DC-DC converters, how much power do we really need

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zeva
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DC-DC converters, how much power do we really need

Post by zeva »

Hi all,

A few times lately I've been reminded of the need for EV builders to migrate towards all weatherproof components in EV engine bays, and DC/DC converters are one of the main culprits. Historically most people have used AC/DC battery chargers "off label" as DC/DC converters - IOTA being a common example - and while they work fine and don't cost much, they're far from water- or dust-proof.

For my MX5 I used an IOTA DLS-45 and had to put it in the boot to save it from getting wet/dirty, but with the pending rebuild I'd rather have a weatherproof DC/DC in the engine bay and keep the boot area clean.

Australian company Amtex make some very nice little sealed DC/DC converter modules, but the appropriate input voltage range (85V-185V DC input for me) are only available up to 300W (http://www.amtex.com.au/power_pdf/PH.pdf).

I had always been led to believe the 12V system could draw up to 500W in most cars, but had never really verified the hearsay. So I just went and put a clamp meter on the battery lead in my MX5 and put the headlights on high beam: 12 amps. So just 150W for the MX5 headlights on high beam. I imagine they're the largest load for the 12V system by far? (Excluding people with ridiculous audio systems..) It seems 300W would be PLENTY for my little car.

I'm wondering if anyone else has any data on the 12V system in their vehicle? I'm curious to know how others compare.
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Post by a4x4kiwi »

Hi Ian,

Here is a current analysis I did when sizing my DC-DC
http://a4x4kiwi.blogspot.com/2008/06/12 ... ments.html
My ute doesn't have anything that would be different from a normal car.
46A worst case is what I measured.
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Post by zeva »

OK you've inspired me to be a little more rigorous with my testing too:

Quiescent current, key off: 0.4A
                                  key on: 3.0A

Headlights low: 10.2A
                    high: 11.4A

Park Lights: 2.5A
Brake lights: 3.6A
Rev. light: 1.2A
Hazard lights: 8.6A (50% duty so 4.3A avg)
Indicators: 4.3A, 50% duty
Wiper: 4A while on
Vac pump: 3.9A
Fan 1,2,3,4: 2.6, 4.4, 6.3, 9.1A

So hmm worse case scenario, quiescent plus headlights on high, hazards on, wipers on, fan on full bore (surprised how much this used!) foot on brake and in reverse (implausible scenario but let's go with it).. 3+11.6+4.3+4.3+9.1+4.1+1.6 = 38A.

It would have to be said that this is an unlikely scenario where it may be reasonable to rely on the battery to supplement the power? BUT I guess it would be best if our DC/DCs were rated for worst case scenarios.
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Post by acmotor »

~23A covers absolutely everything on in red suzi (but then that is not much !).

300W (13.8V x 22A) may suit many small vehicles.

Aircon and spot lights will push up requirements as continuous loads, and electric vac pump as intermittent load. 500W would cover more bases in that case.

Reality is though, You are not always in reverse with foot on brake, hazards on as well as high beam, fan, door open, horn sounding, emotor fans on etc. Surely the 12V aux battery has to do some work some time ? Image
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Post by acmotor »

Hoops, I guess you don't have a rear window demister to add in ? Image

You added hazards and indicators. I thought they were the same globes ?
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Post by Peter C in Canberra »

Something to factor in is that you should have a 12V backup battery in parallel with the DC/DC converter output. If for certain short periods you are exceeding the rated output of the converter I think you would just be drawing the excess from the battery. Once you stop reversing with hazard lights on at night in the rain while sounding the horn you should be back under the rated output and recharging the battery. I think you only need to make sure the converter output is sufficient to somewhat exceed any loads you could have running together for an extended time.
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Post by acmotor »

Peter, that does raise the point of aux battery recharge current. It must be added into the list.
If say 20Ah 12V aux battery is recovering from a small discharge it can pull 10A or more for a few minutes.
Perhaps not an issue if the DC-DC is simply current limited. The 12V system will just run a volt or so low for a few minutes while things recover. Image
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Post by Peter C in Canberra »

In the case of the Iota DLS 90 (I assume the same for the 45 Amp version) my spec sheet says it is current limited. So, as suggested, it will deliver the rated current at a slightly reduced voltage if faced with everything turned on and a partially discharged battery.

I have the 90 amps version to cope with my heater which hopefully reached its final iteration yesterday: [13.6V x 13.6V / 0.4ohm] x 2 = 925W or 68 Amps. Just enough for warmish air on recirculating last night at 10.30PM when it was barely above freezing on the way back from a National Electric Vehicle Festival planning meeting. Not enough power for warm air to emerge if heating outside air. It will have to do because I am not pulling the dash apart again!
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Post by acmotor »

Ah yes, the 1kW 12V heater. You may be in the minority with that luxury though. Image
Useful feedback on its function.
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Post by evric »

Talking about hooking up DC/DC converters to aux battery... How do you limit the max charge current to the battery; say 9Amps for a 33AH deep cylcle?
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Post by acmotor »

Fair question. Basically in red suzi I don't. But then there is only 20A available to go to Aux battery and with 13.8V DC-DC output I've not seen more than 10A go to the Aux battery in tests when I first put the system together.
Battery internal resistance and wiring resistance probably limits things as well. Although the aux battery probably goes over rated recharge current for a few minutes.
With 90A DC-DC there is more current available.
But then my ICE oil machine has a 130A alternator that I guess pushes that sort of current on fast idle just after starting back toward battery.
A burst of 3C charge is not really going to worry the aux battery ?

But there you go Hoops. Another feature for the EVworks DC-DC converter... aux battery current limiting !
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Post by Peter C in Canberra »

acmotor wrote: But there you go Hoops. Another feature for the EVworks DC-DC converter... aux battery current limiting !

In my car with the 90amp converter I have the 90amp fused output go to a solid state circuit breaker from Jaycar (~30amps from memory). It has two threaded terminals. On its input from the converter I have the lug for the +ve cable to the 12V heater so the heater is only protected by the converters current limiting and output fuses. The output lug of the solid state breaker has two lugs on two wires that had connected to the original car battery. IE all the car's original wiring that has been retained comes off here and goes via its original fuses. A third lug at this point goes to the new 12V stuff and the back up 12V battery. IE the charging of this battery is limited to 30amps minus whatever else the car is taking the time (apart from the heater).
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Post by evric »

Doesn't a 30A circuit breaker "break" at about 60A?
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Post by acmotor »

Peter, surely nearly all the 90A can go to the 12V aux battery short term (between 10's of seconds and minutes) The 30A CB will pass that.
Not very different to normal ICE startup operation as pointed out.
weber found a 63A CB tripped at 300A in 6 seconds.

If you get a chance, run a clamp meter over the aux battery wiring at a suitable time just for info.

(I don't expect it goes over 20A for more than seconds though, so not a problem)
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Post by Peter C in Canberra »

Actually, I doubt that much current goes in or out of my back up battery. The battery just sits there and I think the DC/DC converter keeps up virtually all the time. Certainly I see the current meter on the high voltage system responding instantly to varying 12V loads.
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Post by acmotor »

Yes,yes, but the current in question is that which flows into the aux battery when its SOC is down. e.g having run some system items such as headlights when the ign is off. It is likely to get dragged up to 13.8V quite forcefully.

The question is how much current and is there a need to limit the recharge current given the small aux battery size.

My impression is it may not be a problem. I have not seen one with my small DC-DCs. Just wondering if there is high current with the larger DC-DC.

evric asked the question originally.   Image
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Post by Tritium_James »

If it's a small gelcell type battery then they usually have the max recharge current printed on the side. The 7Ah one in the Porsche says max of 2.5A recharge current. I don't know what happens if you exceed that, but I suspect it gasses and wears out really fast.
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Post by coulomb »

13.8 V is below the gassing level (at sane temperatures; at 60°C as sometimes happens in cars, it may well be a different story). The internal resistance of the battery will also limit the current somewhat. If it's discharged after a moderate load, the internal resistance will be higher than when fully charged.

However, there may be localised heating. I would think it would be minor, but I'd really like to know too. This might be a reason not to use the smallest gel cell that barely meets the safety requirements (so many minutes of flashers, etc).
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Post by Peter C in Canberra »

evric wrote: Doesn't a 30A circuit breaker "break" at about 60A?

You may well be right. I really just wanted something that would sit ahead of the various circuits and fuses on the 12V system that would limit to somewhat less than the converter's 90A output. IE I had a big disparity between the 12V heater requirement and everything else. Also the Jaycar solid state breaker was auto resetting and made a convenient binding post so I didn't need to have more than a minimum of connections.
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Post by Johny »

Interesting issue. You know that you are not going to have to bring the Aux battery up from under about 9 volts because if you did, the main contactors wouldn't close to engage the DC-DC converter anyway. That's assuming no jump starting Image

I haven't done any tests yet but there was a problem here using Jaycar SMPS in our portable radio repeaters where, if the battery was too flat, when the system was plugged into the mains (240 VAC), the power supply just shut down. The net result was that the power supply could not bring the battery up from flat - very embarrassing. The supply was 20 Amp and the battery was a 7Ah AGM.

The current spike would not last longer than a few seconds IMO. Could damage occur in that space of time?

Required test. Take a 7Ah->20Ah AGM down to 10% SOC (or the minimum to provide just enough voltage/current to close main contactor) and measure the current curve when connected to a high current charger/voltage source.
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Post by coulomb »

If this turns out to be an issue, then perhaps what we need are not DC-input power supplies, but DC-input battery chargers. Though you'd possibly want one that didn't do an absorption or equalise stage, just bulk charge (CC) and straight to float (CV). These are almost as available and almost as cheap, I would think.

Ah, but these would assume no load on the battery. I guess what you want is a high power battery charger, but one that you can tap into the output before the current shunt that feeds the battery. So it will adjust its current output to supply the load, plus a gentle charge for the battery. When the main contactor is out, the auxilliary loads still work through the current shunt inside the charger.

Alternatively, you could possibly hack the voltage sense in the power supply to restrict the voltage to such a level that the battery doesn't get too much of a blast.

It's starting to look a little non trivial. Let's do that experiment and see if the current seems excessive before attempting this.
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Post by Peter C in Canberra »

Hmm. I have my DC/DC converter hooked up after the main traction circuit breaker but before the contactor. Consequently the DC converter has power available all the time to supply lights or anything else when the car is not running and the battery would not charge or discharge unless the DC converter had failed or more than 90 amps were required (rare or never).

From the above I gather that some must have the converter supplied after the contactor. Having set up the contactor according to the Curtis manual that would have the 12V supplied from the backup battery power every time I took my foot off the accelerator and recharging only when I had my foot on. Is that what most people do? In that case I can see the back-up battery would be more active, would need to be larger, and charging currents might matter.
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Post by Johny »

Peter C in Canberra wrote: Hmm. I have my DC/DC converter hooked up after the main traction circuit breaker but before the contactor....
No, that's right Peter. That's the situation that we are discussing. When the main traction contactor closes, is the current too much for the little battery when it has been reasonably discharged by parking lights, radio, etc.
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Post by Tritium_James »

Does the curtis make the contactor open every time you lift off the throttle? Weird...
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Post by antiscab »

yeh, the idea is faster reaction time if the controller powerstage fails full on.

one of the many quirks (flaws?) of the curtis design.


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