I've seen quite a few people coming to this site with search queries identical to this article's title. Since there is nothing explicitly written anywhere on how to charge two 12 volt batteries with a single 12 volt battery charger, here you go. Really, it's quite simple, but there are a few "best practices" to keep in mind.
The first thing you need to do is to take the batteries out of the vehicle. It's not actually a strict requirement, but it's a very good thing to do. You don't need to mess up your vehicle electrical system, and if you've got two batteries, who knows how they're connected, what they power, if you have a charging isolator, or other complications.
Now that the batteries are safely on your workbench, connect them together in parallel. Take a set of jumper cables and connect the positive terminal of one battery to the positive terminal of the other battery. Do the same for the two negative terminals. Remember to keep things simple - the red goes to positive, the black goes to negative.
By connecting the two batteries in parallel, we are allowing the charge between them to equalize - if one battery is charged more than the other, then the excess charge in one will be driven to the other, until the charge (measured by voltage) is exactly the same on each battery.
Be sure not to connect a completely dead battery to a fully charged battery. The amount of differential voltage can tend to send a large amount of current (amperage) to the dead battery, causing a significant amount of heat to build up. This is a bad thing and should be avoided. If you have a completely dead battery, charge it on its own before connecting the two together.
Finally you need to connect your battery charger. Now here's the trick: don't just connect the charger directly to one battery. Instead, connect the positive lead from the charger to the positive terminal on one battery, then connect the negative lead on the charger to the negative terminal on the second battery. This is a small detail, and in truth makes only a bit of difference.
Turn on the charger, and let it do its thing. Remember to select the correct amperage on your charger for you battery type (although not all chargers give you the choice), and also remember that you'll need twice the time to charge two batteries in parallel as you would need to charge a single battery.
We hope this helps - have more questions? Ask below and we'll see what we can do!
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what about 12 volt in series (24volt) charging with a 12 volt charging system (alternator or battery charger)
Rex, as far as I know, you cannot charge at 24 volt system with any 12 volt charger or alternator, without the help of a transformer that would convert the 12 volt charge to 24 volts. In practice this would get quite complicated, and also expensive.
Take a look at our post “24 Volt Battery Equalizer Install” for some info on converting 24 and 12 volts.
Rex,
If it’s two 12v batteries connected in series to make one 24v battery, then you can still remove them from the vehicle and connect them together to charge them in the same way that you would charge two 12v batteries that are normally connected parallel.
Robin,
While working in the north we had a auxilary power unit that had two 12 volt batteries in series to create 24 volts for use in the helicopters. There were two battery chargers installed which charged one battery each, I believe. I am presently trying to confirm this as I would like to build my own. Would you happen to have any insight?
Thanks,
Rob
Hi guys, as an alternative is it possible to charge a 12v battery with a 24v alternator/charger
Craig, yes you can charge 12v batteries with 24v chargers, but you need two batteries connected together in serial rather than in parallel. See our post on connecting 12v batteries in serial here.
Thanks Robin, what i am really asking is can you charge ONE 12v battery with a 24v charges. i would imagine you may need a step down etc?
rgds
Craig, you’re right you would need a transformer to change the voltage from 24 to 12. Remember that when we say “24″ or “12″ volts, these are nominal voltages. The actual charging voltage might be different. For example a 12 v automotive alternator might put out 13.5-14 volts when charging; i.e. the output voltage of a transformer matters.
The other issue I would be concerned about (this is now beyond my understanding of electronics) would be for the charger to know when to to stop, or when the battery is fully charged. If you have a transformer in the middle of the system, the feedback voltage of the battery would be buffered and it might not work.
I would look really carefully before doing this! Take a look at this article on 24v battery equalizers if you haven’t already – it might be what you’re looking for.
thanks for the advice Robin. will look into it further.
Hi,
To charge a bank of 24 x 12v wet flooded car batteries (the largest being 100amp and smallest being 40amp what type of battery charger is required in terms of power, can they be charged together or will the size differences be an issue and are they still wired up pos to pos and neg to neg regardless of the ammount of units?
Many Thanks
Hi, is it possible to connect 2x 12v batteries in series, to power an 24v device, and simultaneously connecting 2x 12v chargers (one to each battery)? So basically it will be a “permanent” setup where you plug the 2 chargers in for charging, and the 24v output into the device after charging, without having to remove cables each time. Thanks for any help.
12v charger(1)
/ \
/ \
|—- +12vBAT(1)-
| /
| /
| /
| /
| +12vBAT(2)- ———-|
| \ / |
| \ / |
| 12v charger(2) | -
| 24v output
|—————————| +
Ooops, hope this looks better:
00012VCHARGER(1)000
000000/0000000000
00000/00000000000
|—-00+12VBAT(1)-000
|00000000/000000000
|0000000/0000000000
|000000/00000000000
|00000/000000000000
|0+12VBAT(2-0———–|
|000000/00000000|
|00000/000000000|
|00012VCHARGER(2)0|– -
|0000000000000000024V OUTPUT
|————————————– +
Still not….hope you get the idea
Nice try Derick! The answer is yes, it would work fine as long as your chargers are smart enough to know when to charge and when to stop – like an alternator does in a car. Otherwise you run the risk of overcharging the batteries.
I’m interested in connecting 4 12 volt batteries together so that I get a 24 volt source and 2x the capacity.
Is it best to connect to pairs of in series (2 sets of 24V) in parallel, or should I connect them in parallel first and then in series?
Also, what will it be best to charge this 24v system with a 24V charger? If each batter is 7 amp-hours, what amperage of charger would be ideal?
Thanks!
Steve, check out this article on parallel and series battery connections. You’ll see from the diagrams on the article that there is no real preferential order in how they are connected.
The size of the charger is more related to the type of battery rather than the battery capacity, i.e., sealed lead acid, AGM, NiMH, etc. This is more complicated and worth another article. In the meantime you should do some research on the net first. I’d really appreciate it if you write back what you find!
Hope this is helpful to you – let me know if you have more questions.
I have a 24 v. trolling and a two bank 12v charging system. I will need to hook up the 2 12v batteries in series to get 24 v. Can I still hook up the 12v 2 bank battery charger + and – to each battery for charging?
Mark
Mark, that should work. I have a 24 volt Land Cruiser and have had the 2 12 volt batteries, connected in series die. Of course no is around with another 24 volt car, so I jumped the batteries with 2 cars and 2 sets of jumper cables – each 12 volt car connected to each of my batteries. This is the same thing as you are asking.
please please help..
i have a 600va prolink ups.
the built in battery is 12v7ah/20h.
the battery gone dead (now it is 4.28v)and i am not smart enough to revibe the batt.
so i purchased new one 12v 12ah SLA to get more backup though i wasnot sure.
i could not get more than old 12v7ah so i purchased another 12v12ah.
now my ups has two 12v12ah SLA batteries cost 3000BDT.
i connected those in paralal to get 12v24ah(wrong?)
but after 24 hrs of churging it got nothing but only 5 min of laptop power via ac adapter.
i think batteries are not churging.
the transformer in the ups is 13.26 and working.
what to do?
i am unemployed and money really matters to me(never mind)
thanks in advance.
Drubo
You have connected the batteries together properly to get 24 Ah total.
However, I don’t know how the charging system in your UPS operates. The best advice to give you is to talk to the manufacturer. Hopefully they have an engineering department who will talk to you. I’d be curious to know their opinion on installing higher capacity batteries than come stock.
Sorry I can’t be of more help!
what was the OCV when you took them off charge? Basically SLA batteries if deaply discharged should be charged at 1/10th of their rated capacity in your case 12amp = 1.2 amp this depending on the OCV can take up to three days as the first 24 hours is absorbed into surface charge and the chemical reaction that transforms soft lead sulphate back into sulphuric acid is initially what takes time. Fully charged batteries will read a min of 12.6v and 12v 12amp units will produce approx 1/2 amp per hour over the specified 20ahr rate, now measure what your appliance is drawing and you can work out the time scale of available power, but without an OCV as a foundation their is no direction to go, this applies to all SLA and GEL units. Please follow my instructions and guidlines and the batteries (unless they have a shorted cell) will opperate correctly.
I have a ups that works on four 12v car batteries. Connected in series these batteries contribute 48volts that enable me to boost 240v on other end feeding my home appliances. Normally these batteries take current from local power supply and keep them charged for emergency use. During power failure the stored current is used for home use. Is there any way i could use your solar panel to charge these batteries, if yes please advice.Bookmark
How many watts are the solar panels? they start from 5w and upto 15w are basically a battery maintainer, you would require a minimum of 40 – 50w to actually maintain this bank of batteries and if they were deaply discharged it would take a approx 48 hours in direct sunlight to even register a charge, solar panels will not charge unless they are over 100w and then it still depends on direct sunlight, they are a battery maintainer so dont believe the hype about them, see for urself!!!! get a 5 stage battery charger that is compatible for bank charging as most 12v chargers can not function correctly unless it is connected to a single unit due to the different OCV each battery will give, they must have an independent circuit cut off switch that will register a specific rate. Hope this helps, just dont want you to waste your money ,
I have two deep cycle batts. for my camper trailer. I used to take one batt at a time and hook it up to trailer while the other gotput on charger. Now i have a solar battery charge controller alongside a 1000w continuous power inverter, and what i believe is a 35w solar panel. Approximately 30in wide by 40in long.
Can’t i simply attach both batts,in parallel of course to my trailer leads along with the wires from the charger controller so they trickle charge while im at work..?
We have just started a business where one product requires 24 VDC to power a hydraulic power unit, i.e. a 24 VDC motor close coupled to a hydraulic pump. I do understand how to create the 24 VDC package by connecting (2) 12 VDC batteries in series. However, the customer wants to maintain constant charge on these batteries by permanently connecting ‘smart’ 12 VDC battery chargers to each of the batteries. First of all, can this be done and just left alone? NOTE: Very intermittent use and primarily in the summer (boat lift). Can the batteries be left as connected for use with the battery chargers permanently attached? Will using the batteries for power damage the battery chargers?
Thank you!
You can only use 12V chargers if you disconect the batteries from each other, if you leave them wired in series then a single 24V charger should be used. As long as the charger(s) are a 5 stage digital processing unit with a final maintaining charge curve then they can be left on for months on end. However on no account must the batteries be put into service with the charger(s)connected, not even for a short while it is highly dangerous to do this.
Regards,
Regards,
What would be the best way to connect a single solar panel to 2 12v 9AH gel batteries without messing anything up? They are in a portable surveillance system one powers the camera and one powers the DVR. One suggestion I have received is to split the output of the solar panel and put a diode between the panel and each battery. The battery that needs the charge will take it. Is it really that simple?
Thanks for any help!
Hi Matt,
You wont mess anything up and can connect a variety of ways, you could also use a split charge realay. But it realy all depends on the wattage of your solar panel, anything less than 75 watt will be maintainer not a charger, also is it direct sunlight, these factors untimately affect the next stage which is your power demand verses your solar panel capability. It all depends on the power of your panel and that will tell the best way to connect it up.
i have 3 12 volt deep cycle batteries hooked in series for a 36 volt trolling motor on my pontoon boat. i would like to draw off 12 volts from one battery for a low amperage load. Can this be done without creating any problems.
will my charger stop charging if I have 3 deep sicled batterys hooked together? Will it know when to stop? It shows 90% charged. I guess I have to unplug each one to test the voltage?
you can charge batteies in banks fine as long as your chager is adequate, best is a smart charger,a three of five stage. Deep cycle batteies require an end float charge or they will rarely reach 100%, these chargers will let you know when optimum capacity is achieved.
I have 4 x 12V batteries for my boat. One for the starter and accessories, one for an engine-mounted electric motor (these first two are located together in the stern), and two that are use to make a 24V system for my bow-mounted trolling motor (these two are located together in the bow). These are wired independently to the unit with wires directly from each battery. It is only a 17 foot boat, so the battery pairs are not too far away from each other. I just purchased a 3-bank on-board charger. I would like to somehow use it to charge all 4 batteries. Can this be done? If so, how? I do not use the bow trolling motor batteries much, so they should not need too much charging at any time. I’m wondering if I can charge these two at the same time even though they are connected to the same end device?
Thanks.
this is a question
I have a single 12V charger and would like to charge a 24V system is this possible?
Not possible. If you have 2 x 12 v batteries conencted in series to make 24v system you must disconect them from each other then you can use the 12v charger on each individual battery, otherwise you will need a 24v charger.
why cant you hook 1 charger to each side of the batterys with a double y format with a diode large enough to take the chargeing in the possitive side of each cable so there is no back feed to the charger?
can I connect two 12 volt batteries to the same 12 volt
can altenator or does this put pressure on the altenator
Im assuming the alternator is sized correctly, i.e 60ah battery and around 60ah alternator. As long as you connect both batteries in “parallel” this will keep them at 12v pos to pos and neg to neg(make sure not to connect them in series)
Both batteries must also be the same size.
I understand the 2 battery parallel charging concept but how do you connect your device to this setup so it draws on both battery’s at the time? In my case its a 12v trolling motor.
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Can I charge 3 batteries in parallel at the same time.
hello. i have one qus, if i have my solar generator (solar pannel with charge controller) connected to my 2 batteries connected in parallel, now can i connect another battery to these battries which will take power from grid to charge these battries and if the grid supply goes off, the solar charger will continue to charge.
CORRECTION
hello. i have one qus, if i have my solar generator (solar pannel with charge controller) connected to my 2 batteries connected in parallel, now can i connect another CHARGER to these battries which will take power from grid to charge these battries and if the grid supply goes off, the solar charger will continue to charge.
I have a 24 volt ele trolling motor. with 2-12volt battries how do i hook thrm in serise to make 24 volt with a on board charger