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Discussion Starter · #1 ·
I am just getting started and I'm looking for options for my battery and charging. My design runs at 500v-600v, so regardless of what battery technology I use, I will need a way to charge this very large number of cells in series.

Can anyone briefly explain how this is done? It is necessary to generate one very large charging voltage, or is it more common to charge each cell, or small parts of the battery separately with multiple isolated chargers?

What role does the BMS play in charging?
 

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I am just getting started and I'm looking for options for my battery and charging. My design runs at 500v-600v, so regardless of what battery technology I use, I will need a way to charge this very large number of cells in series.

Can anyone briefly explain how this is done? It is necessary to generate one very large charging voltage, or is it more common to charge each cell, or small parts of the battery separately with multiple isolated chargers?

What role does the BMS play in charging?
Hi cat,

With questions like these, I recommend you stop and rethink your plan. High voltage like that will kill you in a heartbeat, or somebody else. For a beginner on a first BEV, start small. Like a cart or bike. 48 volts is relatively safe and nonlethal. Or if you insist on converting a car, keep it under 200 V. Everybody makes mistakes and draws a spark or two. You want to learn from those experiences, not die.

major
 

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Discussion Starter · #3 ·
I suppose my main question is whether the common solution is:

1) charging several BMS managed packs in series from a single high voltage supply
2) charging each BMS managed pack separately with a number of isolated supplies
3) design a very large (100+ cell) BMS as a single unit

Any examples of how large packs are wired would be appreciated. I suspect an understanding of how a BMS operates internally will help so I'll look into this.
 

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take a look through that johannes thread, he has already been down the path you are on for the most part.
 

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Discussion Starter · #5 ·
take a look through that johannes thread, he has already been down the path you are on for the most part.
That's a good point! I haven't read the thread in full, it seems like quite the epic. I will get on and read it. I know johannes's inverter has a boost controller to generate a charging voltage, so I'll look into how this is balanced across the cells in his setup.
 

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See glad i read this. My design is calling for 700v. Maybe I'll build it as two 350v packs and switch to series in scramble mode.

Sent from my SM-N910V using Tapatalk
 

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I am just getting started and I'm looking for options for my battery and charging. My design runs at 500v-600v, so regardless of what battery technology I use, I will need a way to charge this very large number of cells in series.

Can anyone briefly explain how this is done?
I echo Major's sentiments. Start with a lower voltage design. There are special considerations for higher voltages, and it gets difficult and expensive to find things like DC-rated fuses over 600 VDC. An arc at 600 VDC can maintain itself until the conductors are separated by 600 mm (two feet). So it's really important to design such that an arc won't start.

So the following are just for education.

It is necessary to generate one very large charging voltage, or is it more common to charge each cell, or small parts of the battery separately with multiple isolated chargers?
It can be done all 3 ways (one, few. many chargers), and various designs use all three.

I assume that production vehicles charge in one long string, or perhaps two.

I was involved with a 720 V nominal MX-5 (Miata). We could not even find a reasonably priced charger that high in voltage. Elcon/TC chargers of the time stopped at 417 V, just enough for half the pack. We have about 8 contactors to break up the pack when not in use, so there is a maximum of 120 VDC in any battery box (ELV, Extra Low Voltage, can bite badly, but is essentially non lethal). When charging, all the contactors are turned on except for the mid point join, so there are two floating 360 V nominal halves. There is no need for 720 V between any two points (~ 830 V at the end of the charge) while charging. We also have two DC/DCs, one on each half-pack, so the 12 V battery can be kept charged while the main pack is charging.

Others swear by a large number (10-20) of smaller chargers, perhaps 12 V or 24 V nominal. You get redundancy that way, and maybe some economy of scale (buy a lot of chargers for a moderate discount).

What role does the BMS play in charging?
In the MX-5, the BMS tells the charger to back off when any cell in the entire half-pack is getting too high in voltage. We run a control loop so the charger current starts to taper off as the worst-stressed cell heads towards a target stress. Stress can be from over-voltage or over- or under-temperature (it's not safe to charge LiFoPO₄ cells under freezing).

Without this protection, hundreds of cells in series would be unmanageable, in my opinion. The anti-BMSers will no doubt disagree. [ Edit: we also designed our own celltop BMS for this car, and it took over half the build time to design, develop, and build. ]
 

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Discussion Starter · #8 ·
It gets difficult and expensive to find things like DC-rated fuses over 600 VDC. An arc at 600 VDC can maintain itself until the conductors are separated by 600 mm (two feet). So it's really important to design such that an arc won't start.
I'm glad this came up. I've been reading about contractors and fuses and I can see that indeed switching off a DC circuit under load / fault conditions will be a challenge.

I'll look into reducing my voltages. I'll also have a good read through your MX5 design as this seems to have a fair bit of good information about this!

A lot of my reason for planning high voltage was to cut down on conductors. It hadn't occurred to me that insulators would such be a large part of the design / build.

Standard industrial motors will require about 560v. I'll definitely look into options for rewiring / rewinding to bring this down to something closer to 200. Will definitely still need to be a lot of care taken over breaking the DC circuit under load though.

Thanks!
 

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Discussion Starter · #9 ·
Others swear by a large number (10-20) of smaller chargers, perhaps 12 V or 24 V nominal. You get redundancy that way, and maybe some economy of scale (buy a lot of chargers for a moderate discount)... We have about 8 contactors to break up the pack when not in use, so there is a maximum of 120 VDC in any battery box
This would be my preference. I like the idea of keeping the pack broken down when not in use, and using a smaller BMS+charger on each unit, then using contactors to bring the battery up to voltage when in use.
 

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Last I checked 600VDC contactors do not have terminals spaced 2 feet apart. ;)

They are typically spaced 1.5" apart, with a piece of plastic separating the two terminals to increase the creapage distance above the 2.3" limit in free air.

Inside the contactor the distance between the contacts are much closer, but they are switching in an inert atmosphere.
 

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Last I checked 600VDC contactors do not have terminals spaced 2 feet apart. ;)
That would be because they're not designed to stop an arc once started.

They are typically spaced 1.5" apart, with a piece of plastic separating the two terminals to increase the creapage distance above the 2.3" limit in free air.
You need to go a lot closer than 1.5" to get the arc started. In fact, it's about 0.6 mm in air, a thousand times less than maintaining the arc.

You can get away with a lot less than 1.5" on PCBs etc for 600 VDC, depending on conformal coatings, expected amount of comtamination, and so on. A typical terminal block rated for 600 VDC will have terminals separated by perhaps 0.2" (CAT II situations). That means you won't get flashover in air from 600 V with up to 4 kV of transients.
 

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Contactors are designed to stop an arc, they have magnetic blowouts, but that is in a different atmosphere...

I was talking about the contact distance in free air.

Anyway, here is an extreme example 1000V, 400A fuse, (10000A interrupt rated).

http://www.littelfuse.com/~/media/e...es/littelfuse_solar_spfr_spfrhv_datasheet.pdf

Using the 1mm per volt rule, this would be over a 3 foot fuse!!!

Using the 10mm per volt rule, this would be a 4" fuse.

Looking at the datasheet it looks like they have a about 5.5" between the external metal contacts. ;)
 

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I was talking about the contact distance in free air.

Anyway, here is an extreme example 1000V, 400A fuse, (10000A interrupt rated).

http://www.littelfuse.com/~/media/e...es/littelfuse_solar_spfr_spfrhv_datasheet.pdf

Using the 1mm per volt rule, this would be over a 3 foot fuse!!!
I believe that the arc due to the fuse opening is contained completely inside the ceramic (or whatever) of the fuse, quenched by sand. So there is no ionized (conductive) air to carry the arc. But if you drop a tool across conductors with 600 VDC across them, then the arc starts in air, and if the appropriate conditions are met, as is likely in an EV, that burning air will continue to offer a few ohms of resistance until the separation reaches some 600 mm. Not something I'd like to experience close up.

Attached is a slide from the presentation linked to a few posts back. It shows the horizontal arc length in air being over 500 mm for 500 V from about 30 A to 2000 A, and at 125 V, the length is over 100 mm from about 30 A to just under 1000 A. I added the two lines at 500 V and 125 V.

[ Edit: I chose the 500 V and 125 V lines as being very roughly the average of the flattish part of the curves for 500 mm and 100 mm arc length respectively. ]
 

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Ok, I give up...

Your random power point slide on what it takes to MAINTAIN an arc across a distance definitely trumps all my real world examples of contact spacing.

I will leave it to you to inform all the vendors of contactors, fuses, motors, controllers, inverters, connectors, etc... that they have been doing it wrong all these years. ;)
 

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Your random power point slide on what it takes to MAINTAIN an arc across a distance definitely trumps all my real world examples of contact spacing.
You're missing my point. Terminal post [ Edit: was Contact] spacing isn't about maintaining [ edit: or breaking ] an arc. My point is about how scary things can get if they go wrong. The important thing is to try and make as sure as possible that they don't go wrong. An analogy: I'm warning about how bad it is to burn your house down, so be careful with fire and use appropriate materials and precautions to prevent a fire in the first place.

I will leave it to you to inform all the vendors of contactors, fuses, motors, controllers, inverters, connectors, etc... that they have been doing it wrong all these years. ;)
Again, these contactors, fuses, etc are not attempting to provide distances that would quench an arc once started [ edit: at their terminals; obviously it's different inside]. They are about making sure there is no flashover with dirt and grease over the years, and that's orders of magnitude different from stopping live arcs. The spacing on terminals is fine, assuming that you don't drop uninsulated metal tools on them. If you do, you could start an arc that won't stop, as Plasma Boy found out the hard way. The basic principles get more and more important as the voltage increases, which is how this thread started.

Plus of course, there is the basic danger of electrocution, and even burns and eye damage, all of which hazards get more extreme with higher voltage.
 

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We're talking about EV's here, which have batteries capable of short circuit currents in the thousands of amps. Coulomb is talking about the classic tool-drop accident across high voltage battery terminals, where there is nothing to limit the current but the resistance of the arc itself, which is typically around 5 ohms.

As this paper shows,
http://www.neiengineering.com/wp-co...c-Models-and-Incident-Energy-Calculations.pdf
from the results of many different experiments, once started by the dropped tool, a 500 V battery can maintain an arc out to 500 mm with a current of 100 A. That's 50 kilowatts being dissipated as heat and light in that arc. And notice that it won't even blow a mid-pack fuse because that's about the same as the motor power it's designed to provide.

Everyone involved with batteries over 48 volts should read this story of such an accident with a 336 V battery.
http://www.evdl.org/pages/plasmaboy.html

If John Wayland had been able to throw bags of dry sand onto the arc (from a safe distance) he may have been able to put it out.

But of course, 28.35 grams of prevention ...
 
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