Prelude conversion project - some questions

Technical discussion on converting internal combustion to electric
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jonescg
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Re: Prelude conversion project - some questions

Post by jonescg » Sun, 06 Jan 2019, 20:34

I have finished the rear pack subframe. Well, I think it's done. Seeking advice, but I think this will be sufficiently rigid and plenty strong to support a 120 kg battery pack. If push comes to shove I can add additional rails which bolt back onto the main frame rails.
Subframe done.jpg
Subframe done.jpg (66.73 KiB) Viewed 359 times
I mounted an MDF mock-up of the floor of the battery pack and clamped it in place. Then I went around and drilled 26 holes at 6 mm diameter. I wasn't sure if I would use Riv-nuts or weld M6 nuts to the top of the rails, so I welded the M6 nuts anyway. If for some reason this doesn't work out, I can still put Riv-nuts on the underside. The next step is to metal-prime and paint it black.

As for the battery itself, I will use the MDF as a template for the holes which will go through the aluminium composite sandwich panel. As amazingly light and strong as this stuff is, it's got some quirks which if not done properly, make it worse than useless. All holes need to be hollowed out and potted with epoxy, then drilled again. Normally you would use oversized washers but the clearance is rather tight, so I may end up using countersunk screws.

Before I attempt to bolt it up, I need to make the fiberglass lid (upturned tub) for the pack. I figure this should be fairly straight forward, and hopefully I can get away with using a selection of cloth and cling-wrap over the battery as the 'mould'.
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Re: Prelude conversion project - some questions

Post by jonescg » Thu, 10 Jan 2019, 15:36

After watching the Weber Auto video https://www.youtube.com/watch?v=_ILkLUE3Zxc on the three coolant loops on the Chevy Bolt, I'm starting to reconsider my approach to cooling the battery with a radiator.

The Bolt EV uses a liquid coolant circulated through the battery pack, however it doesn't pass the coolant through a radiator at the front of the car - instead it chills the coolant exclusively with the aircon refrigerant via a small (much smaller than mine) heat exchanger (18:26 on the video linked above). I was wondering why they do it this way, when a radiator would be able to get the temperature right down to ambient.

Well sometimes ambient is more than the battery wants, and it needs more cooling power. But passing coolant through a radiator at the same time would only serve to heat the coolant up if it's above 30'C on the day. Moreover, if the aircon is employed to chill the coolant via an exchanger as I plan to, the heat from the Aircon condensor will also serve to heat up the battery radiator - a bit like piping the hot exhaust air from a home split system back into the room you're trying to chill.

So what's the best option here?

1. Abandon the aircon heat exchanger and just cool to ambient?

- Key drawback here is that ambient is probably still well above the battery's happy zone. Also, if the people inside the cabin are feeling warm and turn the AC on, the condensor at the front of the radiator will start to shed heat, which in turn heats up the battery radiator behind it, further preventing the cells to cool down.

2. Exclusively chill the battery with aircon?

- Chevy and Tesla do this, so it's not too crazy. It does mean the aircon system will see a greater workout, even on a mild day.

3. Introduce a battery radiator bypass loop when it's very hot.

- This means leaving the approach as I have drawn it, but including a bypass loop which cuts the radiator out of the picture once the aircon compressor kicks in. So it cools to ambient using the radiator up to about 27'C without the aircon, but above 27'C, the aircon compressor kicks in and passes refrigerant through the heat exchanger, further chilling the battery. But, it also cuts the radiator out, meaning mildly warmed coolant leaving the battery pack is not warmed up by the ambient air - coolant simply circulates back to the refrigerant heat exchanger again. Sounds complicated, but would be less energy intensive than using the aircon only.

Any thermodynamics experts here care to chime in?

The maximum heat generated by the battery will be 6-8 kW for a few minutes. Otherwise, it's more like 700 W most of the time.
Maximum heat generated by the motor, inverter and charger is going to be about 1-2 kW, on a bad day. So we're not talking about a lot of heat, but getting rid of efficiently it would be nice.
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Re: Prelude conversion project - some questions

Post by francisco.shi » Thu, 10 Jan 2019, 15:51

I am having the same issues.
I considered using the air con to chill water which can then be used to run thru the cabin cooling or the battery. Each curcuit would have a pump. This way you can control the two temperatures independently. In certain cases you can use the cabin as a heat sink.
The heat from the evaporator will run thru the heater in the cabin and then thru the external radiator so I can use the air con to provide heating.

The only problem with this is there is no heating for the battery and if the weather gets very cold the battery could end up too cold.
For now this is what I will do because in QLD the weather doesn't get cold enough.

I did consider running a hot water circuit to the condenser and run either the hot water or the cold water thru the battery or cabin depending on requirements but I think this getting a little complicated. The coolant for the hot and cold circuits would be the same I would just run the coolant thru the one with the right temperature.

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Re: Prelude conversion project - some questions

Post by jonescg » Thu, 10 Jan 2019, 16:04

I'm not too worried about heating the battery - like Brisbane, Perth is pretty mild and it will warm up in operation before too long. I can choose to not circulate the pumps below 22'C.
One option could be to put the battery radiator in front of the aircon condensor since it's shedding less heat?
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Re: Prelude conversion project - some questions

Post by Johny » Thu, 10 Jan 2019, 16:09

It's my understanding that Tesla runs the battery heat control system even when the vehicle is off. I have found that the amount of time that the battery pack sits in an over temperature environment while the vehicle is not in use (parked) WAY exceeds any time when it is in use.
So, IMO, if your are not going to run air-con cooling all the time (even when vehicle off) I don't see much gain in cooling with it - just cool to ambient.

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Re: Prelude conversion project - some questions

Post by francisco.shi » Fri, 11 Jan 2019, 04:16

As far as I understand one of the problems with the Nissan Leaf was that it could not cool the batteries below ambient. So if the ambient temperature got too hot it would cook the batteries. If the car is parked under the sun it could potentially get hot enough to damage the batteries. So what I was going to do is use the air conditioner to cool the battery and the electronics but only if it got too hot. My guess is that the cabin temperature needs to be within the limits of what the battery is going to need. So if you need air con chances are the battery needs it too and if you need to heat up the cabin then the battery and electronics can provide some of the heat just by not running the compressor.
The benefit of all this is that you only need one radiator so you don't need to worry about one radiator heating the other one up. The condenser runs close to 100°C on the inlet (worse case). I do not have enough data to do any better for the moment.
Another thing I was planning to do was to put solar panels on the roof and a retractable section to cover the front windscreen when parked to act as a shade. I can get 6 250w panels which I think would be enough to keep the air conditioner running to keep the battery cool without draining the traction battery. I just haven't worked out if it is worth the extra weight.

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Re: Prelude conversion project - some questions

Post by jonescg » Fri, 11 Jan 2019, 10:06

I finally got around to testing the cooling system on this battery module. To be honest I'm disappointed, but it's not all bad. Let me explain.

First up, this is a 4.4 C discharge on a cell which is nominally 1-2 C and 5 C burst - so I'm fairly hammering it. At 330 amps, the busbar linking the two half modules is not rated for this sort of continuous rate (neither are the cells, but that's not what I was testing).

Secondly, the cells were still warm from the day - they have a specific heat capacity of about 2000 J/kg.K so they retain heat fairly well. The cooling loop was bringing them down to ambient slowly before I started the run, hence the higher starting temperatures for the cells.

Finally, the thermal epoxy application on this particular module was poor - the cells to the rear of the module have very little contact with the potting material, while the ones at the front were OK. This is evident in the difference in temperatures of the two probes I had in the pack. I applied the thermal material with a spatula, rather than squeezing beads out with a gun like I did for the others. So there would definitely be better contact and heat transfer in the later modules.

Still, the delta T on the inlet and outlet was at best 0.6 'C with a flow rate of 3 litres per minute.
temp plot.JPG
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So at the point where the cells are as hot as they can be, the water in the cooling loop was removing heat at a rate of about 120 W. At this point the cells are generating about 1000 W of waste heat, so they get hot fast. They cooled down once the load was removed at a rate which was OK, but when I turned the pumps off for the night and went to bed, the rate of cooling was not much slower.

I might try to cool the busbars above with airflow as they are in direct contact with the core of the cells. It may end up being as effective as the liquid cooling :cry:
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