How to charge my lead acid batteries

I'm 15 years old, and I'm making an electric bike. I have two sealed lead acid batteries. They are both 12 volts, 12 amp hours. I also have a car battery charger that is capable of outputting 10 amps, or 2 amps. Instinctively, I think 10 amps is too high to charge my batteries. What about 2 amps though? And obviously I would be charging these 12 volt batteries individually, not in series or parallel.
Will this charger work if I set it to 2 amps? Are there any precautions I need to take, like unplugging it after a certain amount of time, or only charging the batteries when they're at a certain level, or something?
Thanks, I'm new to large batteries and charging. Not new to electronics, but I am not experienced with batteries much.

You should be fine on the 2A setting. Charge them as soon as possible after riding. Try not to run them completely dead, as this drastically reduces the number of cycles they last.

You could charge them in parallel at the same time if they are close in voltage when you combine them. This is usually kind of a pain though, as you have to connect/disconnect them every time.

I used to have a similar setup and charged one at a time. Just don't forget to charge both of them.

In some cases, it is possible to use two chargers at the same time, one on each cell. The trick here is the chargers need to be isolated on the output or you'll burn something up when the second one is connected. This can be tested with an ohmmeter. Any charger with a 2 prong AC plug will be isolated.

They can be charged in parallel? And I was planning on disconnecting them from the bike anyway. And i only have one charger. What is the lowest voltage I should ever runt gm down to? And lastly, how do I know when to stop charging? Thanks!

Don't go below 10.5V. Stopping at 11V would be better. Never let them sit discharged or they will sulfate (google it) fast. If you have a smart charger, let it charge until it cuts off. If you have an old style dumb charger, stop charging when the amp meter drops to 0. Typical charge voltage is 14.5V and old style chargers just keep pumping voltage into the battery and will burn it out if left on too long. It would be best to parallel charge them. Otherwise one will sit discharged while the other one charges. That is not good. If you're running a 24V system, you should consider a cheap 24V charger.
http://www.ebay.com/itm/24V-2A-Electric-Scooter-Battery-Charger-For-RAZOR-E100-E200-E300-E125-E150-E500-/350591864320

Alright I'll parallel charge them. I don't want a 24v charger, I was just given this car battery charger and would like to use it, but thanks. So I simply connect the batteries in parallel and charge the two? Is there anything I have to make sure, like they're at the same voltage first. Well I guess if they're run in series there's no reason for one to get lower than the other... But any advice? Just stick them in parallel and pump in 2 amps until the current reaches 0 or the charger stops?

They should be close enough to the same voltage at the end of your run to parallel them before recharging. But get a voltmeter so you can check from time to time. If one wears out and the other doesn't, they will be at different voltages.

There are now 12v smart chargers for cars, and old school not so smart chargers out there. If you have the not so smart kind, then you should not leave them charging overnight. Once you see 13-13.5v after the charger is disconnected, go ahead and stop charging.

A 24v 2 amps smart charger should be very cheap on ebay. So getting one when you can is not a bad idea, especially if you have an old style dumb charger.

Here are pics of my charger. It's the smart kind, right?

That charger should be good.
Charge each battery separately overnight then you'll know both of them are fully charged. After that they should stay roughly the same state of charge so making the parallel connection should be OK.

Yes, if one battery was dead and one was fully charged, you could get destructively high currents when they are connected in parallel.

The biggest problem I've seen is accidentally reversing a connection when changing from series to parallel. Clearly mark the positive and negative wires. Using some kind of connectors that prevent a misconnection is a good thing.

Most motor controllers will cut off around 18-19v. Under heavy load, the battery voltage will sag a lot. When you stop the motor, the voltage will bounce back up. When the motor gets noticeably slower in use, its a good indication you've fully discharged the batteries and should stop.

New batteries might take a few cycles to reach full capacity. Try not to over discharge them on the first few cycles, as this can damage the cells.

Max charge is 3.6amps each. If you put them in parallel your max charge current will be 7.2amps.

Use the 2amp setting, and put them in parallel to simplify charging. How old are your SLA? SLA are not very good, aged SLA are not good at all.

If they are completely dead they will take 12Hours to fully charge at 2amp charge rate.

Just leave them on the charger as it has auto shutoff.

Do you have an ebike yet? Most ebikes are 36V+, but I've seen a few at 24V.

Also here is a picture of the stuff it says on my batteries. They are brand new, I just bought them for this bike a week or two ago and they have never been used except for like 5 minutes to test my motor, so that hardly counts given they are 12Ah. 14.4-15v cycle charging, 13.5-13.8v float charging, I guess float means the voltage for when they're already charged and it's keeping them charged?

Also in response to el_walto, I don't technically have an electric bike yet but I have about 85% of the pieces. I'm building it myself as a fun project (I'm in high school) and almost everything is from eBay. The motor, bike, brakes, key switch, batteries, throttle, and charger are here, I just need the controller and some spade connectors I ordered to arrive. It is a 24v system though, the motor and controller that is.

Liam.great98 said:

Also here is a picture of the stuff it says on my batteries. They are brand new, I just bought them for this bike a week or two ago and they have never been used except for like 5 minutes to test my motor, so that hardly counts given they are 12Ah. 14.4-15v cycle charging, 13.5-13.8v float charging, I guess float means the voltage for when they're already charged and it's keeping them charged?

Click to expand...

Basically it means when charging they go up to 14.4. They are then full. A smart charger will then cut off and not charge them again until they fall under1 13.8V or so. A dumb charger will just keep them at 14.4 and if left on long enough will wear them out.

^^ At least that is how i understand it.

SLA batteries do not last a great number of charge cycles, to get the most of them, try to keep them charged as often as possible, like right after you are done riding.

Is there any harm in charging them while they aren't really worn out at all? When I got them from the store they were charged to 13v, and aside from running them for 5 minutes or so they havent really been used. I know people say that it's bad to charge your phone battery (li-ion, lipo, ni cad, or nmh batteries) if it is only slightly discharged. However I've noticed several difference between tiny phone batteries and big lead acid batteries... I'm guessing it's fine for me to charge them at any state of discharge?

Phone batteries and your batteries are different chemistry and need completely different treatment.
NEVER leave lead-acid batteries discharged. Even after a small ride, recharge as soon as possible. Altough you don't need to top charge them every time. A car battery is fully charged all the time and they last for years. It does not harm a SLA to be fully charged all the time, on the contrary it is good to them.
I have one more doubt about your charger: it is not too smart. Real smart SLA-chargers have five phases, your charger has only two but switches off automatically.
Anyway, your SLAs will last only a short period of time. When you are serious about e-biking you will soon switch to lithium.

Yep, that's a charger that will shut off, instead of just keeping on trying to push 2 amps all night.

I wasn't sure, you might have had an old charger from the flea market for all I knew.

If you wish to charge only one battery at a time, you need only connect it to one battery at a time. They can still be on the bike, hooked up to the battery wiring.

I know lithium is better, but these were easy to get for someone in my position. I'll make the switch eventually. My charger is a bit old but it has never been used even once. I'll try charging one of my batteries that's almost already charged this afternoon and since it's mostly charged, it should reach full charge before tonight so i can see if the charger shuts off. So I don't even have to worry about monitoring voltage or current since it shuts off, right? I just have to avoid running them below 12v

Most of those chargers can be left on indefinitely and will switch to a 'float mode'. It may even be good for the batteries to leave them on the charger for long periods. To be sure, measure the battery voltage after it has finished charging. If it reads something under 14v, it should be OK to leave on for days at a time.

I used lead-acid for many years since I could get them for free as pull-outs from medical equipment.
Lithium batteries are way more expensive (new), but have several times the energy for the same weight.
You can get a feel for how your bike runs with the lead acid cells, then imaging having 3-4x more range for the same size battery. Now days, I'm starting to see lithium batteries in the junk pile, so things are changing...

If your charger has a green and red light, when it turns green it's off. As said above, off for smart lead chargers is actually a float mode, that is good for sla's to have overnight.

The type of lead charger I was warning about is from way before you were born.

Today I charged both of my lead acid batteries individually. I had it on the 2 amp setting. When they were fully charged the green light came on and the needle was at 100% charged. I measured the voltage before charging with an analog DC meter and it was 12ish and post-charge it was 13ish. The one I've been testing stuff with took longer to charge, which tells me somethings working properly.
Also on my charger, there is only one meter and the amp gauge is the same as the charge indicator. So if you look at the pictures of the charger in a previous comment, you'll see how it looks. I noticed a strange thing happen though. When green LED came on indicating a full charge, I left for a minute and when I came back the green light was off and the gauge showed the needle a tiny bit away from 100%. I watched for about 30 seconds and then the green LED came back on. I kept watching this time, and after about a minute the green light turned off and the needle swung down to 50% and then you could see it moving and it took a few seconds and went back to 100% and green light. At this point I disconnected the battery (just because I was cleaning up, not because anything was wrong) but I assume it would have kept going through this cycle of saying charged and then the green light turning off and back on.
Is this phenomenon common? Is it my "sort of smart" charger's way of maintaining a float charge, by turning off an on or something? This is also my first time charging these batteries so could this be something to do with the characteristics of lead acid batteries and how they need to be charged a few times before they can hold their full capacity?
Unfortunately since my bike is incomplete (waiting for the speed controller to ship) I have no way of running my batteries and charging them a few times. Because if I run the bike 20km or so until it is noticeably lower power, and charge the batteries, and go through this cycle a few times, I'd know that obviously the batteries are getting power from somewhere. But like I said, I am unable to do that with an incomplete bike. As a matter of fact, the bike is in a repair shop getting a new crank so it's gone until friday.

Also about those "dumb" chargers that are older than me, that hardly sounds like a charger more like a 12v power supply with an ammeter!

The charger is trying to maintain a full charge.If it's turning back on every few minutes, your battery isn't holding a charge properly. You might try desulfating it if your charger has a recondition mode. I've had varied results doing this, but it's worth a shot.

Brand new SLA need to be cycled a few times before they hold their top charge.

So the reason it kept jumping down to "not charged" was because they're still new and need to be charged a few times? Also I just was doing some research and found that there's some rule about charging at 1/10th the capacity. So for me, 12Ah divided by 10 leaves 1.2 amps. So I'm supposed to charge at that current? My charger only goes as low as 2A, so should I ONLY be charging these in parallel? Because then I'd have 24Ah which, divided by 10, leaves 2.4 amps which means my 2A charger isn't over the limit.

Don't worry about the charge current. Even with a single battery it will be OK.
I have a similar 2A charger and when the battery first reaches "full" it is not really full and will continue to cycle on/off for a long time. Even after a week on charge, it will still cycle but very slowly as the batteries have a certain amount of 'self discharge' which will cause the charger to kick in periodically. This is normal.

Ok, thanks. It's always good to hear from someone who has practical experience with almost an identical situation as what I'm asking about. I read somewhere that charging AGM batteries (which is what I have) above the specified current can cause them to sulphate and/or cause them to hold less charge. I believe they said a 12Ah battery charged at too high currents could turn itself into a 6Ah battery. But I trust you know what you're talking about, and charging my batteries in parallel doesn't even violate the 0.1C rule. So I believe I'm good.

I agree, that you will need to break in the batteries gently, and then they will develop their full capacity.

Once you get up and running, take some very short rides, like less than a mile. Then fully recharge them. Do this about 6 times before you start using them more.

One full discharge will be needed sometime, to get a feel for what your max range at a certain speed is. Then try to aim for no more than half that. But of course, at times you will simply need to use it all rather than pedal home. Do try to plan your trip to use less on most rides.