Batteries in Parallel

[mettleramiel]

I know that this is probably an extremly amateur question but here it is. I have an electric scooter which has 4 12ah batteries in series to make 48volts. I know that having one or more bad batteries in your bank can ruin all of the others so I want to know that if I add a second string in parallel, but all I have on hand are some 8ah, will I ruin my other ones? My concern is whether or not hookin them up together will bring down the capacity of my 12's down to the 8's.

[AbyssUnderground]

Its best not to mix different Ah batteries because as you state it will affect the larger ones. The smaller Ah batteries will discharge more quickly than the 12Ah batteries so you will damage them quicker than the larger ones.

[Bruce S]

Sounds like you might have one of the Extreme 508 units?

Those batteries in them are of low quality, plus the connections are wrought with cold solder joints. I know I am dealing with one this very moment :->

What AbyssUnderground said is true, it would be bad to mix them, NOT the end of the world but very hard on the 8Ahr units, any possibility you have enough 8Ahr ones to swap out the other ones? The ones in your scooter are the 6-DMZ-14s by chance? if so find some good panasonic ones, you'll be glad you did.

Since I have a ton of Sub-C NiCds I plan on building up a 48V 24Ahr bank.

Will follow up and let you know.

If you can send up a pic of your battery bank and we can possibly swap trials.

I'm copying pics now, will send one up in here soon

Hope this helps;

Bruce S

 

[Bruce S]

Here's what the battery bank may look like.

Hope this helps

Bruce S

[mettleramiel]

No, it is a Scooterteq 500zx and the batteries are of a med-high quality, I got the scooter used, so I don't know if they were the ones that originaly came with it. I fix power wheelchairs for a living and the same brand that is in my scooter is in our chairs. They usualy last people an 3-5 years of daily use. The connections aren't too bad either, all of them clamped, solered and shrinkwrapped. I also have another 36v scooter that I have already added a second string to. It works great and gets me almost twice the distance that I did before. All of those batteries that I added were already 12ah as well. I just put them under the seat which I wasn't using anyway. Just the right amount of room.

Right now, for my 48v scooter, I have the second string in a tackle box, so I can hook it up to the other batteries through the charging port when I need the extra distance. I did it this way, so it wasn't permanently attached since I suspected that higher ah batteries might bring them down (actualy, I thought that the lower ah batts would bring down my higher ones) but my wife doesn't like this set-up as the box sits right at her feet, taking up too much room for her. I think it is an ingenious set-up, myself.

Bruce, how do you like your scooter? Personaly, I bought one for my wife and I ended up loving it so much, that I had to go out and buy my own. We take those things everywhere, hardly even use the car anymore. Everybody asks us everything about them, can't harly go a day without someone stopping to inquire about their specs. I think we may have even sent a few sales here and there. Too bad it's starting to get too cold outside. Gonna be a long winter.

[ghurd]

I say go ahead and mix them like you have planned.

Not only will it not be terrible for the 8AH, it will help the 12AH.

It wouldn't be as ideal as one string of 20AH batteries, but you knew that.

The 8AH will NOT drain faster than the 12AH.  They will be in parallel.  It will take exactly the same time for the 8AH and the 12AH to go from 12.7V (full) each to 11.9V (empty) each.

Say the thing runs 1 hour with the 12AH batteries, it is draining them at C/1, or 12A.

That is hard on batteries, naturally. Completely full to completely empty in 1 hour!

The motor still will only drain at 12A with both sets paralleled.  But the capacity is now 20AH.  The drain is now much less related to the capacity.

Now the batteries go from completely full to completely empty in 1 hour and 40 minutes.

Three good things come from that...

The battery abuse is less because the drain is reduced to C/1.67 and the batteries will have more life cycles.

The rated battery capacity will be higher because AH rating goes up as the drain goes down.

To the store and back will leave the batteries at a higher voltage, meaning less deep cycling or DOD.

Just because the draw is 12A doesn't mean it will take 6A from each battery string.  That would mean the 12AH batteries take 2 hours to go dead, but the 8AH batteries are dead in an hour and 15 minutes.  They are parallel and the voltage will always be the same.

A bad battery in one string will drain the other string because the voltages must be the same.  The good string tries to charge the bad string.

Kirchoff.  What a guy!

G-