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Old 05-13-2015, 03:59 PM   #11 (permalink)
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Haven't forgotten, just had some busy days including a double birthday to attend/organize/endure.

I'll just drop a pic of where I'd like to drop my pack:



Under the aluminum shield is the OEM NiMH hybrid pack.
The black plastics are the vent channels of the forced battery cooling.

My parallel pack will not likely have active cooling - it should be at room temperature when I put it in, and it will probably just discharge at a not too high rate; nonetheless I'll drop in a temp sensor.

I run around these days without the boot floor (which is surprisingly heavy!) to this is pretty much what I see each day, save a blanket to cover it up and some small bags. Not having the floor and foam filler substantially enhances the cargo space

My parallel pack should be small enough to fit in the cavity behind the OEM battery cover.
I intend to fabricate a containing structure, with the necessary main switch and connectors.

I worried about what happens if I get rear ended and the cavity gets squeezed, pushing the extra cells into the OEM pack ruining both - but then the Fit Hybrid, which is technically identical, has a much shorter trunk and no cavity at all... so if that's not essential there I probably should not worry.
I'll leave some space unused just in case.

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Old 05-13-2015, 04:22 PM   #12 (permalink)
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Headway makes some smaller cells that would suit you quite well I think. They have a 10Ah, 12Ah, and 15Ah version. I'm not sure where you can get them in Europe, but its worth a look.

Here is a US distributor:
Lithium LiFePO4 Cells - Headway Headquarters LLC | LiFePO4 Cells, LiFePO4 Battery Packs for EV, Lithium Iron Phosphate Battery

The price is quite a bit lower than the options you've posted.

Edit: found a possible source.

http://www.litrade.de/shop/Akkus-Zub...hparam=headway
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Old 05-13-2015, 04:39 PM   #13 (permalink)
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Thanks - I have seen the headway cells before, but they are actually more expensive than the GWL 10Ah cells - rightly so, as they can handle somewhat higher currents.

I have been looking at A123 pouch cells for the same reason; those are relatively light so I could increase capacity without losing portability.

But I don't expect a very high current.
I figure that if I can get the cheapest LiFePO4 cells that can produce 20 Amps continuously I'm good. It rules out A123 cells and probably Headway cells too.
I may be wrong in this though. I'll keep an open mind.
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My new favorite mineral is tinnunculite which forms when falcon poo reacts with the gases from a burning coal mine. Yeah.
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Old 05-13-2015, 04:44 PM   #14 (permalink)
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Ah, yes I missed the GWL cells. That looks like a nice inexpensive option.

You'll want something to protect your pack when you charge it. I don't think you'll have any issues with discharging because of the car's BMS won't let your voltage get that low.
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Old 05-13-2015, 05:11 PM   #15 (permalink)
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True - nevertheless a string of 36 cells could get damaged if the voltage over it drops to from 117 Volt (fully charged) to 107 (average OEM pack voltage), if one of them is discharged deeper than the rest.

Bottom balancing the cells once in a while should prevent that.
But then I'd need to charge them on the string while monitoring each cell like a hawk, as LiFePO4 voltage tends to hold steady during charge and then suddenly go through the roof, damaging itself, when full.

I might pair each cell with a small Lithium-ion cell to buffer the overcharge differences.
When the pack is nearly full the voltage over all cells will rise more or less simultaneously, so the charger could just have a voltage cutoff.
But I'm scared of what would happen if a Li-ion cell fails.

I could use a BMS. But I don't have any experience with those and stories of packs killed by a faulty BMS make me wary.

If only I could just buy a black box with one dial to set the desired voltage and another to set the max current and hook that up...
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My new favorite mineral is tinnunculite which forms when falcon poo reacts with the gases from a burning coal mine. Yeah.
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Old 05-13-2015, 05:43 PM   #16 (permalink)
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Personally, what I would do is make my own BMS. I designed a simple one back when I had my enginer kit in my Prius. It is based off an arduino and using a few celllogs which measure cell voltages. The charging side worked great, I never finished the discharging side of it. However, a simple amp hour counter gauge would work fine, just not be automated, you could switch it off once you use 7-8Ah.

Here is the link where I had worked on it:

http://ecomodder.com/forum/showthrea...tem-20445.html
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Old 06-11-2015, 05:02 PM   #17 (permalink)
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I would not build this pack nor buy parts for it without my better half's approval.
So I'd better tell her about my plans...

I just did and luckily, she's supportive provided it is removable (for oversized batches of holiday luggage), safe obviously and thoroughly designed.
So, green flag! Thank you mrs. Red*, love you!
(* I dare not call her mrs. Devil as that might just jeopardize the matter at hand )

Now, things to do.
What cables to use between the OEM and portablepack terminals, what connectors (I need some kind of docking terminal, obviously), what cell undervoltage/overvoltage protection and monitoring stuff?
Choices...
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My new favorite mineral is tinnunculite which forms when falcon poo reacts with the gases from a burning coal mine. Yeah.
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Old 06-11-2015, 06:01 PM   #18 (permalink)
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I'd only recommend using voltage as a control for charging. For discharging, I'd recommend counting the amp hours out of the battery. The reason being is charging is very consistent. You charge at say 10A every time you plug in. When discharging, the load is not consistent. When you get a high load, your voltage sags. When the load lightens, the voltage comes back up. Thus, you never really know what your state of charge is while discharging from looking at voltage. Lithium cells make this even worse due to their very flat discharge voltage pattern. The best way I know of is to look at how many amp hours you've pulled out of the pack.
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Old 08-07-2015, 08:59 AM   #19 (permalink)
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Decision time

I have an appointment to look at some cells and stuff next Monday. Time to spend the revenue of my last motorbike.

Most likely setup now is 36 10-12 Ah LiFePO4 cells with BMS and chargers, cables, fuse and whatever the guy talks me in to.
I still like the triple 36V bike battery, but he has advised me against it without giving detail. I'd like to know why even if just to learn from it.

I might replace the ailing 12V battery too while I'm at it.
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My new favorite mineral is tinnunculite which forms when falcon poo reacts with the gases from a burning coal mine. Yeah.
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Old 08-07-2015, 09:45 AM   #20 (permalink)
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Cool. Keep us informed.

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