would never get to that sort of voltage as it's highly damaging to the LiPo cells, so I'd say the designers set that voltage as a last ditch cut-off or it's a function of voltage differential between source battery voltage and voltage regulator aka voltage regulator drop-out.

Time to get coding then...

 

[tony_g]

definately, i dont know why they set it so low but 3v cell is too close to the point of no return lol.

my projects list seems to be having another growth spurt, ah well plenty to do and keep me occupied lol.

 

[srnet]

I want a lipo monitor that will give me custom warnings based on what i want to keep an eye on after my esc cutoff let me down.

Indeed so.

Good Lipos are not cheap.

What is wanted is a configurable warning that tells you when your Lipos are at risk, not when their life has very likley already been shortended.

 

[tony_g]

after this little incident of discharge i definately want to be alerted long before its too late to keep my pack safe.

individual cell monitoring for imbalance once a certain point is hit is what im going to be keeping an eye on with an audible alert so i can hear it when out crawling as well as ending my run when the overall voltage is higher than what the esc decides is the stopping point.

i dont want to have to buy another 5000mah pack sooner than i need to, my "hobby budget" gets stretched as much as it can at the best of times

 

[BeanieBots]

What you want to do should be easy enough with a simple potential divider arrangement as already described.
The current drawn will be insignificant compared to your model.
It's then just simple maths to determine each cell voltage. Resolution won't be great but it doesn't need to be.
I have built an analyser for up to 6S packs where I used op-amps to give the PICAXE a true differential input (with 3v offset) but you should not need that level of complexity.
Not sure I'd even use a PICAXE for 2S. Just a basic comparator.