Battery charging and discharging

I have a bunch of 12V batteries to use with my kids' powerwheels. I have a 14Ah, two 9Ah, and 13 or so 5Ah. 
 

I have some questions:

1. Some of the time I've wired two of the little ones in series for funner times but a couple times when pulling the discharged set I've noticed one being considerably hotter than the other. What would cause that?

2. For charging I've always used a couple of old APC desktop UPSes. Is it okay to rig the batteries up in parallel for recharging the whole lot at once if they're not all equally discharged or the same "health"?

3. I have like 7 that discharge is just a minute or so. Are they recoverable with those battery "fixer" devices that crank it up to 14V for the coated material to drop off?

Thanks for your help. BTW yes, the Power Wheels Dune Runner gearbox will shred in a day or two of 24V, but the offbrand Suzuki 450 is still running strong.

Alright, I have made a pyramid scheme charging setup out of wire nuts and F2 connectors. Unless someone tells me otherwise, I'm going to hook it up to 4 batteries and see what happens.

I would dig around here https://kidcars.tv/modifiedpowerwheels/

I don't know much but with rc batteries they need to be both very similar batteries and also in a very similar state of charge to charge multiple ones at the same time

Poop. I'll have to buy something to repair them the best it can and try to determine which are similar.

Regarding one battery getting hotter than the other on discharge, it was suggested to me that the second battery in line gets hotter because it has all the electricity of the first battery going through it as well as its own electricity. Does that make any sense two people who know stuff?

Ahh, a "desulfater" is what I'm after! Now to pick the best. I'd take any suggestions. 

I have a fleet of Power Wheels with aftermarket SLAs. I charge our four 12v 12ah batteries in parallel all the time. The first thing I do at the end of the day is hook all four batteries together in parallel and let them sit overnight. By the next morning they are all at the same charge level. Then I hook a normal 12v car battery charger to them (one charger clamp on each end battery) and alternate charging them at 2 and 10 amps. I run the 10 amp charge until things warm up, then I back them down to 2 amp til they cool down.

No idea if I'm doing it right, but I've got several years use out of this batch so far.

slefain said:

I have a fleet of Power Wheels with aftermarket SLAs. I charge our four 12v 12ah batteries in parallel all the time. The first thing I do at the end of the day is hook all four batteries together in parallel and let them sit overnight. By the next morning they are all at the same charge level. Then I hook a normal 12v car battery charger to them (one charger clamp on each end battery) and alternate charging them at 2 and 10 amps. I run the 10 amp charge until things warm up, then I back them down to 2 amp til they cool down.

No idea if I'm doing it right, but I've got several years use out of this batch so far.

This makes sense to me, and hey, if it works.

Me, I'm doing it wrong.  If I want to charge two batteries simultaneously, I've got two battery chargers.

Thanks for the tip on leveling the charges. I just bought a desulfater so I can restore some of the old ones. I'll report back

I got a $35 NOCO Genius 1 or something like that. So far it doesn't seem to have done squat to 3 bad batteries, incl one I topped off with distilled water. 
 

Slefain is basically de-sulfating them in a slightly GRM way, but you should get a real desulfater or two and do it "correctly" because you need pulses of electricity to shake material off. That sulfation is likely why one battery got hot, it just has too much built up junk and plenty of internal resistance to electrons flowing.

Once you've done that, use those 5Ah and the two 9Ah; obviously use only the 5s with other 5s, ect. You should have no issue using a UPS to batch-charge them like you're thinking.

Just know about the "Peukert effect" when you use lead-acids; basically when you combine them together their overall capacity drops slightly because of wacky physics between the moving electrons.

In reply to GIRTHQUAKE :

Cool, thanks. Ordered another one last night that's supposed to display status updates and stuff. Here's hoping. 

Other update- the kids did test laps around the house and I pulled and combined the hot batteries from each and yes, they were the slow ones. I marked them for repair. So far I've seen no marked difference after attempting to repair them with my second desulfater device, but pairing up batteries that do NOT get hot has yielded some fast action. Maybe I'll try a third desulfater and see if there's any improvement. I'm just not getting a lot of positive vibes from the Mrs about spending more than $30. I have a $120 car battery to repair so maybe I can justify a high end one for that. That NOCO garbage is marked for return to Amazon. 
 

Finally shredded a gearbox in the Power Wheels branded Dune Racer. Meanwhile the off-brand Suzuki R450 (or similar) ATV is still kicking. I've fried a few 40A fuses but the gears are working well. 
 

If they're not liquid cells- just AGMs- then de-phosphation will likely only go so far because the sulfate can't be physically removed from the cell.

With liquid acid electrolytes, you can boil the plates by hitting them with huge DC currents (from a welder typically), boiling and shocking the buildup off the plates physically. It's pretty metal, but you can't do that in AGMs.

They're all sealed lead acid. AGM is some mat thing, right? I'm going to check the fluid levels in them and see if that would help. 

In reply to P3PPY :

Yeah, if they're sealed they're likely AGM and you can't alter their acid level. I think golf cart batteries and other "power" cells for UPS might have a few you can alter the acid in them but then you have the risk of tipping and spilling acid.