# Charging 2 SLA batteries in series?



## Daravon (Jun 27, 2011)

Is it a problem to trickle/standby charge two sealed-lead-acid batteries that are hooked up in series, as if they were one big 24V battery?

I have a Hagerman Bugle phono preamp that runs off two tiny 12V sealed lead acid batteries. The batteries are in series arranged with their junction as the 'virtual ground', so they look like a +/- 12V supply to the amp. 

Right now, I wait till the batteries go dead, and then charge the batteries one at a time using a taper charger, to 13.6V. This is inconvenient, and it's abusive to let the batteries run dead, so I would like to rig up a continuous charge system. For the power switch, I have a 3-pole dual-throw switch, with a charge port hooked up to the opposite throw as the amp, so when the switch is turned off, the batteries are connected to the charge port. So basically if I hook up the charger full-time, the batteries will be connected the charger when the switch is set to "off" and they will be connected to the amp when it's set to "on". I can't conceive of a double-ended battery charger, but I could charge both batteries as one big 24V battery. Is there a problem doing that?


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## 45/70 (Jun 27, 2011)

Hi Daravon. Provided you have a 24 Volt Pb charger, it should work fine. I charge 2x 12 Volt AGM batteries that way all the time.

If you think about it, most Pb batteries are just that, batteries. That is, most are usually a group of individual cells in one container, eg. 3x 2 volt cells making up a 6 volt battery, or 6x 2 Volt cells making up a 12 Volt battery. So, if you think about it, in your case, you're just adding six more cells to make a 24 Volt battery, as far as the charger sees it anyway.

Dave


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## Daravon (Jun 27, 2011)

That's what I thought, but initially the two batteries will probably not be in exactly the same state of charge. I suppose it will eventually equalize but I wasn't sure.


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## 45/70 (Jun 27, 2011)

Well, I figured your amp would likely drain the +/- of each "battery" equally. Then again while I'm no stranger to audio amps, I'm no expert either.

I'm sure it's not going to be any worse than when a car with a dead battery is jumped by a car with a good battery. Think of it that way. I'm pretty sure you'll be OK. The thing you wouldn't want to do, is charge different kinds, or capacity of batteries this way. You'd no doubt run into trouble then.

Dave


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## VegasF6 (Jun 27, 2011)

I wouldn't compare this to jumping a car battery, that would be a parallel connection. I have seen people trickle charge lead acid batteries in parallel and I would not recommend it at all (and I told them so, but of course, they knew better than me  ) The real problem in that case is there is no current limiting device between them if they a different state of charge. 

Series connection however is just fine. More and more often lately the RV and Camper crowd are running 2 6V series batteries when it used to be 2 12V deep cycles parallel. If they are kept together as a pair all the time you don't need to take any special precautions at all. If not, and one is more fully charged than the other, I think I would suggest a gentle trickle charge. In series they will see the same amount of current no matter the charge rate, but I don't know how termination would be affected, that's why I suggest the gentle charge to equalize them. They wouldn't just equalize on there own of course connecting them together.


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## Murray B (Jun 27, 2011)

My lawn mower has two 12V SLA batteries in series and it charges them just fine. This is not the same problem as with your pre-amp.

First problem I see is that a typical SLA float voltage is about 2.3V per cell. You should check the data sheet of your batteries to confirm the float voltage. This means the charge voltage should be about +13.9 and -13.9 Volts. Will the pre-amp handle that sort of voltage?

If so, then it is important to consider the amount of ripple from the charger if the pre-amp is to be used while charging. Keep in mind that the batteries have a floating ground. If there is another piece of equipment connected and it's ground goes to house ground then your pre-amp will think the connection between the batteries is at ground potential. If the charger then tries to pull the other battery terminal to ground then there will be a problem. Its sort of like crossing the streams with proton packs. 

If not, then it may be possible to purchase a DC-DC converter but they are not cheap.

Overall it will probably be better all around to buy the Hagerman power supply kit. It is only $39 and will save a lot of grief over the long run.


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## 45/70 (Jun 28, 2011)

VegasF6 said:


> I wouldn't compare this to jumping a car battery, that would be a parallel connection.



Yeah, you're right Vegas, a bad example perhaps, as that is a parallel setup. Just the same, and what I was pointing out, is that the voltage potential difference is extreme in that situation, and it does work.



> I have seen people trickle charge lead acid batteries in parallel and I would not recommend it at all (and I told them so, but of course, they knew better than me  ) The real problem in that case is there is no current limiting device between them if they a different state of charge.


I don't believe there is any problem with parallel charging Pb batteries, I could be wrong though. I think Pb cells act similarly to Li-Ion cells when charged in parallel, with the cells being able to equalize with relative ease, compared to say, NiCd, or NiMh cells. Pb and Li-Ion also have a very similar CC/CV charging algorithm, with the only major difference being the "float charge" at the end of the Pb charging cycle, I believe.

Murray B, I'm not ignoring you. You seem to be way ahead of me here. As I said in my previous post, I'm familiar with preamps and amps because I use them, but am certainly no expert on the subject.

Dave


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## Mr Happy (Jun 28, 2011)

If the two 12 V batteries are the same, and if they are each discharged more or less evenly, and if you disconnect them from the pre-amp before charging them, and if you charge them with a proper 24 V charger at the appropriate rate, then it should be fine. Otherwise, YMMV.


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## VegasF6 (Jun 28, 2011)

45/70 said:


> Yeah, you're right Vegas, a bad example perhaps, as that is a parallel setup. Just the same, and what I was pointing out, is that the voltage potential difference is extreme in that situation, and it does work.


 
It does work, mainly because you are putting a big load on the battery to start the car and then un-hooking the jumper cables. But there is such a sudden in-rush of current to the lower voltage battery I wouldn't want to do it for very long, nor as a practice. Just in emergency situations.


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## Lynx_Arc (Jun 28, 2011)

many UPSes charge two 12v batteries in series


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## Murray B (Jun 28, 2011)

45/70 said:


> Murray B, I'm not ignoring you. You seem to be way ahead of me here. As I said in my previous post, I'm familiar with preamps and amps because I use them, but am certainly no expert on the subject.
> 
> Dave


 
Okay Dave, I should have expressed myself more clearly. The Hagerman power supply is for + and - 15 Volts which is good news. They also provide a schematic of the power supply. The only problem is that they use 7815 and 7915 voltage regulators which are the wrong voltage to charge your batteries. What you really need are a 7813.9 and 7913.9 which nobody makes. What I would do is use a 7812 and 7912 and put three silicon diodes in series on the reference pins to make them work as a 7813.8 and 7913.8. Of course I am not advising you to do this and anything you do is done at your own risk.

For an SLA battery the float voltage is that voltage that can be continously applied to the battery to charge it without harming it. These batteries are very simple to charge and there no more to it than applying that regulated voltage. If you check the specifications there is also a higher maximum charge voltage that will charge the battery faster but can damage it if left applied after full charge is reached. Fancy chargers will charge at the higher voltage and then switch to the float voltage when finished. There is no reason to use the higher voltage if there is no need to charge the battery faster because the float voltage will charge it just fine as long as you don't mind waiting overnight.


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## Daravon (Jun 28, 2011)

> The Hagerman power supply is for + and - 15 Volts which is good news. They also provide a schematic of the power supply. The only problem is that they use 7815 and 7915 voltage regulators which are the wrong voltage to charge your batteries. What you really need are a 7813.9 and 7913.9 which nobody makes. What I would do is use a 7812 and 7912 and put three silicon diodes in series on the reference pins to make them work as a 7813.8 and 7913.8. Of course I am not advising you to do this and anything you do is done at your own risk.



Wait; what? What's this about power supplies and 7812s? Instead of doing the diode trick (which is pretty smart btw), I would just use LM317 regulators, which are adjustable. 

The Bugle amp is just some op-amps; it can run on +/- 9V to +/- 15V. I have no power supply; I'm just using the two SLA batteries. The amp has no mains plug; it just runs on batteries, and the virtual ground is tied to the rest of the stereo system through the ground connection on the RCA jacks...I think. As I explained in the parent post, the batteries are isolated from the whole system when the switch is off. The charge port is connected to the opposite throw of the dual-throw switch. 

It will probably be hard for me to find a +/- battery charger, so I would rather just charge both batteries at once. The problem is to do that I need 27.2V voltage source. I think even laptop power supplies are not that much voltage. If I could find a 30V power supply, I think I could use an LM317 to regulate it down to 27.2V. Then whenever the amp was turned off, both batteries would be connected in series to the 27.2V. When the amp was on, the batteries would be disconnected from the charger and connected to the amp.


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## CKOD (Jun 28, 2011)

Murray B said:


> Okay Dave, I should have expressed myself more clearly. The Hagerman power supply is for + and - 15 Volts which is good news. They also provide a schematic of the power supply. The only problem is that they use 7815 and 7915 voltage regulators which are the wrong voltage to charge your batteries. What you really need are a 7813.9 and 7913.9 which nobody makes. What I would do is use a 7812 and 7912 and put three silicon diodes in series on the reference pins to make them work as a 7813.8 and 7913.8. Of course I am not advising you to do this and anything you do is done at your own risk.
> 
> For an SLA battery the float voltage is that voltage that can be continously applied to the battery to charge it without harming it. These batteries are very simple to charge and there no more to it than applying that regulated voltage. If you check the specifications there is also a higher maximum charge voltage that will charge the battery faster but can damage it if left applied after full charge is reached. Fancy chargers will charge at the higher voltage and then switch to the float voltage when finished. *There is no reason to use the higher voltage if there is no need to charge the battery faster because the float voltage will charge it just fine as long as you don't mind waiting overnight.*




A ~14.x volt (Consult the battery datasheet for proper voltage!) "topping" charge helps reverse sulfation that a 13.8v float charge wont do. While floating at 13.8 wont really harm anything, service life could be extended with periodic charging to a full topping voltage. 

And saying lead acid is simple to charge is a great big can of worms  You can run into topping charges, temperature compenation, equalizing charges for balancing cells, pulse charging, etc. You could use a something a simple as a transformer + rectifier with poor regulation so the voltage sags under load to keep the current from being excessively high, upto a microcontroller powered monster.

All that being said, 

2x http://www.batteryspace.com/leadacidsmartcharger10afor12vleadacidbatteryworldwideuse.aspx of these to ensure that the batteries are charged properly if they dont discharge at the same rate. The output should be isolated from the input, as there is no ground pin, and having the output - and the AC neutral together would be a big safety hazard (especially on a wall wart type unit where you could plug it in upside-down  )


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## Daravon (Jun 29, 2011)

I've actually decided to not use continuous float charging--mostly because the charge port on my amp is on the front, so I don't want a charge cable wrapping around and sticking in the front of the amp all the time. It's no burden to charge the thing every 10 records or so if I can just plug the charger in and leave it for a day or so.

I found an unregulated 30V, 400mA printer PSU in my junk last night. I soldered up an LM317 as a voltage regulator--I figure the 400ma rating of the PSU will be enough current limiting. I will probably set it at 27V=13.5V/battery for the first charge and just leave it hooked up a couple days to even out the batteries. After that, if I want to choose a voltage that I can use to charge them fully, but won't harm them if I leave them plugged in for an extra few hours/days, would that be about 29V=14.5V/battery?


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## CKOD (Jun 29, 2011)

Float charging wont "even out"/balance the cells on the sealed lead acid batteries, the voltage isnt high enough. A topping charge may do it some, but an "equalization charge" is even higher, and causes the battery to gas(or at least the overcharged cells, the undercharged ones reach their fully charged state) Not a problem on VRLA/flooded batteries, and even acceptable in small amounts on AGM cells if they were designed for it, but it'll trash gel-cells quickly. Consult the datasheet of the batteries (or at least post the make/model here ), as the voltages depend on chemestry of the plates, cell construction (VRLA, AGM, Gel, etc...) actual gel cells (where the acid is gelled) cant take any gassing without hurting capacity, as it forms bubbles that hold the gel away from the plate. AGM can take some, and VRLA/flooded can obviously gas just fine, provided you dont lower the water level, and the voltages vary for each. 

Also, for your LM317 circuit, you can use an extra resistor on the feedback divider, so that you can use a switch to short out/bypass the extra resistor, and lower the voltage of the charger. (or use trim pots to make it easy to set the 2 different settings), that way you can have a float setting and a 'topping charge' setting. Just be mindful of the switch. maybe a red gaurd/cover for a toggle switch i.e. missle switch cover for it?


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## Murray B (Jun 29, 2011)

Daravon said:


> Wait; what? What's this about power supplies and 7812s? Instead of doing the diode trick (which is pretty smart btw), I would just use LM317 regulators, which are adjustable...If I could find a 30V power supply, I think I could use an LM317 to regulate it down to 27.2V. Then whenever the amp was turned off, both batteries would be connected in series to the 27.2V. When the amp was on, the batteries would be disconnected from the charger and connected to the amp.



The Hagerman schematic is for a double ended power supply that provides +/- 15 Volts which is too much for 12 V SLA batteries. Modifying it to produce +/- 13.8 should allow the amp to be used while charging. Since emergency lights have used float chargers to maintain their cells since before I was born, and I'm old enough to remember where I was when JFK died, I'm pretty sure that float charging would work here too. 

If the batteries are completely out of circuit while charging then the solution is simpler. The 317 is a great regulator and have used it to make simple adjustable power supplies. There is no reason that it would not work as well as a 78XX in this application but the fixed regulators are simpler to use. [By the way, using diodes to change the voltage of fixed regulators is a common trick that goes back decades and I did not come up with the idea.]

One thing about SLAs that I have never found in the literature of any manufacturer is that most, if not all, SLA batteries will form an insoluble form of sulfate when left partially discharged for any length of time. This means if they are not recharged soon after use the capacity will significantly decline over time. 

It is also important that you find the manufacturer's data on your batteries and follow their charge recommendations rather than just taking my advice. 

Anyway, good luck with your project.


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## Mr Happy (Jun 29, 2011)

Murray B said:


> One thing about SLAs that I have never found in the literature of any manufacturer is that most, if not all, SLA batteries will form an insoluble form of sulfate when left partially discharged for any length of time. This means if they are not recharged soon after use the capacity will significantly decline over time.


I'm surprised you have not come across that. I had thought it was widely known and documented. A general rule of thumb for lead acid batteries is that they should be kept fully charged at all times and should not be used in applications where they are regularly discharged. Even the "deep cycle" kind of battery is only partly resistant to discharging and will still have a shortened life if subjected to such treatment. (This means, for instance, that those cheap halogen spotlights that have an SLA battery are designed with the wrong kind of power supply and the battery is guaranteed to fail sooner rather than later.)


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## Murray B (Jun 29, 2011)

Mr Happy said:


> I'm surprised you have not come across that. I had thought it was widely known and documented.


 
It is documented by 'battery guys' which is how I knew about it but I have not yet seen it mentioned in any manufacturers' literature. It is great that some companies are including it now because it is a simple way to dramatically extend the life of the battery. I have seven years on the batteries in my cordless electric mower largely because I charge it after every use.


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## hannyB (Jun 30, 2011)

Provided you have a 24 Volt Pb charger, it should work fine. Lifeless battery got your motor? Do you've any idea what is taking place with that lead-acid device under the cover? Why batteries go dead and other tales. Because a battery is self-recharging, the earlier attempt to crank the engine got the process rolling. The hydrogen gas buildup created on the surface of the carbon rod that contacts with the liquid sulfuric acid inside the battery disrupts the reaction necessary to produce electrical power. Once the gas dissipates, the battery appears to come back to life with the next start attempt.


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