# Charging SLA Battery in car



## Ready (Jun 16, 2010)

I have a 7ah SLA battery. Can I charge it directly from the 12v cigarette plug? I want to build a 12v cigarette plug to alligator clip adapter and just hook them up to the battery. I know I would have to watch and not leave it there all the time. Will it harm the battery?

Thanks for the info.


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## cottonpickers (Jun 16, 2010)

I wouldn't recommend it. 

A lighter socket is usually 10amp , and if the sla battery has a low voltage you could easily exceed this as the current rushes through the wiring to equalise (as current wouldn't be limited). This would likely blow a fuse or possibly melt wiring. 

There also the chance that you will short the clips with the same result. Better solution is to fit a female socket on the sla and get a 'lighter-lighter charger' which will limit current and be fused.


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## core (Jun 16, 2010)

I would second that: Don't do it.

Even a "lighter-lighter charger" probably isn't going to be current limited to the point where it's appropriate for a little 7Ah battery. (If you don't care about your SLA battery, well that's another matter.)


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## alpg88 (Jun 16, 2010)

you really don't want to charge 7ah battery with 10 amps, usually sla of that capacity charged with 500ma for 18 hours, slas don't take fast charge well. 
boosters that have 7-15ah batteries inside can be charged with cigarette plug, (car chargers are included in the box) but usually there is a big sand resistor in there to limit current, or in good quality boosters circuit board.


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## Lynx_Arc (Jun 16, 2010)

one other thing even worse.... charging an SLA while running the engine it would be like a 100 amp alternator trying to charge a 7amp battery. Think about charging a nimh AA at a 30 amp rate or a 4 minute fast charger and you will get the idea.


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## core (Jun 16, 2010)

Lynx_Arc said:


> one other thing even worse.... charging an SLA while running the engine it would be like a 100 amp alternator trying to charge a 7amp battery. Think about charging a nimh AA at a 30 amp rate or a 4 minute fast charger and you will get the idea.



Well the 100A alternator isn't a problem, but the increased voltage (say 14.5V instead of 12.6V) will result in a higher charge current on top of it all, yes.


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## Lynx_Arc (Jun 16, 2010)

core said:


> Well the 100A alternator isn't a problem, but the increased voltage (say 14.5V instead of 12.6V) will result in a higher charge current on top of it all, yes.



I realize you won't probably won't get 100 amp into it but... if the battery for some reason is damaged and shorted it has the potential to draw a lot more current at 15v than directly off a car battery at ~12.6v.


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## march.brown (Jun 16, 2010)

Ready said:


> I have a 7ah SLA battery. Can I charge it directly from the 12v cigarette plug? I want to build a 12v cigarette plug to alligator clip adapter and just hook them up to the battery. I know I would have to watch and not leave it there all the time. Will it harm the battery?
> 
> Thanks for the info.


 Can't you just use a 12 volt charger or a trickle charger at home.

Some people with caravans use a system that will charge a second 12V battery that can be used to power lights etc in the caravan ... They use a diode in the circuit to prevent the caravan from discharging the vehicle battery ... Perhaps a visit to a caravan shop will provide you with the necessary parts.
.


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## core (Jun 16, 2010)

march.brown said:


> Some people with caravans use a system that will charge a second 12V battery that can be used to power lights etc in the caravan



Caravan? I actually had to look that one up just now. (Caravan == Recreational Vehicle for you USA people) Yeah, the setup you're talking about I would call a "house battery". I'm not sure any RV charge controllers can be set to such a small current limit though. I could be wrong.

To the original poster: I think your best bet is to just get a compact hobby charger (they all accept 12V input) and do it right. What's that little Accucell going for now, maybe $20? 

How do you charge your SLA at home? It's quite likely whatever you use to charge it runs on 12V, no?


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## Lynx_Arc (Jun 16, 2010)

march.brown said:


> Can't you just use a 12 volt charger or a trickle charger at home.
> 
> Some people with caravans use a system that will charge a second 12V battery that can be used to power lights etc in the caravan ... They use a diode in the circuit to prevent the caravan from discharging the vehicle battery ... Perhaps a visit to a caravan shop will provide you with the necessary parts.
> .


you could use one but there is a chance it charges at too high of a rate to be good for the SLA in the long run as it may be designed for charging large lead acid batteries instead of smaller ones.


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## 65535 (Jun 16, 2010)

I can't imagine you don't have at least ONE 12V wall wart, it it puts out 100mA-1A you'll be set. Just stick it on there over night, THough I'd do it outside, I've smelled SLA's leak not nice.


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## core (Jun 16, 2010)

Hrmmm...

Are we talking about needing to charge a 7Ah SLA *from a vehicle* or do you just need a way to charge it period, and you thought going out to the car would be the easiest way?


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## SilverFox (Jun 16, 2010)

Hello Ready,

I do it all the time...

I have a jump pack that has a gel cell battery in it. I use the compressor to charge up my tires. After using it a few times, I check the voltage and when the battery gets low, I hook up the 12 volt adapter and charge the pack battery while I am driving.

Your battery is smaller than mine, but if you watch things it should be OK. Mine tests out to 16 Ah, and it takes 4 - 5 hours to charge it. I estimate the charge rate to be in the 2.5 - 3 amp range. This may be a little high for you battery, but other than reduced cycle life you should be OK, as long as the charging system on your car is in good condition.

Do I recommend doing this? NO. Look at all the reasons that others have posted to understand why. However, if you pay attention to the charge it is possible to do it and do it with some safety. A good place to start thinking about this is to evaluate what would happen if something went wrong, and take measures to contain any possible damage.

Tom


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## core (Jun 17, 2010)

SilverFox said:


> I do it all the time...
> 
> I have a jump pack that has a gel cell battery in it. I use the compressor to charge up my tires. After using it a few times, I check the voltage and when the battery gets low, I hook up *the 12 volt adapter* and charge the pack battery while I am driving.



Tom, have you assembled your own charging method for your jump pack, or what exactly are you connecting? Running 12V to the _jump pack_ through its _intended charging jack_ is a whole different ballgame than directly connecting 14+V directly to the terminals of a SLA battery!


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## Ready (Jun 17, 2010)

Wow so many answers. Yes I was wanting to charge it from the vehicle while driving. I forgot about the high amps being a problem. 

I don't have a hobby charger for them yet. I have been using a foldable solar 12v 6w panel to keep them charged. 

I do have wall warts but I guess I would have to watch them and not leave them on there too long, since they don't have any trickle capability, is that correct.

I had been looking at this one:
http://www.batterymart.com/p-12v-1a-sealed-lead-acid-battery-charger.html

But I also wanted to have a way to charge in the vehicle to top them off before I get to a campsite or something.

Thanks for all the replies and help.


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## SilverFox (Jun 17, 2010)

Hello Core,

My jump pack has a 12 volt adapter that attaches directly to the battery. One option for jumping a vehicle is to simply put the jump pack in the vehicle and hook up the packs 12 volt adapter to a 12 volt adapter in the vehicle. This put the jump packs battery in parallel with the vehicles battery. You wait for the charge to equalize between the two batteries and then start the vehicle.

The size of the wires and fuses in the vehicles 12 volt adapter circuit limit the amount of current that flows when using this method. It is quicker, and sometimes more effective, to use the heavy gauge jumper cables built into the jumper pack and hook them directly to the battery. After the vehicle starts, if you leave the jump pack hooked up, its battery will be charged by the vehicle charging system but there isn't usually enough room to leave the jump pack hooked up while driving down the road.

The jump pack also has a charger built into it that runs off of household AC, or you can charge its battery using another battery charger connected to either the jump cables or to the 12 volt adapter.

The jump pack battery charges to 14.1 volts when charged from my vehicle, and 14.4 volts when charged from its built in AC charger.

Tom


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## Illum (Feb 25, 2013)

Sorry to bump this, but SilverFox mentioned charging his jumpstarter directly from the car, care to elaborate the setup? 
I'm currently trying to figure out how to float [again, not charge but float] about 18AH worth of SLAs directly from the car. Currently the concept is simple, A fuse, a TVS, a 3A forward diode feeds a capacitor bank filled with everything from 0.01uF to 3300uF. That then feeds a 9-32Vin 15Vout Cincon DC/DC converter. 
Since the converter is only 10W at 15V, two 1N4001 diodes are used to drop the voltage to around 13.4V. On paper it seems plausible, but on the car an array of problems is causing it not to work as intended. Namely the noise interference which I suspect is radiating from the converter.


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## SilverFox (Feb 26, 2013)

Hello Illum,

My jump starter has a 12V plug receptacle built in and a cord that allows me to plug it into the cars 12V system. I simply plug it in and drive to top things off. I do NOT leave it hooked up beyond the time it takes to drive to my destination.

If the battery in the jump pack was discharged, I would first hook the jump pack to the cars battery with the engine off and let the two batteries equalize. This takes 15 - 20 minutes. I would then start the car and let it run for about 10 minutes. This would put a "quick" charge on the battery in the battery pack. This raises the voltage to a point where current drops off and there is no problem hooking up to the 12V receptacle in the car would present any problems.

After this "quick" charge, driving to my destination completes the charge.

Tom


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## Norm (Feb 26, 2013)

Remember in the old days, we used a bulb in series with the charging circuit to limit current?

Norm


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## VidPro (Feb 26, 2013)

I do the same here that silverfox related, mostly when I *have to*, on the go. When I am home, i would use a normal charger or hobby charger. I have a Male-Male car cord, straight wired with fuses on both ends, so if there is any Tip (+) to ground connection that occurs, it is fused for it.

Then, I just connect it via the cigar lighter socket, and manually terminate. the cig socket is good for 10A usually, as long as the other battery is not Way low, it shouldnt be a problem. there is a lot of wire to get to the socket itself so there is already some resistance.

One of the cords I use has a Led light that is in the car cord connector, that one is much better, with the tiny led Indicator, I can tell if the connections are made, and if the fuse is broke or not. Another trick was to put one of those Car Cord Automotive testers on the end of the cord. That end plugs into the battery, and can give an estimate of the voltage or charge state.

This year, china has a Cig lighter automotive tester that actually shows a Voltage, would make a good end of a car cord. that and those cheap voltmeters that will run off the power that is already there. with stuff like that, seeing the charge state and voltage would be much better.
With a 6-10foot charge wire, and via the resitance in the car cig lighter already, if the "metering" device plugs into the small battery it will mostly read the battery itself while charging. so although it is reading Both the car voltage and the battery voltage, it is still closer to reading the battery, and works ok. If i then disconnect the One side going to the car, I can read the battery only. 

the one i used before was the radioshack auto alternator charging thing , has 3 lights used in combo, red =low , red+yellow =med , Yellow=ok and green was charging or charged. Simple quick Interpretable idea of the charge.
A Pic of that one here http://www.google.com/imgres?imgurl...a-MK-jriwKvkIGwCQ&itbs=1&sa=X&ved=0CDcQrQMwBQ

And something like this , is what i would use now http://www.amazon.com/dp/B007CT6510/?tag=cpf0b6-20 Just as example pictures because they are not so easy to find using search http://www.amazon.com/dp/B0092KVYGI/?tag=cpf0b6-20


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## Illum (Feb 26, 2013)

SLAs are best floated at 13.8V, my car puts out anything from 10V to 15+V depends on what its doing, at ignition, idle, compressor click on, compressor click off, etc.  I've once made a male to male connector by soldering two male connectors onto one wire. On one side I modified the fuse compartment with a big fat resistor (1 ohm, 5W I think :thinking:] in series with a salvaged 1N5822 diode, I figured I can keep the jumpstarter battery well juiced and alive should I ever need it.  When I did need it I found the jumpstarter battery boiled dry and molten plastic in the connector where the resistor resided. I never did it again, but when I bought a new jumpstarter the first thing I did was to rig up a charger plug that bypassed the existing port.  I used a dedicated battery charger [1.5A for the 18AH battery] as opposed to using the cheesy unregulated wallwort that puts out...oh I don't know... 500mA? That battery was very happy for about 2 years, then I let someone borrow it and whoever used it drained the battery completely flat and neglected to tell me about it. 

I remember that Norm, come to think of it back then "precision" isn't really given much emphasis. Common lightbulbs were used as NTC thermistors just because they work, and if one blows prematurely you would serialize two on the next try. Epic voltage droppers though 

This is what I have in mind, a rough sketch, based on the available parts on hand. the biggest problem is probably going to be isolating electrical noise. Namely, preventing noise coming from the car to interfere with the step-up converter and preventing the noise generated by the step-up converter to interfere with the car. I'm sure a coupled inductor on the incoming side might curb some noise. A varistor or a PTC thermistor for inrush current protection might not be a bad idea, but I have no idea how to calculate what parts I need. Simple really, let the car voltage fluctuate while the output maintains at 15V. put everything in a fat aluminum box, install a couple quiet fans, and it should be good to go. 






one of these will be integrated into the mess and that alone draws 40mA, so estimated max charging current is 600mA until the converter heats to about 70C. http://www.alton-moore.net/graphics/desulfator.pdf If the battery is pretty full to begin with, its resting voltage should be quite high,that should prevent the converter from "injecting" current into the battery correct?


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## Genius (Jul 9, 2014)

Sla Battery's on board your car /van/truck. hi there folks Just a quick one to total put a lid on this one . Yes you can charge a 7 Ah gel battery from your cigar port . I have 25 years in electronics and 15 years automotive Recky mech . 
I have Hooked in to my service van Two Booster packs one is a dual mega boost 800 12/24 volt system Anderson pickups and the other is a single 20 ah AGM snap on Model for small stuff and backing up ecu's. I Also have running off the same system a second 
Sealed ultima / optima . (Yellow top) This is hooked Directly to the main starting battery Via Smart Relay 250 Amp rated master fuse. dual controller for both units also installed displays volts amps. the second battery the Yellow top has a 4 way Cigar socket
hooked to it via 30 amp fuse and is fed with a stretch of 4 mil twin red/black . this some might say is overkill but in my job overkill has to be So it's failsafe , now hooked in to that I have two led inspection lead Jedi light's my single SLA Snap on pack and that
leaves me one spare socket for my Compressor or my mag charger / head torch charger my snap on pack is constantly attached to the main system this item was purchased from snap on in 2008 it to date In its life has probably done now 1500 jumpstart's and countless ecu backups while I have the main battery out for what ever reason . The Bigger Booster I have mentioned in this post has it's own dedicated ciggy port that's hooked direct to the First battery from a spare master tap in the second fuse box . 
I also have two large Anderson pick ups front and rear hooked to both main battery's for 35mil jump leads . oo:

these units are charging when the engine is running sometimes it's running for 15 hours or more depending on the night and I recently did a full drop test on both booster packs and they are still fine . a ciggy lighter hooked in to a 3 amp fuse before the cell is 
fine . it will slightly decrease the over all life of the cells but not by much as my snap on unit is over 5 years old now and my mega boost is 2.5 years . SO YES U CAN


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## IonicBond (Jul 10, 2014)

The reason you can attach an sla directly to a vehicle cigarette lighter socket is that it will accept current very fast, the voltage of the sla itself will rise very quickly, and thus do the self-limiting of current on it's own, aka the "absorb stage".

But this is not perfect for a direct attachment, and the jumper pack manufacturers usually spec something like not exceeding 4-5 hours max doing it this way. They are also usually in the 18ah range or so, you are using 5ah. Thus I would limit it to no more than 1 hour.

You may also be exceeding (almost guaranteed actually) to be going well beyond the "maximum inrush current" rating, which for most general purpose agm's, is about 0.25 to 0.3C. For a 5ah battery, that would mean a max of 1.5a!! With the direct-connect, you will initially exceed that, but for a VERY short time as the sla terminal voltage will rise very quickly, and self-limiting of current will occur.

But this is not something you want to do on a regular basis. You'll be cutting down cycle life, and also creating "hot spots". Thus every once in awhile, if you charge normally, grab an IR pointing thermometer and scan around the case and terminals. Basically you are just beating it to death and getting away with it, until the short cycle life catches up and you merely replace it.

Still - 5ah direct connect? I'd be VERY careful!

If that weren't enough, we have another problem with the fast-charge direct-connect method: Peukert's law. Basically, the faster you charge and discharge, the less efficient that process is. In other words, when you charge directly from the vehicle, when all is said and done, your 5ah battery may only have 2.5ah true capacity due to the fast charge. Going further, since you don't want to take any lead acid battery past 50% discharge for decent cycle life, let's cut that in half to having a little more than 1ah of *usable* capacity. Thus direct connect is really only suitable for emergencies just to get something into it. Also, because of Peukert not really allowing for a full charge, that means your battery is going to sulfate, and reduce capacity every cycle. If you do the direct connect often, be SURE to use an ac charger at the proper rate to truly get a full-charge into it once in awhile!

Be sure to use a charge controller with that solar panel. Otherwise, especially with this small battery, when charging it will try to rise to meet the voltage of the solar panel itself at the end of charge, which for a "nominal" 12v panel is usually around 17-22v. Not good since you don't want to take an sla agm beyond about 14.4 to 14.7, especially if it is not temperature-compensated. A simple inexpensive yet quality charge controller like a Morningstar 4.5A "Sunguard" would be perfect for your use.


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## phaseform (Nov 20, 2021)

Why not just use a current limiter circuit?

250W 10A Step Up Boost Converter w/ Current Limiter for Arduino DIY Power LED


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