Batteries: some hot, some not. What gives?

[PaleRider]

Alright, dumb question, maybe. Hope this is the right forum.
This morning I ran a nominal 54 volts -- measured 59.4 at the start of the ride.

At the end of the ride -- immediately post ride -- I measured 51v.
Pulled the pack apart and found some batts with significantly (2v) lower voltages than others, some hot, some not.
The general trend seems to be that the warm ones are the ones with the higher remaining volts.
After resting all the batts for an hour I restrung in series and the pack was back up well over nominal voltage at 57 and change.

Anyway, I'd guess this means that the hot ones are
A) better batteries,
B) working harder,
C) compensating for the weaker (lazy) cool ones with low voltages?

Am I reading things right?

 

[Ypedal]

What chemistry ? how many AH .. and what model controller/motor ?

 

[PaleRider]

yikes... sorry, that was dumb.

SLA, clyte dc brushed 409 in a 700c wheel, no controller, 35 amp toggle after a 30 amp fuse. I don't know how many amps I'm pulling. The only meter Id be willing to throw in series and ride is rated for 10 amps. I'm thinking Id let the smoke out if I tried that, so I haven't.

The batteries are a little mismatched. I've got two 4.5ah 12v in parallel for nominal 9ah. The rest are 12v 7ah and 6v 7ah. They're decommissioned UPS batteries, so no doubt their conditions vary, I don't really know by how much. the ones that got really hot when I tried to charge them (or were at 0 volts when I got them) have been set aside. I'm not using those ones.
The bottom line about my batteries is that they're free, I've got quite a few, and I have an arrangement where I get a few more every day. I'm trying to put together a pack using the best ones.

I don't know how many watt hours I'm using per km, but probably somewhere between 10 and 15. Worst case scenario lets call it 15. I've got a 10km ride. (BTW, the thing is on less than half the time)

So 150wh/54v... I'd need 2.7ah. I should be able to get that much out of my batteries, no?

 

[bbsux]

Oh god, I don't know where to begin...

 

[Ypedal]

In a situation like this..

If it gets the job done and gets you to work.. giv'em hell and just go baby go ! don't worry about the heat factor.. ( different age cells and different capacity including what would be considered unusable bricks won't all play nice ) ..

Considering what they cost you it's all good !

Now.. if you had paid 500 $ for a battery packs.. i'd give you very different advice !! hehe

 

[PaleRider]

Alright, thanks! My attitude is definitely "go for it".

So the batteries that are getting hot are the ones doing the work. They'll be kept. I'll consider the ones that end up cool, low voltage as parasitic and toss them. I'll probably try to figure out how to load test them to confirm my thinking.

Guess I'll find out what works and what doesn't by doing.

(Right now the plan is to back off to 48v nominal and bulk up on the amp hours. At 60v the bike wants to go too fast and I have to work too hard pedaling to keep up with it, and the end result is twofold undesirable - I'm beat and so are the batteries. I need an ammeter so I have some idea of what I'm drawing and how to best deal with that.)

 

[Ypedal]

It's a 2 man job.. but get someone to throttle the bike with the whel off the ground and apply the brakes to load the motor.. while this is happening test the voltage of each battery with a voltmeter..

Anything below 10v is worthless.. between 10 and 11 is ok.. 11v + is good 12 to 13 is excellent and i don't think you will have any in this range ! lol..

The hot one's are working too hard and the cold one's are likely the best and one's to keep but the loaded test will confirm this.

 

[fechter]

I'd guess that the batteries that are getting warm have a higher internal resistance. In a series string, the current is the same, so heat generation will be a function of resistance.

As the batteries warm up, they will tend to develop a higher terminal voltage just due to the heating. I don't think you can automatically conclude that the warmer ones are better or worse than the cold ones, they just have a higher resistance (which is generally bad).

Load testing is good. If you don't want to use the motor/brakes to load them, find another load like a car headlight or an old electric heater to put across each cell while you watch the voltage.

 

[Lock]

I am sooo fed up w/PBA, and actually just placed an order for a LiFe pack, but yesterday I was offered free about 50lbs of SLA's as "used" UPS batts. 5Ah and up... 10-12Ah max I think they were. And these guys get `em regularly. They get zip for these used bats when they turn `em in for recycling (and would I like to take all their old UPS batts off their hands in the future?).

So I *was* kinda wondering what sorta shape the little UPS batts end up in when the UPS electronics decides they need to be retired. Have they really been exercised to death? Or just sulphate w/time? Does the UPS circuitry just decide "it's time" based on months in service period? Are these actually really deep cycle batts in design?

Just curious if anyone here knows UPS boxes and how they treat their batts.

Have to agree w/the sentiment "They're free, what the hell, burn`em up!" hehe Ya know, `s long as they're headed for the recyclers anyway...
tks
loCK

 

[deardancer3]

I am sooo fed up w/PBA,

So I *was* kinda wondering what sorta shape the little UPS batts end up in when the UPS electronics decides they need to be retired. Have they really been exercised to death? Or just sulphate w/time? Does the UPS circuitry just decide "it's time" based on months in service period? Are these actually really deep cycle batts in design?

Just curious if anyone here knows UPS boxes and how they treat their batts.

tks
loCK

There are quite a few different scenerios for swapping UPS batteries. Some make sense and detect a a truely faulty battery, others are "as long as your are there, 3 batteries are due to be swapped in 6 months and the dirve is 35 miles so just swap them now" to "swap everything you can think of , this customer is weary of problems".

Half of the batteries turn out to be just fine, but finding which half is too expensive for paying skilled labor on the clock.

So just charge, load test and keep the best.

my two watts worth.

dick
(ex service manager for remote location computers)

 

[fechter]

Some places will change the batteries on a time basis (2 years, 5 years, etc.) to maintain high reliability. Other places run them until they fail, in which case they are usually fit to be recycled. You might have a mix of both. I can usually get a pretty good idea of condition by measuring the voltage. Anything that is outside of 12.4 to 12.9v is usually not worth bothering with.