The nimh battery build...

I think I would have the same problem I am having now with that battery. Even if not 8ah is too small a reserve to get many places.

I think I figured out what the problem is. In my sleep last night, or at least while in bed, I remembered having read something from a lithium battery dealer about what size battery to buy for the motor you are running. I think there is a max amp discharge as well. I think the batteries I used have a lower max discharge rate than I need. So if I had done a little more research I could have done the same thing, but successfully. i would guess that the max discharge rate for this battery is about 10to 15amps. it should last forever though lol.

Now I know from a dead stop, the drive system I have, pulls at least thirty amps while getting up to speed. Sla batteries will drain all the amps your system ask for right up to the point that they self destruct. I know it is a 25 amp full load drive from both the rating and the fusing I had to use when I was building up to the point it is now.

So if tis battery is self limiting to say 12 amps, it isn't enough to accelerate the bike. But it would be enough to run it half throttle under light load which is about what it is doing. I am going to need to do some research, but I bet that is the problem. So to run a bike that draws 25 peak amps, I probably need a battery with a peak drain of 30amps at least.

So today I'm going to test to see if I can use this as a range extender in a hybrid battery combo pack with sla batteries for their peak amp feature. I have no idea what the right cells for a full modern battery pack would be, but I do know that we can build one. How much money we would save I don't know.

Someday the basic 1.2 amp cell will come along and we can configure our own packs.

To a present day thought. I already run the bike trailer configuration so the battery pack weight is a factor for me after a certain point. It would be nice to have a pack under thirty pounds. wich would also fit in a manageable container. I have more testing to do on the batteries I have.

well I got my benchmark for the hybrid test. I ran a 2.5 )approx) test run with a 1.3 drop in voltage on an sla battery pack. At the moment I am charging them, so that I can test them in a hybrid situation. it should be interesting at least.

I'm not sure exactly what happened yet so I'm going to tell you what happened without any firm results. The bike ran better than it has in a while. It was strong all the way. I had some trailer trouble, so I might have had a little more drag this trip. I'm also not positive the sla pack was fully recharged

Anyway the good news is that both packs read exactly the same at the end of the ride. They are sharing amps and it is not adversely effecting the bike's performance. Tomorrow I will make the same ride but with full packs for sure.

I'm a little bummed out. I ran the bike over the same path with both the 12ah sla pack and the 10ah nimh pack in parallel and the voltage drop was exactly the same. It made no difference except that the nimh pack dropped to match the voltage of the sla pack. I couldn't tell that it did anything at all.

I am going to set up a 1 mile test today with a 7ah battery pack. I am doing that just to see if the nimh pack is stronger will it make a difference. I can build a 20ah nimh pack for about sixty more bucks but if it isn't going to do anything why bother.

OOOPs never mind I had a bad sla cell in the sla pack. It read 12.23 while the other two cells read well over 13v. So back to the test track when everything gets charged again. All my posts combined are a little like a bloody diary. I just realized that. Sorry if I'm boring you guys, but it helps me to think. Maybe I should just start a blog somewhere as a diary and leave you guys in .

I could call it diary of a mad ebike builder and boy am I mad right now. Two days totally wasted on a bad cell in a sla battery pack.

You definitely have a point, the nimh start to gain voltage immediately I have already noticed that about them. As to the other I swear I feel bad about all the posts that I made which really are like the old seinfeld show all about nothing at all.

well that's pretty much why I admit to all the mistakes. I don't mind seeming a bit stupid if it helps someone else to avoid the problem/ Most people want to appear smart but me, how I look to other people doesn't really effect me. That comes with getting old I think. I just hate to go back and read what sounds more like a diary than a casebook/

All day yesterday I tested repacked my sla batteries to get them ready for testing. I thought I was ready before but found that I wasn't really. A bad battery in the string of sla batteries was skewing everything in both packs.

I have decided that I will try to test with a 7ah sla pack first. it should show the most dramatic voltage change the soonest of all my packs. I built it yesterday.I should get a pretty good change in voltage after only a couple of miles. It will also recharge the fastest for the nimh as a parallel power source.

So Day two of parallel battery testing begins.

The very first successful test of the Nimh battery. The bike ran like a pig with the 36v 10ah battery pack alone. So I decided to give it a try in a hybrid battery pack. I ran a 7ah battery pack, which I threw together using old batteries. I rode the bike on a measured mile real world ride. It needed a lot of pedaling at the end and the batteries read under 13v each. Some more than others of course.

I recharged the pack and added the Nimh pack in a parallel configuration. The bike had full power, which it would not have with the nimh pack alone. It had full power for the whole ride and the batteries read higher on the meter. Over 39v total for the pack.

So I can toss the lightweight Nimh battery onto the bike should I need a little extra push on a ride. I do on a ride to the lake, the sla pack will come in limping, this will help that a lot.

Now I had to toss several of my older sla batteries. Most of them go back to when I bought the hub motor so they have been charged and recharged over three hundred times, probably closer to 400 to 500 times.

I have one 17ah set that are fairly new maybe six months old no more. I made a pack from the best of the worn out batteries in the hopes I could use it in case of emergency. The point is that I am going to be adding to the nimh battery pack I think.

Next time I'm going to build with C cells instead of aa. I can build a c cell 9ah battery with just ten c cells. By not having to parallel a lot of batteries I can charge each 9ah set individually and not have to worry about balancing them with a circuit. I can do it manually or use an inexpensive smart charger.

I have a 10ah battery now, so I think I will add two 9ah c cell batteries. Which will give me 36v 28ah. It is my hope that that will allow me to draw at least 25amps from the battery pack so that I can use it as a primary pack. I need 60 batteries to do the build...It looks like it will cost me about $130 to do the build... Of course I would like for it to be fifty bucks but I just cant make that happen. This is the place I plan to purchase the next batteries.. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320510724927&ssPageName=STRK:MEWAX:IT

I don't know how the performance will compare to lith but for about two hundred and fifty bucks, I might have a 36v 28ah battery pack. The price is for sure better. if I were starting over I would definitely go with all c cells. Oh well live and learn....

Well it looks like I can build a 36v 28ah nimh battery pack for under $250. if that lipo sells for less then 400 with shipping please let me know. the one I saw at 36v 30ah was over 700 bucks. the one they showed the link to in this thread was 12 v 20 ah for a hundred bucks. okay that would be would be three hundred for a 20ah which is pretty complicated battery pack. One dead cell and you have to find someone to rebuild it. A dead cell in mine and I just have to buy a five dollar replacement cell.

I have been building or trying the total pack, and since i know that it will work in a hybrid pack I don't have any qualms about spending the hundred and thirty bucks to build the full pack.

We will see what is what, but to be honest, I like to build stuff myself then I know what is in it and how to fix it. Then D cell is disproportionately expensive. the cost per ah is best with the double a but the balance issues raises it's ugly head. The c cell is the compromise, If the d cell was 20ah and not too expensive it would be the way to go for sure.

I spent a lot of time trying to talk myself out of making the final push to a Nimh battery pack. I think I have all the channels in my brain moving in the right direction to explain everything.

I had some problems with the original 12v 12ah battery. It over heated when I tried to use it as part of a hybrid battery pack. Now I think I understand why. Now this is not very scientific but here is what happens with the different type batteries I'm pretty sure.

both kinds of batteries are like the gas tank on a lawn mower. When you charge them, it is like putting fuel in a gas tank. The difference is in how the fuel leaves the tank.

[b[Sealed lead acid [/b]batteries are like a gas tank with no restrictions. The throttle calls for gas and it just pours out till it fills the carburetor . No governor at all just wide open pouring out.
nimh battery/] is more like a gas tank with rust in the fuel line opening. no matter what the throttle says it will only allow a smaller but steady stream of fuel to move to the carb. But if you use a bigger tank there is more pressure and the stream of fuel is larger. pretty much why they tell you to buy a higher capacity battery if you have a high amp drawing motor. One of you guys who knows more about batteries can correct this little analogy but it is how the whole thing feels intuitively.

So when i had the single nimh battery inside the larger sla battery pack, the sla batteries were pouring fuel (amps) through the nimh battery forcing more amp through it than it would allow out, therefore producing heat. When I used a complete ten hour battery pack, producing current at it's own rate, in parallel to the sla they worked perfectly. So thats the overheating came from, trying to force too much current through the nimh pack. Since I won't be doing that again, the problem is solved.

So I can solve the running like a pig problem with more ahs in the battery pack. There is no heating problem with the nimh, if they are not mixed with sla batteries in the same pack.

Now as to the charging thingie. If I use c cells at 9ah per cell. I can make a 36v 9ah battery with no parallel cells. i am going to reuse the 36v 10ah I have already but if you make a battery from scratch you would have three batteries of 12v 9ah running in parallel and then three strings of them running in series to make 36v 27ah. All you have to do to avoid bms circuit is charge each of the 12v batteries separately. Then check them for voltage after a cooling off period. Too much trouble, then this project will probably work by just charging the pack up and hope for the best. I think you can buy a bms circuit that would work easily with this since there are so few cells. Regardless the project should work so I am going forward with it. I am ordering the c cells when I finish writing this.

actually I think without the bms the ability to charge each 12v non parallel cell will be an advantage. Also being able to read each battery without taking the while thing apart will help with finding bad cells and replacing them. The aa cells I built now seem like a mistake. the C cells would have been easier to manage but hindsight is always better than my foresight. The new C cells are on the way so in a few weeks we will have new information for the grist mill.

Well I got an email the small batteries should be here in one to three weeks. So I will be finishing up the battery pack shortly thereafter. We should all know something in a month or so about how the diy pack works out.