Is this dangerous?

[RooGM]

Hi everyone,

This is my new 14S 2P battery configuration I have installed on my MP4.  Right now I have to charge every pack individually, which takes a long time.  I want to make a wiring harness to charge the 4 x 5S batteries in parallel.  The wiring harness would just be one connector going to the charger and 4 connectors wired in parallel going to the batteries - that would be the main charge wires (the balance plugs would all be wired in parallel two on another harness).  Normally this is no problem when the batteries are separate as I charge my RC heli and plane batteries in parallel this way all the time.  But now that these packs are wired into the bike like the below diagram, permanently it's not working for me.  When I tested connecting the negatives of battery one and two together I got a big spark.  The harness would connect the negatives of batteries 1, 2, 4 and 5 together and the same for the positives, ending in a single negative and single positive, ie. a simple parallel wiring harness to charge all the 5S batteries at once.  But the spark tells me I cannot do this while the batteries are wired up in series in my bike (even with the output terminals not connected to anything) Is there a way I can solve this?

The below image is how the batteries are currently wired up in my bike.

[Bikemad]

Yes, it is very dangerous to connect different packs in parallel while they are still connected in series as you are effectively shorting out each of the packs (hence the big spark!  ).

If you look carefully at your diagram below, you should be able to see that by connecting the negative terminals of packs 1 and 2 together you are effectively shorting the negative terminal of pack 1 directly to the positive terminal of pack 1 as it is already directly connected to the negative terminal of pack 2 via the interconnecting wire.

Connecting the positive terminals of packs 1 and 2 together will effectively short circuit the positive terminal of pack 2 direct to the negative terminal of pack 2 which is already directly connected to the positive terminal of pack 1 via the interconnecting wire.



Before you can connect the packs in parallel for charging, you must first disconnect the series connections to prevent any short circuits from occurring.

Similarly, connecting the balance lead connectors in parallel while the packs are still connected in series will also result in a short circuit through the balance leads.

To charge packs in parallel, they must be identical in voltage, chemistry and number of cells and there must be no series connections between any of the paralleled packs.

To run packs in series, there must be no parallel connections between each of the packs in the series.

Hopefully you will be able to understand at least some of the above.



Alan
 

[RooGM]

Hi Alan,

Thanks, now I realise that I completely overlooked the simple short in the circuit and I'm scratching my head thinking "how could I not have seen that?"  The way I have my batteries wired up in real life is as per the diagram but not nearly as neat in arrangement, so it was difficult for me to picture exactly what paralleling the batteries was doing.  Thanks for pointing out what my apparent stupidity missed 

I don't know how everyone else using RC lipos in their bikes charges their batteries all in one go - maybe it's common just to charge them individually like I'm doing.  It's quite a chore though.

I have now added another problem in that I ever so slightly pierced one cell in one of my new packs while working on the battery box and I'm trying to decide if I can still use it.  It's just a tiny scuff about 1mm by 1mm but it does appear that it may have slightly breeched the silver cell plastic, though it doesn't have the tell-tale pierced-lipo smell and isn't leaking any perceivable fluid.  It charges normally and voltages of all the cells in that pack are the same.  It also runs fine on the bike in my brief test.  I have sealed it with some JB Weld epoxy Steel Stick and some thick Gorilla repair tape but I have to say I'm nervous about it.  I am planning on putting the bike on the stand out on a concrete surface tomorrow and running the motor while observing from a safe distance until the battery is down towards LVC to see what happens under discharge.

Thanks again for the above explanation.  I really should have noticed that myself.  I must be getting older.

[Ron]

I can't really help with the punctured cell.  My advice would be - "when in doubt throw it out".  I bought the hardcase lipo from hobbyking to help avoid that sort of problem.  Even with the hardcase I almost worn a hole thru a battery case from vibration inside my battery box!

I have 12s worth of lipo that I bulk charge using a meanwell NES-350-48 power supply.  This charge arrangement can be dangerous (as in fire, etc) if not handled correctly.  I check the balance of my cells with a battery medic before and after charging.  You can search endless-sphere.com for more info on bulk charging and decide if it is right for you.

[Ron]

There are a couple of chargers that will do 14s lipo packs in one go.  The downside is price.  http://www.thunderpowerrc.com/Products/Chargers_2/TP1430C  There is also a Hyperion EOS 1420i but it might be discontinued.

[Thuktun]

I was running 4 SLAs in series last year and charging in parallel. Changing all those connections everytime I wanted to charge my battery got old real quick so I installed a bunch of DPDT switches that were off in the middle so that I could turn them all off then on in the opposite direction without any shorts. You could probably do something similar, I considered doing it again for two 7s10p packs I'm running now but I have a BMS coming in the mail so my current setup is only temporary. The problem will be all those balance leads. Some sort of rotary switch that can turn them all at once perhaps? You'll have so many switches though the switches will be as big as the pack lol. My SLAs had 5 switches, 3 for the series/parallel switching and 1 for power and 1 for precharge. You could manually connect and disconnect the balance leads, that'd save a lot of complexity and space.

[Thuktun]

These are the switches I used and how it was wired but only showing 3 cells, also a couple of the wiring harnesses. This thing was a pain to setup. All those wires were squeezed into a tiny space.

When delivering current to the motor 2 poles are in use so as not to melt the switches from too much current and when charging in parallel it only needs 1 pole. Very important to use switches that turn off in the center and to turn them all off before switching from series to parallel and vice versa!!!