Upgrading from 36V to 48V

[lava]

I have an Amped Bikes 500-750W direct drive hub motor and 36V/48V compatible controller, running at 36V. I want to upgrade to 48V, and just wanted to do a sanity check before I spend any money. I did a search and found a thread on where to buy batteries:
http://endless-sphere.com/forums/viewtopic.php?f=3&t=24362&p=352769&hilit=cheapest#p352769

If anybody has anything to add to that, I'd appreciate it.

Also in my searches I saw some threads about people needing to change their controller settings to get better performance when changing battery voltages. Is this something I need to do? I am not sure my controller even has that capability.

 

[hardym]

It should work. Yes, your controller should have some adjustment to change between 36V and 48V. This may be an internal strap or an external set of wires that needs to be connected or disconnected. The straps/connectors may change both the low voltage cutoff (lvc) and change the internals of the power supply. You may need to find someone to crack open your controller to identify how to make the change.
Mark.

 

[lava]

I see this pack on the BMS website:
http://www.bmsbattery.com/48v/496-48v-10ah-lithium-ion-electric-bicycle-battery-pack.html
Says it has a 15A continuous discharge current.

On gocarlite.com, the same battery:
http://gocarlite.com/Electricbikekitproducts.php?view=productPage&product=18&category=
But that one says it's enhanced for 20A continuous discharge current.

Doug at gocarlite has been extremely helpful and provided a lot of good advice and support ever since I got my kit from him. He said that if I bump up to a 48V battery, the motor would use about 1100W. Doing the calculation, that's just under 20A of current draw. So it appears that I'd need a battery with a 20A discharge capability.

So, here's where I'm confused: I'm pretty sure the current limitation is not inherent to the cells themselves, so what's in there that's imposing it? Is it part of some protection circuit? And, how can I modify the first battery I listed to increase the ampacity?

 

[dnmun]

this battery is less than $500 for a 48V20Ah pack delivered. the ad says the sale lasts for another hour:

http://www.ebay.com/itm/LiFePO4-48V-20AH-Battery-5A-Charger-BMS-Rechargeable-Scooter-Sea-7-8Week-Outdoor-/140793235954?pt=US_Rechargeable_Batteries&hash=item20c7ee49f2

rather than use an inadequate battery, maybe get one big enuff to do the job, seems like a better deal than what you were looking at for half the power, but not sure how long it takes for you to jump on it.

 

[lava]

Thanks, but I don't need/want a battery with more than 10Ah capacity, but I do want to have 20A discharge current capability. Granted, the battery in the link you posted can discharge at 45 Amps, but at 20Ah capacity it's way more than I need and would be a significant weight penalty.

In the links I posted above, I am still not entirely sure where the 15A and 20A discharge limit comes from as I'm pretty sure the individual cells can do way more than that. Is there some kind of battery management board in there limiting the discharge current?

 

[neptronix]

Thanks, but I don't need/want a battery with more than 10Ah capacity, but I do want to have 20A discharge current capability. Granted, the battery in the link you posted can discharge at 45 Amps, but at 20Ah capacity it's way more than I need and would be a significant weight penalty.

In the links I posted above, I am still not entirely sure where the 15A and 20A discharge limit comes from as I'm pretty sure the individual cells can do way more than that. Is there some kind of battery management board in there limiting the discharge current?

Then a lifepo4 is not for you; sure, a 10AH pack can output 2C ( 10AH x 2C = 20A ), but it will give you voltage sag and make tons of heat doing it. It won't last long either under such abuse.

These lifepo4 packs are overrated about oh, 95% of the time. Never run a battery at it's continuous maximum the entire time.

For lifepo4, you would want a 15AH at a minimum. or you could alternately get some A123 packs from cell_man, which have very high output and good safety. The other option for high power / small size / low cost is RC Lipo, which is a whole 'nother bag to discuss as they aren't safe if mistreated.

Be warned that 10AH is not a lot of capacity though anyway, especially on a motor that will hit upper 20mph figures on 48v. That will gobble the power down at the higher speed.

 

[dogman dan]

If you want higher power with less weight, then you need to look for cells that can do that. A123's for lifepo4 for example. Or go to the RC lipo for crazy high discharge rates. Then you can carry only 5 ah if it's enough range for you.

2c lifepo4 would work fine for awhile, but later on as the pack ages the cells will become unable to handle 2c anymore. At that point, that's when the 15 ah pack starts to shine. Once you put a 10 ah pack able to only do 1.5c on a 20 amp controller, it's doomed. But the 15 ah size pack still has 23 amps in it at 1.5c.

So what we are saying is that with the least expensive 2c cells, it IS inherent in the cells themselves that there is a limit. 10 ah of 2c is being pushed to its limit at 20 amps. Batteries pushed to thier limit every cycle die early.

 

[lava]

Thanks for the info guys! What is different about the A123 cells that allows a higher discharge rate?
Are A123s more expensive than standard LiFePO4 cells? EDIT: just checked out cell_man's website; looks fairly pricey. Would you say it's worth the extra cost? Are the performance and cycle life that much better?

 

[Tommy L]

Thanks for the info guys! What is different about the A123 cells that allows a higher discharge rate?
Are A123s more expensive than standard LiFePO4 cells? EDIT: just checked out cell_man's website; looks fairly pricey. Would you say it's worth the extra cost? Are the performance and cycle life that much better?

I've been testing LiFePO4 cells for over two years now. You get what you pay for. Sizing is very important.
You must know what you will be asking the pack to do and size it either by adding more amp/hrs or having higher "C" rate cells.

I've currently used Headway 38120HP Red Cells with success and I also use A123 also with success.

Read and learn as much as possible and it will save you money in the long run.

My commute is only 10Km/6Miles, but I bring 0.8KW of energy with me. Enough for to and from work at only 50% use.
I also bring my charger to work and charge with only 25% energy used from my pack. If I can't charge then I've used 50% of my
pack on the return trip.

This is how I like to size my pack. I avoid deep discharges, and I balance as often as needed. Yet another topic.... "BALANCING"
and knowing exactly what that means and know each cell.

I have Headway and A123 for sale on my website: www.rawvelocity.com

If you maintain and treat your cells correctly, they will apparently last 5 to 10 years if you keep the DOD (depth of discharge) on the low side.
In saying this, they are not expensive at all.

Tommy L sends..... \\m//

 

[lava]

Thanks for the heads-up, Tommy. I also am only doing half-discharges on my commute, as I can plug in at work.

I looked through the stickied schematics thread but it was kind of overwhelmingly long. I am now considering building a 48V A123 LiFePO4 pack; can someone direct me to a schematic of a typical arrangement with BMS? I'm an electrical engineer so I am pretty sure I could pull it off without much trouble, but I would like to see a schematic to really get a sense of the cost savings vs time investment.