The nimh battery build...

[deacon]

cold morning update.
I charged the batteries for the first time last night. The ending charge was 13.8 or so... This morning the charge was 13.4 and 13.5 still respectable amounts. I'm going to take the bike out for a test ride and more calculations after the sun comes up and the temperature rises. Yes I was up at 6:30. I was too excited to lay in bed,

I don't think I got a full charge on the batteries last night. They were up close to 14v but they never even got warm. Not getting warm is the ideal I know, but I would expect them to get a little warm after they reach full charge so I probably didn't get a complete charge on them.

I know that the standard on sla is anything over 13.25 is considered full charge, I wonder what it is for Nimh. I couldn't find it on a quick search of google.

My other related project is to keep searching for the best possible container for the batteries. I am sure I can find something better to the carry them on the bike.

 

[jdcburg]

Deacon - here is a quote from the Wikipedia NiMH page. I also included part of the section on overdischarge: "Discharging - Under a light load (0.5 ampere), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts; some measure almost 1.5 volts. This voltage falls rapidly to about 1.25 volts at 10% depth of discharge (DOD) and then remains almost constant until the cell is over 80% discharged. The voltage then falls rapidly from about 1.2 volts down to 0.8–1.0 volts at which the cell is considered "flat" in most devices. Mid-discharge at a load of 1 ampere, the output is about 1.2 volts; at 2 amperes, about 1.15 volts; the total effective differential internal resistance is about 0.05 ohms. Nickel metal hydride batteries provide a relatively constant voltage for most of the discharge cycle... Over-discharging - A complete discharge of a cell until it goes into polarity reversal can cause permanent damage to the cell. This situation can occur in the common arrangement of four AA cells in series in a digital camera, where one will be completely discharged before the others due to small differences in capacity among the cells. When this happens, the "good" cells will start to "drive" the discharged cell in reverse, which can cause permanent damage to that cell." -jd

 

[deacon]

Well I think now you hit it on the head with the mix discharge rates ect. The bike at 27ah still runs like a pig. After the first time it ran like a big, if you remember I tested a Hybrid system. I hooked the nimh pack up with a sla pack and the bike did excellent and the power usage was down.

My next test of the day will be a 24v setup with (2) 7ah sla batteries and two of the Nimh batteries. I would like to test the range since the 7ah alone will only do about a mile. That combination will drop the weight on the trailer significantly. Probably enough to mount them to the bike or a least build a much smaller trailer.

I also have two 12ah batteries i can match up with the Nimh to test. What I learn from them I can take on to the 36v batteries should I decide to go that way. When I tested the hybrid pack before I found it ran just a little better with the nimh probably because they tended to keep the sla batteries hot longer.

I hate that the nimh runs the bike so poorly but it does. So life goes on. So now the thread becomes about a sla nimh hybrid battery. pack. I Think that the nimh would probably work fine if I found the right batteries but the cost of those would probably be the same as going with a factory made lithium pack.

One thing is I am free now to work with smaller ah packs of the Nimh. If they significantly improve the pack range, I can probably rebuild the nimh batteries to 18ah 36v using only the c cells. That would make the management of the batteries simpler for sure. There just might be a bright side to this after all.

 

[deacon]

JD I reread your post just to be sure and I metered the batteries when I came in at 13.2 volts. That should be full charge or close to it. There is no reason for the bike to perform so poorly.

If each one of the cells discharges .5 amps then ten cells to a 12v line should discharge 5 amps per string.. There are three strings to a battery so that would be 15amps which is about about half what it be running at to be "normal" . I am just not going to build more batteries lol.

I'm going to try the hybrid system today and see what it does.

 

[jdcburg]

Deacon - That sounds very strange that the batteries would come back with a full charge yet have no power. If the motor is drawing too much for the packs, they should discharge very quickly. Have you checked/changed controllers, wiring, connections, etc? How about running the motor no-load and checking the voltage at the terminals? I found that my SLAs showed 24.5V after a ride when they felt dead, but when I ran the motor with the wheel off the ground the voltage immediately dropped to 21.5. Also you could try running it no-load and timing it until the batts run down. Just a few thoughts. As I mentioned earlier, the Prius uses NiMH so there must be something good about them - jd

 

[zabac70]

@jdcburg

As I mentioned earlier, the Prius uses NiMH so there must be something good about them - jd

Those cells are like lap top batts , but somewhat better (better discharge ratings and overall construction and with tons of regulating electronics). The problem with NiMH AA cells that deacon uses is their discharge rate "on demand" - which means they can't produce enough power in a short period of time when they are asked to (going uphill or start from a stand still). The cure for this is over sized pack (Ah I mean) with some (electronic) management to distribute energy demand evenly among the cells (more cells - less individual strain on the each cell ). Deacon doesn't want to have any relationship with electronics so he have problems with his pack and there is little to conclude just from measuring the voltage. If he could , somehow, manage to measure current (Amps) from individual cells in real time (while driving and power fading) , he would see the root of the problem. The thing is , when NiMH's are under a stress (more power is drawn than they can produce) they tend to worm up and get hot , which increases internal resistance of the cells so even more energy is wasted in overcoming that resistance, which increases heat and so on , up to a point when cell chemistry change its attributes temporarily (extremely low flow of electrons between the electrodes) or permanently (change of polarity) - 0.5 A per cell is far cry in that moment...Also , deacon uses SLA's in the same pack , which draws energy faster then motor itself (to level up voltage between two different batt. chemistry and capacitance ).

 

[deacon]

I had this long post written but decided not to bother. The nimh battery project was not a roaring success but I will find a way to use them. I think I can use them like an extra gas tank in a car. One tank feeds the other kind of thing.

As for the rest of it, this is a hobby. I do it because I enjoy the experimenting period. It's like i said before some people fly cross country others drive. It's the trip, not the end that is where the fun is for me.

I did switch it out for some hot sla batteries and they ran just fine. It's got to be the batteries just wont empty out *(on demand amps) like an sla. The big deal with all these batteries is range so they give up their charge more slowly. That I am beginning to learn. But like I said with a little luck I can use them to replace the draw down from my sla battery pack. I would never buy them again unless they just last forever as a reserve fuel tank kind of thing.

 

[jdcburg]

Well I hope you keep on experimenting. Many people argue that friction drive is not viable, yet you and others continue to disprove that theory. If the NiMH packs simply provide enough power to keep the SLAs from sagging when they need the juice, you may be able to go with smaller lighter SLAs and still get the range/power/speed you need. That's essentially how hybrid autos work - the gas motor kicks in to boost the batteries when needed.

Deacon, after you finish with the battery experiments, how about making a rhino drive using a multiple speed bicycle hub so you can have more power/speed choices. The old Sturmy-Archer 3-speed hubs I used on my front wheel drive experimental ebike had ratios of about 5:4, 1:1 and 3:4, giving me speeds of about 12 to 18 mph at top motor rpm. I bet you could somehow lace or weld the hub to an appropriate size scooter wheel or rim. There are probably lots of old Schwinns and Huffys in the thrift shops - jd

 

[zabac70]

Deacon , I know how and why you do it - we cleared it some time ago. My earlier post was for jdcburg (shortened version of whole thread)

 

[deacon]

JD actually that is a good idea. I was thinking next I might use a childs 12.5 inch wheel as as a drive just weld a #25 sprocket onto the existing one. I could have a bike shop redo the rear wheel of a three speed if i see on around. have them change the rim to a 12.5 rim. Now that would be interesting.

On the nimh there is one hope left. I might not have gotten the batteries frully charged since I didn't want to over charge them. So today I have had them on several hours and they aren't even warm. I don't know enough about how these things act yet to know what I have done. This is the last hope for them as a solo power source.

I have a direct drive motor with s six inch wheel I plan to work on today while those batteries cook. I guess I'll finish this cola and get to it.

 

[deacon]

I charged the pack all day yesterday. I charged it until it got warm so I know it was full capacity. It came off the charger at 1.39v which is about what they say it should be. They said in the article JD got for me. 1.4 to 1.5 so 1.39 is pretty darn close. Over night they dropped to 1.36 Which is within it's operating range but it is also where it was yesterday when it wouldn't pull the bike up a hill.

So today's plan later in the day, is to load the big sla pack and the small nimh pack on the trailer. Try to start out with just the nimh pack and see if it will pull the bike. Also check the packs voltage between the start and if and when I give up on the pack. Then to hook both packs together and see how that does. I want to drain the Nimh pack to base charge if possible. Then charge it all day again.

If I have to go hybrid pack, and I'm almost a 100% sure I will, the real test will be a trip to the lake and back. Last two times I did it, the batteries were dragging because of all the hills going out there. If the two packs play well together, i should get back with a sound power source. If so I can start working on what the best combination of batteries is and what the effective range is ect, I did try this with the 12ah set of batteries and it seemed to be a difference but I didn't try to quantify it because I still had hopes the batteries would run as a primary source of power.

 

[jeff96]

I guess it's one of those things; if it was easy, everyone would already be doing it. It would be interesting to see if the output of the nimh batteries improve after a few cycles.

I'm not sure if this is a good idea, but I would take a single nimh cell, hook it up in series to a multitester set to amps, and short circuit it just long enough to get a reading. There is a small risk of overheating or burning the battery, and if the output is >10 amps then a multitester will die to prove it. On the plus side, you would get an idea of how many peak amps you can get out of a series.

I know you've put alot of work into assembling the packs, but I'm a little suspicious of the tin foil connections inside them. Soldered tabs would make more reliable connections

 

[jeff96]

just had another thought; I believe you have the batteries wired in 12V series, then you series the 3 12V packs to make your 36V supply? Would you be able to discharge more peak amps if the individual batteries were wired in series to produce 36V.

 

[deacon]

I doubt it jeff. I charged the pack all day yesterday until it got warm. Then this morning I took off all the fancy wiring and just hard wired everything for todays test. It still has the battery output problem. This morning I could feel when it hit the wall at about half throttle it maxed out.

However here is the MAYBE good news. I hooked the whole 36v pack up in series with my sla pack (hereafter to be known as slak) and it shows promising first results. I have gone three miles with very little battery use from the nimh pack (hereafter to be called the nkpk..... The sla showed it down about a volt. So I will check it after a thirty minute or so rest to see what the voltage is. Will the nkpk raise the voltage of the slak battery. The nkpk has more ah so it might share them with the sla pack. That would be kind of neat.

 

[zabac70]

just had another thought; I believe you have the batteries wired in 12V series, then you series the 3 12V packs to make your 36V supply? Would you be able to discharge more peak amps if the individual batteries were wired in series to produce 36V.

It is a same thing - 3 * 12 V or 30 * 1.2 V. Deacon arranged it that way because it was easier for him to make pack(s) that way. Resistance of wires is close to none and doesn't really affect performance of the battery pack.

 

[deacon]

the way I wired them also allows me to check the charge on each string of 12v to balance them if they need it.

The slak is 17 ah the nkpk is 27 ah I think that tonight i will down grand the skak to 18 ah to balance it out a little and to allow me to wire the reminder into a 36v 10 ah nkpk to boost a 12 ah slak.

Of course that will depend on the results of todays tests.

Also have to say this in all honesty, this pack might work with a more efficient bike setup. A light weight bike with a motor that pulled about ten amps and which had pedal along gearing might work pretty well with this battery. My outfit pulls 25 amps and needs all of them and more. Since the bike seems to want to run at about half capacity on the nkpk, I bet it would pull a fifteen amp motor with some pedaling just fine. I live in some very hilly country as well.

 

[zabac70]

Deacon , you start to sound a bit disappointed lately. You can still use those NiMH to refill your SLA and that is an improvement on itself (more range without adding more heavy SLA's). When you finish this experiment , you may try to borrow some NiCd packs (before you buy them) from battery operated tools and hook it to your setup. Trust me , it'll heal all the grief you got with NiMH.

 

[jeff96]

It is a same thing - 3 * 12 V or 30 * 1.2 V. Deacon arranged it that way because it was easier for him to make pack(s) that way. Resistance of wires is close to none and doesn't really affect performance of the battery pack.

It is the same thing in voltage potential yes, but the issue is drawing enough amps out of the pack to drive the motor.
I have some old AA nimh cells that I did an experiment on.
I hooked the test leads of my multitester directly to the poles of the battery in 1 increments of 1 to 4 batteries in series. These are old mismatched batteries, so the results are
1 battery: 149 to 187 mA
2 batteries series (3V): 217 to 275 mA
2 batteries parallel (1.5V): 175 to 205 mA
3 batteries: 301 mA
4 batteries: 316 mA
Unfortunately, 4 batteries isn't enough to compare amperage output between a parallel circuit and a series circuit. My theory is that fewer parallel circuits may lower the battery resistance and allow a higher number of amps to be discharged from the pack.
I'm suggesting that 3 12V 27Ah packs with 3 individual parallel circuits may have more resistance than 3 36V 9Ah packs in parallel. Either way it's the same number of batteries, voltage and aH rating.

In the end though, it sounds like you can't lose. The sla batteries provide the juice to get the motor going, and the nimh tops up the sla

 

[deacon]

that is the theory but in practice im not sure just yet. The nkpk ran down way too soon,. so I found a bad cell in one of the strings and replaced it. Now I need top re-charge the whole thing and start over. This actually is fun.

The testing worked just the way I hoped it would. I could test each string of each battery together and separately. First find the lowest leg of the pack then go down the batteries till I found the one that was two volts lower than the rest. Took it apart and found the bad cell.

The neat things was i didn't have another cell, but I had some left over AA cells. I put four of them together in a parallel circuit and put that whole glob into the string as if it were one battery. It worked just fine.

I cut the pack down to 18ah so there are fewer batteries to deal with. Funny thing is the bike came in limping after five miles which is what the slap would have done, but the voltage was high in both the slap and the nkpk. I guess that one bad cell was the problem, unless the bike is drawing amps that aren't showing in the voltage, but that is unlikely.

As far as down goes not really... I had tested the aa before I spent the big bucks on the C cells. I knew that it was a possibility that i would wind up with it doing just what it has done. So im not surprised just a little disappointed it would have been nice to have less weight on the bike/

 

[deacon]

I have no idea if this will make a difference, I am going to have to check it tomorrow but I know why the bike came in with power but running badly. The tension on the drive wheel was way way too much. I should have known but then I'm just not as bright as I pretend. I am going to try the lightweight batteries again with the tension reduced then if that won't fly, I'm going to set them as I did today. There to help out the slap.

The sla will deliver all it has even if the wheel is binding. I bet that is why the bike ran so bad. It may or may not be enough to let the nkpk work but it's worth a try. It will at least give me more range. I might even be able to pedal the bike again lol.