Chargers and Charging Strategies

I think we would all prefer to spend more time flying and less time charging batteries, I know I would.  I spend $ to try to make this happen.   When I started flying electric, it would take days to charge up enough batteries for a single flying session since I only had one charger.  It sure seemed like a lot of effort to charge the batteries needed for a single flying session.  While earlier, a 4 flight session would be considered good,  I now consider 6 – 8 flights a good session. 

I started with a single Astroflight 109D, then bought a second one.  I liked the idea of controlling the charge current directly. At the time I was flying several A123 packs in addition to LiPos and I modified one of the 109Ds to do either lithium or A123 chemistries by adding a switch based on a tear down I found on the web;  worked well.  Basically this was one charger for a single battery (single flight).  Balancing was a separate operation with a separate box. 

My next step in the charging game was when I bought the FMA Direct 10S charger with two outputs.  This gave me two batteries at once and the balancing was built in, a nice charger improvement and procedural simplification. Two batteries at a time cut charging time in half.  Also the 10S brought some nice telemetry on the cells as it notes the battery impedance as well as an adaptive charge current algorithm.  I set up a loose leaf binder with a page for each battery and recorded the impedance, final voltage, date and time to charge for every battery and every charge.  This book is getting pretty thick now.  I can see trends as the batteries age and degrade showing different final voltages within a pack and increasing impedance over life.  Later I bought a second 10S which doubled my productivity again to 4 batteries at once. 

Using dual 10S chargers has sustained my needs for a while.  Since all the batteries that I consider good will charge in 30min or less, I could do 4 packs in 1 hour per charger or 8 packs per hour.  So, in an hour or two I’m good to go.  This is a big improvement from earlier times. 

I’m getting a pretty good pile of batteries.  I try to use a battery size for more than one plane, sometimes I’m successful but sometimes not.  My partial list of batteries I use consistently now follows.  I have others as well but don’t fly them as often.

Tim's active batteries


runs on

made from

number of sets

71" slick


2x 4S


68" velox




51" slick




41" edge












Until recently, most if not all the chargers have been 12v input which allows them to be driven from a 12v battery in the field or the 12v output of a power supply that plugs into the wall.  As the power capability (watts) has increased on the newer batch of chargers, many have migrated to also accommodate the use of 24v as input.  This doubles the watts out for the same current since watts = amps * volts. 

A Note on Power Supplies.  When I redid my home power supply, I setup to have the capability to source 24v.  I moved from my single 30A supply (12v * 30A = 360 watts) to dual 12v supplies at 47A in anticipation of the next increase in charge capability.  This dual supply gave me the capability of sourcing 1128 watts (12v *47A *2 ).  See Aug 2011 newsletter or Tim’s Tips page on the website for these details (Tim’s Home Power Supply).     

The motivation to move up from the single 360w supply came when I started doing 8S packs for the 70” class planes.  I could run dual 10S chargers on 4S2500 packs ok with the 360w supply (51 slick).  But 4S4000 would kick out on undervoltage occasionally which meant that the supply was inadequate to support the current demanded by the charger.  The output voltage sags because more current is being demanded by the charger than the power supply can deliver, so time to upgrade the power supplies.   

I had been eyeing up the new FMA Direct Power Lab8 for some time now.  This unit is advertized at delivering 1344 watts and a 40A charging current.  I just received it in the mail. 

Two Charging Solution Paths

Fundamentally, we want to charge more batteries in the same or less time.  There are a couple of ways to go at this.  We can charge more packs in parallel with a given charger or we can increase the power delivered from the charger or we can do both.

One easy and inexpensive way to do this for the smaller batteries is to use a “fan out” cable with the charger you have today.  This allows you to use the capacity of your charger that might not be consumed on the smaller packs. 

Both parallel and serial charging cables are available from several vendors.  I found some inexpensive ones at EP Buddy.  The motivation to using these cables is you can charge more batteries at once using the balancing connector path.  For example, you could use a combo of serial and parallel to make several 3S1350 batteries appear to the charger as a single larger pack.  You would sacrifice the telemetry the charger provides on a per cell basis, but this is a small concession.

 Note that, using this method, the charge current through the balance cable is the limiting factor which I have seen listed at about 4A. 

Parallel charging cables

One method is to use parallel charging cables so that instead of your charger doing one battery for each output it has, you can do 4 or maybe even 6.  This happens because the current from the charger divides between the parallel connected batteries.  I just got some of these parallel charging cables from EP Buddy.  They are rather inexpensive ( a  6 up, 3S cable is like $6).  

6 Up, 3S Parallel Charging Cable for XH Connectors

Following is a pic from their website showing an icharger 206B that has 2 outputs charging 6 batteries at one time.  I like it!  

Parallel Charging Cables in Use

This is a nice inexpensive way to lessen your charging time using the charger you have now.  It is most effective on the smaller batteries since there is likely unused capacity in the charger when doing this size battery but it also works on the larger packs. 

This scheme works well while you are under the power capacity of the charger and within the balance connector current levels.  For a 10s this would be about 350watts or 4A whichever is less.  If you’re below both of these numbers, you will see the battery group charge in about the same time as a single battery pack.  A nice up side.  

Of course, going to parallel charging the next limit you run into is the size of the power supply or charger capacity, whichever is less.  Either will limit the current that is delivered to the batteries.  For example, my FMA Direct 10S is about a 350watt charger.  So, doing 8 3S1350 is about the same as doing a single 6S5200.  I usually set the charger for 2C charge which is about 10A in this case with the 6s needing 24v.  So, I’m expecting 240 watts from my 350 watt charger which should be ok, knowing there will be some losses along the way in the charger and the power supply.  So, we’re ok on overall power, but the balance connector is the weak link at 4A max.   I can charge 1 batt at 2.7A which is 2C, or 2 batts/output 2A each (1.5C), or 4 batts 1A each (0.7C).  As the charge capacity decreases the time must increase for any given maHr.  That is, the constant here is the maHr which is the charge delivered to the battery.  

There is an extra step in the process when using parallel cable charging.  Before charging you need to let the batteries equalize to the same voltage/charge level for some amount of time, then you plug the group into the charger.  This makes all the separate batteries look like one big battery to the charger.  A convenient process is to take the parallel cable to the field with you, fly a pack and plug it in.  Then fly another pack, plug it in, etc.  This way while you’re flying, the batteries are equalizing and when you get home or later you can charge the group. 

Another way to look at this is that the parallel charging cable scheme is limited by the current rating of the balance plug of about 4A.  So, if you want to charge at 2C, the largest battery is a 2000maHr for a single battery.  If you are willing to charge at 1C, then you could do two 2000maHr batteries.  So, you need to be careful to stay within the capabilities of the balance connector since this is the only connection to the batteries.

This method could be used on a 6S5000, but the charge current would be 0.8C (4/5) for a single battery, or 0.4C for two 6S5000 with proportional increase in charge time.  Clearly for this size batt you need to be able to use the main power connector, not just the balance connector in order to achieve a shorter relative charge time.  Recall, my goal is to charge in shorter time.  Although it might be that charging a group of batts at 0.7C gets them all done quicker than individually.  

A similar time degradation occurs with 4S2500 batts although not as severe.  For two 4S2500 in parallel on a single 10S output, you can charge each batt at 2A which is 0.8C (2/2.5), or four 4S2500 for the charger.  The upside is that you could charge 4 batts instead of 2 with the 10S, 2 batts per output.   So, there is a bit more up side at this level compared to 6S5000. 

Ok, how about 3S2250?  For two 3S2250 on one 10S output, each would charge at 2A and 0.89C (2/2.225).  So, you could charge 4 at once (2 per output) but at a time penalty.  I suspect this method would be faster than doing only 2 at a time individually though, so there is some upside here as well. 

The Next Step

While my dual 10S setup has served me well, the bigger batts require more power to charge in a shorter time.  So, I wanted more than a combined 600 watts distributed over two chargers.  The FMA Direct Power Lab8 should get me to the next level.   

With the higher capacity chargers, there is a more evolved method beyond the parallel charging cables and this is called a Paraboard.  The Paraboard uses both the balancing connector and the main heavy wire connections, so there is no balance connector current limitation as with the parallel charging cables. 

The Paraboard can take up to 6 batteries in parallel for a single charge.  Industrial strength charging!  Here is the bundle I bought with the Deans type Paraboard from FMA Direct.  Since I’m migrating to EC5 connectors for my bigger stuff I also ordered an EC5 Paraboard from EP Buddy ($16). 

Power Lab 8 combo with Paraboard

EC5 Paraboard.  One note on the EC5 Paraboard regarding connector gender is in order.  From the days of Deans, I always used the female side on the battery.  My thinking was that with less exposed metal on the Dean’s connector itself, the chance of shorting to something unintentionally was reduced.   I see many other people thinking the same way.  On my first EC5 install on a couple of 6S5300, I carried through with this philosophy.  I have been dragging my feet on changing out 8 4S4000 (use two for 8S on my 71” slick) because I had a working scheme, and was a little lazy.  I have been accumulating enough EC5 connectors for a while to do this.   

When I looked at the picture on the EC5 Paraboard online, it showed female side mounted on the board.  This was against my existing “female on the battery side” philosophy and would require the male EC5 on the battery.  So, guess I’ll change my philosophy for the EC5 because of the more insulated housing.  So, in this case, my laziness paid off!

Another note on the EPBuddy power board vs the one from FMA Direct is in order here.  While the connections are essentially the same to do the charging, the FMA Direct ParaBoard also incorporates fuses for each battery which I think is a good idea.   

My plan is to use the PL8 at home on 24v and make a box for my dual 10S chargers with parallel charge plugs for the field.  In both cases, parallel charging should lessen my charge time and increase my flying time.  A step toward goodness. 

Happy flying.

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