Anyone else struggle with how batterie percentages and volts work in their first year?
what?
Like how if your batterie is at 50% thats not good, at all
Yeah, assuming you are using a battery beak you generally want above 120% battery and good status before using it in a match.
Exactly, i found that extremely confusing.
FRC is pretty harsh to batteries. You want your battery to be at full capacity for the start of a match so that you can draw at the current levels robots want to be able to draw at for the whole match. As you get into lower percentages of the battery it still has charge, but it won’t be able to supply as much current so things like accelerating your drive train will cause the voltage to drop enough to start seeing brown outs. You start at >100% due to surface charge on the battery terminals. It isn’t really significantly more charge, but it’s a good indicator that your battery is really fully charged.
@moderators Is this able to get split to a separate thread since it doesn’t seem to start with a quote?
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i think this is a valuable resource but shouldnt do it in the sacred quotes thread
^
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Done
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Thank you!
Presumably you’re asking about a curve like the one below (which isnt’ the same as an FRC battery, but similar)? The “percent of charge” is not linear with voltage on most batteries, including lead acid because of the chemistry involved. Instead, the curve is going to have 3(ish) zones:
- A quick drop from 13.something to ~12.5V. This is considered “surface charge”, and doesn’t increase battery capacity.
- A flatter section where the voltage stays relatively constant between 12.5V and 11 or 12V
- A “fall-off-the-cliff” zone where the voltage drops off below 10.5V very quickly. Discharging the battery beyond this point can cause permanent damage to the battery.
There are a couple of confusing extra details:
- The curve changes with discharge current. The nameplate capacity of FRC batteries is 18AH, but that’s given assuming you only discharge at 9A (aka “0.5C”). FRC robots can discharge as high as 200-300A - much higher! So we usually get a lot less juice out of a battery before the voltage drops below 10.5V. That’s why the chart above has different curves for different amperages.
- Because the battery has some internal resistance, the voltage drops (sometimes called “sagging”) when drawing current/power. You can’t really measure the state of charge until you stop, at which point you’ll see the voltage rise back up to whatever point it is on the curve.
- The relationship between voltage and state of charge changes with battery age. Lead Acid batteries in particular can’t handle very many high-current, “deep discharge” cycles. Even after one season you’ll see batteries hit their 10.5V cliff earlier than they did at the beginning of the season. They’ll also suffer more voltage sag during discharge since the internal resistance gets bigger as they age. I’m sure there’s some chemistry reason, but I’m not familiar with the details of why.
Does any of that help?
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Running test will help understand the curves shared by @nuclearnerd of your particular batteries. They all start out as fully charged on a battery beak, but some take a dive very quickly.
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Annoyingly the Battery Beak manual doesn’t list what formula it uses to convert voltage to “percent charge”, so I can’t tell you exactly at what point they call a battery “50%” charged. But they do have a pretty good description of “surface charge”, and why “120%” doesn’t really have more energy than “100%” as reported by the Beak.
To OP’s point, I also don’t know why the Battery Beak is calibrated such that a 100% with-load capacity (compared to 120%) is considered undercharged or a value representing an old/worn battery. This seems to just be a matter of proper sensor calibration. LiPos have similar voltage sag behavior under load but their capacity checkers do not try to infer that battery is overcharged by 20-30% of normal.
Here’s the middle of a long thread on battery testing and my quest for the tools to evaluate battery capacity without too much time and $$, and a way to immediately identify a failed cell battery.
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According to this commonly used MK ES17-12 battery spec sheet the full 18Ah capacity is the 20 hour discharge rate of 0.9A, not 9A.
I also find it interesting that the maximum charge current specified is 5.4 A.
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If you’re doing a standard CBA test at 7.5A to 10.5V (as recommended by AndyMark), how many Ah should a “perfect” 18Ah MK ES17-12 test at?
The standard answer is 12Ah or more in that test denotes a “good” battery. Below that, only use it in practice matches or for demos.
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Our best batteries typically ran 15Ah. After a healthy amount of use they end up around 15.5Ah or so. I’m not sure what a “perfect” battery will look like but, indeed >=12-13 Ah should make a good battery.
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Measured Ah varies with the test current. Real FRC loads are pretty heavy. I like to test at 50 Amps to be more realistic and ALSO to get the test done faster. The CBA tests take a LONG time.
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