# 432V battery pack



## charliehorse55 (Sep 23, 2011)

I think you're talking about a contactor. The EV200 is perfect for this. It can safely switch DC busses of up to 900V.


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## PStechPaul (May 1, 2012)

You might need a bit more than 432V nominal for the inverter to work properly without tripping on undervoltage, which is usually about 400VDC for a 460 VAC inverter. 36 x 12V SLAs in series will drop to 400V when they are at 11.1 VDC each.

You will also need a fuse for overcurrent and short circuit protection. Don't rely on a circuit breaker or contactor, and make sure it is rated for the DC voltage. 

Another way to do it is to use multiple isolated DC-DC converters with 12V (or ideally 48V) inputs, so you can use less expensive lower voltage DC fuses and contactors on the low end, and you can shut down the inverters in case of a problem and you will have no more than 48VDC on the vehicle, which is considered "safe".


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## EVmot (Sep 19, 2010)

Thank you guys for reply.
I found this ev200 on ebay for quite reasonable price. 

DC/DC
- maybe it's not cost effective but it great idea and safer that original design.
btw. can you suggest some dc/dc converter. I dont want to buy any of Chinese 
stuff for this matter. There are bunch of them on ebay but i didn't test any of them.

Did you try to test this multiple dc/dc connection in praxis ?

EVmot


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## PStechPaul (May 1, 2012)

I have used a DC-DC converter on my small tractor project, with a 2 HP induction motor, and it worked OK, but my custom design failed, and I have yet to use my converted Chinese inverter. I have one that is 120VAC (170VDC) at 1000W and another which is 220VAC (270VDC) at 1500W. But the first is really a 400W continuous and the second is probably 800W continuous. You can see my adventures at:
http://www.youtube.com/watch?v=kTsHYm5Y78U

I have been working on a better DC-DC design for 24 or 48 VDC input to 300 or 600 VDC output, at about 1000W continuous output. For my small tractor, that's about all I really need. I think I will be able to build these for about $50 worth of parts, and a complete module with 4x12V12Ah SLAs including charger, BMS, and protection, would be about $150 for 560 Wh nominal. So a 10 kWh system for a car would be about $3000. Probably twice that for LiFePO4.

I've been busy with other things but I will probably return to this project in the Spring when I can do more work outside on my tractor. This will be an "open source" design and I'd be happy to give you more details. Meanwhile, you may be able to use the Chinese inverters especially if you modify them for DC output as I have done. I have a thread with the details on this forum.
http://www.diyelectriccar.com/forums/showthread.php?t=89691&highlight=inverter


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## EVmot (Sep 19, 2010)

http://www.diyelectriccar.com/forums/showthread.php?t=89691&highlight=inverter
**
I have done this but the problem is that there no real power in this 
inverters. I got 233V on the capacitor side and inverter was rated 1,5KW (3KW peak ). When i switch to the second gear , inverter was shut down. 
Maybe some 8KW inverter would give some reasonable power but i doubt it.

On the other side if i go directly from the battery the i can use as much as the battery can give. Or course, for limited amount of time. The motor is winded to 1750 rpm's so at 200 HZ i can easly reach 110 km/h and the motor can pull full power. Till now i dint use more then 9KW.

The second part of the project is to rewind the motor to 2500 rpm's at 50HZ.


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## Frank (Dec 6, 2008)

For the DC/DC go to Vicor,


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## major (Apr 4, 2008)

EVmot said:


> The second part of the project is to rewind the motor to 2500 rpm's at 50HZ.


That would require a motor with 2.4 poles. You better stick to integer multiples of pole pairs. Use this relationship:

RPM = frequency in Hz * 120 / (# of poles) 

This yields the synchronous speed. For an induction motor the actual speed will a percent or so different due to slip.


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## PStechPaul (May 1, 2012)

A major problem with the usual automotive inverters is that, especially for higher power units, the incoming power connections are much too small. A 1200W inverter needs 100 amps at 12V for rated power, and that requires cables of #6 or #4 AWG, while those supplied with the inverter are no larger than #12 or #10. Even the connectors are not rated for such current, being usually binding posts of some sort. These inverters also shut down on undervoltage of about 11.5V which is not unusual for 12V SLA batteries under heavy load.

You can get 48V input inverters up to 5 or even 10kW but they are rather expensive, and still need a lot of current. So I think it is better to use multiple isolated DC-DC converters purpose-built for VFD drives and vehicle traction motors. It should be fairly easy to connect the outputs in series and/or parallel to get the voltage needed for 230/460 VAC motors.

As for the motor rewinding, you probably want to use a higher pole count for overclocking. 1750 RPM at 200 Hz does not sound right, and as Major pointed out, neither is 2500 RPM at 50 Hz. Generally you want to limit overclocking to about 4x or 200/240 Hz, and motor RPM is best kept under 5000 RPM. Efficiency will drop at higher drive frequency. Higher pole count usually means somewhat lower power to weight/size ratio especially for smaller motors and 6 poles or more. If you can handle the extra weight it is good to use premium efficiency motors which have more copper and possibly thinner laminations of higher quality steel.


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## EVmot (Sep 19, 2010)

Thank you all for replay.
Just a little remark....
1750 is at 50hz not on 200.

Frank mentioned the Vicor company.
I was looking on they web side and it look that thay have bunch of ready made solutions for such purposes. Quite interesting.

OK. so i have two options.
To go with contactor and additional voltage/current protection or to go with DC/DC version.


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## PStechPaul (May 1, 2012)

1750 RPM is typical for a 4 pole ACIM at 60 Hz with synchronous speed of 1800 RPM. 

I searched the Vicor website but found no DC-DC step-up converters anywhere near the specifications required. And a general web search also yielded nothing useful. So it appears that the options are to buy DC-AC inverters and modify for DC output, or build one from scratch. It's pretty simple, really, and I built one using a circuit similar to the following, but with a PIC to generate the PWM and a half-bridge MOSFET driver:










I also may use just a single capacitor, or several in parallel for more current. This also can be constructed as a FWCT bridge as I have done before but I think this design is better and does not need the two halves to be as well balanced.

Using the batteries directly is probably the best option, and certainly the least expensive, and it facilitates regeneration. But it requires a higher degree of precaution for safety purposes.


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## EVmot (Sep 19, 2010)

Great !

Am working with microcontrollers too so if you can send me schematics for dc/dc version with mcu, that would help a lot. If not, i can try with the one you send.

To be honest am not sure if this could "transfer" enough power to the motor and how much is the lost in the conversion part.

E.g. if am driving 90 km/h i would use around 8kw, maybe less because the car with battery has 800 kg. Could this power be passed thru dc/dc converters ? What if i would like to use full motor capacity.. 15kw ?

I haven't use dc/dc approach like this...ever, so am a bit sceptic but i will try it if you think that this could work and on such high power.

*But it requires a higher degree of precaution for safety purposes.*
** Btw. what kind of protection would you suggest ? I have few in mind but your idea is very welcome.


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## PStechPaul (May 1, 2012)

For a high voltage battery pack I would recommend the usual DC rated fuse (perhaps more than one) and the high voltage DC contactor (Kilovac et. al.) Also, the wiring from the battery pack should be enclosed in a protective sheath like conduit, and colored orange as a safety warning for first responders in case of accident. There may be other precautions dictated by local code.

It would be even safer if you could have each series connection controlled by a contactor, or at least each series string up to 48 VDC nominal. That could be very expensive using the Kilovacs, but I think it would be safe to use 60 VDC rated relays along with a capacitor across the contacts so that they would not immediately see full current and the voltage across each contact would remain at a safe level while they opened. In your case, with 15kW maximum at 400V, the current will be just 37.5A maximum under normal conditions. A 30 or 40 amp relay may be sufficient. The relays should be set up so that they have perhaps a 200-500 mSec delay on opening, which should give enough time for the fuse to blow under short circuit or extreme overload, and the Kilovac should cut the current otherwise. Thus the individual cell/battery connections would be opened (and closed) under conditions of no load. All bets are off if the fuse fails (which is unlikely if properly sized), or if the Kilovac fails to open. But I think these relays would provide an extra measure of safety by limiting the available voltage on the vehicle to 48 volts or less when shut down.

As for the capability of the DC-DC converter, there are inverters rated at 5000 to 10,000 watts, and this circuit is essentially the same. In fact, you can probably add more primary MOSFETs in parallel and wind a higher power transformer, or use multiple transformers in parallel, to make a 10 kW converter from a smaller one. As long as the MOSFET driver(s) can handle the gate load, you can make it as big as you want. I would rather use the PIC which can be programmed for various frequencies and also used for other purposes such as overload sensing and parameter display. But it is also possible to use a PWM switching controller like the SG3526 or SG2524 or TL494 or TL598 or IRS2153. That's probably what you'll find in the automotive inverters. 

And computer PSUs have essentially the same circuit except with high voltage primary and low voltage secondary. I have gutted an old PSU and rewound the transformer to step up from 12/24/48V to 150/300. So you could make a converter with 600 watt modules to get 15 kW with 25 of them, and you can get them for free at computer recyclers and/or the dump. But you can get a ferrite core and coil form good for about 2000W for about $10-$15 per set, and probably $5 worth of magnet wire.


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## EVmot (Sep 19, 2010)

It looks like we are on the same track 
Firstly i bought 6 x 12V/85aH SLA battery's and i place them under rear seat ( the steel box is under the seat ). Then i was thinking to make the dc/dc inverter from 72/450 and this was working fine but with limitation.

This is the basic schematics that we have used but it didn't worked over 48V. The transformer was one really big toroid and on 200hz you should get 8kW. Unfortunately, we give up on this project because after the months of testing we could not make it work on 72V. Everything over 48V was producing sparks and pcb burnouts. we didnt try with higher frequency and ferite core.



> I have gutted an old PSU and rewound the transformer to step up from 12/24/48V to 150/300. So you could make a converter with 600 watt modules to get 15 kW with 25 of them, and you can get them for free at computer recyclers and/or the dump.


**  Damn good suggestion !. Newer thought of that..This could solve the whole problem. Btw. did you make some other changes accept transformer rewinding ?

1.) Would it be any problems if i would use different power rating PSU's. E.g. 420 / 550W ?
2.) Can you share winding calculations for 72V input ?I have 72V lead acid battery charger so i can use this in final charger design
3.) So you think that making one 1kw ( or higher ) converter based on PSU, shouldn't be any problem.
This could be the only real solution for me because i dont have place for more then 10 psc.


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## PStechPaul (May 1, 2012)

What you have looks similar to what I made for my first attempt. I also used a toroid transformer and wound it for higher frequency and power. I used 600 Hz and 1200 Hz, and I actually tested it under minimal load up to 16 kHz and it seemed OK. This was a core rated at about 200 watts at 50/60 Hz and about 0.2 V/turn. So I figured 2 V/turn at 600 Hz and I used 8 turns of #10 AWG for the primary. I was able to fit 100 turns on the secondary to get about 200 VAC with 12VDC input. I used a voltage doubler to get about 320 VDC and it was able to drive a 2HP 3 phase motor. Then I made some improvements and used 24 VDC at 1200 Hz and a directly rectified output to get about 300 VDC for the VFD. This also worked OK, but the MOSFETs shorted, probably because I did not have a precharge system for the large capacitors.

I don't know what output transistors you are using, but remember that they will normally see twice the supply voltage, so at 72 VDC input it will be 144V. There will also be transients with much higher amplitude peaks. You probably need an RC snubber, and you should use TVS diodes, rated below the maximum breakdown voltage, across the transistors to reduce the voltage and absorb the energy of these transients. You can run it for a short time and check their temperature to see if they are doing anything. If they get hot quickly, you need to use snubbers to reduce the transients. It is also possible to dump these high voltage spikes into a capacitor and use a small DC-DC converter to pump the energy back into the batteries and primary capacitors without wasting it.

You probably don't need the feedback for your design, as the output does not really need to be regulated, and the push-pull circuit works better with a 50% duty cycle and minimal deadband. The output will just be a fixed multiple of the input voltage, and you can wind the transformer for whatever you need. 

As for repurposing scrap PSUs, any power rating can be used, and you may be able to get as much as 1000 watts out of a 600W unit by increasing the frequency and/or providing better heat sinking and forced air (or liquid coolant). You might even be able to reverse the transformer and use appropriate transistors for the primary, especially if you find a PSU rated for 12VDC at high current.

You can see my DC-DC converter in this video:


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## EVmot (Sep 19, 2010)

Very nice !

1.)
What would happen it the input voltage droops under 48V,
let's say, on 37V. Would the dc/dc work or not ?

2.)
Can i "charge" the dc/dc converter to 50V ?
Would it work normally ?

3.) To be honest am not quite sure how can PC transformer coil keep up with so much power.
E.g. if you have 500W * 10 pcs you will get 5KW and this much power would be transferred over PSU transformer.
Do you think that this power would be separated/divided correctly on each DC/DC converter ?


Evmot


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## PStechPaul (May 1, 2012)

The DC-DC converter simply steps up the input voltage by a fixed ratio, much like an AC transformer. And, like a transformer, the output is subject to a load factor which may reduce the output by 10% or so (at which point there may be 10% losses as heat).

The DC-DC that I have recently built will work from about 10 VDC to 60 VDC. The low limit is determined by the undervoltage trip of the Hi-Lo MOSFET driver. Otherwise it could work as low as 5VDC or even 3VDC if the MOSFET gates can still be driven. The high limit is determined by the circuit components, particularly the output MOSFETs, but also the capacitor ratings, and the linear voltage regulator I use to obtain the 12 VDC for the MOSFET gate driver (I use an LM317HVH which can operate up to 60V input).

The circuit is designed for nominal 12, 24, 36, and 48V batteries, which can range from about 15% low (1.75 vpc) to 20% high (2.3 vpc). It can be made more efficient, or for other voltages, by tailoring the components and design elements appropriately.

If the multiple DC-DC converters are identical (or match within a few percent), and the batteries are identical, the outputs will match and each PSU will handle an equal share in parallel. If one unit is low, either because the battery voltage has dropped or for other reasons, the output diodes will simply stop conducting, which will reduce the load on that unit while increasing the load on the other, which eventually should bring them both into balance.

OTOH, you can connect units in series (with perhaps 120-170 VDC outputs), in which case each unit will supply the same current but at different voltages. So you can create a series/parallel output connection such as 4S2P similar to smaller battery packs.

The major disadvantage to such simple DC-DC converters is the lack of regeneration capability. But I would propose using enough capacitance on the DC link to absorb the energy of braking, and then using one or more switching type battery chargers to transfer this energy back to the batteries at a lower rate. The large capacitors will also provide high current for short term acceleration without overloading the DC-DC converters. 

It may even be better to add a small high C-rate Li-Ion battery pack to assist with such short-term energy transfer and storage. 

The energy required would be something like 20 kW for 10 seconds, or 200,000 watt-seconds (Joules). The energy of a capacitor is 0.5CV^2, so you would need 2*200,000/400^2 = 2.5F. Very possible, but expensive, probably one or two thousand dollars. Otherwise you would need 20000/400 = 50A surge current rated lithium cells, which could be met with 5 A-h 10C pouch cells. You would need about 120 of them, perhaps $1000 worth. Maybe only 2 seconds worth of energy is needed, which cuts the costs down to a few hundred dollars.


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