Schottky Diode Parallel Battery Connector

[jha07]

I saw this thing and it got me thinking.

http://www.ebikes.ca/store/photos/Diode25A.jpg

I plan on having two battery packs connected in parallel. But I was just going to plug them together. What is the purpose of connecting them through a schottky diode? I don't really see any reason for it. Wouldn't the diodes prevent the two packs from reaching a balanced state of charge? That seems like a negative to me. There must be a good reason for using it.

 

[Tiberius]

This is what you need to do if paralleling two or more packs. If the battery packs are different state of charge there will be cross currents without the diodes, and these currents could be large. You don't want them reaching a balanced state. With a diode combiner you can even combine different type and capacity packs.

This is exactly what I have done - see the thread on Torq with battery mod climbs hills. I should acknowledge an email exchange with Zev at ebikes.ca in which he confirmed it was a good idea. They have a packaged combiner for sale but I built my own 40 A, 4 into 1 combiner with off the shelf diodes.

The reason for Schottky diodes instead of ordinary ones is that they have a lower forward voltage drop.

Nick

 

[jha07]

Is this what is happening? With two packs connected using diodes, which ever pack has the higher voltage will be supplying more current. This continues until the voltage of the two packs are about the same, then the current draw will be 50/50.

I can see if you connect two different types of batteries together, with different capacites and voltages, you wouldn't want any cross currents.
But if I use two identical packs. Why wouldn’t I want the two packs to self balance? Isn’t that a good thing? I can see it been a problem if I connect a fully charged pack with a drained pack. But if both packs are charged, wouldn’t it be fine?

 

[Tiberius]

Because there is only the internal resistance of the batteries stopping it, the initial cross current could be large. In theory, if the voltages on the packs are always very close, and you never get a cell failure, and don't discharge it until the cell reversal point, and... But why not play safe? Its a protection feature with very little downside.

Nick

 

[JamesC]

Is this what is happening? With two packs connected using diodes, which ever pack has the higher voltage will be supplying more current. This continues until the voltage of the two packs are about the same, then the current draw will be 50/50.

Nick

Please would you add a little to your other excellent thread regarding the mechanism of the Schottky Diode combiner.

When using a NiMH battery to cream off the moments of high current draw, thereby reducing the pressure on the Li-ion main battery, is the concept based on the Schottky allowing a current flow from the NiMH only when the voltage supplied by the Li-ion droops below the NiMH voltage ?

I am not clear as to whether a Schottky Diode works as an on/off device, or allows proportional use from both batteries, based on the voltage.

Thanks
James

 

[john]

James,

A diode allows current to flow in one direction but not the other. You put a diode on each battery so that current can flow out but not in. Initially, current will flow from the battery with the highest voltage. As current flows, this voltage will drop and current will start to flow from the other battery. This should work quite nicely with the battery most able to supply the current at any one time being used most.

 

[Tiberius]

James,

It is exactly as John says. The diode is a one way valve, and there is one for each battery. Its not an either-or switch, but a combiner. In practice, with a heavy current draw, the load will be shared between the batteries. The voltage on each battery will drop, to a level at which they are equal and both delivering current.

Nick

 

[jha07]

I guess the main reason I don't want to use a diode is because I want to charge in parallel as well. I don't suppose there is a way to do that with diodes.

 

[JamesC]

James,

It is exactly as John says. The diode is a one way valve, and there is one for each battery. Its not an either-or switch, but a combiner. In practice, with a heavy current draw, the load will be shared between the batteries. The voltage on each battery will drop, to a level at which they are equal and both delivering current.

Nick

Thanks - I was being feeble minded there.

I will revert to the other thread on improving the performance of Li-ion.
James

 

[Tiberius]

I guess the main reason I don't want to use a diode is because I want to charge in parallel as well. I don't suppose there is a way to do that with diodes.

hi,

There's a thread where we discuss exactly this. I think its in the Battery FAQs forum. It explains how I rigged up a parallel charger that would cope with the batteries at different states.

Nick

 

[jha07]

I read that thread, and I'm trying to think through the process. But I'm still a bit confused.
I assume when you charge, the direction of the diodes must be reversed. Otherwise, nothing would happen. This means you can't hardwire the diodes in place because you have to change them between charge and discharge. Is that what you're doing?

And you need the bulbs to "short" the diode so the charger can see some voltage which is needed to start the charging process. Why does only one bulb turn on before you start charging. Shouldn't both turn on since both packs are at a higher voltage than the charger?

If the diodes are suppose to stop cross currents due to voltage differences in the packs, but you add bulbs to allow the charger to see voltage from the packs, won't the cross current just travel through the bulbs? Doesn't that just defeat the purpose of the diode?

 

[Jeremy]

You just use a second set of diodes for the charging circuit and all is well. There is no need to switch anything around, as long as there is both a charging connection (via the set of diodes arranged such that they allow current to flow into the battery packs) and a discharging connection (with diodes arranged the opposite way around). The only limitation is that the positive discharging and charging connections must be separate, although a common negative connection can be used.

jeremy

 

[Tiberius]

If the diodes are suppose to stop cross currents due to voltage differences in the packs, but you add bulbs to allow the charger to see voltage from the packs, won't the cross current just travel through the bulbs? Doesn't that just defeat the purpose of the diode?

The bulbs have a high resistance so only a small current flows. I was thinking of resistors until I happened upon the right bulbs. They also give an indication of what's going on.

Nick