Tuesday, December 6, 2016

So lets talk about Batteries. Vol 1.

So let’s talk about batteries.
Many things about batteries depend on each other. Most people flying race quads are going to be flying 1300-1800mah 3 or 4s battery packs. I’m going to assume you know what mah are, and what 3s and 4s mean.
Chemistry is what really limits our batteries. Reaction area, and speed of the atoms and molecules. Reaction area, for LiPo’s works out to “physical size”. Speed of atoms is “literally” the definition of temperature. The warmer something is, the faster that reaction can happen.
LiPo batteries are built of thin layers deposited on big sheets of polyester. (I think it’s polyester) Some Lithium, Some carbon, a membrane that’s impregnated with an electrolyte. Those sheets get rolled up or folded up into the shapes we’re accustomed to seeing. If you’ve ever seen a bunt up battery, you can see those layers peeled apart. Durable batteries have thicker layers. Durable batteries have higher C ratings. We’ll come back to C ratings in a few moments. Batteries with more capacity, have bigger sheets of battery. These directly relate to the size of a battery pack.
For my 9xr ratio, I had a 2c 2200mah 3s pack, that was smaller than 1500mah 20c 3s pack. A 45c 1500mah 3s pack, is about the same size as a 2200mah 20c pack. This is due to the tradeoff of cell area, and cell durability. “Stronger cells” are thicker.
Now we need to talk about abuse. And abuse is something that we do ~very well~ in the multirotor community. Tiny batteries, steep pitch, and multi blade props, four 20-30-40 amp ESCs and 2000kv+ motors. (High Kv essentially means “low turn” for the r/c car guys.)
Battery packs beyond their capacity and number of cells, are rated on the “C” rating. C is “the capacity of the battery” The “C” rating is a multiplier of the capacity of the battery. This lets you do some math to figure out how to treat, and what is considered abuse of the battery. There’s going to be a separate charge rating for both charge, and discharge. Typical limits for charging are in the 2-4C range. That’s 2-4 times the capacity of the battery.
For instance, if we have a 1300mah battery pack, we could safely charge it at 2.6 to 5.2 amps. Slower charging is always better for a battery. To a limit… I don’t have any research handy, so I’ll say you don’t want to charge LiPo at less than 1/10C. This relates back to our bag of chemicals, the slower we charge, the more time atoms have to do their thing. The slower we ~need~ them to move, the less likely they are to damage their surroundings. If you exceed the speed that those chemicals can react, ~things~ start happening. For instance, electrolysis.
Electrolysis is the splitting of chemicals via electricity. Generally, it’s water, and that’s how we get puffed packs. When you push a pack to hard, and you get water splitting into H2, and the O2 then oxidizing other parts of the pack, you end up with a puffy pack, and less reaction area due to oxidation.
Let's stick to abuse. Charging, or discharging quickly, causes heating. Moderate heat can be a good thing. Batteries can “have things done to them” faster, if they’re warmer. To a limit, either by generating vapors, or, let's say, fire.
When charging, or more importantly, discharging, heating comes from within the pack itself. While we’re used to the idea of batteries providing power, they also consume some power. All parts of an electrical circuit, have some resistance. That includes LiPo batteries. This resistance is called “internal resistance”. For LiPo batteries used in commercial situations, which frequently see ~very~ cold temperatures, the procedure for starting involves a two step process. First, is “knowing” the cart/truck/plane won’t start, and applying the starter for a few seconds. This puts a high load on the battery, warming the pack. Then, you wait a few seconds, and try again. The engine would start on the second attempt, because the warmer battery could provide more current without the voltage sagging under load. Some r/c car people have taken to preheating their LiPo packs with some very high C charging. We’ll come back to voltage sag.
Now i’m making it sound like batteries being warm, is good. Unless you’re having peak current issues, it isn’t. Batteries age in relation to their temperature. The warmer they are, they faster they lose capacity and C rating. It’s better to keep batteries cool. Cold even. But not frozen, as that can cause the electrolyte to freeze and directly damage the cells from the inside.
In the past, the internal resistance of batteries were there to save us. You could short out most NiCad, or NiMh batteries and nothing would happen. They’d get warm, and finish discharging. No ~real~ drama, excepting some of the very latest NiMh cells, funny, that was discovered by the r/c car guys too. Maybe we should stay away from r/c car people. Do the same with LeadAcid, and you might get some steam… Their discharge would be self limited by chemistry and internal resistance. This, is not the case with LiPo. The internal resistance of LiPo packs are often in the miliohm range. This means we need to do current limiting somewhere else. Sometimes we don’t….
And when we don’t, because a lot of us use OSD’s, we’re aware of “voltage sag”. Voltage sag is what happens when we get ahead of the chemical reactions that restore the voltage of our batteries. Voltage matters to us, because our motors are a long series of wire coils. And the speed at which we can energize those coils, is directly related to the available voltage.
Back to cell abuse, deep discharge of LiPo batteries is a problem. When the voltage is ~very low~ in a LiPo cell, some of the copper used in the battery can dissolve in the electrolyte. This makes it more conductive, and makes it more prone to runaway. It’s also irreversible.
So how do we stop our packs from dieing? Keep them somewhat charged, so they don’t dissolve into themselves. Keeping a battery charged also has it’s downsides, when you charge a battery, you’re forcing the battery into a state it does NOT like being in. That is, it holding the positive and negative poles as far apart, electrically, as it can. So the lower voltage you can store it at, the better. But batteries also self discharge, so you need to have “some” buffer. The generally accepted voltage is 3.6v, for storage. Finally keep them cool when stored. Higher temperatures increase self discharge rate, and the damage the cells do to themselves while stored.
I ~think~ I have all of that right. I didn't cover balancing or cell choice. But that gets into an even longer discussion.