Diodes and charging question…

[Neophyte]

With my newest e-bike almost complete I've decided to order a couple of batteries from Li Ping plus some diodes. I intend to go the diodes route in parallel connecting two 48V 20Ah packs.

And with this in mind I asked Li Ping about the involved wiring and whether the packs could be charged while remaining parallel connected [ charged separately ].

Li Ping replied by sending to me a wiring schematic showing the diodes placement and orientation on the positive discharge wires.

Additionally, he wrote…

Hi Gary,

If the discharge wires are connected in parallel with the diodes, it’s fine to charge them with two chargers. The diodes are just avoiding the current between the two packs through the discharge wires when they’re connected in parallel. So, if the diodes are connected, it’s ok to charge them together.

Best regards,

Ping

And the question I have relates to the fact that there are both positive and negative discharge wires on each pack and only the positive of each plays host to a diode. And, so, is it feasible that charging while the packs are still parallel connected could result in potentially damaging cross currents via the negative wires?

And with that asked here's a passage from additional material provided me by Li Ping...

If you don’t want to use the diodes, it’s feasible.

However, please make sure these two packs are in same charge level when you
connecting them in parallel.

For example, if one pack is 50% charged and the other is 90% charged, the 90%
charged pack will charge the other pack when you connecting them in parallel. The
charging current in a short period of time could be huge and harmful to cells in both packs.

The easiest way to avoid it is to fully charge both packs before connecting them.

Also, please disconnect the packs when they’re charged, although they’re charged
through a separate pair of wires, because one pack could be charged higher than the
other. If they’re connected together in parallel, the higher pack will charge the lower
through discharge wires. Furthermore, if some of the cells in the lower pack have been full,
they could be over-charged by the higher pack, because there’s no over-charge protection
on the discharge wires.

Thanks for any/all help.

 

[dnmun]

do like he says. charge both packs to the same voltage before connecting them in parallel. if they are identical packs then they should function identically and you do not need to put any diodes in either line. you can use a resistor to connect them together for the first time so the current flow is controlled. if it gets hot right away then the packs are still not close in voltage.

you can charge them both together in parallel too. if one BMS detects overvoltage and shuts off then the other pack can continue charging until it is fully charged.

if one pack has a dead cell then when the two packs in parallel reach the same fully charged voltage, then one of the packs will have multiuple cells that have been overcharged to higher than normal voltage, on average, but the BMS will still protect that pack.

 

[Wurly]

I hope you don't mind me chipping in here.

I have 2 ping packs wired together with schottkys. I leave them connected at all times, but i charge them with two separate chargers through the charge wires (BMS) yet still have packs connected to the discharge cables via the diodes.

I think Ping answered with the first reply. The second reply seems like he thought you wanted to not use diodes.

On your second question, do you mean charge both packs with one charger linked to both packs or use two chargers wired independant of each other?

One of my packs is older than the other and takes a good 20mins longer to charge which would indicate to me, that they are being charged independantly.

I can't say i've noticed any problems with cross currents......how can it, if you use two chargers?

You have me thinking now :?

 

[dnmun]

you do not need diodes to connect them in parallel. period. just be certain they are charged to the same voltage before connecting them in parallel. i think i already covered it, not sure why it is so hard to understand.

you do not need diodes

you do not need diodes

you do not need diodes

diodes make heat and rob power, they will catch your pack on fire if they are in the bag with the pack.

you do not need diodes.

 

[Neophyte]

dnmun,

Appreciate the input.

Though I've read several instances of your diodes perspective during my time here so I'm familiar. And you are right as I, myself, have recently gone the parallel route without diodes. Two BMSBattery 48V20Ah packs. And all has been fine. No theatrics. No drama. No pyrotechnics. No such. The packs charged separately after disconnecting them from one another.

This time, however, I chose to try diodes. A pair of 48V20Ah packs from Li Ping. The diodes coming at a cost of $7.00 each. More or less an experiment. A little comparison test. We shall see.

While on the subject of charging you wrote...

you can charge them both together in parallel too. if one BMS detects overvoltage and shuts off then the other pack can continue charging until it is fully charged.

So I parallel connect the charge wires and hook up a single charger?

As for the prospect of the diodes heating up to the point of causing a fire?

Both battery packs will be housed in bags. One in the front triangle and the other on a rear rack.

I'll have to monitor the diodes carefully.

Will be interesting.

Wurly...

Thank you for the reply.

Very helpful.

And at $7.00 each do you imagine that the diodes being shipped to me by Li Ping are of the Schottky variety?

What I do know is that they are rated 100V90A. Don't know the form factor.

As for charging?

The original plan was to charge the packs separately but if, as dnmun says, I simply have to parallel connect the charge wires I will go that route and use just one charger. Much simpler.

And thanks again for the input.

 

[dnmun]

that's correct. connect the negative from your charger to the C- lead from each BMS. if one pack has a cell go to HVC, the BMS will stop charging it and continue charging the other one. the diodes are completely irrelevant. the need for diodes is a holdover from when people combined nicad and SLA or SLA and lifepo4, not for combining two identical lifepo4 in parallel. money has nothing to do with it, they are just irrelevant for this purpose. period.