NIMH 36V 10Ah pack questions

Hi there this is actually my first post on this website. I am planning on building a NiMH pack out of 30 size D cells with a 10ah each cell. Now I know you can spot weld them all and create a 36V array and just use one plug but how does it works to balance out the cells. Do NiMH cells need to balance out? ALso some people get away with just building them in 3 separate 12V packs and using 3 individual chargers.

On a side note anybody tried the ebay cheap chinese size D NiMH batteries, I know where to get the tenergy ones but I was wondering if anybody has tried the ebay ones?

NiMH autobalances as it charges, which is why it gets hot. I strongly suggest you do a bit of reading about NiMH before you go down this path, as not knowing that already means you may not have all the info you need to do this safely.

As for capacities of various NiMH cells, they are all overrated. Only ones I know by brand name that are any good are SAFT, I think it is. F-cells. Typically 13Ah and should actually get you that at high-C-rates.

I doubt you'd get even half what the average D-cell advertises, at ebike C-rates.

In NiMHs, balancing basically occurs at the end of the charge when the cells that are not yet fully charged catch up to the cells that are. The charged cells overcharge but this is relatively harmless in HiMH cells so long as the charge rate is low. Balancing can be an occasional task. It is best to balance at, say, C/10 perhaps every 10 to 30 cycles. To save time, you can terminate a fast charge early (i.e. don't wait for delta-peak termination) and change to C/10. The first charge is best at C/10 or C/15 but much of this depends on the cell construction. YMMV.

The best D cells available can do 8ah ( 8000 mah ) , i've had a few packs of " Nexcell " brand cells that worked ok on a 20 amp controller but voltage sag is always significant, a 36v 10ah ping with BMS and charger would be a better buy if you have the right ebike motor/controller for it..

Do not waste your money on ebay nimh " Tenergy " cells... you are throwing your money away and wasting your time....

I second tenergy is bad. I went with their "2900mah" AA cells from all-battery before (not ebay), and I was registering 2200-2400 mah on them, if I recall. I'm sorry if you already started with this plan, but if you can still back out of it, I [although a noob] think you should use a different cell.

Ok so NIMH isn't the best choice for building a 36V pack but at $100 out the door for 30 D cells wouldn't this be a better option then using three 10a SLA batteries? Please let me know as I too am planning on building a pack for my ebike.

it's not that nimh canot work, it's just that all nimh cells are not created equal.

see :
http://ypedal.com/Chaos2.htm

The cells above got the job done but just barely..


A 10ah pack is quick and easy to build, a 10ah pack of D cell nimh ( assuming you can source proper cells, and the tenergy cells are not it .. ) you still need to have spot welded tabs and a proper charger... a bunch of cells with springs will not work out, it has been tried before and is by far not the best option....

Pay a little more and get something that works imo..

you have gotten some very good info here. Let me put it a different way;

NIMH has a higher internal resistance as it gets hot, discharged and at high currents.

To avoid this, use voltage rather than current, to get your wattage.

If you go 72v nimh at an 8 am max discharge rate from fairly decent set of cells, you could get about 600 watts from reasonably priced cells. then charge with two 36v chargers.

But if you try for 600 watts from any but the best cells at 48v, you will get into the high resistance area, making for almost thermal runaway.

So voltage is the key to success for NIMH. But that high voltage on some motors tempts one to go really fast, then again going for higher discharge at higher speeds. but if you can resist that speed temptation, or current limit your pack, higher voltage NIMH can be a fairly good value for the folks with reasonable expectations.

d

I appreciate all the info, I'm on a budget and all I want is something that's a little lighter and will out perform the 10ah SLA pack I'm using now. I would love to just buy a ping lifepo4 36V 20ah pack and be done with it but I don't have $400 to waste on that. All I need to know for right now is if a 36V NIMH pack will give me better performance (longer run time, few more amps since with SLA batteries u get maybe half the rated amp/hours at high discharge rates). If I will get this from the NIMH pack then spending $100 is worth the upgrade, if not I will just keep using my SLA pack.


Thanks!

exactly what motor/controller/bike would this be going into ?

SLA can put out higher amps, it's nothing for a 10ah sla brick to output 100 amps for a few seconds, but nimh would not even come close without melting into a pile of goo ( if you are lucky ) ..

Is it possible for an 8ah pack of nimh to deliver more energy than a 10ah SLA ? yes ( capacity.. 10ah sla gives aprox 5ah usable... 8ah nimh is good for up to 7ah if used properly )

Is it possible for an 8ah pack of nimh made from the wrong cells to deliver more energy than a 10ah SLA.. no

Will the performance( acceleration from a dead stop ) of nimh pack be better than SLA.. not likely...

Will the nimh weigh less ? .. yes

Will nimh be smaller ? yes

all that is null and void until we know exactly how you plan to use this pack.

If you use a 250w kit, and weigh 150 lbs or less and pedal full time using the motor for ligh duty assist.. get the nimh...

if you want a 500w kit and weigh more than 150 lbs, have hills to climb and are not a big fan of the pedals and sweating while you ride.... stick to the SLA..

tmxl69 said:

Ok so NIMH isn't the best choice for building a 36V pack but at $100 out the door for 30 D cells wouldn't this be a better option then using three 10a SLA batteries?

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That would depend on *which* D cells and which SLA. There are some great SLA out there and some really crappy ones, and the same goes for NiMH, with double on the crappy.

With the NiMH, the catch is that many (probably most) of teh cells out there regardless of physical size are relatively high internal resistance, so when you draw ebike-level currents out of them, then:
A) they get hot faster, with a lot of voltage sag, which both prevent getting good performance from them
and
B) they don't give you nearly the rated capacity, so a 10Ah cell might only give you half that, or some other significantly-less-than-rated amount (because the rating is at a much lower current draw, like maybe 1/10 capacity (C), or less).

If they're really cheaply priced D cells, it's a fair bet that they're in that class, and not going to give you what you expect. Use them in a flashlight or something, which is probably what they made them for, and they'll likely work great. Maybe even in cheap power tools, though not as well or for as long.

I can't remember for sure the name of the brand of cells in the packs I have from Deardancer and Ianmcnally, but I think it is Sanforce, and another good one is Saft (not sure if you'll find the Saft for sale anywhere, though). Those cells are good, and perform well at high C-rates. Anything else I couldn't vouch for.


Some SLA are great, too, like the ones made for powerchairs and the like (as opposed to the typical UPS SLA). As Ypedal says, they can often put out huge currents for a very short time, like during motor startup, which NiMH D cells sure won't do in my experience. 17Ah and 20Ah SLA I had on CrazyBike2 were able to supply 130A+ long enough to taco my chainrings, rip apart the chain, destroy the rear rim and bend the rear axle as it pulled it out of the dropouts before the controller exploded, when I had a derailment of the chain on a powerchair motor drivetrain experiment. :lol: The Nimh D cells would likely have been unable to even bend the chainring; the F cells might've managed that but little else.

oh, and forgot to add....

you have zero.. well.. ok.. maybe a 5% optimistic chance of buying 30 cells that will arrive all in working and fairly matched condition.....

You will have to buy extras to filter out the runts, how much extra depends on how old and how " good " cells you can get your hands on.. they have to be capacity matched and internal resistance matched to have a proper working pack.. no small feat.

expect tp spend hours testing, cycling, assembling a pack, that if done with crappy cells will all be a huge waste of time !

Well maybe I should just stick with SLA for now then. The kit I installed was one from ebay (came from China). It is a gearless 500W front wheel hub motor kit, came with everything except the batteries. I believe the controller is 15-20a.

The NIMH batteries I was wondering about I also found on ebay, they are from Hong Kong. Here is what the item description says about them:


Terminals: w/ solder tabs
Voltage: 1.2v
Chemistry: NiMh
Size:6cmx3.1cm
Amperage: 10000mAh

High quality D size 10,000 mAh high capacity nickel hydride metal (Ni-MH)rechargeable batteries with tabs for easy soldering
Ultra long running time with this 10,000 mAh high capacity
30Amps Continuous High Drain Capability (3C High Discharge Rate Means Lower Impedance)


So there's probably no way I will see a 30amp 3C continuous discharge rate on these?

give us a link to the cells.. one of us may recognize them.... but it's not likely that it will actually do 3C... plenty.. PLENTY>. of false specs on ebay..

Here is the link to the cells on ebay:


http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180646221563&ssPageName=STRK:MEWAX:IT

very hard to tell from the photoshopped picture they posted, but the spot welded tab looks pretty small, if.. big " if " the cell was capable of 30 amps, it would likely melt the tab.. sorry to sound so negative here but i canot stress enough how much a waste of time and money this would be.... if you have money to burn and time on your hands i assure you we would all love to see the results of an attemps and i'd be tickled pink to be proven wrong about this.... buuuuuuut.... i strongly encourage you not to do it.

These for example :
http://www.nexcellglobal.com/EbikeBatteries.htm

are proven cells, they do work, but notice the small print
( and this page is OLD.. may not even be relevant anymore )

quote "
a. 0.1CmA charge 16Hrs then 0.2CmA discharge to 1.0V.

b. The final specification is subject to our sales confirmation. "

to get the spec capacity they are discharging at 0.2c .. so that's a really slow discharge and nowhere near ebike discharge rates of 1C to 2C that you need..

I emailed the seller and he said that "botton" top cells have a lower discharge rate and would not be good for an ebike setup. He said that the cells with the solder tabs are flat-top cells and are capable of higher discharge rates of 3C.

I'm going to take your advice anyway and not attempt this as I'm sure you have more knowledge and experience with batteries then the seller does.

Thanks!

As I pointed out I posted this thread to gather info. Now that the info is here I might not buy a set at all. I ordered two 5ah 5S packs from hobbyking to test out in series for my bike. I also ordered a charger total it was 150$ but still I am gonna fit this on a very light aluminum bike with front chromoly forks a 250Watt front geared kit from bmsbattery. For now my commute is about 3 miles each way so I think that if I assist it all the time I might get away with it. ALso I live in florida and my commute is pretty much flat.

Migueralliart, let me know how those packs you ordered for your ebike work out. Thanks!

tmxl69 said:

Migueralliart, let me know how those packs you ordered for your ebike work out. Thanks!

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I got the Lipo batteries for my mountain bike. I am now building a commuter bike like I said it is a lightweight frame with a chro-moly front fork no shocks. Ordered a 24V front geared kit. About the NIMH I ended up ordering from a reputable US seller so I got 50A rated NIMH cells and once I get em here I am gonna take em to battery+ a friend of mine works there and is going to do the spot welding himself for 20 cells , already got the shrink tape and 10 gauge for the conectors. So in the end its a 50A rated pack 10Ah 24V which is more than enough for my FLAT 3 mile commute.

Here's the link to the batteries; http://www.batteryspace.com/nimhrechargeablecelld-size12v10000mahflattop50arate1pc.aspx