Quote:
Originally Posted by MecaNaught
In researching a higher Amp hour battery for off season robots, I've
found a LiFePO4 battery that is the exact form and size as the normal 18AH lead acid batteries. However, I am not experienced with the specifications, and I'd rather avoid frying any of our boards. Anyone know if these would be safe replacement? (Only for non competition use of course)
Normal Lead acid:
http://www.power-sonic.com/images/po...12_Sept_10.pdf
Magical Lithium battery:
http://www.power-sonic.com/images/po.../PSL-12200.pdf
Infact, I'm surprised FRC doesn't use these batteries in competition, but perhaps they're too new. |
There's a few reasons why first doesn't use Lithium batteries and it really isn't surprising. I work as a engineer in the drone industry so I've gotten pretty comfy with the world of lithium batteries.
First, have you looked at the cost of anything like that?
So lets look at that battery you picked out, most of those types of bricks are made up from round cells welded/soldered together, if you scroll down on the datasheet that pack uses a IFR26650EC cell, and it uses 4 in series 7 in parallel so a total of 28 cells. So that cell being what looks like a no name, and so is the pack I couldn't find a price but you can look at the price of the cell
http://www.batteryspace.com/lifepo4-...ssed-ndgr.aspx
That's $7.50 a cell for a no name brand (more for an "brand name") so with 28 cells that brings the cost up to $210 before cost of manufacturing the case and packaging and putting it together/etc. Add all of those you can be looking at $300+ per battery. So if FIRST made us buy these batteries... we'll we would need a lot more fundraising... compared to the cost of $40 each for our SLAs.
Second, the PSL-1220 pack is rated for a MAXIUM discharge of 20amps...... think about that for a second. On you're first robot you should be using at least 4 of the 40amp breakers and probably a few of the 20/30amp breakers. Meaning that you're robot has the potential to draw 200-300amps burst down to 50-100 amps contentiously depending on how you're robot is designed. So this battery would be woefully unable to keep up.
To explain father, I like to generalize that they are two categories of lithium batteries, power types and energy types.
Power batteries are types that can output large number of amps and handle surges of current without damage, but they lack capacity compared to something of the same size and weight as a energy battery. This is what you need to power a robot, or say a gokart, or a tesla, or drone/RC plane/etc.
Energy batteries, are batteries that can pack a lot of capacity into a small size/light weight package but lack the ability to output a large number of amps without permanent damage. These types of batteries are useful for powering things like your laptop, or smart phone or computer packup, etc. Those types of devices do not demand high current nor do they have surges of current. This is what the PSL-1220 is.
Now to supply a first robot's need of power you need obviously more of a power type of battery, something that can handle large current draw without damage. That would be something like this:
http://www.a123systems.com/Collatera...ta%20Sheet.pdf
A123 26650 cells are LiFEPO4 power cells, that ONE cell can output 70 amps! But notice how it only holds 2.5AH compared to the 3.3AH of the cells used to build the PSL-1220 battery (which can only deliver a few amps each). A 12v pack built of A123 batteries with the proper hardware would be a GREAT replacement for the first pack, able to deliver crazy amounts of power, be a good deal lighter but it will literally cost hundreds of $$ for each pack at retail pricing. You'll also notice on the data sheet for the SLA battery it's rated for 180amps for a 10sec burst, and because of the SLA chemistry you can safety go over this number for even shorter periods of time.
Thirdly, a few more downsides of Lithium batteries compared to SLA is that Lithium batteries need to have there cell voltages monitored very closely. Use up to much of the battery? permanent damage. Draw to many amps out of the battery? permanent damage. Charge it to much? permanent damage/fire. Charge it to fast? permanent damage/fire. So it's pretty easy to ruin your $$$$ lithium pack (and I've seen high school students abuse the SLAs more then they should). A lot of modern day electronics that use lithium battery packs have a BMS (battery management systems) a bit of electronics and software designed to keep you from doing any of those things to the pack. But for something that can handle the level of power from a FIRST bot it will not be cheap. The SLA batteries we use are very durable, you can kind do whatever you want to do them and they keep ticking and they are designed to be used to start motorcycles so they can handle a large surge of amps and because they are Sealed generally pretty safe. They do degrade kinda quickly usually after 150-300 full cycles (depending) but are so cheap this isn't really much of an issue.
Fourthly the safety aspect...
Now LIFEPO4 is just one type of lithum batteries, it's also probably the safest type of so. But lithium batteries can vent if you short them for to long, overcharge them, puncture them, crush or dent them and etc. What happens when a lithium polymer (a bit more potent the LIFEPO4) battery decides to vent or is helped along the way?
https://www.youtube.com/watch?v=1wW_36XIupw (and that's a 4ah 12v battery now imagine what a 20AH going off would look like).
So TDLR: The PSL-12200 is only rated for a 20amp output which is no where near enough to power a first robot, maybe okay for testing of bench top electronics and free spinning a motor. Lithium in general is really expensive compared to SLA and various types of lithium (generally the cheaper ones) can be dangerous and should not be trusted in the hands of high school students...... (I have been at a high school level mini-battlebot event where a student had a lithium battery go off in his face.....).