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November 14, 2012 AT 7:28 am

ASK AN EDUCATOR! – “Is it safe to use Li-Po batteries in series?”

Mnova asks:

I would like to know if it is safe to use Li-Po batteries in series in order to provide a higher operating voltage. I was reading the Adafruit Learning System write up on Li-Ion and Li-Poly batteries and I got a bit confused.
The ALS says the following about charging Li-Pos in series:

“”This is also discouraged because the battery wont be able to be charged in a balanced manner. You should purchase a lithium ion pack that is preassembled.””

I would like to know, if it is OK to use the batteries in series and then charge them individually.

Great question!

Batteries are purchased in two configurations, as individual cells (i.e. an AAA, AA, C, D, 18650, etc) or as a pack. Individual batteries do not need to be balance charged as the charger will regulate the voltage and current input to the battery and turn off when complete. The Adafruit Learning System’s statement is based on the fact that some battery chemistries do not “self-balance” when charged in series. In contrast, chemistries like NiMh and NiCd can be charged without balancing.

If you purchase a Lithium based battery pack containing 2 or more cells, it should have secondary wires that are connected to each respective cells positive and negative terminal. Each wire is shared with its neighbor, so a 2S pack would have 3 wires, a 3S pack would have 4 and so on. These taps allow for “fine tuning” the current and voltage entering the pack during charging and maintains a constant voltage over each cell. Make sure you have a battery charged that is designed to charge multiple lithium cells and that it contains this balancing capability. Below is an example of what can happen if a LiPo battery is improperly charged:

In order to prevent a disaster like this, periodically examine the physical state of your batteries. They should not feel squishy or appear ballooned and make sure you store them in a fire-proof container. I happen to use a old coffee can for mine. If you find out they are damaged, bring them to your local hobby store for recycling.

There is a great website that provides quite a bit of information regarding the proper handling of most battery chemistries.

I hope this has helped answer your question, have fun with your LiPos and be safe!

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“Ask an Educator” questions are answered by Adam Kemp, a high school teacher who has been teaching courses in Energy Systems, Systems Engineering, Robotics and Prototyping since 2005.


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6 Comments

  1. This doesn’t answer the question at all. Is it okay to DISCHARGE in series, then CHARGE individually?

  2. This answer is specifically for ‘RC’ lipoly batteries – where each cell is UNPROTECTED. Thus, a separate wireset is needed for each battery to properly charge it. If you have protected cells, you can discharge them while they are connected in series (although its always better to get a packaged set for more consistent performance). All separate cells (protected and non-protected) should ALWAYS be individually monitored & charged – essentially that’s what an RC pack does. If you want to charge them together in series, get a pre-packed and protected battery.

  3. Thanks. The idea that I’m essentially walking around with a live explosive in my pocket freaks me out sometimes. After reading reports of proper (Apple) battery packs just blowing up for no reason, I’m disinclined to be too adventurous when assembling my own projects. :-)

  4. Still, nobody has answered the actual question, which is whether or not it is safe to DISCHARGE Li-Po batteries in series.

    The answer is yes, but with two caveats: First, the must be of the same capacity, and second, you must monitor the voltage on each battery, and shut down your circuit when either one drops below its minimum level. I don’t know exactly what this level is, but it’s somewhere between 2 and 3 volts. The reason for this is that when two batteries discharge in series, if they don’t have EXACTLY the same amount of charge (in coulombs), then as soon as one is dead, current from the other will try to charge the dead one IN REVERSE. Which will certainly damage the lower-capacity battery, and probably be a safety hazard as well.

    If you’re using batteries with built-in protection, this will shut down your circuit automatically at the end of discharge of either battery.

    If you’re not using protected batteries, and you don’t include discharge detection and automatic shutdown, then the answer is NO, it is not safe to discharge Li-Po batteries in series.

  5. oops – I see that Adafruit actually DID answer the question. My bad.

  6. I’m an avid RC helicopter enthusiast, and have a large range of electric heli’s in my fleet (up to 2m rotor diameter!) These require their lithium polymer batteries to be cared for far more than most as my largest heli will draw in excess of 6-7kw from a 14 cell pack.

    Now when making up a pack in series, you should generally try to use matched cells (where the internal resistance of each cell is equal). If they are not matched, you can end up with a very expensive firework! This is because the cells with the higher impedance will discharge at a slower rate, meaning their voltage remains higher. And when the voltage difference between the cells becomes too much, it generally spells trouble.

    Now when charging, I would try and keep the cells together and charge them as a single pack using a lipo specific balance charger. The charge method is slightly different depending on the charger. I personally use a Revolectrix Cellpro 10s charger which provides the bulk of the charge current through the packs main discharge leads. While it is doing this, it monitors each individual cell voltage and ‘balances’ the cells by boosting the charge on cells with a lower voltage. This is, in my opinion, the safest method of charging the pack.

    Other cheaper chargers balance the packs using a discharge method. This is mostly the same as the cellpro charger, but, instead of boosting the low cells, it discharges the high cells. Now this is fine if the pack is pretty much balanced when you put it on to charge (cells within 0.2V of each other) as it will keep it balanced, however if it is outside of this range, I would use the cellpro’s method as it will bring the really low cell up in balance before continuing with the rest of the charge.

    Sorry if that’s just a load of waffle, I do my best to explain things, but never great at phrasing it :)

    Also, as an extra safety level when charging, have a look for a lipo safe bag. This will essentially contain a lipo fire should one occur (can be seen demonstrated on youtube.)
    I always use one, and always charge my lipo’s on a concrete floor. I have *touch wood* never had a problem with any of my batteries as I make sure they are looked after properly, but I never take shortcuts with safety on them. Follow all this, and its almost certain you won’t be bitten :)

    The TL:DR; Follow the advice and get a premade pack (the cells in these will be matched, so you will almost never have a problem), get yourself a decent balance charger and a lipo safe bag. Decent quality, cheap lipo packs can be found here: http://www.hobbyking.com
    I use the Turnigy nano-tech packs for a lot of my heli’s, and they are brilliant. They also cell the cellpro chargers and lipo safe bags ;) (perhaps something for you to look at adafruit!)

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