Multiple SLA battery charging station

Discussion in 'The Projects Forum' started by nickorossa, Nov 30, 2010.

  1. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    Hi

    The number of battery powered ride on vehicles that my kids have is increasing, along with the batteries to power them. The batteries are either 6v (12ah) or 12v (12ah). Links to the data sheets of the two batteries I'm "standardising" on are below.
    12v/12ah http://www.yuasaeurope.com/images/uploads/eu/downloads/datasheets/NP/NP12-12_English.pdf
    6v/12ah http://www.yuasaeurope.com/images/uploads/eu/downloads/datasheets/NP/NP12-6_English.pdf


    My starting point is one of these laptop based power supplies; primarily because I have one lying around not doing anything and secondly because I don't really like the idea of me messing with circuits with a mains voltage :eek: The supply should is capable of 6a@15v, so I think that should allow up to 4 charging channels without too many issues.
    http://www.maplin.co.uk/Module.aspx?ModuleNo=43515#spec

    From an earlier thread, I had a reference to this diagram http://forum.allaboutcircuits.com/attachment.php?attachmentid=21844&d=1281922846 which I think gives me a good basis for a charging circuit. Looking at teh diagram; I see three stages. The first being the input source and a "bridge rectifier" arrangement with 4 diodes. The middle section being the charging circuit and the third section (labels RSbat, BAT and RPbat) being a representation of the battery internals for the simulation (confirmation required?).

    I think I can duplicate the middle section 4 times, each taking a feed from the same laptop power supply, and each having a set of leads that would then go to the battery to be charged.

    Looking at the middle section, to be honest I don't really understand how it works :confused:; however I'd like to add the following to it.
    1. A rocker switch to switch between a 6v and 12v recharging (3 position with centre being off)
    2. An LED to indicate that the battery is charging
    3. An LED to indicate the battery is charged and in float charge


    My end point would be to have a small box that I can plug the power supply into, with 4 charging circuits each having a rocker switch to control off/6v/12v and LEDs to indicate charge stage.


    Any advice and help you can give me on this would be appreciated.

    Thanks.

    Nick.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    A standard car battery charger with a 2 amp slow charge and an 6 or 8 amp fast charge should handle a dozen or more parallel connected batteries with no problem (12 Ah or smaller) They cost 10-20 dollars on sale

    Do you just want to have the fun of building it yourself? :)
     
  3. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    Hi

    Thanks for the quick reply. My intention is to have a suitable setup that even the wife can use it when I'm not there! A tall order I know, but thats the intention. :D

    Unfortunately living in the UK means some things are not as cheap as they are in the US!


    Looking at the data sheets again; I don't think the 6v/12 switch is necessary as the charging characteristics are the same for both batteries. Don't know how I missed that first time around :mad:

    Nick.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    A few sets of red and black plastic marked alligator type clips would allow even a 'caveman' :) to be sure of polarity ID.

    In my honest opinion, despite the price of the small charger, it will be the cheapest and quickest 'easy' method available.

    Unless you are seriously determined to build it yourself, then, suggest you get ready for a surprise when these nice folks start telling you everything you will need to get/buy to make it.
     
  5. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    I don't mind doing it myself, and I probably have most of the components in the garage.

    I've been looking at various forums about charging SLA batteries and there appears to be varying opinions about the correct/best way to charge them.

    Nick.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The charge circuit you found that I drew up is really rather limited in current output, as the LM317 is a linear regulator with 1.5A maximum output current. Since it is a linear regulator, there is a large amount of power dissipated within the regulator itself if the input-output voltage differential is high.

    Also, the circuit you found is merely meant to be a float-level maintainer (~13.7v), not really for cyclic charging. It would eventually charge the batteries, but it would take quite awhile to do so, in the meantime generating a fair amount of heat.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    When you said that "you probably have most of the components in the garage", what sort of parts DO you have?
     
  8. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    Hi

    As far as building the circuit in the original post, I have the LM317's, the transistors, the capacitors and most of the resistors. I know I'm missing D5 and R3.

    In terms of what components I have generally, well I haven't specifically inventoried everything. I probably should at some point :rolleyes:

    If there's a better way to achieve my end game; please point me in the right direction.

    Nick.
     
  9. k7elp60

    Senior Member

    Nov 4, 2008
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    I'm a little confused. Both data sheets are for 12V batteries, the first one is for a 12A and the second is for a 6A. Yet you list 6V and 12V batteries.
    Do you have all 12 volt batteries or do have some 6V batteries also? With the power supply listed the 12V batteries could be charged with a linear charger without to much heat, but to charge the 6V batteries it would take a lot of wasted heat.
    The simplest charger would be one that charges in the standby voltage as it generally can be left on for extended periods, and will not overcharge the batteries. For a 12V battery the voltage would be between 13.5 and 13.8 volts, and 1/2 half of that for 6V batteries. The cyclic type chargers are usually more complicated as they monitor the charge voltage(near 14.5 volts) and also monitor the charge current. When the charge current decreases to a certain amount the charger is either stepped back to standby charge voltage or disconnected from the battery.
    Connecting discharged batteries of the same voltage directly in parallel with each other is not a good idea as one battery my never get fully charged. An excepting to this is if the batteries were the same date code and originally put is parallel for service.
    I too can help with battery charger schematic's
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I can see that. :)
    BZZZZZT! Error, error!
    Both batteries are rated for 12AH.
    The 1st datasheet is for the 12v batteries, the 2nd is for the 6v batteries.

    Yep.

    The schematic of mine he linked too would require a number of mods to make it compatible with 6v batteries. For one thing, the temperature compensation would be double what's required for a 6v battery.

    It's not really capable of attaining the up-to-3A charge rate specified in the datasheet. Besides that, it would also make a good room heater.

    This is essentially correct. Optimal float voltage is around 13.65v at 25°C for his 12v batteries.

    Actually, during the "bulk charge" phase, the battery is charged at limited current (<=3A) until the terminal voltage is reached; for the 12v batteries the terminal voltage is 14.5v @ 25°C. Then they can be charged indefinitely at the float voltage (13.65v @ 25°C).

    Connecting discharged batteries of the same voltage directly in parallel with each other is not a good idea as one battery my never get fully charged. An excepting to this is if the batteries were the same date code and originally put is parallel for service.[/QUOTE]

    Connecting batteries in parallel can result in very high current flow, overheating and fires/explosions if one battery is fully charged and another deeply discharged.

    Even if they start off being the same voltage, if one battery develops a shorted cell (a very common failure mode) it will prevent the other battery or batteries in parallel with it from becoming fully charged, leading the remaining to early failure. This can get expensive in a big hurry.

    Have at it. I'm still recovering from being sick for the last several days.
     
  11. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    Hi

    Thanks for the reply. Now I'm now a bit confused.

    According to the data sheets both batteries (and even the 4ah version) versions all have the same voltage charging specifications, but differing current charging specifications; so I'm not sure why you both refer to halving the voltage unless you think the data sheets are inaccurate.

    I was hoping that by having multiple charging circuits I would avoid the issues of charging multiple batteries in parallel.

    Essentially my end game is to have one socket in the wall with the ability to charge multiple batteries at the same time with independent charging circuits for each.

    Thanks for your help.

    Nick.
     
  12. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I can 'feel' the simplicity of this project from here. :)
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Go back and look at the datasheets you posted links to in your initial post.
    The first battery is 12V, 12AH
    The 2nd is 6V, 12AH.
    You even stated that you wanted selectable 6v/12v charging.

    If you have something different (other than those two exact batteries), please tell us.
     
  14. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    Hi

    The two data sheets specify the same charging voltages; even though the battery voltages are different; unless I'm missing something.

    So do the charging voltages need to be different or not. If not; then the switchable voltage becomes unnecessary.

    The 12v12Ah and 6v12Ah are the batteries that I have.

    Regards.

    Nick.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    I missed that before; yes, the datasheet for the NP12-6 specifies a 13.65V float for a 6V battery, which is an error in the datasheet!

    You should write to the manufacturer and ask them why they specify floating a 6v battery at twice it's rated voltage. Maybe they'll send you some free batteries. :) They need to correct the datasheet.

    They must be correct for the voltage rating of the battery. The NP12-6 datasheet is not correct.

    OK, good. The source of the confusion is the manufacturers' fault for the error in the datasheet, and also mine for overlooking the charge voltage specifications to begin with.

    Sorry about that. I'm still somewhat "under the weather" with this lousy cold/flu.
     
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  16. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    Hi

    Thanks for clearing that up. I'll speak to the manufacturer and see what they say about the effects/damage to a battery being charged at the wrong voltages due to their data sheets.

    Going back to the project itself; a switch to change voltages would be required for the differing battery voltages.

    If the circuit I included previously is not suitable as a starting point do you know of any that are? I searched the web, and I couldn't find anything worthwhile.

    I had a touch of flu a while back and it certainly wiped me out for a few days. I hope it doesn't hang around too long for you.

    Thanks for your help.

    Nick.
     
  17. k7elp60

    Senior Member

    Nov 4, 2008
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    I think it would be possible for nicarossa to use one 12V charger for both 12V batteries. If he connected some low value resistors between the output of the charger to each 12V battery. If he used 2.0Ω resistors at 2W that should be okey. In addition to the resistors I would insert a diode in series with the resistors on the output with a current rating of the total current, if the charger doesn't have one in the output circuit. This would prevent the batteries from being discharged in the event the mains voltage fails. The output voltage of the charger would have to be increased to overcome the drop of the diode and the calculated drop of the resistors(0.24V)
    It is possible to use another 12V charger to charge the 6V batteries, by replacing the resistors with some 6V incandescant bulbs that have the rated current.

    I have been sucessful in charging both SLA and flooded lead-acid batteries in parallel a number of times. I use new batteries of the same manufacture and same ratings. I currently us Trojan T105 golf cart batteries, and have used Powersonic and Yuasa SLA batteries a number of times.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Is this really your best recommendation?

    Did you even read what he wants to use for the chargers' supply?

    Did you not read my cautions about connecting batteries of different charge states in parallel?

    Are you huffing canned air?

    He'd be better off making three copies of the schematic he originally found - but that leaves the 6v battery out in the weeds.
     
  19. k7elp60

    Senior Member

    Nov 4, 2008
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    The circuit he has chosen to use limits the current to a little over 100mA.
    If he changes that resistor and mounts the 317 on a reasonably sized heatsink he can charge them faster. Yes there is a heat situation for the regulator.
    Personally I think you are acting just like a marine corps sergant, who always thinks he is right.
    For your information I have charged or supervised the charge of hundreds of SLA batteries in parallel.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, and at that rate, it will take a 12AH battery roughly 12/(.1*.75) ~= 160 hours, or 6-2/3 days to become fully charged from fully discharged. Now were R4 changed to 413m Ohms (0.413 Ohms) then the maximum current output would then be 1.5A (1/4 of the 6A available) and charge time would then be 12AH / (1.5*.75) ~= 10-2/3 hours, a C/8 rate, or half what the batteries can support. Keep in mind that the batteries are for powering children's toys, and children generally are not noted for having a lot of patience. These batteries will support a C/4 charge rate, or 3A. At that rate, they would become fully charged in about 5-1/3 hours. However, that would mean a re-design of the circuit and adding pass transistors, as a single LM317T can only handle about 1.5A with excellent heat sinking.

    "that resistor" meaning R4.

    The heatsink should be rather large, preferably copper and preferably have forced-air cooling. The fan should be AC powered, otherwise the output of the 6A converter would be diminished quite a bit.

    A rather large one.

    You expected otherwise? ;)

    I'd rather be right than lucky.

    For your information I have charged or supervised the charge of hundreds of SLA batteries in parallel.[/QUOTE]

    There ARE situations where several or dozens or even hundreds of lead-acid batteries are connected in parallel or series-parallel arrays, such as when being float-charged in storage, in UPS banks, even in large trucks. However, in these situations the batteries are installed when fully charged, and are maintained all at the same charge levels.

    In our OP's situation, his children have a number of battery-powered toys, and although it's pretty likely that the batteries will be exhausted when the kids finally connect them to be charged, the same voltage is not guaranteed by any way, shape or form. Connecting two batteries of same labeled voltage but different charge levels will result in high current flow until the charges are equalized; with resulting power loss and increased internal temperatures, which shortens battery life. Using resistors will limit the current flow externally, but will waste power when charging.

    Additionally, if any one battery develops a shorted cell (a very common failure mode) it will prevent any cells in parallel with it from becoming fully charged, leading to their early failure. This can and does happen, all too frequently.
     
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