Quote:
Originally Posted by Ecky
Hugely useful post, thank you IamIan.
I also need to factor in weather / temperature. If I'm going to be using an EV when it's 20-30 degrees below zero, I'm definitely going to be using a heater, and range will already be reduced by virtue of lower temperatures.
Is there any reason I shouldn't have the battery modules split, to help distribute weight? A double 24kwh Leaf pack could provide what I need, but I don't think I'd want all of that weight in the back.
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Welcome ... happy to help.
Cabin Heating does use additional energy .. but compared to traction energy usage it is a small % .. you can make it an even smaller % influence by using targeting heating ... For example .. heated seats .. or heated heat pad ... more directly applies the heat energy to the target it is needed .. you .. Trying to heat yourself by 1st heating the cabin air is a far more energy intensive method of getting you warmed up .. ie it takes more wh/joules to accomplish the same effect on you.
Usually one of the biggest hit to BEV's in cold weather is the traction batteries themselves being cold .. For example .. The A123 cell in my PHEV battery pack at -20C the colder batteries have reduced their 'usable' capacity to about ~60% of what they have available at +20C .. from around ~2.9kwh down to about ~1.7kwh .. and if that weren't bad enough .. the colder weather also increases the cell's internal resistance .. a +20C battery pack might have down around ~200mOhms ... but it will be up to about ~600mOhms by -20C ... soo less usable kwh of energy .. and less usable kw of power .. That still work for my application ... but it is an effect that is still important to keep in mind when designing a BEV conversion ... to make sure it meets your needs.
That is the reason why Some OEM BEVs .. like Tesla's actually deliberately spend energy .. while charging and even not while charging to warm up the battery pack itself ... of course the specifics will vary from specific battery type to specific battery type .. but ... you can test these things... by doing test discharges while the battery is in your home freezer.
You can also ...like heat plugs and such used to get ICE temps up .. you can do similar things in the winter to get the Battery temps up .. not crazy hot .. but just prevent them from getting soo cold it has a serious effect on the vehicle's utility / usefulness.
Cold air is denser and will increase your wind resistance .. but it is a smaller impact than the 'cold traction battery' effect ... for example:
Looking at the Previous 'normalish' around ~60F air temp numbers.
The Gen1 Insight Need to supply to wheels:
~8.80kw to sustain 60MPH @90%Eff = ~9.77
~5.95kw to sustain 50MPH @90%Eff = ~6.61
~3.87kw to sustain 40MPH @90%Eff = ~4.3
~2.42kw to sustain 30MPH @90%Eff = ~2.69
If all else equal .. but air goes down to say 0C , the above changes to about
:
~9.54kw @60MPH @90%Eff = ~10.6 -- Roughly ~8% more
~6.37kw @50MPH @90%Eff = ~7.1 -- ~7% More
~4.09kw @40MPH @90%Eff = ~4.5 -- ~5% More
~2.51kw @30MPH @90%Eff = ~2.8 -- ~4% More
Yeah ... it's a hit .. maybe as much as ~10% or so ... but the cold battery is the much much bigger potential hit ... cold air and cabin heat combined together would still fit int eh ~25% buffer from before ... but not usable kwh capacity loss from a cold traction battery.
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The good thing about distributing the weight is for handling and such .. trying to get it the final back to near OEM-ish 4 wheel front back , side side distribution is ideal .. depending on final physical sizes weights and such .. you might have to make a few compromises .. but near back to OEM 4 wheel distribution would be ideal.
The other aspect of weight distribution is to try and keep your center of gravity / center of mass .. as low as you can .. good thing about going with the gas tank idea you had .. Higher Center of mass = bad
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If you can keep the Traction Batteries themselves from getting that bitter cold ... the previous ~25% buffer will handle the minor bits of colder / denser air ... etc.
If you can't keep them from going down to those bitter cold -20C temperatures .. you will either need to vastly increase the ~25% buffer ... or , do opportunistic charging .. or it might not meet your cold weather needs.
The batteries do have a fair bit of thermal mass .. which will help some ... but only goes so far.
Of course you could do the brute force approach ... if you double your capacity .. up to about ~60kwh when warm ... even in -20C you should still have plenty more than about ~35kwh usable .. but .. that would probably be too much weight with Leaf Cells ... Maybe if you did something like a Tesla pack which would be about ~500Lbs of batteries+ the extras for wiring , pack structure , etc .. Before adding that much more batteries .. My 1st direction instead might be some of those efforts to keep them from getting bitter cold ... just like we make efforts to get and keep the ICE warm in cold winter too .. a little cold has little effect ... it's generally only down in the real bitter cold that there are large / significant performance effects ... ie when the traction battery itself (not air temps) is down bellow 0C.
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One other note about 'all that weight in the back' ... Still true all the stuff above but .. just keep in mind ... MikeD did drive around for some time with all the OEM weight stuff in the back ... +a several hindered pounds of lead acid battery in the spare tire well on top of that ... + the weight of the 5th wheel ... and it's hydraulics .. and electronics .. etc ... soo it isn't like the frame won't take it ... it's more like the suspension system would need beefed up to handle that much more ... and the car will handle very differently if you had that much weight there.
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Of course the increased Li resistance in cold weather is actually a good thing .. in a way ... the act of using what usable kwh of energy it will give ... that will itself help some to heat up the battery .. generally speaking most members of the Li-family are very energy efficient ... up around ~95% or so (depending on conditions) ... but even that 5% of say ~20kw of power to move the car ... is still all by itself an automatic about ~1kw of heating exactly where you want it on cold traction main battery pack ... inside the battery pack itself .. if the charger also did a little pack heating of it's own while it was plugged in .. the batteries might not themselves ever really get down to those bitter -20C temps.
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Just food for thought ... about ~25% increase in the spring rate of the springs ... ~1900Lbs +~25% = ~2,375Lbs ... that's a fairly easy part.