# 18650s minimum voltage



## okashira (Mar 1, 2015)

Why? 
Just buy some of the neptune loggers if that's waht you want.


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## GoElectric (Nov 15, 2015)

Mmmmmm, what R they?

(I looked)


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## okashira (Mar 1, 2015)

http://www.speedict.com/


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## GoElectric (Nov 15, 2015)

Hi John, 

Mmmmmmm. Wow. Wireless. So simple. Awesome. And the price is only $90??!!

I do see a couple of problems though (emailed them). How to monitor more than 15 74P cells? And are there alarms? My cells won't all be in the same place. Don't have an android phone either.

Regardless, I'm still not sure what voltages to use. 

Anybody have any suggestions (with reasons)?

Jim


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## okashira (Mar 1, 2015)

Just buy 1 per two modules?

Anyway, why not just buy a ZEVA BMS??
We get a small discount and can program it for you


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## Moltenmetal (Mar 20, 2014)

MiniBMS has the right voltages for nmc cells and is $12.50 per board, with a $45 headboard to replace your PLC. Isolated, low current draw on each cell, open source and very well tested. Why reinvent the wheel?


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## Karter2 (Nov 17, 2011)

Most Lico/Lipo, ICR,NMC, etc 18650's seem to have a manufacturers min voltage of 2.5 v or there abouts......but if you look at the discharge data, there is very little capacity left below 3.0 volts .....so you wont gain much range by hanging on down that low.
BUT.. You will miss out on significant capacity if you do not fully charge to 4.2 v !


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## riba2233 (Apr 29, 2015)

Moltenmetal said:


> MiniBMS has the right voltages for nmc cells and is $12.50 per board, with a $45 headboard to replace your PLC. Isolated, low current draw on each cell, open source and very well tested. Why reinvent the wheel?


It may have started as open source, but it's not anymore. And it's still expensive, I have 42 cells, so with added VAT and shipping it's close to 800 USD for the set.


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## riba2233 (Apr 29, 2015)

Karter2 said:


> Most Lico/Lipo, ICR,NMC, etc 18650's seem to have a manufacturers min voltage of 2.5 v or there abouts......but if you look at the discharge data, there is very little capacity left below 3.0 volts .....so you wont gain much range by hanging on down that low.
> BUT.. You will miss out on significant capacity if you do not fully charge to 4.2 v !



But you have to set LVC to 2.5 V because of various unpredicted situations (low soc, high load, emergency...) and cold weather. LVC is only a safety feature used to save cells, not something that should be relied upon to tell you when to stop driving.


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## Moltenmetal (Mar 20, 2014)

The ZEVA does look a little cheaper, at $135 AU (~$135 CDN today) for a module which will do 12 cells, plus a $75 headboard- and it gives you Canbus if that's what you want. The ZEVA is probably nicer for packs like the Teslas that GoElectric is using just because it's a centralized single board for 12 cells rather than individual celltop boards that you'd have to mount somewhere.

Either way, I doubt GoElectric is going to get his cell monitoring solution below ~$12/cell, even forgetting about what he'd be paying for a PLC- which likely isn't hardened sufficiently in environmental terms for an auto environment anyway. Which again begs the question I asked before- why re-invent the wheel, other than as a fun project?


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## GoElectric (Nov 15, 2015)

Hi - glad to see some more responses. Always a reality-check. 

First, voltages: Thanks. Yes, 2.5V usually means your cells are probably at less than 10% SOC, except when they are supplying a big load. I can ignore short-term drops via the PLC, but I think I will go for an alarm at 2.8V, just to stay farther away from full discharge. I can set the ignore function to say, 30 seconds or even a minute to get past heavy load situations, but will pick-up cells which really are at 2.8V under a normal load sooner. Eh? 

Now to the bigger question of "Why bother?" It is tempting to just buy the stuff alright. Zeva would work best for me, about $CAN500 plus shipping and tax for the cheapest option, which is okay. Even Orion is okay when you consider the total cost of the project.

But I'll bet there are elements which are over-done in most projects, just because the DIYer just wants to. Hopefully because that is one of his strengths. That could be enough-said, but I do appreciate the feedback I get on here, so I will add the following, so you can see it is kind-of a natural fit:

1- I seem to be in the bottom-balancing camp, so don't particularly need or want top balancing - Tesla cells especially (74P);
2- I am also building a control-system for keeping the batteries warm, which is how I got started on the PLC in the first place. I'm not aware of off an off-the-shelf system;
3- I have two guys helping me just because they are interested: an EE with 25 years experience designing and building electronics and an instructor in Automation and Control at the local tech institute. So, it should get done right;
4- Having a PLC on board can be darn useful for all kinds of things in the future;
5- I am an EE as well;
6- I want to start a business doing this. So once I get it figured-out, I can make money on each, control battery temp and still charge less than ZEVA etc.... 
7- As I said, I may replace the PLC with an Arduino at some point.

Good point on survivability - do you know what certification is required?? These are industrial 12V controllers. I don't know if an Arduino would be reliable....

Thanks for sane and rational responses. DIY!


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## Moltenmetal (Mar 20, 2014)

You're not the only person going with their own DIY route- but if I were doing this as a business, I'd want to be able to buy the parts rather than to be the only supplier of some custom thing that you have to order by the hundreds to make it economical, and then be short a few when one of your good customers needs a few replacements etc. etc.. I'm sure you will keep it safe, but I doubt it's going to get a lot cheaper than what's already out there.

As to top vs bottom balancing...(yawn)... You're going to want to stay well away from the bottom of the pack if you want it to last anyway, and the Ah meter on your car is the gauge you'll live by when driving. As long as you have a good Ah gauge to avoid the bottom, plus a BMS of some sort to shut off the charger when any cell goes into HVC, cell balance doesn't really matter all that much in meaningful terms- assuming you only draw from the entire pack in series and don't do anything too crazy.


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## GoElectric (Nov 15, 2015)

Hi. 

Thanks MM. Understood. I may change my mind, as it is easier to buy off the shelf stuff, especially BMS. If there was a battery monitoring system like the one for bikes Okashira recommended (with alarms), THEN I would go for it. 

Ah - there was the bridge suggested earlier, yes. I pitched it to my EE and he said he would find "something better" which is where this is going - I'm okay with that; better than the bridge, but I'm not knocking it.

Keeping batteries warm - I just don't see something plug-n-play out there. It has to be fluid circulation.

Anyway, all will be trial and error. Things evolve.


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## Karter2 (Nov 17, 2011)

> - I seem to be in the bottom-balancing camp, so don't particularly need or want top balancing - Tesla cells especially (74) ....


...odd that you should choose to follow Tesla with their choice of cells, and pack configuration,.... but then reject their much tested and market proven methodology of cell/pack management .


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## Moltenmetal (Mar 20, 2014)

I agree completely, Karter. Tesla apparently monitors voltage on every group of paralleled cells, ie. Each of which acting as a single cell of larger capacity. I think that's what your original plan was, GoElectric, and that's what I'd do.

The Lee Hart bridge is a simple and inexpensive balance check, but I wouldn't substitute it for a real cell level BMS. The real benefit of a BMS is worry free charging to near full useful pack capacity. That's a wonderful and valuable safety feature for an expensive pack.


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## GoElectric (Nov 15, 2015)

Hi. Well, there is Tesla alright. Nobody knows what they really do, except that they have two charge modes, and I can do that with the PLC. I am having a Tesla BMS board sent up so my EE can have a look at it tho. Yes, I suppose they are doing top-balancing, but EVWest is not - why?? 

As I said, things evolve. I'm listening. The PLC has other uses too, so could always add a top-balancing BMS. Can someone tell me the benefit of top-balancing?? I can't figure it out, simple as that. Perhaps if one or more of the 74P cells fails?? Perhaps that is the reason? Drift? This could happen because of temperature variations, but I am going to circulate coolant something like 10 minutes out of every hour, so all the cells in all of the packs should stay at the same temperature. If need-be, I can circulate all the time (10W).

All cells will be monitored.

Thanks.


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## GoElectric (Nov 15, 2015)

I re-read everything. Thanks again for the useful replies. This is a DISCUSSION board.

I realised afterwards if one of a 74P cell dies, top balancing still won't do anything - I'm still limited by the lowest-capacity "cell."

Oh - perhaps not exactly. If I can fully charge the other cells, then if the bad cell is at a low (possibly damaging) SOC, the other cells will be above that. By a factor of 1/74... but it could be 2 or more 18650s which go. I guess this is the over-all benefit of top-balancing. The trade-off is cramming all that charge into the cells is not good for them either?

(What DOES Tesla do?)


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## okashira (Mar 1, 2015)

GoElectric said:


> I re-read everything. Thanks again for the useful replies. This is a DISCUSSION board.
> 
> I realised afterwards if one of a 74P cell dies, top balancing still won't do anything - I'm still limited by the lowest-capacity "cell."
> 
> ...


False that what Tesla does is not known.
*THEY DO THE SAME AS ANY REPUTABLE MANUFACTURER OF LITHIUM ION PACKS. LIKE SERIOUSLY, 100% OF THEM ALL DO THE SAME THING, THEY EITHER BALANCE AT THE CV PHASE OR THEY BALANCE DURING AND AFTER CHARGING TO MATCH ALL CELL VOLTAGES AT NEAR 100% or 100% SoC, OR 90% SoC OR WHATEVER THE MANUFACTURER SET AS MAX CHARGE OR THE CURRENT MAX USER CHARGE SET-POINT IS.
THERE MAY BE DIFFERENCES IN HOW OFTEN THEY BALANCE OR THE ALGORITHM, BUT THEY ARE ALL THE SAME END RESULT WITH SOME SLIGHTLY BETTER RESULTS THEN OTHERS.*

The only people who bottom balance are:
1. People who buy crap chinese cells for their EV's and pro-port like they are as good as any OEM cell from Japan, Korea or USA.
2. People on DIY electric car who read what other people on DiY electric car who watch JACK RICKARD shows - a guy who pro-ports to be an expert but he is very far from it.
3. People who use NO BMS whatsoever, don't keep a very balanced pack, don;t charge above 90% SoC, and they want to be able to run their pack down hard and not worry about totally damaging a cell when it drops to 0V or negative voltage.

Note that none of the following are on the above list:
*FORD
PANASONIC
MAKITA
MILWAUKEE
A123 SYSTEMS
GS YUASA
N.A.S.A.
SPACEX
CHEVROLET
BOEING
LOCKHEED MARTIN
LG
SAMSUNG
TESLA MOTORS
NISSAN
DEWALT
APPLE
MICROSOFT
BMW
FERRARI
VOLKSWAGEN
MERCEDES*
....on and on and on....


I am sorry for the yelling, but yes, it really is yelling because I am tired of reading this stuff.


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## GoElectric (Nov 15, 2015)

Hi John,

Wow. Well, I just wish I UNDERSTOOD why top-balancing is so good! Somebody somewhere must have written a paper on it or something. It must be something to do with drifting capacities; something I can't imagine.

Maybe I will change my mind. 

Sorry if this irritates you - not my intention.

jim


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## Karter2 (Nov 17, 2011)

GoElectric said:


> .....I just wish I UNDERSTOOD why top-balancing is so good! ...


We are not all Einsteins ! Not everyone can understand the details of all things technical.
Sometimes we just have to take a lead from those who specialise in certain areas and have done the hard yards to understand those things we dont. ( EG.. Weather forecasts, Stock market,..... top balancing , ?....etc !)
There is always a risk you get bad advice ( EG ... Weather forcasts, Stock market, etc ! ) ..
.......but when there is a huge concensus of sources all in agreement on a subject, ( re, okashira's post ),... then i for one am comfortable to follow their lead.


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## Sunking (Aug 10, 2009)

GoElectric said:


> Wow. Well, I just wish I UNDERSTOOD why top-balancing is so good! Somebody somewhere must have written a paper on it or something. It must be something to do with drifting capacities; something I can't imagine.


I can tell you why manufactures use Top Balancing, not that hard to figure out. 

From a product liability, uninformed public, technical architecture, and feasibility is the only way they can go.

Top Balance technically is very easy, fully automated requiring no knowledge of the user, and defensible. 

Look on the flip side, Bottom Balance. No real good or easy way to Bottom Balance a pack except initially when the cells are received before they installed and placed in service. Once in service they cannot easily be re-balanced. You pretty much have to remove them, place them in parallel and repeat what you did initially before they were installed.

To Bottom Balance the user has to know his chit. Joe Blow public is stupid.

So from a commercial perspective, Top Balance is the only logical choice. Does not mean it is best way for the battery. In aerospace applications, Top Balance and BMS as you know it is not used. That would take up space and add weight. They Middle Balance. Military and Government can afford to pay for thousands of cells, then the engineers balance and match them to 1% or less. Does not matter where they balance them initially. All the cells arrive at full charge and fully discharged at the same time.


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