NiMH Thermal Balancing

I have a theory for Nickel Metal Hydride (NiMH) chemistry that I call Thermal Balancing. This is how I charge my NiMH packs.
I use a cheap 2.5 amp 36V SLA charger. http://cgi.ebay.com/Battery-Charger-Electric-Scooter-Bike-36V-2-5A_W0QQitemZ300200312172QQihZ020QQcategoryZ11332QQcmdZViewItemQQ_trksidZp1742.m153.l1262
It seems to work fine so I figure it’s time to share it.

I prefer to construct a 30-cell NiMH pack using C-size 5000 mAh cells in series. This creates a 36V (nominal) pack with a max voltage of about 42.3 volts
(1.41V per cell is needed to achieve full charge in each cell). I buy Powerizer or Tenergy button-top C cells for about $0.60/Whr on ebay.
http://cgi.ebay.com/12-Pcs-C-Size-5000mAh-NiMH-Rechargeable-Batteries_W0QQitemZ330251292816QQihZ014QQcategoryZ40975QQtcZphotoQQcmdZViewItemQQ_trksidZp1742.m153.l1262
I solder them together (a no-no? I say WTF). I often draw up to 30 amps from this pack with only very mild heating of the cells (another no-no?).
And sure, these are cheap NIMH batteries. These are the cheapest NiMH cells I can find.

The 30 cells fit perfect in a $6-RadioShack plastic project box (quite rugged),
http://www.radioshack.com/product/index.jsp?productId=2062284&cp=2032058.2032230.2032276&parentPage=family
(radioshack pic is wrong btw, see project box pic here ... http://98.131.176.65/endless-sphere/GEDC0405.JPG and http://98.131.176.65/endless-sphere/GEDC0404.JPG)
which, in turn, is almost the exact same dimensions of a typical 12V 20AH SLA battery. http://cgi.ebay.com/Battery-Pack-for-RBC11-APC-Replacement-Battery-12V-20AH_W0QQitemZ310066549256QQcmdZViewItem?hash=item310066549256&_trksid=p3286.m14.l1318
The project box fits perfect in my Grubee (4 x 12V-20AH) battery rack or at the bottom of a typical bicycle rear saddle basket (WALD)
(or on the back of my chopper e bike http://endless-sphere.com/forums/viewtopic.php?f=6&t=4160&st=0&sk=t&sd=a#p62316).

During pack construction I install a 50°C (122°F) Klixon thermal switch embedded in the pack. A separate charging lead is connected to the Klixon and the positive terminal of the pack.
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2911

These packs are my work horses and are MOST reliable and have the best energy density per mass going! So bring it on!

Knuckles! .....NiMH? ES EV ppl are all drunk on Lithium these daze. Go away.

hehe

kidding

Knuckles said:

...these are cheap NIMH batteries. These are the cheapest NiMH cells I can find."

Click to expand...

Ya see "cheapest" is a moving target depending on your horizon (time frame) and what your options are doing...

When you buy it and how many times you have to fiddle with it - replace and recycle the pack or part thereof (and where do the materials *really* go in this recycling process?) and what it costs every time ya swap things out are the long term benefits and costs.

So Knuckles, ya know I am only curious why you bring up NiMH `cause recently I spent a Thou'($) or so on Lithium. How does NiMH do in the -20°C btw?
;->
tks
Lock

Actually these are the most robust, dependable, cheapest, maintenance free, self-balancing, highest energy-densisty packs I have yet to build!

If it ain't broke ... don't fix it. AH's are AH's. All prejudice aside.

LET THE THREAD BEGIN!

Knuckles said:

During pack construction I install a 50°C (122°F) Klixon thermal switch embedded in the pack. A separate charging lead is connected to the Klixon and the positive terminal of the pack.

Click to expand...

That is brilliant and simple, also old skool. I'll probably shamelessly steel this idea and call it my own. (assuming I make another NiCd/NiMH pack)

Regards,
Marty

Take take take ... that is the point of this forum. I have yet to get a dead cell. This surprises me because I figure everyone would say it is a silly, stupid non-complex solution.

BUT IT WORKS! And I struggle to understand why!

Lock said:

How does NiMH do in the -20°C btw?
;-> tks Lock

Click to expand...

Ha Ha. Leave it plugged in and it is a non-stop battery "warmer" and always at FULL charge.
He He back at you! It is all about thermodynamics and heat transfer (I think). Adiabatic v. Isothermal.

Some thoughts on your charging scheme;

Right now you would probably get better pack longevity if you decreased the thermal cutout temperature about 10F. Have you tested for number of cycles? Bet your range would be the same.

If your pack is starting its charge cycle at about 95F after a nice sumer ride, then a rise of 20F is about what I see as my charge terminaton for my NIMH delta d and t adn all that.

When the ambient starts to drop and your pack starts a charge cyle at about 65F, your cutoff will be to high.

another cheap easy way to do this more better, (if that is a ral phrase or a Yogi-ism) would be to use an adjustable thermal cutout and have a digital thermometer lead buried in the pack, set the thermal to 20F over.


another way is to use an amphour meter to log the discharge, then calculate the amount of constant current charge needed x115% and set a timer to allow just that.

Just some thoughts from another NIMh old fart, milking the last out of an old technology while the Lithium addicts stumble over thmselves to spend theri last $1000 on batteries.

Thank you, Thank you, Thank you. YES! I want the 40C Klixon. BUT ... Nobody will sell it! Oh how I have tried to get 40C Klixons. Any thoughts on where to find a lower temperature thermistor?

btw charge lead is different than the discharge lead. When I race on these bats they will get VERY warm (he he). I just don't GAF!
These are so cheap to make and they keep going and going and going (and they seem to self balance!)

PS. So cheap to buy and simple that I really don't care about pulling 30 amps from a lil box with a 5AH rating. Sub-C has a 5-mohm rating and these can do 50 amps!
I build these lil sub-c babies too! NiMH is far from dead technology IMO.

PS I like to put my bats in the freezer too to charge them faster. Do the same thing when on the job doing construction with Dewalt 18V NiCD powertool bats.

Also get this ... the Klixon 50C is rated for only 5 amps. When I use a 2.5 amp charger the Klixon shuts off at like 110F! The more powerful the SLA charger, the COOLER the bats will stay at full charge!
WACKY!

Man ... I have so much more to say about this. But it is fundemental that the NiMH packs be built as 'bricks' where the temperature of the pack can uniformly distribute and be centered around the thermal cut-off charge switch. It is a thermodynamic (charge - discharge) process. This is why (I think) it works so well for me. And, btw, the bats are always cooler than those cheapo NiMH single cell chargers that let the NiMH cells go way over 130F! This is a win win NiMH solution. But why do these cells self balance? That is a major point of this thread and I have a wacky (idiotic theory) of why this occurs!


calling all fechters ... calling all The7s ... and EVERONE ELSE ...

The internal resistance goes up as the cells get fully charged ( and they produce heat ). vs Lithium where the voltage will shoot right up quickly once they are full and toasts them..

You can ' overcharge ' Ni cells to a certain degree, without killing them. A " Trickle " charge can be given over a lond period of time and this will ballance a pack after a few cycles if it has been sitting for a while.

Still.. the heat thing. and the self discharge.. make Ni a thing of the past. gimme lithium plz.

Ypedal said:

gimme lithium plz.

Click to expand...

I love lithium too! (my doctor prescribes it ). But $/$ and simplicity do have their merit.
yes they do (no they don't) yes they do (no they don't) yes they do (no they don't) yes they do (etc.)

ps ... discharging NiMH is endothermic (at low amps) so this tends to cool a NiMH pack.
see ... http://www.hardingenergy.com/pdfs/NiMH.pdf

"When a NiMH cell is discharged, the chemical reactions are the reverse of what occurs when charged. Hydrogen stored in the metal alloy of the negative electrode is released into the electrolyte to form water. This water then releases a hydrogen ion that is absorbed into the positive electrode to form nickel hydroxide. For NiMH cells, the process of moving or transporting hydrogen from the negative electrode to the positive electrode absorbs heat and is therefore endothermic. Heat continues to be absorbed until the cell reaches a state of over discharge, where a secondary reaction occurs within the cell resulting in a rise in temperature."

F'n'A bro ... I got the cheapest most reliable system going. Plug and play ... for da masses. Hey, SLA is like almost 200 years old. NiMH is FAR FROM DEAD! It will kick Li ass any day of da week!
(Is Faraday dead ... I think not! He is alive and well in living New Jersey right now! Time for my medication ... mommy!)
nothin wrong with 'easy peasy' just like 'Brooks' says ... Shawshank style ... 8)

Man I am digging all the LI justifications here ... KEEP IT COMING plz

and Keywin thinks a big drop in LiFePo4 is coming real soon ... :wink:

Like I said ... I love lithium ... see my first dead a123 cell here ... http://endless-sphere.com/forums/viewtopic.php?f=14&t=2498&start=270#p79767
I have yet to have a dead "old-skool" NiMH cell ... Oh well ... my bad.

It's NiMH-rats, not old farts if u pls Mrs. Frisby.
I agree that there is a definite crowd mentality going on that seeks to pressure anyone that considers going NiMH that they must be brain-damaged or something.
I certainly hope NiMH isn't an old technology & is perfectly viable & well suited as an ebike entry/mid level power source.
In fact, with the new LSD I think NiMH has finally matured & is just hitting it's stride.
Or rather it would be, were it not for a certain oil company holding back the chemistry.
I would dearly love to see LSD the only flavour as the standard NiMH, with D & F size Eneloops that would scale up to 16 & 24 Ah respectively, which of course anything over 10 Ah is strictly verboten.

Knuckles said:

These packs ... have the best energy density per mass going!

Click to expand...

Have to disagree a bit there... maybe.
I just picked up five of these 6-cell 7.2V/5Ah *sub*C Venom Packs to serve as a small parallel add-on range-extender.


While it's certainly at the other end of the cost spectrum @ $1.50/Wh, that's partly cuz they come pre-assembled into a pack paying for the added labour along with added bits & pieces I don't need but basically that's the price to be paid to get the newest tech with the highest specific energy NiMH available, (85 Wh/kg by my calculation), however I may have tracked down a source in HK for the individual cells @ $1/Wh.

Lock said:

How does NiMH do in the -20°C btw?
;->

Click to expand...

I can field this one.
I routinely do -20°C without battin an eyelash.
WOT top speed is down 1 or 2 mph of the restricted 20 mph max.
Range drops by 5 miles from the typical 18-20.
Winter b4 last I wanted to test out how much more performance tapered off @ -30°C since this is about the absolute bottom limit according to the temp graph in the Saft manual.
Top speed was only 15 mph & range basically cut in half.
I don't think I'll be routinely torturing my NiMH like this, but it's nice to know I can still count on them if my LIfe were to depend on it, should I ever get snowed in without a lick of beer in the house.

Thank y'all for sharing... the Prius packs are starting to show up and they look very tempting.

One new member has just acquired a pack and also needs charging info.

Been using NiMh for several years since I retired my Nicads! Like the electronic shutoff of Astroflight, but this using just Delta T sounds kool! Be advised that U need really good NiMh (like the Prius ones) to even come close to a Ping pack for power at equal voltage! Mine (Batteryspace) suck in that regard! Thw A123 will eat them alive,IMHO!
otherDoc

docnjoj said:

Been using NiMh for several years since I retired my Nicads! Like the electronic shutoff of Astroflight, but this using just Delta T sounds kool! .....
otherDoc

Click to expand...

If you are talking what knuckle is doing, its not even Delta T, its absolute T. If you are talking my crude proposal with an adjustable thermal cutout, i guess that is delta T. I saw that adjustable thermal switch at Pep boys some 5 years ago, wish I had picked one up. KNow I am stooping to using a low F value thermal switch and placing it a little bit away from the heat source (cells). Adjustable?- yes.. Precision?....in a dollar store sort of way.

This thread was started about NiMH Thermal Balancing, and has been requested by the thread starter to get more data on that; Dont know if those of us not adding value to NiMH Thermal Balancing are guilty of thread hijacking or not.
guess will leave that up to knuckle.

Anyway, Each chemistry has its advantages; some folks have mastered the art of dealing with SLA and getting the most out of it while figuring out how to not have the weight be a big deal. I like the way one can just parallel another string, no diodes, no BMS, just connectors.

Nimh i think has recently gotten ahead of itself on price- It was nice when I could get a good 24v pack (just assembled cells) 12 ah, capable of 15 amps with little sag for $250.

I have always had a lot of satisfaction staying with "mature products" , increasing the usefulness and service ability long after the shine is of its rose. Too many times customers invest in something and they feel their money is wasted when the next doodad comes along and there is no more support for what they bought.

So it does an industry good to be able to say.. "Look we still have folks experimenting and supporting SLA, NIMH, NiCad, Brushed motors and controllers,. This isn't Betamax video. "

Anyway, you can't get the big power out of NIMH using Current like some other chemistries,, power comes from volts here.
So you have to figure out how to charge effectively at higher voltages with easily available chargers...,

so, with that, we now return you to the thread topic... NiMH Thermal Balancing.

d

I've been checking out specs of different LiFePO4 and I've noticed that their energy density (Wh/kg) varies quite a lot from one manufacturer to another. Of course some are made for high output power (watts) instead of high density, so this might explain part of the differences we see between LifePo4's.

We hear all the time "LiFePO4 have XX% higher energy density than NiMH", but I think this is genericaly used by all lithium manufacturers, and is NOT always the case... compare before believing!

An example: I am about to order a lifepo4 pack from ping which I calculate as having an energy density of around 90Wh/kg. Now, I have here in my workshop a whole bunch of NiMh packs (3-4kWh!) that have an energy density of 85Wh/kg... and they are old tech (6 years or so)! Of course internal resistance of these packs make them quite bad for heavy loads, but I'm thinking of making a hybrid lifepo4/Nimh setup for long travels were the nimh's wouldn't ever see over 0.5C discharge max.

On a side note about internal resistance, it is also wise to compare this when getting batts, because I've also noticed large differences from one company to the next... and they also are not always up to spec (like the headways that claim 3mohm, but really seem to be about 7-8mohm, per cell, per 10Ah). I am working on a table to compare different companies battery pack offerings, because the game seems to be very confusing right now. It seems we are entering a "battery rush" with these artificially high oil prices and our (hopefully) growing enviromental conscience.

I hope I didn't go off subject too much here, but my point is as Knuckles points out: NiMh is not a dead tech! It has just been crippled by the GM/chevron fiasco blocking the big cells. And beware of any situation where everyone is rushing into the same direction... it creates hype.

I will be trying your charging method, Knuckles, but maybe use a very simple PIC setup to monitor temperature rise in each seperate NiMh pack - it would then be easy to use any type of thermistor instead of a fixed temp switch, and also be able to adapt to different ambient temps by using a simple dT/dt scheme.

Just my 2 X 0.01$ worth! Slap me if you want knuckles... :wink:
Pat

if you have your nimh cells on a thermostat then balancing occurs naturally because when the cells reach full charge the heat raises and cuts off the charge. the charging will be switched on/off and the cells will stay at 50*C. this has the effect of balancing the cells because the average current is reduced to a trickle charge.

I will be watching this hybrid thread closely cause Im about to purchase LiFePo4 and have NiMh that hate to pull big amps! Keep the info coming, please!
otherDoc

For charging NiMH ... This is my guide ... View attachment nimh-graph.pdfFor Info on the Klixon ... and (this will really piss folks off) ...
I am now charging my 36V NiMH packs with a 48V 2.5 amp (very cheap) crazydude SLA charger.
I guess I am a bad bad boy.

THERMAL BALANCING ... The tech you love to hate. f'n'a

http://en.wikipedia.org/wiki/Bump_%28Internet%29

I like this idea, nice and simple, real simple and reliable - it is the thermal condition that causes the problem and this solves it simply . I have some expensive chargers but still don't trust them, some have thermal monitoring, some don't, even when they have there is only one sensor. This method would allow the addition of multiple protection within packs and open up the ability to charge in parallel too without the risks/complexity of a full BMS but still allow some the dynamic charging abilities of these chargers to work unhindered.

Whoever mentioned those single cell fast chargers has a point too - their temperatures are way up - the risk with a pack is obviously a lot higher and it is far easier to go into thermal runaway because their is very much less ambient cooling.

Looks like Sensata are the parent company.

45 DegC (4MM) series thermal cutout loks to be available:
http://www.sensata.com/files/battery-4mm.pdf

Here's the Distributor contact page - can but ask what the minimum order quantity and who can supply.
http://www.sensata.com/contact/contact-battery.htm

Found these equivalents. Might even get some samples + postage.


Baozhu Electric Applianes Co.
http://www.thermtector.com/probig.asp?id=1

Maximum contact ratings10K(4A/12VDC 3A/24VDC 3A/120VAC 2A/250VAC)
Opening Temperature 30~155℃ <-------------------- Lower temp range!? -------------------------------------------------

Third Party Certificates:
VDE UL CB CQC
Product Sample Availability Policy:
free below 10 pcs samples but transport charge
Order Lead Times(Days):
7~10days
Minimum Order Quantity:
NO

Trumpet said:

Found these equivalents. Might even get some samples + postage.

Baozhu Electric Applianes Co.
http://www.thermtector.com/probig.asp?id=1

Click to expand...

The web page says "under 25mOhm resistance"... I sure hope the one you want to use is way under that!

-EDIT-

I checked your klixons and they also have quite high resistance, between 20 and 60 mOhms. It all depends how many of these you need per string I guess, but it will bite you at high power levels during discharge, because they are still in the pack, right?

ZapPat,

That is why you use a separate charging feed - another positive lead dedicated to charging - say with 3 cut-outs per string (at 25mOhm each) charging at 2A will create a voltage drop of 0.15V, as long as it is consistent (which it will be pretty much) it won't affect the charging process no matter how fancy the charging method.

Trumpet said:

ZapPat,

That is why you use a separate charging feed - another positive lead dedicated to charging - say with 3 cut-outs per string (at 25mOhm each) charging at 2A will create a voltage drop of 0.15V, as long as it is consistent (which it will be pretty much) it won't affect the charging process no matter how fancy the charging method.

Click to expand...

So you have a thermal switch every how many cells for this to work well? I guess to have good thermal contact with the switch, round cells would have to be in groups of seven (this gives a hex shape cluster). Any more than this per group and the switch will too be far from some cells, no?

While I agree that the extra resistance is not really a problem while charging, while discharging it will hit your efficiency and performance. What I don't quite understand here is how your "extra positive lead used just for charging" will manage to bypass ALL your thermal switches... Unless you bring out two leads from around each switch, and short them together when you are discharging the pack. Am I missing something or what? for a 48V pack, that's about 7 clusters per string, so 7 thermal switches, and thus about 14 or so wires to use as bypasses during discharge. And this is when you only have ONE string... imagine two or three!