I know that in RCs we rarely if ever use Li-ion batteries but I get asked about them on my youtube cahnnel/facebook from time to time because my interest in electronics was born out of my love of RC which means I tend to find myself using all manner if batteries for all sorts of reasons these days and its amazing how many of them (99.9%) can be charged/cycled with a good CCCV charger. Thats the type of charger that our LiPo chargers are in case you werent familiar. Anyway, someone asked me about some Li-Ion packs they were trying to build and I ended up writing a fairly lengthy response so I figured I would also share his question and my response here incase any of you might happen to need this info or were maybe just curious.
@Rolex I was thinking that maybe this post would be good to add to the sticky about batteries if you think it warrants it.
Question:
You seem to know about these batteries let me ask you this. I have 4 - 18650 batteries connected in parallel per cell. I have 12 cells connected in series starting with negative from cell 1 to positive in cell 2 and so on. Should be 4V x 12 cells = 48V. 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48V. Why do I have = 72V? Thanks
Response:
Well first lets address the power ratings for Li-ion batteries. The nominal charge of a Li-ion battery is 3.7V per cell. Their peak charge, in the case of a new battery, will be 4.2 V/C. Occasionally you'll get a cell or maybe a brand that will have a max charge of 4.1V/C but if you measure it with a multimeter it will most likely end up being closer to 4.2 than 4.1V/C but anyway, I digress. The maximum discharge point of a Li-ion cell is about 2.8V/C although you should be able to use it down to about 2.6V/C without damaging the battery. Deeply discharging the cells will damage them and in rare cases can cause them to catch fire, leak, deform, etc. Li-ion batteries that are not going to be used for extended periods of time (6 months or more) should be stored at about 60% of their max charge which is right around their nominal voltage. This is generally accomplished with a CCCV charger (let me know if you want more info on CCCV chargers) that will have a "storage" mode that will charge or discharge the battery down to the appropriate storage level.
Now that we have the basics covered we'll tackle the series and parallel problems. Connecting batteries of any kind in parallel increases the mAh rating (or capacity) of the battery pack. In your example of 4 cells wired in parallel you would start with the nominal voltage rating of the cell (3.7V in this case) and lets say they are 1000mAh capacity for the sake of the math being easy. In this example youve taken four 3.7v 1000mAh batteries and created a 3.7V 4000mAh battery pack because when wiring in parallel the only thing that increases is the mAh or capacity rating.
When it comes to wiring a pack in series its basically the opposite of parallel. In other words you add the voltages but the mAh value stays the same. In your example you have twelve 3.7V, 1000mAh, Li-ions wired up in series so your pack would be 12 x 3.7V and the mAh would stay the same so you would have a pack that was 44.4V, but still only 1000mAh.
**Footnote 2** List of nominal, max, and minimum volatges for various common battery types.
Type | Nominal | Maximum | Minimum |
NiXX | 1.2V/C | 1.5V/C | 1V/C |
LiFe | 3.3V/C | 3.4V/C | 3V/C |
Li_ion| 3.6V/C | 4.2V/C | 2.6V/C |
LiPo | 3.7V/C | 4.2V/C | 2.8V/C (a lot of people will typically say anything from 3.0 to 3.5V/C but one of the battery technicians from www.onlybatterypacks.com told me that 2.8V/C is completely safe and that real damage doesn't start until about 2.6V/C. That being said my recommendation is no lower than 3.0V/C since this is a proven safe number and still lets you get the vast majority of the capacity out of the pack. I've never had an issue using 3V/C)
@Rolex I was thinking that maybe this post would be good to add to the sticky about batteries if you think it warrants it.
Question:
You seem to know about these batteries let me ask you this. I have 4 - 18650 batteries connected in parallel per cell. I have 12 cells connected in series starting with negative from cell 1 to positive in cell 2 and so on. Should be 4V x 12 cells = 48V. 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48V. Why do I have = 72V? Thanks
Response:
Well first lets address the power ratings for Li-ion batteries. The nominal charge of a Li-ion battery is 3.7V per cell. Their peak charge, in the case of a new battery, will be 4.2 V/C. Occasionally you'll get a cell or maybe a brand that will have a max charge of 4.1V/C but if you measure it with a multimeter it will most likely end up being closer to 4.2 than 4.1V/C but anyway, I digress. The maximum discharge point of a Li-ion cell is about 2.8V/C although you should be able to use it down to about 2.6V/C without damaging the battery. Deeply discharging the cells will damage them and in rare cases can cause them to catch fire, leak, deform, etc. Li-ion batteries that are not going to be used for extended periods of time (6 months or more) should be stored at about 60% of their max charge which is right around their nominal voltage. This is generally accomplished with a CCCV charger (let me know if you want more info on CCCV chargers) that will have a "storage" mode that will charge or discharge the battery down to the appropriate storage level.
Now that we have the basics covered we'll tackle the series and parallel problems. Connecting batteries of any kind in parallel increases the mAh rating (or capacity) of the battery pack. In your example of 4 cells wired in parallel you would start with the nominal voltage rating of the cell (3.7V in this case) and lets say they are 1000mAh capacity for the sake of the math being easy. In this example youve taken four 3.7v 1000mAh batteries and created a 3.7V 4000mAh battery pack because when wiring in parallel the only thing that increases is the mAh or capacity rating.
When it comes to wiring a pack in series its basically the opposite of parallel. In other words you add the voltages but the mAh value stays the same. In your example you have twelve 3.7V, 1000mAh, Li-ions wired up in series so your pack would be 12 x 3.7V and the mAh would stay the same so you would have a pack that was 44.4V, but still only 1000mAh.
**Footnote 2** List of nominal, max, and minimum volatges for various common battery types.
Type | Nominal | Maximum | Minimum |
NiXX | 1.2V/C | 1.5V/C | 1V/C |
LiFe | 3.3V/C | 3.4V/C | 3V/C |
Li_ion| 3.6V/C | 4.2V/C | 2.6V/C |
LiPo | 3.7V/C | 4.2V/C | 2.8V/C (a lot of people will typically say anything from 3.0 to 3.5V/C but one of the battery technicians from www.onlybatterypacks.com told me that 2.8V/C is completely safe and that real damage doesn't start until about 2.6V/C. That being said my recommendation is no lower than 3.0V/C since this is a proven safe number and still lets you get the vast majority of the capacity out of the pack. I've never had an issue using 3V/C)
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