Do I need a Battery Management System?

[frank]

Wanting to know if a bms is needed if I make a pack of (12v 18ah x4) in series to make 48v.

Thinking that the bms is used primarily for the charging, am I wrong in thinking that, reason I ask is that I will be buying a charger to charge the batteries as well, and I was thinking of buying a charger that would do the 48 all at once, and thinking this is where the BMS is used, otherwise I'll charge them in parallel as 12v 18ah, will make 2 wire harnesses 1 for the bike with Anderson connection and 1 separate for the charging.

[Leslie]

Is it SLA? If so maybe not. 

If it is LiFePo4 not really, but you need to set your charge voltage properly and balance occasionally.. 

If it's a  a different lithium pack indeed you do.

If you have spent your money on a SLA pack it wont be lasting you very long. If you haven't yet spent money a 10ah pack will beat an 18ah SLA pack pants down. 

Pretty sure the 48v 10ah lithium pack were getting better range than my 48v 24ah SLAs.  Your only allowed to about 40% discharge for an SLA, any lower it dies fast.

Recommended is 25 to 15% and anything over .5-.8 C double kills it. 

18 ah SLA pack I estimate 2 mths of regular riding and it will start cutting out,  Usually the used SLA batteries are still workable, I use them for my power tools, and added to my solar battery bank  but they slump under heavy load and cut out.  Useless for EV.

http://www.ritarpower.com/products.aspx?catid=19&pcatid  These were the best SLA's that I could work. 

Ritar EV series, lasted 6 to 8mths, costed me $400AU 4x12v, and max range I would push was 24km occasionally.  A good LI pack for $100 more will last you 2+ years and get you anywhere between 35 to 70km range depending on how much you pedal how good a deal you get, and how hard you ride it.

My 48v 15 ah Lifepo4 pack weight approx 9kg.  Is almost 2yo, an amazing range 40-80km range, doesn't have a BMS and still runs like it's new..

My 2 SLA 24ah, weighed 30kg died in 6 mth the other 8 mths had a 24km range consider its it life span and continued to wreck the tires and bike stands throttles due to added weight and it falling over occasionally, more brake pads, bad connection poles, one connection shakes loose lose once and that battery is fuddled..  Became a money hole and constant choir to keep it running.

[frank]

Thanks Les.

Yeah was going to go the cheaper sla, was thinking that they wouldn't last as long or keep a charge up for a long haul, have had some experience with solar and know from those batteries what the charge is like under load, sla need to be constantly on a charge just to keep up to that peak performance.

The added disadvantage with sla is the wieght , but was thinking that if I did make up an sla pack, would be better getting a battery that has a higher AH, 18 to 20, then at least if they give me 10AH or anything near that would be a sight better than my 24v 10AH, and all of this would only be good for my daily around town errands.

I live on a second floor and have to carry my bike up and down, so weight is a factor and will need a detachable pack or back pack, plus my whole reason for getting another form of battery pack is for a long hilly country ride well over 100kms, from Lismore nsw to near out past Casino half way to Tenterfield, well over 100kms to our property, that's got a solar system to charge from, for the return, think I'll need more than 4 batteries for that kind of kms, lol.

If BMS isn't too much of an issue.
My thinking was to get 4x12v batteries and give it a go, can make them into a 48v or 36v packs, 3x12v for a 36v, for a town bike.

But by what you have said made me think, its not going to matter how many batteries I have to make up how much volts to the AH, when they all get to that point of less charge that will be all she wrote, because sla has a limitation of needing a peak performance percentage.

If anything can always make them into a ups for my computer.

Nah bugger it, if I'm going to cash out on a battery I'm better off getting a LiFePO4 unit, in the long run less fuggering around.

Thanks mate.

[Cornelius]

With 18Ah SLA's, do not expect more than 15-20km ride time before LVD cutout. (Low Voltage Disconnect)
The problem with lead acid are that the voltage drop are too great when loaded with larger currents; They're not designed for greater load than C/10 before the voltage drops significantly...

That said, I myself started with 18Ah SLA's (x3 for 36V)... Great fun to get started with the GM motor, but I soon invested in a Lithium battery... 21kg's and riding distance of 15 km was getting tiresome...

Edit:
You do not need a bms for SLA's; just charge them up properly for LEad acid; 14.4V x4 in your case...