SLA battery SOC and more!

[Mugrunty]

Hello,

I decided to post this information here instead of where it used to be in the POB Battery meter post. It was too off topic! I took a few screenshots of the programs I made for the TI-84 plus calculator. It can tell you the Estimated State of charge for your POB lead acid battery based on open circuit voltage. I also made a little program to estimate runtime on a 7Ah battery depending on the current draw. It compensates for the decreased capacity you get when you put large loads on Lead acid batteries. Take your POB for example. That unit draws about 3.6 ish amps after warming up IIRC. You punch that in your calculator... 7/3.6 and you get 1.94 hours. Obviously thats not what you get with your POB. Now, you use this extra cool formula and you would get 1.25 hrs (1hr 15min). Very close to the actual runtime of that flashlight! (~70min)

I'll post the formula that the program uses.

If current draw is < 1/20C or 350mA, just divide the numbers normally

If current draw is > 350mA, use the following formula:
RUNTIME (hrs) = 5.8488 * I(^-1.203)

Just plug your current into I.

The SOC formula is a really nasty polynomial equation so I wont post it unless someones really interested.

I got all this data from BB battery datasheets, me using excel to form equations and graphs, a pdf on Valve regulated batteries, and the David linden Handbook of batteries book. Here are some screenshots of the programs. Too bad I cannot host the programs somewhere.

:grin2:

 

[lctorana]

Really fascinating.

Always wondered about the capacity rolloff with increasing load, now I have sdomething mathematical to play with.

I know it's only one specific example, and one cannot generalize from the specific, but a start is a start.

Thanks!

 

[Mugrunty]

Yeah, it's definitely a little weird. For now, that formula only works for the 7Ah type. :)

Eventually, I'm going to put these equations into a microcontroller. Then I'll build up a little board with a 2-line 16 character display, control MOSFETS, sense resistors, and a button or two to try and squish into the POB. Either that or just a box for now. Then, I'll be able to see a fairly accurate representation of how much juice is left in the pack. Not only that, But It'll give me runtime estimates, current draw, voltage, % charge remaning, Time remaining to fully charge battery, and all that Jazz. I'll be using the ATtiny 861 chip, a 0.020 ohm resistor, two MOSFETS, and that awesome display. The MOSFETs give me the capability of over-discharge protection/over-charge protection. For now I'll be using the SLA, so the over-charge protection isn't really needed. But If/when I switch over to li-ion, it will be ready. Plus, li-ion's charge/discharge efficiency is close to 100%.

The programming is whats going to take me a while. I'll get there eventually!