Mixing Amps in a Battery Pack

In the general case, no, that's not a good idea. If you keep the discharges low then it should be acceptable.

One should always strive for balance .. in this case, balanced voltages across all batteries in a battery pack. The issue will come towards the end of a discharge.

One way to harm a battery pack is when one battery is close to being depleted while others are not depleted. If the user continues to use the pack, the battery that's almost depleted is in a state of deeper discharge than the other batteries. The deeper a battery's discharge the greater risk of damage to the battery. Also in the case of a pack with misbalanced batteries, the batteries with the higher voltage end up working harder than the ones with the weaker voltage.

It would also make charging that pack more complex. The best way to charge such a pack is with two chargers, 1x 24v charger for the 2x12v15ah batteries and 1x12v charger for the 18ah battery, or else just go with a straight bank charger setup. But if you charge that with a 36v charger the 18ah battery is going to skew the determination of when charging is finished and you run a risk of the 15ah batteries being regularly overcharged.

I found this at:

http://www.zbattery.com/Connecting-Batteries-in-Series-or-Parallel

The last paragraph on the bottom of the page -

"Important notes: When connecting batteries in a pack there are some important things to keep in mind - - Find out the requirements of your application. For example: Don't double the capacity on your Power Wheels vehicle if you're not supposed to...you could burn up the engine. Follow the recommended guidelines for your application. - Don't use two different chemistries when connecting a pack. Usually the voltages will be different, but more importantly the charge rates will be different and the capacities may be different, thus resulting in a shortened life span. - Try to match capacities as much as possible. When connecting batteries in a pack you should try to match the capacities as much as possible to avoid discharging one battery quicker than another. A pack operates at a combined voltage so your one cell that discharges quicker will likely discharge deeper than it may be able to recover from."

David's explaination above is more comprehensive...

Harlow

I have done just what you're proposing, using Rikiman's suggestion of bank charging, and not ever fully discharging the pack. Harlow's link is accurate. You'll add 12V to your battery pack's total voltage giving you 36V. Make sure you setup can handle the increased voltage.

If you're trying to add more range, just add 2 more 12V batteries in parallel (not series), and charge and use as normal. Just be sure your batteries are reading equal voltage when you connect the two packs.

My thoughts are that you should use your existing batteries, purchase an additional 12V 15Ah and wire it in series with your other two. Buy a 36V charger and a 36V controller and run your S650 motor at 36V. In a few weeks, or maybe months, when your range has dropped to just a few miles, you can replace them all with a brand-new matched set of three. Of course, the same thing will happen to them, unless you bank charge, or install Equalizers.

I'd suggest, at a minimum, you monitor each battery with a Digital Multimeter, or voltmeter, and watch how much they begin to vary from each other. Without some way to keep your pack equalized, you are looking at a shorter life.

I bought a complete Schwinn S750 (2006 model with chain drive and 750W motor) from craigslist. Paid $80 and all I had to do was put in batteries. Runs great! I had several 12V10Ah SLA's lying around that had been removed from other scooters (In a pack of 3, there is usually one "revivable" one). I cycled them on the bench with a power inverter to discharge, and a Vector 12V charger to re-charge. I also ran them through a few desulfate cycles. I put three in the new S750. Each was a different brand from a different scooter and a different age. I bank charge them in parallel, and discharge in series. I have gotten surprisingly good performance from these old "discarded" batteries. Sometimes, I think they are even getting a little better. I get higher readings after a charge, and after a discharge, on a fixed route that I take every day. I don't know how long they will last, but they are as good or better than when I started a month ago, and I cycle them twice almost every day.

Contrast that to the Schwinn Stealth 1000 that I bought in May and needed new batteries by July (two mile range). I replaced with (3) 17Ah and they were dead in 3 months. The common denominator? String charging with the stock 36V charger and no provision to equalize between batteries.

Let us know what you do, please.

Harlow

Just wanted to follow-on to my last post with another example.

At the end of August, I picked-up a 2005 Schwinn S600 from craigslist. It had originally been a Christmas gift for the 12 year-old daughter. She rode it about 2-3 times in 3 years. I'm guessing it has less than 5 miles on it, judging by the tires. The original batteries were long gone, of course, and they had replaced them with the OEM "Bag O' Lead", and it was dead too. I'm thinking the "new" batteries had been cycled maybe twice, and had sat for over a year, until it was time to clean out the garage. I don't know if it was fully charged after it's last use, or whether it was stored partially discharged.

I removed all three batteries and measured their resting voltage:

12.57
9.34
10.59

How's that for a variance?

it's seriously crappy.

Yeah, that is indeed crappy.

SLA 12V fully charged and new should get something like 14.xV.