36V SLA - expected voltage after charging?

[yoyoman]

I bought a used scooter with 3 SLA 12V12Ah batteries, which I hope to repurpose for an electric bike.

What should their voltage be right after charging? I started the charge with them at 12.03, 11.5 and 11.66, and right now they're at 12.66, 12.25, and 12.31 for a total of 37.22V. They have been charging for 2 hours, and the batteries are not the slightest bit warm. What voltage should I expect or hope for?

Is it OK to let them charge overnight, or does that depend on the charger? I'm just using the scooter charger connected directly to the batteries.

 

[Anonymous]

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[yoyoman]

So far, at I guess half the charge time, I am at 12,94, 12.48 and 12.52 for 37.94 total (these are being charged in series). I hope the increase in voltage is a good sign that they will be usable!

Hmm, the green light on my charger is lit when connected to the batteries and is more strongly lit when I plug into the wall ...

 

[Anonymous]

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[D-Man]

My old ones go about 13.1 once they are pulled from the charger and sit awhile.

 

[Rassy]

YoYoMan, because there is a fair difference in the voltages, and they have probably been abused, I would recommend you charge each of them with a 12 volt charger (such as a car battery charger with a 2 amp setting). Then you can find out if each individual battery will hold something around 13 volts. Once they have been equalized, and any bad ones replaced, then using the 36V charger in series should work okay.

 

[fechter]

The voltage just immediately after charging is not a good indicator. Let them rest for 12 hrs, then measure. Good batteries will generally be between 12.6v and 13.0v.

A better measure is to place a load on the battery (car headlight, big resistor, actual motor load) then measure the voltage under load.

 

[yoyoman]

I don't own a car charger, and I'd prefer to limit spending.

Can I discharge the highest voltage battery to a level close to the others, thus somewhat bringing them into "balance", and then continue charging them in series?

 

[D-Man]

You can. Just monitor the voltages so they don't go over 15 volt while charging. I sometimes use a 12 volt spotlight to draw some juice out of one. A couple hours of trickle would be good after they are charged as stated above. Usually the charger gets pretty hot when charging. After it goes green, it will cool down if you have a normal 3 stage type charger. When the charger goes cold, that means its not charging anymore or not doing much. I'd leave it on the charger till this happens.

 

[yoyoman]

Being impatient, I decided to try out the batteries on my first ride. Although I have 3 batteries from the scooter, my $50 craigslist purchase is only a 24V brushed motor. So, I took the two highest batteries and tried them out. I live in a hilly area, and going downhill, I probably should not have used the assist to go faster be/c on the way back, it ran out of juice on the climb. I just measured, and the batteries are at 12.34 and 11.75.

Yup, this is definitely not looking like an optimal charging setup with all the batteries being so far apart.

Once I get them more balanced, what are the dangers of using all 36V on the bike? I've read the motor can probably handle it, but will there be any advanced signs that my controller can't handle it, or will it just go up in a poof of smoke?

I do have the 36V controller from the scooter, but I'm eyeing that and the my1018 motor for some other crazy project.

 

[yoyoman]

ok, some last stats before I get a chance to balance them.

Final charging voltage: 13.69, 12.72, 13.08
After a night unplugged: 12.73, 12.34, 12.58

Not quite in the "Good batteries will generally be between 12.6v and 13.0v" range for all of them, but maybe that will change when I balanced them a bit.

 

[D-Man]

Those batteries were bought 2 years ago and a BB BP12v-12ah was 25.99 then. Now its at least $42 for the same one if you shop around and its their entry level battery. Probably going to switch brands next time as BB is a rip-off now. Almost as much as a regular car battery. Their EVP and EB are even more. Thats a joke.

 

[TylerDurden]

Crystalyte 408 Go-Hub / 20 amp controller /Bontrager Hank 26x2.2 Slicks
Sla Batteries: BP series 42v 12ah - 3155 miles on $95 total battery cost.

Do you know how many miles you have to bike to get the price down to $1 per gal?

A few thoughts:

3155mi @ $95 = .03/mi

.03 * 20 = .60/gal (assuming 20mpg)

3155 *.6 = 1893mi @ $1/gal

Or... if 800mi = $2.5/gal, then 2000mi = $1/gal (assuming the batteries survive; and D-man is, well.. Da Man. )


@ yoyo... usually it's just poof. gone. no warnings. If you bust-open the controller and get the FET and cap ratings, you might have a better idea if it's gonna live or not.

 

[yoyoman]

Oh, if busting open the controller will help, I'll try to open it up later and take some picts.

It's a puny sized controller, and inside the big cap says 50v 1000uF, and the FET says Texas Instruments LM339N. Does that help, or should I snap a picture?