I've looking into using some Li-ion cells wired in a series for a project, and I have a question regarding charging them. Obviously there are many options for single and multi cell configurations; however, I am having trouble understanding the process of charging cells that are in series because pretty much anything I can find online references charging multiple cells that are in parallel. I would like to use three 18650 batteries in a series to achieve the voltage necessary for my build and integrate the charging system into my PCB layout, but I guess my question is: If they are in series, would I charge them as if they are one cell or still use a multi cell charger? I've read about balancers as well, but am still unclear as to their application in this configuration. Can someone please shed some light onto this subject? I apologize for my lack of knowledge regarding this stuff... this is simply something I've never had to work with in the past.
Thanks in advance for any info you may be able to provide!
When LiPo Cells are in series, they need to be individually balanced which explains why LiPo packs with cells in series will always have EXTRA wires. These are attached to a balancing connector and make sure that the batteries all accept charge the same way and that one battery is not wildly different (charge wise) from the others.
I personally use this device to charge my series connected 2S and 4S LiPo's.
I personally think that it might be easiest to design your solution around one of these types of chargers than try to recreate a charging solution on your own. (I say this based on your query and implied gaps in knowledge, but I don't mean to imply that you CAN'T come up with your own on board solution) The real issue is that most charge controller scenarios involve parts that only come in Surface Mount packages... making development and learning that much more difficult.
Thanks for the reply and the info! I'll read over everything and I'm sure it will help! As a side note, this is for a beta prototype for a product that will be going into production soon. It was originally using a 9v, but I've recently made the decision to switch to a non removable battery setup.
"I've recently made the decision to switch to a non removable battery setup. "
Like Apple. Didn't prove very popular.
With regard to Lipo batteries, any setup that does not account for the fact that it is still not a good idea to turn your back on a charging Lipo is doomed to failure.
Charging batteries can get messy. From what I understand, it is never wise to charge batteries in series. I would look at having a dedicated parallel line that is used to charge the batteries and another line that is wired in series to operate the device. A boost driver is another option so that you can get the needed voltage out of one battery.
The switch from a removable to built in battery has many reasons, so I won't go into why in this post.
I think I'm going to use a single cell and a power path management/charger from TI that seems pretty awesome, then use a step up to achieve the voltage necessary.
Another question that someone may be able to shed some light on:
I need 3 different voltages for different stages of the circuit... 9, 5, and 2.8. would it be more efficient to step directly up from the battery's 3.7 to 9v, then sort of cascade down to 5, then 2.8? Or is it better to go from 3.7 to 5, then up to 9v and down to 2.8 from there?
I would suggest looking at the convertors efficiency rating and the power drain from each method if battery life is your main concern.
mdlockyer77:
3.7 to 5, then up to 9v and down to 2.8 from there?
Cascading convertors could introduce further losses.
I would suggest lipo to voltage with separate converters would be better although you do not say the current requirment of each rail
Yeah, well try prying two cells apart with a sharp pocket knife while holding it in your left hand. You could say it's a grown-up version of "hot potato" , when the tip punctures one of the cells.
It was actually not intentional. At the time I didn't know that puncturing a cell caused it to ignite. To an observer it looked like a box of wooden seft-starting matches igniting in my hand.
I dropped it on the ground in the parking lot and watched it scorch the pavement. It's one of those mistakes you only make once.