# BMS and charging, and "EV Works"?



## Rocky_Electric (Feb 1, 2013)

I just can't figure it out. I see a lot of stuff saying you need the BMS, yet nobody sells anything like it. At least not many people do. I don't really even know what to look for with them either. All I care about is being able to charge and discharge 15 or 30 LiFePO4 batteries for a go-kart thing safely. The batteries are expensive, I'd rather not destroy them.

So what do you do? Well these look simple and easy, http://www.ev-power.com.au/-Thundersky-Battery-Balancing-System-.html









They are pretty hard to find. I read through this thread and found a link that had them, 
http://www.diyelectriccar.com/forums/showthread.php?t=669

BMS thing: http://www.evworks.com.au/index.php?product=BMS-CM160-V6



Ok so here's what I am asking I guess. If I buy let's say 15 of these batteries, 








http://www.evworks.com.au/index.php?product=BAT-LFP40AHA

And 15 of those small BMS things that I linked to above, hooked up the 15 batteries in series for 48 volts, could I just plug this charger in without ever worrying about an over charge?








http://www.evworks.com.au/index.php?product=CHG-KP4810EL

As well as never worry about over-discharging them during use?



I hope this all makes sense. I am super excited about all this and am drafting out what my plan is for this "gokart" that will be more like a small custom aerodynamic vehicle <500lbs. Part of the plan is having maintenance free batteries. Simply drive till the batteries are shut off by the BMS if desired, or let the charger be plugged in for 24 hours and not worry about a thing.


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## Ziggythewiz (May 16, 2010)

Rocky_Electric said:


> Simply drive till the batteries are shut off by the BMS


Do you only fill your gas tank after it's empty?


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## Rocky_Electric (Feb 1, 2013)

Ziggythewiz said:


> Do you only fill your gas tank after it's empty?


No, but I guess my point is should I be worried about over running the batteries even with those small BMS's installed to each battery


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## Ziggythewiz (May 16, 2010)

I don't use a BMS, but if you do it should be to protect your batteries if you mess up, you shouldn't rely entirely on it.

With lithium you need an AH counter to know how much juice you've used and therefore how much you have left.


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## Rocky_Electric (Feb 1, 2013)

Oh ok, considering you surely have many more cells than I do, maybe I shouldn't concern myself so much with using a BMS? Afterall, it adds 13 bucks a cell + 50 for the master unit. 

So are these examples of the units to what you are referring to? And I can gauge how low my batteries are from that and stop running the vehicle as they approach the limit of how much juice (amps) they have?

http://www.evolveelectrics.com/E-Xpert Pro.html











http://minibms.mybigcommerce.com/products/EV-Display.html
(image too big)


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## Ziggythewiz (May 16, 2010)

If your cells are well balanced and you use them within the recommended operating range they shouldn't need any other intervention for a long time. If you use some form of half-pack monitoring you'll be able to detect any issues before they're a real problem.

I use JLD404s. You'd need a shunt from there as well, total cost ~ $90. It will let you know how much you've used and also has two programmable relays that can be used to limit or stop the vehicle.


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## Elithion (Oct 6, 2009)

Rocky_Electric said:


> could I just plug this charger in without ever worrying about an over charge?.


No.

A BMS is just a paper weight unless it is allowed to control DIRECTLY charging and discharging.

Installing a BMS and not connecting is to shut off the charger and the load is like installing a burglar alarm and not bothering to connect it to a phone line: it doesn't do you any good if the alarm detects a break-in, as it can't call the police.

So:


Yes, the EV Works BMS is a fine analog BMS
You MUST connect it so that it shuts down the charger whenever the BMS detects that ANY cell is fully charged
You MUST connect it so that it shuts down the load whenever ANY cell is fully discharged
If you aren't able to buy an EV Works BMS, please know that the MiniBMS is electrically the same as the EV Works BMS (that's a story for another day) and is more available. But, if you're in Australia, do not buy the MiniBMS because EV Works has an Australia patent that the MiniBMS allegedly breaks.


Here is a selector of all Li-ion BMS.


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## Rocky_Electric (Feb 1, 2013)

Ziggy, 
It makes more sense now. That panel meter is something that you would have on-board to measure how many amp/hours you have burned through. So long as you balance each cell before hand and don't over discharge/charge your batteries there is no need for a BMS and you can determine this by using the panel meter with a shunt. Without having used the unit and reading that it has a relay and alarm functionality, I assume it can be used in conjunction with a solid state relay to turn off the charger at full charge. Or even to, say , disable the throttle to prevent you from over discharging accidently. That's my understanding of it. 

Elithion,
I noticed you're based in, Boulder. I am too! I didn't know about the miniBMS system. Considering I am in the USA and that it is cheaper, it sounds like a great option. However, I thought the purpose of the BMS was to stop charging full cells and continue charging the other cells until each cell is balanced and full, as well as disable the batteries from dis/charging after certain point


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## Elithion (Oct 6, 2009)

Rocky_Electric said:


> So long as you balance each cell before hand and don't over discharge/charge your batteries there is no need for a BMS


So long as you don't allow thieves in your house there is no need for a burglar alarm. ;-)



Rocky_Electric said:


> I didn't know about the miniBMS system. Considering I am in the USA and that it is cheaper, it sounds like a great option.


Indeed, an analog BMS is great for some users (it's easier to grasp the concept) and for simpler applications. 
Note that an analog BMS knows there's a problem, but doesn't know what or where.

Others prefer a digital BMS, which knows not only what the problem is, but also where and by how much, and reports it.

It's mostly a matter of preferences, as the cost is awash.




Rocky_Electric said:


> I thought the purpose of the BMS was to stop charging full cells and continue charging the other cells until each cell is balanced and full, as well as disable the batteries from dis/charging after certain point


With a single bulk charger, what you describe (bypassing the full charger current around the cells that are full) is indeed possible. That takes balancing loads that can handle the full charger current (those who tell you that that's not the case are forgetting what happens when a pack is badly unbalanced, and the charger starts at full current).

In practice, though, any off-the-shelf BMS shuts down the charger for a bit, as the BMS discharges the most charged cells a bit (with a smaller load), then the BMS restarts the charger. (Except for the MiniBMS, that doesn't restart the charger, until you unplug and plug back in.) Detailed description.


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## Ziggythewiz (May 16, 2010)

Rocky_Electric said:


> I assume it can be used in conjunction with a solid state relay to turn off the charger at full charge. Or even to, say , disable the throttle to prevent you from over discharging accidently. That's my understanding of it.


Yes, one JLD404 has 2 relays with 2 setpoints each, and both NO and NC connections, so you can have the relays stop charging and limit or stop discharging, or just about anything else V, A, or AH related.


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## albo2 (Oct 4, 2011)

The Ex-Pert pro is all you need it has a high voltage and low voltage alarm, you can use the relay to switch the charger off, I use the amp hour counter and rarely go below 80%, I also have a timer on my wall socket I have an app on my phone I punch in the amp hours I have used it calculates the time I need to charge, simple but a completely separate form of redundancy, so I can't possibly over charge
1. The charger has a charge profile.
2. The Expert-pro is set to shut off the charger.
3. The power is shut to the charger after a set time
You really don't need a BMS just bottom balance and keep an eye on how many amp hours you have consumed.
I check the voltage of my pack every morning using the Ex-pert pro it's the same all the time, if a cell fails I will notice it.


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## Elithion (Oct 6, 2009)

albo2 said:


> I can't possibly over charge
> 1. The charger has a charge profile.
> ....


Ah! If it were so simple!
Please read: CCCV chargers: a false sense of security	

A charger with a profile works if and only if the pack is balanced, and only if it is top balanced.

With a bottom balanced pack, a CCCV charger (with our without a profile) will cook the lowest capacity cells.

With an unbalanced pack, a CCCV charger (with our without a profile) will cook the cells with the highest voltage.

With a BMS, a CCCV charger does not need a profile; also, the BMS protects the cells with the highest voltage by shutting down the charger before they are cooked.


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