Hello all! I have came across some question regarding using LIPO battery. Sorry for asking a question which is out of arduino topic. But somehow my application is eventually using an arduino to work it.
Question, can I directly short-circuit a LIPO battery by using a resistor across the +ve and -ve terminal?
For example, I have a high wattage resistor, and need it to generate some heat from it by supply current across it. If i short it with a LIPO battery, will it damage the LIPO? or will LIPO explode?
From basic calculation, V=IR, 12V = 6A x 2ohm. Hence, 6A drawn from LIPO battery is workable right? If my LIPO battery could support 25C with a 1000mAh capacity.
If theoretically fine, then will it over draw the LIPO voltage? I read that if LIPO voltage is too low, it might caught fire.
Electronics guru, please shed some light with me.
Thank you.
Yes, you can put a resistor across the terminals.
How long do you expect to draw 6A from a 1AH (1000mAH) battery? Minutes?
Yes, overdraining LiPo batteries is bad for their health. Many battery packs have protection built in against overdraining and overcharging. Check if the one you plan to use has that. Fire is certainly a risk if not - do you recall the Sony laptop battery recall, news reports of Apple product fires, or the Boeing 7x7 fleet grounding after fire caused several emergency landings? I think all were overdischarge related.
Thank you CrossRoads for the advises.
So you mentioned that most of the battery packs have protection built in, but how do we identify that?
IF, without the built in protection, can i just make one voltage monitor for it? Example, drain the battery voltage drop until 3.3V and stop supplying?
pwillard:
Well, if your battery gets warm and puffy... it's the kind without protection. So maybe start small and build up to the load you want to use.
Well, according to this site http://www.tjinguytech.com/charging-how-tos/the-variable-c
your 1000mAH battery is only going to be 1C, not 25C, so getting 6A out of is doubtful to start with. Read the LiPo basics that are part of the same site.
You'd have to read the datasheet for the battery pack, or ask the distributor/manufacturer to find out about built-in protection.
A battery pack like this is likely to have built in protection. I can't find much in details tho.
CrossRoads:
Well, according to this site http://www.tjinguytech.com/charging-how-tos/the-variable-c
your 1000mAH battery is only going to be 1C, not 25C, so getting 6A out of is doubtful to start with. Read the LiPo basics that are part of the same site.
You'd have to read the datasheet for the battery pack, or ask the distributor/manufacturer to find out about built-in protection.
A battery pack like this is likely to have built in protection. I can't find much in details tho. http://hobbyking.com/hobbyking/store/__9164__Turnigy_1000mAh_3S_25C_Lipo_Pack.html
First you state, "a 6 Amp discharge rate from a 1000mAh LiPo battery is doubtful".
Then you provide a link to a 1000mAh LiPo battery that is rated: 25 Amp Constant Discharge = 25C.
If 25 Amps is possible then how can 6 Amps be doubtful?
I see that LiPo batteries can typically be discharged very quickly like: 20C, 25C, 30C, 35C and faster ...
If the battery is 1000mah X 25C then you can safely pull 25amps at the rated voltage (11.1v for 3S) (like the turnigy battery above)
The problem with guessing if the OP can pull 6 amps from his unknown battery is that we don't know the rating of that battery.
Check the spec sheet to be certain and safe. If you current overdraw a lipo battery then you can get "thermal runaway" which is a fancy way of saying the $h!t will explode and catch fire (literally; first heat will cause the pack to expand like a balloon until the pressure ruptures the casing, exposing the lithium to oxygen, which will ignite almost instantly)
C rating X mah= max continuous amps, which is correct.
By calculation, if i draw 6A from resistor, and the LIPO contain 1000mAH (1AH), it give us about 10mins run time for the resistor to fully empty the LIPO. (by fully, will it damage the battery? will it become lower than 3.3V LIPO safe voltage level?)
So to sum up for the conclusion, can i say resistor discharge across the LIPO battery is possible, under the condition which below the rated C (discharge rate). But, over discharge (which example, left it discharge for couple of hours or even days) will damage the battery, and cause hazard.
Just because the pack is electrically capable of giving you 6amps at 3.x volts for 10 minutes does not mean that you are not going to expereince a thermal runaway situation!!!
MOST LIPOS THAT DO NOT SPECIFY A C RATING ARE NOT EXPECTED TO BE USED ABOVE 1C!
Just becuase you found a pack that *can supply what you need, doesn't mean it won't explode when you do.
I don't know that 3.7v is going to be enough for your application, and you wouldn't get 70watts from a single lipo cell for 10 mintues with what you have proposed so far. You would get closer to 20watts (6Ax3.7v)
First off, I would use LiFe cells, they are less prone to exploding and less finicky when it comes to overdischarging them. LiFe cells operate under load at about 3.3v per cell and need to be charged when they hit 3.2v or lower.
If you are looking for 70 watts for 10 minutes, I would start with a 3s pack (that's 3 cells in Series, so ~11v) rated for 10amp continuous draw or more, and if you needed it for 10 minutes then it would need to be 1200mah or so.
So, you need something like this. That is 3S which will put out 10v @ 2100mah and 30C max continuous draw (63A). That battery will run a 6A load at 10v for 1 hours, approximately. The only problem with LiFe batteries is that it's very hard to *measure when they are almost discharged because their discharge curve is so flat. They run at 3.3v for a long time and then drop off to 3.2v very quickly. You would be best to use a DMM and a clock to figure out what kind of run time you might get, and then only use %80 of that time as expected so you don't overdishcarge the cells. Check out rcgroups forums for more info (that is the HobbyKing forums).
You will also need a charger designed to charge such a pack like this one
I AM NOT RESPONSIBLE FOR ANYTHING THAT YOU DO
DO NOT LISTEN TO ME OR MY ADVICE
interestingfellow:
Just because the pack is electrically capable of giving you 6amps at 3.x volts for 10 minutes does not mean that you are not going to expereince a thermal runaway situation!!!
MOST LIPOS THAT DO NOT SPECIFY A C RATING ARE NOT EXPECTED TO BE USED ABOVE 1C!
Just becuase you found a pack that *can supply what you need, doesn't mean it won't explode when you do.
I don't know that 3.7v is going to be enough for your application, and you wouldn't get 70watts from a single lipo cell for 10 mintues with what you have proposed so far. You would get closer to 20watts (6Ax3.7v)
First off, I would use LiFe cells, they are less prone to exploding and less finicky when it comes to overdischarging them. LiFe cells operate under load at about 3.3v per cell and need to be charged when they hit 3.2v or lower.
If you are looking for 70 watts for 10 minutes, I would start with a 3s pack (that's 3 cells in Series, so ~11v) rated for 10amp continuous draw or more, and if you needed it for 10 minutes then it would need to be 1200mah or so.
So, you need something like this. That is 3S which will put out 10v @ 2100mah and 30C max continuous draw (63A). That battery will run a 6A load at 10v for 1 hours, approximately. The only problem with LiFe batteries is that it's very hard to *measure when they are almost discharged because their discharge curve is so flat. They run at 3.3v for a long time and then drop off to 3.2v very quickly. You would be best to use a DMM and a clock to figure out what kind of run time you might get, and then only use %80 of that time as expected so you don't overdishcarge the cells. Check out rcgroups forums for more info (that is the HobbyKing forums).
You will also need a charger designed to charge such a pack like this one
I AM NOT RESPONSIBLE FOR ANYTHING THAT YOU DO
DO NOT LISTEN TO ME OR MY ADVICE
USE ANYTHING I SAY AT YOUR OWN RISK
Wow, thank you very much for the advises. I do really appreciate that.
But I do believe that I never mentioned i am going to use 3.6V 1S LiPO. And of cause, to get 70+ watts (12V x 6A), i'm clear that i required 3S or above to meet the criteria.
I'm just not sure that do i need any special regulator circuit to draw power from the LIPO to get 70+ watts from it.
Well, you suggested LifePo4 and i think it is good! Will taking consideration in that.
