Sorry, I meant internal resistance but I can’t edit the Thread name to correct it.
Does anyone have information on 2007 12V 17 Ah Battery internal resistance as a function of charge state?
The data sheet says 10 milliohms but this is presumably at full charge. What would the battery internal resistance be at say half charge?
Is the internal resistance affected by by discharge rate?
REgards
Frank
the internal resistance is affected by discharge rate because higher current equals higher temperature which changes the resistance.
are you trying to determine your state of charge during normal robot operation? maybe there is another way that would be easier. using the onboard A-to-D conversion and scaling the software to monitor the battery between 10V and 15V. i think there was an official FIRST post about using external circuit to do this… i forget now, was a year ago when i saw this.
Keep in mind that your battery is considered DEAD when it reaches 12V and needs to be recharged. if you allow your battery to drain lower, like 11V, you will cause damage. you may notice that the next time you charge your battery it does not reach the same voltage off charge, or that it doesn’t last as long when applied to the exact same load.
also, during a match when you are pulling heavy current you will reach 12V. then when you disconnect your battery and measure the voltage it may say 12.18V or something… you might think that its OK to put back on the robot, but this is misleading. under loading conditions your battery may drop below 12V and this is not good for your battery and needs to be recharged.
same sort of thing applies to your backup battery, though being nicad they are more tolerant. 7.2V is the target here.
slloyd
Frank,
There are a variety of factors that change the internal resistance of the battery but for our applicataion, consider the .011 ohm spec to hold true for most of the life of the battery and under most discharge conditions. If you take a look at the discharge curves you will see that the terminal voltage falls below the 12 volts mark almost from the start and you can easily determine that 100 amps flowing across .011 ohms will produce a 1.1 volt drop inside the battery. Remove the load and the battery will return to 12 volts since you are measuring “open circuit” voltage. (Without current flowing, there can be no voltage drop.)
The critical point for the battery is when ‘under load’ the terminal voltage falls below 8 volts for any length of time. It is at this point that the RC goes into stanby mode, the backup battery takes over the processor and all PWM outputs are disabled. If the main battery quickly returns to normal voltage then the RC wakes up and starts to function until the supply again falls below 8 volts. By watching the wiring resistance between the main battery and the RC and keeping it to a minimum, the critical 8 volt point will be less of a problem.
If you watch the battery voltage monitor on your OI you will see that rarely will the battery voltage be 12 volts while running. Get in the habit of having your drivers keep the OI in battery monitor so that they can tell when the RC is getting to the 8 volt point. It is not unusual for the OI to display 10-11 or even less than 9 volts for most of the match if you are running full out. You might even see short periods where the OI will read 5 or 6 volts without shutdown.
Thanks for the reply.
The original question was driven by trying to determine what motor performance I will have at various states of discharge. . i.e. what voltage would I have to derate the motor to to determine rpm and tourque when the battery is under load in a partially discharged state. In our applicaton, our robot lift operates at the end of the match (poorest battery state) and this is going to be a high current effort to liftanother bot in say 5 sec.
I thought the internal resistance inceased as it discharged and hence you got a greater load vs no load sag for a given current when the battery was partly depleted.
The other series losses are the victor or spike and wiring.
I asked the question of Victor and spike on resistance in another thread and have not yet seen the replies. . Victor data sheet does not give an on-resitance spec but the FET they use is 0.012 to 0.16 ohms. There ar 3 FETS in each leg of the H bridge and thus the series/par effective resistance is .008 to .0102 ohms for the Victor.
Stil wondering however if SLA battery has increasing internal resistance as it discharges and what is the curve. i.e on-resistance vs voltage or
on-resistance vs remaining charge. Near total discharge, Iknow the internal resitance gets hisgh as i have seen this. The question is wht is the curve that intoerpoaltes between .01 ohms at full charge to ?? 1 ohm ?? or more at near total discharge.
Regards
Frank
Frank,
Although the internal resistance changes in this battery as it discharges, there is a variety of other factors that make your calculations difficult. Internal resistance will also vary with beginning state of charge and temperature. And the discharge rate of your battery will vary with each match depending on the opponent strategy and the task you take on for your alliance. I generally accept that for the majority of matches with a freshly charged battery at room temp, the 11 mohm spec will hold true throughout the match.
The calculations for the Victor On resistance are correct but generally the ON resistance can be ignored as the internal resistance of the battery and the wiring resistances tend to be much greater. I have used a value of 2 mohm for the Victor for demonstration purposes to show it has some resistance in the belief that the .012 ohm spec is the max ON resistance of production range FETs. Without having made regular measurements of a variety of different robots, I would think that on average, a typical robot main battery to motor would encounter 20-30 mohm including breakers, wire resistance, connectors etc. I have seen many robots that would exceed that value by 100% due to excessive wire and poor crimping.
The Spikes are actually relays in a bridge configuration and again their series resistance is small compared to wiring resistances.
So the simple answer is don’t use the calculations to design for spec parameters since there are too many variables. Expect that if your robot design is fairly efficient, you should still be able to expect the battery to deliver 9-11 volts under load at the end. If it is inefficent, fudge the graph down towards the 8 volt point for your calculations. Remember that the internal resistance of the motor will vary with temperature as well.
A comparison chart for popular controllers is available at http://www.enigmaindustries.com/links.htm