Building Nimh Packs, Thermistors, and charging.

I need advice from you guru's out there...

I am building a new 72v nimh pack consisting of (60) Tenergy Sub-C 3.5ah cells. I want to use a 36v nimh charger so I would like to split it into two separate subpacks.

1) What brand/model/specification of thermistors should I use and where can I order them? (not ebikes.ca, Justin doesn't have stock).
2) If I break it into (2) 36v packs for charging, can I charge both subpacks in parallel from the same charger? I figure the stronger pack will get lightly overcharged, but I am OK with that.

Thanks,
JD

The thermistor might be available from DigiKey or Batteryspace, but it would be helpful to know which type, since they are not all the same.

I wouldn't recommend charging Nimh strings in parallel, but I've heard of someone doing it successfully. The problem is when one string gets close to full charge, the voltage drops, which will cause that string to hog all the current from the other one. At the very least, you should install diodes on each string to prevent charge transfer, but you will still tend to charge one string more than the other one. With really long strings like you're talking about, I think the problem would be less severe, since statistically cells in both strings would tend to reach full charge at about the same time.

It would be best to charge one string at a time, or use two separate chargers. If the string is all series, you wouldn't have to disconnect it to charge half the string at a time. Just install a tap at the midpoint.

3.5 ah is pretty small, so expect quite a bit of voltage sag under load.

If a 72V 3.5ah pack is what you really want, definitely charge the strings in series for the reason Fechter explained. Others have had issues with unmatched cells in long strings leading to severe cell imbalance over time -- 6, 10-cell strings charged by 1-6 12V NiMH chargers will better limit the imbalance. Fully matched and tested NiMH packs like sold by some vendors don't have as much problem with this, but occasionally lose cells to internal shorts too.

Another issue is with sub-C longevity. The RC people at http://www.rcgroups.com regularly pull 30-40 amps from NiMH sub-C's for a few minutes at a stretch and report that by doing so they only get a couple dozen good cycles.

I run two packs of NiMH and ocassionally charge them in paralell. I put the thermosister in the lower AH pack so it shuts down when that pack is charged. I would like to add a diode, any recommendations for a bullet proof diode for 36v?

xyster said:

Another issue is with sub-C longevity. The RC people at http://www.rcgroups.com regularly pull 30-40 amps from NiMH sub-C's for a few minutes at a stretch and report that by doing so they only get a couple dozen good cycles.

Click to expand...

Thanks xyster!

That is unfortunate - do you know if those were high-current subCs? I went to do a search but it required me to register.

For this pack I am using the tenergy 3500mah sub C's which they rate at 10c, and running 3 parallel strings for 10.5ah (rated), figuring I would be at a "mere" 4c for 40a. This is not as damaging as running a single string at 10-12c, but if they only get a dozen cycles at a that rated max capacity I imagine even 4c will shorten pack life. Sigh. I have been only getting 2750mah out of the '3500mah' cells at 1c on my CBA II, which is somewhat disappointing. However, I have only run 3 cycles on them, and haven't been leaving them on for the 'trickle charge' so I may be able to get them up to 80% of their rated capacity.

How about the tenergy '4200' 4/3A cells? Have you read anything on them?

Can anyone recommend a 60v charger? I decided to back my pack down to 60v.

-JD

fechter said:

The thermistor might be available from DigiKey or Batteryspace, but it would be helpful to know which type, since they are not all the same.

Click to expand...

Thanks fetcher!

How would I know 'what type'? This pack will be (3) 60v strings of tenergy 3500mah sub-c cells rated at 10c. If I can't find a 60v nimh charger, I'll break each string into 36/24 volt packs for charging.

-JD

oatnet said:

xyster said:

Another issue is with sub-C longevity. The RC people at http://www.rcgroups.com regularly pull 30-40 amps from NiMH sub-C's for a few minutes at a stretch and report that by doing so they only get a couple dozen good cycles.

Click to expand...

Thanks xyster!

That is unfortunate - do you know if those were high-current subCs? I went to do a search but it required me to register.
....

Click to expand...

I use google to search groups requiring registration to search. Usually their pages remain open to indexing. Use " site:rcgroups.com" after your search terms.

And yes -- all the high-rate sub-C's have been faulted for low cycle life at rated discharge current. I think it's probably the nature of the beast with NiMH in general losing life as a function of current-produced heat. Many posts like this:

http://www.rcgroups.com/forums/showthread.php?s=&postid=553637#post553637
<blockquote>
3. All Sub C NiMH cells have a limited life span. No matter how you take care of them you will gradually loose ~100 sec of run time (measured at 20 amps) after 30-50 high amp cycles. It’s not the amperage per se that kills the run time it’s the heat. The hotter you get the cells the more chemicals get cooked off. As the chemicals get cooked off you lose run time, gain internal resistance and actually gain a bit of voltage.

My racers get new, just matched packs for every national event they go to. After the race these are used a good regional and club race packs for ~3 more months (one 2 day race per month). After that we can’t make run time with them in the modified class.

In R/C cars we average 30amps during a 5 minute race and generally go back to the pits with 30-50 sec of power left in the pack. After 30 cycles (races) we dump before the end of a 5 minute race. I imagine running the packs at anything over 30 amps will make the lifespan even shorter.

The plus side is that even after loosing time you are still ahead of NiCd 2400's on run time and voltage (if you bought good matched packs).
</blockquote>

It's not clear from the quoted text if they are recharging the subC immediately after while still hot, which does kill nimh cells right quick.

oatnet, remember that the C rating is for 0.2C current draw for nimh or 700 ma in your case.
Capacity at a higher discharge rates will be less because of Peukert.
However the capacity will increase somewhat after a dozen charge cycles.
I take it these are new cells, so don't give up yet.
Just like a box of cereal, some settling of contents may occur when nimh have been sitting unused for a time & it takes a few cycles to churn up the chemicals.

I plan on doing the same, using 3 strings of subC because this form factor has the highest specific energy of all the nimh sizes. I figure dividing the load by three should reduce the heat generated in each cell to an acceptable level & give satisfactory cycle life, but won't know till I try. Plus I'm only running a 500W motor on 36V.

It's not clear from the quoted text if they are recharging the subC immediately after while still hot, which does kill nimh cells right quick.

Click to expand...

If you read some of the many other posts over there on this subject, it's clear that the heat simply from drawing upwards of 10C out of nickel batteries is the culprit. Numerous posters report less than 50 good cycles from the best nickel batts at that rate. Paralleling 3 strings to bring current down to around 3C as you're planning may solve the problem.

I have just bought a thermisitor from batteryspace.com. It works.

Does it have a part number?

Toorbough ULL-Zeveigh said:

I plan on doing the same, using 3 strings of subC because this form factor has the highest specific energy of all the nimh sizes. I figure dividing the load by three should reduce the heat generated in each cell to an acceptable level & give satisfactory cycle life, but won't know till I try. Plus I'm only running a 500W motor on 36V.

Click to expand...

Thanks TUZ!

Whose SubC's are you going to use? Have you looked at Tenergy's 4200mah AF which has more mah/g, and more mah/$ than their other SubC's? It doesn't seem to be in stock though and I wasn't ready to wait. They also list a 3800mah SubC, but it costs significantly more than the 3500 SubC, the extra 300mah isn't worth the expense to me.

-JD

xyster said:

It's not clear from the quoted text if they are recharging the subC immediately after while still hot, which does kill nimh cells right quick.

Click to expand...

If you read some of the many other posts over there on this subject, it's clear that the heat simply from drawing upwards of 10C out of nickel batteries is the culprit. Numerous posters report less than 50 good cycles from the best nickel batts at that rate. Paralleling 3 strings to bring current down to around 3C as you're planning may solve the problem.

Click to expand...

Thanks (whew!). I hope 3-4 ends up being OK - I'll definitely keep a watch on the temperatures. Hmm. Can I install a thermistor on the drain wires just to make sure?

BTW, I followed your advice and bought a few sets of Maxxis Hookworm tires. No kevlar belt yet, but I'll get there. I definitely like the tires, and I think they will be even better when I get them on wider rims. These are the new 1-ply version - are yours the original 2 ply? I'd be interested to take them up to 110lbs, but I don't know that my 1 ply version will hold it. On the plus side, everything I read said that these were tough to get on the rims and I was looking for 'Tioga bead wax' from some random post - but the 1-ply version slipped right on.

-JD

oatnet said:

Thanks (whew!). I hope 3-4 ends up being OK - I'll definitely keep a watch on the temperatures. Hmm. Can I install a thermistor on the drain wires just to make sure?

Click to expand...

I think the thermistor has to be wired to some sort of cut-off or monitoring circuit so as the resistance increases with increased temperature, the circuit is either opened, or the rider is alerted. The circuit gizmo gurus here would know a lot more...

BTW, I followed your advice and bought a few sets of Maxxis Hookworm tires. No kevlar belt yet, but I'll get there. I definitely like the tires, and I think they will be even better when I get them on wider rims. These are the new 1-ply version - are yours the original 2 ply? I'd be interested to take them up to 110lbs, but I don't know that my 1 ply version will hold it. On the plus side, everything I read said that these were tough to get on the rims and I was looking for 'Tioga bead wax' from some random post - but the 1-ply version slipped right on.

-JD

Click to expand...

Hey, great JD. I didn't have much prob getting mine on either. 110 lbs psi sounds way too high for such a wide tire. My Hookworms are rated to 65psi, and I run them about 50. I don't know if they're two-ply or not. I bought them new about 9 months ago. After neighborhood garbage-day rides I often extract numerous shards of glass, nails and such. One 2" nail poked all the through the tire, but instead of puncturing the tube, the nail just bent. I rode around on it like that for at least ten miles. No flat! Love those kevlar belts. I understand from my research that a couple of the hard, plasticized kevlar belt brands actually cause more punctures then they prevent. I'd stick to the fuzzy yellow panaracer belts, or something similar:
http://www.universalcycles.com/shopping/product_details.php?id=11244&category=

I've got these on all three of my EV's. I also use thick DH racing tubes.
http://www.universalcycles.com/shopping/index.php?category=599

Can you remind me what you're riding/building EV wise? Usually I remember people's hobbies and rides better than their names...

Let's see...there's that old guy who works at a San Francisco hospital. He's into blowing things up; has the biggest capacitor I've ever seen; rides a Vego with a modded 600W BMC BLDC motor....forgot his name though...

The link to the Thermister etc is http://www.batteryspace.com/index.asp?PageAction=VIEWCATS&Category=868 -

If anyone is in Canada ordering from there try to keep your orders under $20 because they use UPS standard and they will charge you huge brokerage fees ($16 +taxes approx) even on a $21.00 order.

The link to the Thermister etc is http://www.batteryspace.com/index.asp?PageAction=VIEWCATS&Category=868 -

If anyone is in Canada ordering from there try to keep your orders under $20 because they use UPS standard and they will charge you huge brokerage fees ($16 +taxes approx) even on a $21.00 order.

That's cool -- the cut-off circuit appears to be built into the thermistor unit.

oatnet said:

Thanks TUZ!

Whose SubC's are you going to use? Have you looked at Tenergy's 4200mah AF which has more mah/g, and more mah/$ than their other SubC's? It doesn't seem to be in stock though and I wasn't ready to wait. They also list a 3800mah SubC, but it costs significantly more than the 3500 SubC, the extra 300mah isn't worth the expense to me.

-JD

Click to expand...

Powerizer 4200 from Batteryspace.
They have 4500 mah but are almost twice the price & also for only an extra 300 mah.

I should have ordered as little as a month ago, my cost for a hundred cells has gone up almost seventy bucks. But with the Canadian dollar heading towards parity with the greenback if I wait a month or two it should all work out even.

I see that All-battery now carry the 4500 mah as well which wasn't the case just a month ago when I was shopping around, & at much better price than Batteryspace. The appeal of the 4500's isn't so much the slightly higher capacity but that they should be able to sustain the 40 Amp peak drain rate better.

They have 4500 mah that they claim have a higher specific energy than A123's M1

Click to expand...

Let's compare to the emoli, which has a higher energy density than the A123.... Taking their ratings at face value and considering "energy density" to mean both the more common (in EV land) 'energy per unit weight' or the more correct 'energy per unit volume' :

http://www.m2energysolutions.com/products2.html

emoli:
weight: 100 grams
max rated discharge: 45 amps
nominal voltage: 3.8v
capacity: 2.9ah
volume: 37.2 cubic centimeters (26mm X 70mm)
watt-hours: 11
energy density:
110 watt-hours per kilogram
or
0.30 watt-hours per cubic centimeter


http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=1827

Tenergy 4500 sub-C:
weight: 59 grams
max rated discharge: 45 amps
nominal voltage: 1.2v
capacity: 4.5ah
volume: 17.9 cubic centimeters (23mm X 43mm)
watt-hours: 5.4
energy density:
91 watt-hours per kilogram
or
0.30 watt-hours per cubic centimeter


So it's a tie by volume and the emoli wins by weight. Interesting! Probably this NiMH Sub-C does beat the A123 for energy density (I'll leave that to someone else to figure if they want).

You're too fast for me.
I took that out cuz I didn't feel like starting an argument.

I took that out cuz I didn't feel like starting an argument.

Click to expand...

Why the heck not?

xyster said:

I took that out cuz I didn't feel like starting an argument.

Click to expand...

Why the heck not?

Click to expand...

I'm a little leery that the specs are accurate until I get some to verify their claims.

Used the following source in Canada to build a 36V 9AH "D" cell NiMh pack:

The thermal cutoff switch is
"Digikey part number: 317-1083-ND"

thermistor: 10K NTC Thermistor for NiMH / NiCd Packs and Chargers.

Charger end I used to match the Nexcell charger:
NEUTRIK. 3PIN FEM. XLR CABLE CONNECTOR NC3FX
Here's the pinout info for it as supplied by Zev:
"The charging pins are:

1: v+
2: gnd
3: 10k thermistor.

If the resistance on pin 3 is too low, the charger will not start.
It's possible to spoof the thermistor with a 10k resistor. If you do
that, there will be no temperature monitoring, just negative voltage
rollback and absolute time cutoffs. You can get away with it for both
NiMH and NiCad, but for safety it's not recommended."

They also have the ANderson Power Pole connectors you may need.

Supplier:
BNE Electronics
444 Manitou Road, Calgary, AB
B&E Electronics Ltd., Edge Member
Ph (403) 243 7211 x13, Fax (403) 287 1664

Email: bjohnson@be-electronics.com
Internet: http://www.be-electronics.com

Not sure if I understand, you plan to add 2 cells to a 24V pack in series and make it 26.4V?

My 36V Nexcell charger has a fixed output rating of 36V, so although it's output voltage is higher in order to charge, it probably wouldn't do a very good job if I increased the voltage of the pack.

However someone with more experience may know your particular charger, and wether it would work if you could adjust the output of your charger by 3 volts or so to accomadate your new pack.