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48V LTO Off Grid system
#21
They are still relatively new/unproven tech, they are not quite main stream and no one rally knows for sure how long they will last.

The are also physically larger and heavier per watt hour than Li Ion/LFP

As far as I can tell these are the only "down sides"

I have had pretty good success using 2.7v supercap top balancers (like this: https://www.ebay.com.au/itm/263810452104...3156047608) with 5s 12v setups, just make sure you balance your cells before you assemble the pack.
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#22
That may well have been my next question.

I'd read an EV thread somewhere where a guy had bottom balanced and run some prismatics without BMS. The additional part of his technique was that on the first charge he ran around all the prismatics with a voltmeter and soon as the first cell hit charge he mesaured the whole pack and set the charger then to turnoff at that voltage. He asserted that you only needed to check every hundred cycles as the cells only drifted very slowly off their original capacities based on manufacturing differences. Indeed after a year of use he had not needed to change anything.

I was going to try this for a while on the test pack of 5.. simply to observe.. i figure that these cells , nearly the same AH as a large prismatic , are even more uniform.

As for your reference to the super cap top balance above.. could you share a little more insight in how that works? i look at the link and it wasn't immediately obvious how it works.. If you are using that on your cells.. do you have any other BMS functionality or just these?
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#23
I wasn't happy with the voltage creep between the cells, even after bottom balancing, after a single charge the cells were already drifting noticeably.

The top balancers are connected 1 per cell in parallel with the cell, once the cell exceeds 2.65v the balancer activates and begins to bypass upto 1A of current so that remaining low cells can charge, theoretically with a charge voltage of 14.4 (2.88 per cell) you should be able to reach a full charge for each cell in a pack.

I in practice this does work with a balanced pack, I intentionally tried an unbalanced pack and the lowest cell only reached 2.4v will the remaining 4 cells reached over 3v.

Once the cells were balanced the packed charged correctly, each cell reaching 2.8V then stabilising back to 2.65v.

I have also noticed with high current charging the balancers can struggle to keep up, but I have only tried this with the unbalanced pack. The theory is in a balanced pack by the time you are reaching 2.7v per cell the charge current should be low enough to be manageable.

In a 12v pack I am not using any other BMS, on the 48v pack I have a BMS but have not installed it yet.
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#24
Thanks for that.

I found this BMS on Alibaba: https://www.alibaba.com/product-detail/5...3b73uKgxLY . Managed to connect with them today.. in small quantities the 4S (12V) 80A is USD39 and the 20S (48V) 80A is USD56. If I've done my maths right then , at least on the 48V version the price per cell is on par, or a little better than the super cap balancer referenced. I might go this way for now, at least on my test quantity of 5 LTO cells.

After the boat project (12V) I'll likely do the house (48V) so it will all be good learning.. though that said I just found a source of 6A 32650S LiFePO4 near where my boat is for USD2.10 so I may yet do that for the boat. I dont need 50 years of battery for that.

One more question on the LTO BMS. The cells are 40AH. I dont believe I will ever charge or discharge them more than 1C. Therefore an 80A (2C) BMS should be more than enough.. right?

And another observation. When I inquiring on BMS for the LiFEPO4 only one supplier cited lifespan of their BMS , which was 50000 hours. I did not think about it much for the LiFePO4 as 50000 hours is about 5.7 years and probably most of what I'd get out of liFePO4s cycled fully every day. (These BMS things are always on)

But what about LTOs. A 50000 hour lifespan on the BMS wont come close to the life of the cells. It suggests BMS with connectors (at a minimum) is the way to go.. located in in the battery where they can easily be swapped out.
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#25
I am only using the Cap Top balancers on 5s 12V packs with no other BMS.

For the 48V pack I am using this unit under the recommendation of another member: https://www.aliexpress.com/item/Free-shi...69857.html

They have since released a 300A unit also: https://www.aliexpress.com/store/product...59733.html

You are correct in your assumptions regarding BMS sizing, an 80A BMS would suit charge and draw rates anywhere up to 80A. 

If you just want to use the BMS for balancing and cell management and your BMS is undersized you can side step it by connecting the load directly to the pack.

Or you can run the load through a shunt and contactor so that you can still take advantage of the low voltage, over voltage, etc. safety features.
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#26
Ok thanks again.. that BMS you referenced looks way better than what I was looking at.. I take it that it is a Batrium equivalent.. but a lot cheaper.

My latest dumb question goes as follows.  I keep hoping I will stop asking 

If I have a 4s10p or 20s10p and I'm connecting a BMS.. do I need to connect a completely separate BMS to each series string? To this point I assumed that was the case.  The attached image was new to me.. as  a 4s it was connected to 2 p strings also.. can this be true of all bms?  If true then I'd guess you can go to 3p , 4p etc?

Indeed as your reference a new 300A version of your BMS I have to believe this method of connected the BMS is possible for all.

If this can be done , what is the downside? (compared to say having 10BMS in a 4s10p system) simply each parallel set is as weak as the weakest cell?  This would certainly save a ton of $ on what I thought I'd have to spend on my 12V BMS.

(image is just of a bms I found browsing, not one mentioned.. it is the battery config that interests me)


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#27
The Batrium units are far more expandable and customisable than the units I linked but these units suit my needs for a reasonable price.

There is no wrong way to connect a BMS or BMS's's's's, as long as you get the polarity right of course, it's all about finding a middle ground you are happy with between cost, complexity, diagnostics and failure management.

If you look at the picture of our setup you can see blocks of 8 cells in parallel, 20 of these blocks in series give 48v, and I only need 1 BMS and 21 monitor/balance wires to manage all 160 cells. However if 1 cell goes bad I have no way of knowing which cell, the BMS will report that 1 block is constantly out of balance, and I will have to remove and tear down that block and manually diagnose which cell is the issue.

Alternatively I could build strings of 20 individual cells in series then place 8 strings in parallel, I would then need 8 smaller BMS's but I would instantly know about any cell failure as each cell is individually monitored. However now you have to factor in the cost of 8 BMS units and 168 monitor/balance wires.

As I have purchased new cells and checked that each cell is similarly spec'ed when building the packs I am happy to take the chance if less detailed monitoring for lower cost and lower complexity.

This is how most forum members are building their packs, thoroughly testing and spec'ing cells first, then building packs of similar parallel cells, then connecting similar parallel packs in series.

Using smaller cells it is possible to connect the individual cells via fuse wire to protect against individual cell failure.

For the LTO cells you would need to use something like these: 
https://au.element14.com/siba/90-058-05-.../dp/607654
https://au.element14.com/littelfuse/157-...dp/1354645

As we are using new cells we have opted to just use a single larger fuse in the middle of the pack:
https://au.element14.com/siba/90-058-05-.../dp/607721
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#28
Thank you Baron. Your title belies the insightfulness of your answers.
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#29
Those Aliexpress units look like they only can do small current balancing 40mA (=0.04A) - so on big batteries, this may not be enough to keep them balanced
Running off solar, DIY & electronics fan :-)
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#30
(11-05-2018, 08:24 AM)Redpacket Wrote: Those Aliexpress units look like they only can do small current balancing 40mA (=0.04A) - so on big batteries, this may not be enough to keep them balanced

With a little bit of work and some soldering, you can expand some of the Ali boards current ability. I would imagine you just pop off the balancing resistors, add some wires and make another board. If they have through hole resistors, even easier as you can put heavier load on the board.
But, that's just a theory Wink
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