Newb Asks: Why not just boost the voltage? Topic is solved

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Gregski
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Newb Asks: Why not just boost the voltage?

Post by Gregski »

please forgive me if this has been asked and answered a 1,000 times already

QUESTION: Why must we, or why do 100% electric cars run 360 volt battery packs and not 240 for example and just boost the voltage to a higher number?

I have two battery packs from BMW 530e plug in hybrids, (each pack consists of six 60 volt cell modules) they are approximately 360 volts nominal and 9 Kwh each in parallel that would be 18 Kwh however if I was to reconfigure them into three packs of 240 volts that would yield more range wouldn't it, or would it? I recon none boosted 240 volts at what ever amps would not give me the speed and power equivalent of 360 volts at same amps, but that's where the booster converter comes in, no?

please be gentle as both of my parents passed on the electronic recessive gene to me

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Re: Newb Asks: Why not just boost the voltage?

Post by mjc506 »

No. Range is governed (amongst other things, such as weight of driver's right foot) by energy stored. Whether the battery contains (say) 30Ah (amp hours) at 360V or 360Ah at 30V, the energy stored is the same (30Ah x 360V = 10.8kWh kilowatt hours).

Note that kWh is not the same as kW (kWh is a measure of energy, kW is a measure of power). Amp-hours is a measure of how many amps a battery can flow for an hour from a full charge, or how many hours it can flow 1 amp.

Higher voltages allow higher motor speeds with the same hardware (greater electro-motive force to counter the back-emf generated by the spinning motor).

You could use a boost converter to boost the voltage of a lower voltage pack (the prius does this) but the boost converter is not 100% efficient, and is another set of components (a switching power semiconductor and a large inductor at the very least). These components also have limits on the power they can manage.

Batteries do lose a bit of efficiency when dumping (or charging) at a high rate (power - this is down to a greater current passing through the internal resistance of the cells, and losing some of that power to heat), so you might think that reconfiguring a pack to half the voltage but double the amp-hours (ie, halving the current through each cell, quartering the heat generated) and then using a boost converter to double the voltage again might be useful. Unfortunately, conservation of energy kicks in, so energy in to the boost converter = energy out (and by extension, power in = power out). So if you double the voltage out of the converter, you double the current coming in. Plus you have the losses from the boost converter, and you also require heavier cables from the battery as they're carrying twice the current.

The most efficient solution is to increase voltage and therefore reduce current. This reduces thermal losses (Power = I^2 * R). But there are obviously practical limits, like voltage limits for the switching power electronics in the inverter, insulation limits, EMI, etc etc (especially insulation within the motor itself). The result is that most OEMs are ending up around the 360V mark. (Although there are a few pushing 800V...)

Clear as mud? :-)
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Re: Newb Asks: Why not just boost the voltage?

Post by johnfin »

Great answer to a useful question, thanks both.
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Re: Newb Asks: Why not just boost the voltage?

Post by Gregski »

thank you so much for that detailed explanation, sounds to me as if it is a shell game or you rob Peter to pay Paul ie volts up / amps down, or volts down / amps up

so to rephrase a Booster Converter boosts volts (not amps, is that correct?) and since power is power can't be pulled from thin air, it has to come from somewhere, ie the battery pack, so if we have a 100 volt battery pack but want to boost the voltage to 200 (assuming 100% efficiency for the sake of argument) the Booster Converter will still pull only 100 volts from the battery pack but at double the amps (a none Booster Converter will yank by comparison) then the magic happens inside the Booster Converter and 200 volts get passed on to the motor but the amps drop back to half?

so why does Toyota/Lexus do this in their Prius/GS450h respectfully? the only reason I can think of is that it behaves sort of like a Turbo, where it's just a momentary boost of energy for the sake of speed (MPH) and I imagine is not sustainable for an extended period of time, after all the Lexus GS450h boosts 288 volts to 650, that's more than 2x, though knowing electronic specs this could be for as little as one second (right?), and correct me if I'm wrong but I think that Booster Converter tops out at 30 Kw so it normally pulls 288 volts at 104 amps, then to boost to 650 volts it would have to draw 234 amps from the battery pack, no?

as my buddy at work used to say [and we was governmint employees] this makes perfect sense when you're standing on your head

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Re: Newb Asks: Why not just boost the voltage?

Post by mjc506 »

Yeah, pretty much dead on. A boost converter increases voltage, a buck converter reduces it (just convention). So yeah, A boost converter set at 200V will draw double the current from your 100V pack (and a buck converter at 100V would draw half the output current from a 200V pack). You can get DC-DC converters that can supply a voltage higher or lower than source (usually a boost converter feeding a buck converter), and an some topologies are bidirectional (act as a boost converter in one direction and a buck in the other) - the Prius converters are bidirectional (to allow battery charging/regen), the gen3 is a boost/buck too!

Bear in mind that the prius/gs450 are not electric vehicles - The design is really intended as an 'eCVT', with MG1 taking power from the ICE and using that to drive MG2 (advantages are that the ICE can run at its optimum rpm, and there's no gearbox as such. Disadvantages are the losses between MG1 and MG2 but these are pretty small in comparison to running the ICE away from it's optimum in most 'daily drive' situations). The battery is really only there as a small 'buffer', so they could get away with a lower voltage and the boost converter. This was all designed back in the dark ages of NiCad cells :-) That little boost converter can, however, dump 25-30kW into the drivetrain from the battery for a few seconds if the driver floors it, which is helpful!

Of course, more recent designs (including the Outlander and other hybrids/PHEVs) have seen the advantages of a bigger battery pack (a significant electric-only range, lithium cells have better energy and power density, so a 'full voltage' pack can still be a reasonable size and drive a motor quite happily etc etc) so that's how things are moving. I believe the later generation prii have a more capable boost(/buck) converter, but the pack is still relatively (compared to full EVs) small, so will get warm quite quickly if drawing lots of power from them (on the plus side, with a small pack, it'll go flat quite quickly too!). The Outlander operates on 280V(???) directly (no boost converter) and is designed around that voltage...

There are other ways of getting high enough motor speeds. You can change the gearing between motor and wheels (which will require more torque, hence more amps, but the power requirements stay about the same), you can change the construction of the motor (higher kV, rpm/volts - different windings, magnets etc), or you can do some magic in the inverter called 'field weakening', which changes the kV of the motor dynamically by using some current to counteract some of the magnetic field. This is definitely witchcraft :-) but does require additional current (generally, at full throttle, you would run an inverter 'flat out' at max current, so you'd end up with constant torque from 0 up to the 'base speed' of the motor, and then as field weakening kicks in, get a 'constant power' region where torque drops with speed, until max rpm. Of course, that fits in quite nicely with how ICE vehicles operate (acceleration reduces with speed, mostly due to gearing) so that's not necessarily a downside, and you get the full torque of the motor at low speeds where it's more useful (increasing the kV of a motor reduces the kT - torque per amp. If you increase kV mechanically, you lose that torque all the way through the rev range. Do it with field weakening, you keep full torque at low speeds!)
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Re: Newb Asks: Why not just boost the voltage?

Post by arber333 »

I agree. On a side note, outlander components are designed to run with full 96S pack. Its just that Outlander uses only 88S, probably due to space constraints.
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Re: Newb Asks: Why not just boost the voltage?

Post by mjc506 »

Yes, packaging is difficult, especially on the hybrids which have to have all the ICE-related bits too!
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Re: Newb Asks: Why not just boost the voltage?

Post by johu »

Yes, field weakening is definitely witch craft, still not quite mastered by the openinverter firmware :(

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