With more and more RTR kits coming out that require 2 packs to be run in series I thought it’s time to give some info regarding these setups. Running 2 packs in series is not a good setup and is used because it’s more convenient as it’s easier to get purchase lower voltage packs and put them in series. The reason why this setup isn’t very good is because the packs will discharge down to different levels. This is due to the flow of current, the current from one pack must flow ‘through’ the second pack meaning that the current sees virtually double the resistance of the first pack.  As you read below about the ESC cutoff, the pack with the highest resistance will drain the least. The ideal setup would be to run 1 pack at the proper voltage instead of 2 packs in series to get the desired voltage. This is why we offer 6S packs that fit in the Arrma 2018 battery trays as Arrma allows the use of one 6S pack instead of two 3S packs.

  If you are running a Traxxas kit or other kit that doesn’t allow for one pack to be used you need to make sure the low voltage cutoff (LVC) in your ESC is set high enough for the lowest of the 2 packs to not have the cells be discharged down to low. Discharging packs down too low under load will decrease cycle life and increase the chance of cell damage. Once you have hit the low voltage cutoff take the packs out and look at the cell voltages.  If they are below 3.6v per cell you need to increase your cutoff. If your ESC doesn’t have an adjustable cutoff or it’s already at the highest setting then you need to consider running a LiPo Alarm which can be attached to the pack and connected to the packs balance connector.

  An important aspect of running 2 packs in series that is often overlooked is to swap the packs every run. That is to have each pack run on the ESC negative lead and positive lead alternatively. As mentioned above, one of the packs is basically solicited twice as much as the other, by alternating which is seeing twice the work load every run, you are using the packs equally. We recommend keeping packs in pairs when used in series, and by marking each pair 1A-1B, 2A-2B, etc. it is easy to keep track of which pack has been used on which ‘side’ every run.

  If you are running 2 packs in series and one gets damaged it’s best to replace both packs as if you take a new pack and run it with one that has some cycles put through it you will have an even greater difference in discharge between the packs. As a pack gets cycled the mAh drops and the resistance will increase. A new pack with higher mAh and lower resistance will not discharge down as low as the older pack.

  The low voltage cutoffs on ESCs aren’t very accurate as there are a few things that will impact when they will cutoff. The resistance of the pack and the rest of the system will have an impact on how much voltage drop the ESC will have.   For example you have 2 different set of packs with the same mAh but one set has 7mOhms of resistance per cell and the other set has 1.8 mOhms of resistance per cell the set with the higher resistance will have more voltage drop under load. This means the cutoff will kick in earlier and the cells won’t be discharged down as low. If running packs in cool/cold weather they will cutoff earlier then running them in warm/hot weather as the packs resistance is higher when they are cool/cold.  If you are running on a low traction surface like concrete or asphalt your packs will discharge down more before the cutoff kicks in. This is due to less amps being pulled so the voltage drop is less which means the packs will discharge more before the cutoff kicks in. The opposite is running in grass or sticky mud you will pull more amps and the voltage drop will be higher so you will discharge the pack down less. This is why it’s important to have your cutoff set and adjusted based on the conditions you run in. As mentioned above the best way to check and make sure your cutoff  is cutting in at the proper level so your packs don’t get dis charged down to low is to check the cell voltages as soon as the cutoff kicks in you want the cells to be at 3.6v or above.  

 Research and testing has proven that discharging a battery down to 100% which is referred as Depth of Discharge will shorten the cycle life of the battery.  This means the less you discharge down the pack the higher the cycle life will be. The draw back to this is that this reduces playtime. A decent compromise is to try to be around 80% Depth of Discharge. This means you need to try and have 20% of the packs mAh left into the pack after you are done running the pack. Check the pictures below that show three different cell voltages and the perecentage of mAh left into the the cells/pack. Keeping a pack stored fully charged is also bad for the pack so you should store them around 3.85v per cell.   Packs also deteriorate as they age this is one of the reasons why SMC sometimes runs out of packs as we do our best to supply our customers with fresh packs. Not using a pack to try and keep it lasting longer is not going to help. You might as well run out as often as you can and replace it once it has worn out.