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Old 10-08-2019, 05:20 PM   #11 (permalink)
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https://batteryhookup.com/collections/lithium-ion-assembled-battery-modules/products/10-pack-32-85v-2890mah-94-93wh-lithium-ion-batteries-with-bms-and-fire-proof-case-9s
Buying 2 sets of these 10 packs would be under $450 which was quite a bit less than the similar number of watt hours I bought for my Ebike build. I think my 52 volt 14 amp hour pack was like $550. These could be a 72 volt 24 amp hour battery for $450.

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Old 10-08-2019, 09:29 PM   #12 (permalink)
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Quote:
Originally Posted by Daox View Post
Good analysis!

IMO a BMS is pretty much necessary for any chemistry. For my mower which is currently lead acid, it's a simple monitoring system to make sure I don't over-disharge any specific battery. It's not automated, but it is a BMS.

The lipo chemistry I agree would require a much more robust system for safety and battery life. However, lots of the OEMs are using a similar chemistry in their EVs due to the power and every density advantages over the other chemistries. I agree a BMS would add cost, but I also think (don't quote me on this) that there are more and more off the shelf systems you can get that will do what you need. I haven't looked into that yet to verify.
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Originally Posted by RedDevil View Post
You will still damage or destroy LiFePO4 cells if you overcharge them. My LiFePO4 packs have BMSs.
Agree, any battery system should have some sort of BMS, especially lithium, and especially non-LiFePO4 lithium chemistries. I mentioned that the lithium Insights don't use an active BMS because the owners carefully monitor voltage and usually have larger-than-OEM packs that don't get drained as much, so the packs don't spend much time near full or what would be worse, empty.

With LiFePO4 you could probably get away with LED balancing modules and some devices to control temperature based charge/discharge, etc, overvoltage/undervolt circuits, and homemade stuff that would suffice. For non-LiFePO4 I would want an actual BMS.

In the Formula SAE electric formula race car team I am on here at college, we are using regular lithium ion cells in addition to the Orion BMS - a ~$1000 unit. Obviously this is a wholly electric race car, so the power demands are much greater than an Insight or lawnmower would demand, so a Orion probably wouldn't be necessary, but a good BMS for a sensitive lithium chemistry probably isn't cheap.
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Old 10-09-2019, 06:11 AM   #13 (permalink)
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In one way LiFePO4 is even more sensitive than other chemistries. The cell voltage is very strongly tied to the 3.2-3.4 Volt range and varies little between 1% and 99% charged. So you can't walk away for a minute while all cells charging at 3.4 Volts per cell; once full it jumps to over 4 Volt very rapidly, where the damaging takes place.

My BMS starts balancing at 3.65 Volt per cell and shuts down charging at 3.85 Volt.
Charging a pack of 12 Ah cells at 3A, there's just a few seconds in between.
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Old 10-09-2019, 08:19 AM   #14 (permalink)
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A smart charger is definitely required for charging, for that very reason.

Lithium batteries can use coulomb meters to measure SOC more accurately (Lithium Insights use a more sophisticated device called OBDIIC&C). If you keep the SOC between 20% and 90% you should be fine and extend the life of the battery verses discharging/charging them to 0% and 100%.

SOC is fairly easy to measure on lithium batteries with a coulomb meter, versus lead acid where the voltage varies under load. Other lithium chemistries are much more sensitive to things like temperature and charge/discharge rates.
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Old 10-09-2019, 06:03 PM   #15 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
Maybe they will get a bigger selection.
I have played with 18650 cells before and big prizmatic cells.
I prefer the big ones.
I completely agree. However, there seems to be more resources out there now to assemble 18650 cells into larger packs without fancy tools / welders.

These are the two that seem nice IMO.


Vruzend makes these interlocking pieces that can be assembled into whatever size and shape battery you want.

https://vruzend.com/





This guy sells these circuit boards that just pop in 18650 cells like most AA battery holders.

https://kit.com/jehu

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Old 10-09-2019, 07:42 PM   #16 (permalink)
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The big ones I was looking at go together with copper bus bar, drill and bolts.
The 18650 Legos are cute.
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Old 10-09-2019, 09:59 PM   #17 (permalink)
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Quote:
Originally Posted by Daox View Post
Vruzend makes these interlocking pieces that can be assembled into whatever size and shape battery you want.
This is actually a really neat idea. Bookmarked the link for any future lithium experiments I may do in the future (solar powered lithium assist to my Insight, anyone?)

It would be neat if they had some sort of solution for battery pouches too.
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Old 10-10-2019, 08:36 AM   #18 (permalink)
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I agree larger cells are still nicer for larger packs. You never know what they'll have next week though as they get what is available and what they find. Just gotta keep an eye out. They very recently ran out of the red 8Ah Headway lifepo4 cells.

Edit: So of course I go look what they have and they do have the Headway cells back in stock

https://batteryhookup.com/collection...3223103.eac2d9
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Old 10-10-2019, 09:15 AM   #19 (permalink)
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So 8 cells (4 in series, 2 in parallel) would provide 16ah and 400A max discharge current, down to -4 *F and charging as low as 14 *F and cost only $84. Adding the cost of a VRUZEND battery kit at $34.99 brings us to $118.99. Add in some sort of inexpensive LED balancing modules and that's one pretty cheap lithium battery!
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Old 10-10-2019, 10:56 AM   #20 (permalink)
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Unfortunately the vruzend stuff is only for 18650 sized cells. The Headway cells are a much larger size.

The good news is the Headway batteries have screw terminals on the ends. So you can pretty easily make your own bus bars for them.

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