Hi cat,
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.
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.
It can be done all 3 ways (one, few. many chargers), and various designs use all three.
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).
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.
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.
The Rule of thumb is 1mm per 10VDC in free air...
That would be because they're not designed to stop an arc once started.Last I checked 600VDC contactors do not have terminals spaced 2 feet apart.
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.
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.
I assume you meant 10 V per mm there.
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.
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.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.
