Why are BMS (battery management) important?

[morph999]

Hi. I'm back. Man, $1700 for a good kit is just a little much. I'd still like to get a good bike kit and I saw a thread about the Li-poly batteries. I was planning on putting 2 of the 22v together to make 44v. My question is: does this need a BMS? I was planning on buying the 22v zippy packs and putting them in series. It'd be $1000 cheaper to do this if I can figure out how to do it.

What's wrong with just hooking the batteries up in series to the controller without a BMS ?

 

[PWJM]

there was post covering this just a day or so ago, but can't for the life of my find it now....

anyways, the gist of what I read was that its not the end of the world to run lipo's without a BMS, but after several deep discharges and charge cycles the batteries become unbalanced and will lose some capacity (range).

correct me if I'm wrong here guys, I'm sure there's more to it than that.

 

[Drunkskunk]

You need a bms, or balancer during charging. on discharge, it's not critical, ... If you have experiance with Lipo already, and you have a propper way to moniter the battery while riding.

If you don't have experiance, or don't have a way to moniter the voltage and amp draw while riding, then get a BMS.

 

[GGoodrum]

What is most important, with all Lithium-based chemistries, is to make sure the cells don't get over-discharged. If the cells in a pack become unbalanced, you very quickly get in a situation where the overall pack voltage is still higher than the LVC set in a controller, for instance, but you could have one, or more, cells that dump very quickly, as the pack gets close to depletion. That will kill cells. The problem is that you may not notice until it is too late. This is especially true for very high C-rated packs, because the pack feels just as strong 30 seconds from the end as it does in the beginning. If the cells are all balanced, they will all dump at once, so you will instantly feel the power drop. If just one, or two cells dump early, you might not notice soon enough.

The safest way to protect the packs is to use a LVC scheme that monitors each cell, and either cuts power, or tells the controller to cut the load, if any cell voltage drops too low. That way it doesn't matter if the cells are "balanced" or not, no cell can get discharged to the point of damage.

On the charging end, if the cells are all equally matched in capacity, internal resistance and thermal characteristics, a simple pack level CC/CV charger/supply can be used. Good quality packs will start out with cells that are very closely matched, but eventually they will drift apart, and end up with slightly different capacities. Unchecked, these differences can grow to the point that it can become a problem. RC-type balancers can bring the cells back to the same level, but most simply drag down the high cells to the level of the lowest. That's not necessarily the best way. The optimum way to charge a pack is to individually charge each cell. What that does is ensures that each cell can get a full charge, at its on pace. Most BMS boards basically work like individual chargers. Shunts let each channel bypass some current, once its cell is full, which lets the "slow" cells catch up. The newer RC balance-chargers basically do the same thing, which is to let each cell get full at its own pace.

-- Gary

 

[morph999]

I need a battery(s) that is capable of 30 amps of continous current (whatever that means) so I'm not sure that these zippy batteries from hobbycity will work.

 

[morph999]

You guys were right. I've scoured the internet and you just can't find a way to power a 5303 crystalyte without spending at least $800 on a battery. I hope one day the prices come down. For now, I've decided on this kit from ebikes.ca

http://ebikes.ca/store/store_dc.php

the Clyte DC brushed motor. The kit is only $280 without battery. Can someone help me find a good cheap battery for it? It needs 30 amps of continous current. I still don't understand how to read these batteries. It's all very frustrating to me. We need a simple website where everything is listed in layman's terms. I need a 48v battery. Thanks. I only need to go maybe 15 - 20 miles total. I live in the city. This will be for 30 amp controller.

I'm sorry I wasted a lot of your time. When it comes to these things, I'm lost.

 

[dogman dan]

You will need more battery per mile with a brushed motor, though they can be more reliable, controllers cost less etc. For ranges above 8 miles of riding I can't say a brushed motor is a good choice. I found out the hard way, thinking just like you are now. Go with a 408 and a 20 amp controller. Then the battery choices get easier. You won't need such high discharge so you can go a lot farther with cheap sla, or better, nicads, or better, 16ah or 20 ah lifepo4 from pingbattery.com If you got a ping 48v 20ah you would get pretty good speed and at least 25 miles of range at full speed. That much extra range is nice for that day when the wind is 40 mph in your face.

It's a dilemma, If you are willing to ride slower, get 4 sla's and use them till you save up money for the lifepo4 battery of your choice. A 408 ridden 15 mph will go a long way on 4 sla's. Or start with a smaller pack of nicad or nimh from ebikes, and then just add more later, as you get the money. A really good ebike is gonna cost $1500- or even $2500 for a really nice setup with the 5304. But if you go for a 408, you could run it at higher voltage later if you get the 24-72v controller. With a brushed motor, overvolting will make a lot of heat, and on longer rides it may go up in smoke. Fine for shorter rides, but brushed gets real hot after about 10 miles. For city riding, a 408 at 48v should be all the power and speed you need, since you will be doing a lot of stopping at lights and stop signs. Where the 5300 series motors get handy is when you are climbing monster hills, hauling loads, or can ride higher speeds on open roads with no stops. In the city, a 30 mph bike is plenty fast since you have to keep stopping every mile or so for lights.

Other good choices for a brushless motor are ampedbikes.com and hightekbikes.com I'm currenty running an aotema motor from hightek, and find it pretty nice. A bit less expensive than crystalite for the whole kit, but good enough for my 30 miles a day commuting.

 

[dogman dan]

Looking again at the ebikes-ca pages, the ezee kit with a nicad pack is a pretty good bargain. The nice thing about a gearmotor is you can do pulse and glide type riding on flat ground. Perfect for city stop and go. Since the motor freewheels, you can use the motor to get to speed, turn off, and pedal some, then coast. Direct drive motors won't coast near as good. 8 ah nicad will go pretty far used in that kind of hypermile mode. Personally I'm more of a floor it till it melts kinda rider. You may be too if you were looking at 5300 motors. The other nice thing about the ezee is the disk brake compatibility, allowing use on bikes that are not cheapies with rim brakes. Too many choices! Not enough money.

 

[Wile E. Coyote]

I agree with dogman, the ezee is a pretty good bargain.

Morph999 seems to be in the same situation I was a couple years ago when I built my first e-bike. I was a noob and did not want to spend much money on something I was not sure would work. I bought a Crystalyte DC motor and wound up regretting it. The DC motor is next to useless on hills. Don't be fooled by wattage ratings, the DC motor turns most of it into heat!

As to the BMS question, I use Dewalt packs without a BMS on discharge (bypassed internal 15A fuse). I don't recommend this unless you are certain you have more than enough battery capacity to do your commute without over-discharging.

 

[morph999]

what do you guys think about this battery

12v 18ah for $30....huge discount but could I use it? Would it give 18 miles of range?

http://www.amazon.com/Universal-SLA-12V-18AH/dp/B001DL7D1O/ref=pd_sbs_misc_10

I just don't want to get run over by the cars. I live in the city so it's a bit different. It's harder to use a bike in the city than in the country. I've lived in the country so I know. Hell, it's harder to walk along the street in the city, too. Cars don't seem to want to move over, even for pedestrians. Someone was hit just 2 miles from me while she walking across the street.

 

[Drunkskunk]

That battery weighs 13.2 pounds each, and 4 of them would give you a 52 pound battery. 18Ah of SLA would only give you 10 to 12 Amp hours or usable power, due to the way SLA work. Almost no bike rack made will handle 52 pounds continualy, so you would need something custom or expensive. SLA last about a year if you're lucky.


If you try to go cheap on the parts, you will regret it. There is a lot of cheap junk out there.


For what you said you want to do, a basic brushless kit with a 48Volt, Nicad battery would be your best bet.