Charging 12v Battery from 24v supply

  1. Delgado Member

    Messages:
    368
    Slough UK
    Been trying to work out a way to charge the 12v batteries on my pumps from the 24v vehicle supply. I know that it can be done as I've seen someone else do it, but I'm not sure how.

    So, Is there a simple way that I could rig up a charger (permenantly mounted on the vehicle), and what charger would I need?

    Thanks
     
  2. danelectro

    danelectro Member

    Messages:
    402
    Norfolk
    I'm assuming this is a 24v supply from a truck??? If so, I believe trucks are fitted with 2 12v batteries in series (giving 24v), so I guess (in simplistic form) an output in parallel from each of the batteries, will give you a 12v supply, you just need to a couple of cables from your truck to the trailer batteries???
    So if the truck is running, you'll get the truck batteries and trailer batteries charged at the same time......Obviously depending on the output from the truck alternator.
     
  3. tigler Expert on the trivial

    Messages:
    3,931
    Two 12v batteries can be connected in series across the 24v supply. Just in case you don't know what that means, connect a positive of 1 battery to a negative of the other, then connect the 24v supply to the remaining positive and negative terminals.
     
  4. danelectro

    danelectro Member

    Messages:
    402
    Norfolk
    This little diagram I found might help.

    The pic on the right is how your truck is wired, on the left is how you get 12v
     
    • 263battery_hookup.JPG
  5. piman Member

    Messages:
    1,737
    Location:
    Oswestry Shropshire
    Hello Delgado,

    just take 12 V from one of the truck batteries, if you try and connect 12 v from both, i.e. in parallel you'll get a big flash as that will be a dead short across the batteries.
    Alternatively and a better solution as suggested is to join two of the pump batteries in series to charge them up

    Alec
     
  6. eddie49 Member

    Would need some detail on the spec of these "12v pump batteries" to answer this. For example, capacity in AH and whether they are sealed/gel or normal wet car-battery type? You need from 13.8 to 14.7 volts to charge a 12v battery. There are plenty of "24v-to-12v truck converters" on the market, but if their output is a genuine 12v, that will not be enough. Tapping the mid-point of your existing 2 x 12v truck batteries, via a fuse and a switch, might work, but you would need to monitor voltage and charging current with/without the engine running to avoid over-charging.
     
  7. tigler Expert on the trivial

    Messages:
    3,931
    Another option is a split charge regulator as used for Caravan batteries, this could be connected from one of the truck batteries.
     
  8. Shox Dr

    Shox Dr Chief Engineer to Carlos Fandango

    Messages:
    15,534
    Location:
    East Yorkshire
    A lot of battery chargers step down the voltage from 240v mains to 12VDC. I converted one a long time ago. You just need to bypass the transformer in the charger. That way you still retain the electronics in the charger to maintain the correct level for optimum charging
     
  9. Delgado Member

    Messages:
    368
    Slough UK
    Some interesting Ideas...

    The batteries are 19AH 190A, they're small like motorbike ones.

    I'm thinking that charging them straight from the trucks alternator will most likely fry them (but I'm not sure), I'd like something a bit more controlled and the split charge regulators look interesting as does converting a 240v charger.

    Any more Ideas, or any advice on a suitable split charge regulator?

    Thanks
     
  10. madkayaker

    madkayaker Pro sparkey Pro Welder

    Messages:
    13,694
    Cumbria
    you can get converters to run 12V kit from 24V supply that would probability work
     
  11. Justme

    Justme Member

    Messages:
    3,058
    Location:
    Pwllheli Wales
    You can get dc to dc chargers. They come in 24 to 12v flavours & they do run at voltages suitable for charging. Dont tap (IE take off) 12v from a 24v bank as it will imbalance the batteries in the 24v bank. As will fitting the two bats as a second 24v bank & then fitting each one with its own load.
     
  12. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,614
    Location:
    Sheffield UK
    Presumably you mean connect the remaining electronics accross one of the truck batteries to feed 12v into it?

    In general terms I wouldn't suggest connecting batteries in parallel without a split charge relay, proper device for the job, blocking diode or Dr Shox's method because you run the risk of drawing starting current through the charger wiring.

    Also when connecting 2 x 12v batteries to get 24v they should be identical, putting another battery in parallel accross one of them may affect the charging of the truck batteries.

    Edit - just read Justme's post!
     
  13. awemawson Forum Supporter

    Very simple way to do it is put a 12v bulb in series with the battery. Find out what current you want to charge at (usually no more than 1/10th of the AH capacity) and choose a bulb of suitable wattage to draw that current.
     
  14. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,614
    Location:
    Sheffield UK
    Are you saying put that combination accross the 24v or 12v?

    This would be a crude constant current charging circuit, car batteries should be charged at constant voltage.
     
  15. awemawson Forum Supporter

    The current is nicely regulated by the non linear resistance characteristic of the bulb and hence in not constant current. When cold the filament has a low resistance, increasing rapidly as it heats limiting the current. Fairly obviously the combination of 12v battery and 12v bulb go across the 24v supply.
     
  16. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,614
    Location:
    Sheffield UK
    But as the battery charges the current reduces, the voltage accross the bulb reduces, (even if its resistance stayed constant, the effect is magnified by the drop in resistance as the filament cools). The lower voltage drop accross the bulb increases the voltage accross the battery.

    One of two things could happen, I'm not sure which, perhaps a bit of each:

    1) The increased voltage accross the battery will force more current through it, this increase will tend to heat the filament, reversing the above, thus the circuit will stabilise towards a constant current

    or

    2) As the voltage accross the bulb decreases the voltage accross the battery will exceed safe limits.

    Some cheap & nasty NiCd battery chargers used a resistance in series with the battery, if the ratio of the voltage drop accross the resistance to that accross the cell is high, the current in the circuit is near as dammit constant.
     
  17. Delgado Member

    Messages:
    368
    Slough UK
    Some great information there fellas!

    So, if I'm understanding this correctly, Whatever I do it needs to come off the 24v where the 2 batteries connect to the truck, so as not to unbalance the batteries?

    If I connect to this point, then I need to put something (I'm leaning towards a dc/dc charger) in the circuit so that when I try to start the truck engine it doesn't try to draw current from the secondary batteries?

    So, if I go for a dc/dc charger will it drain the truck batteries, i.e be charging permenantly, if so, can I rig it up so that it only charges when the truck is running, and if so how?

    The secondary batteries are 19ah 190A, someone mentioned that I only need a charger that is 10% of the battery capacity, is that 10% of the AH or 10% of the Amps?

    Thanks!:D
     
  18. Wedg1e

    Wedg1e They call me Mr. Bodge-angles

    Messages:
    6,576
    Location:
    Teesside, England
    The way caravan batteries used to be charged (probably still are, I dunno, I wouldn't have a caravan given!) was to use a relay connected to the alternator warning lamp of the car. That way, once the engine started and the lamp went out, the relay switched over, connecting the caravan battery to the alternator.
    You could use a similar system so that 24v is only applied to your 24-12v converter when the truck engine is running.
     
  19. Justme

    Justme Member

    Messages:
    3,058
    Location:
    Pwllheli Wales
    10% of the ah, but its only a guide.

    Dep on bat type you could need more or less. Plus if you have plenty of time it wont hurt to go smaller too.
     
  20. eddie49 Member

    Still not exactly sure of your specific needs, due to not knowing how many of these 12v pumps you have, how often they are used, how low their batteries will get drained, whether you want to recharge them only when the truck engine is running, how long the truck runs versus when is the next time the pumps need to be ready, what capacity and health the truck batteries are...
    A classic split charge system as used by caravanners, is a relay to allow a 12v leisure battery in the caravan to be charged in parallel with the 12v car battery when the car is being driven, and then "split" from the car circuit when the caravan is occupied, to avoid flattening the car starter battery.
    For the specification of your 12v batteries, the "19" is the capacity of in Ampere-Hours ( AH ). The "190" is the "Cold Cranking Amps" ( CCA ), a number mostly used in USA to rate how good the battery could be at starting a car/bike on a very cold day. To avoid overcharging, the recommended charge current in Amps for wet lead-acid batteries is one-tenth of the capacity in AH - known as "C/10" - or 1.9 Amps in your case. At that rate, a completely flat 19AH battery would take 10 hours to recharge. Maybe you won't be driving the truck for 10 hours.... However, the 12v batteries probably won't be totally empty, and also a decent-sized truck battery should be able to push 1.9Amps into a couple of 12v batteries for a few hours when parked up without going flat itself.
    As mentioned, a 24v-to-12v charger is not the same as a 24v-to-12v converter. There are a few discussions about this situation on caravan/motorhome/van conversion websites, and one posting mentioned using "solar battery chargers". These intelligent gubbins apparently take the variable output from photovoltaic ( PV ) solar panels and charge lead-acid batteries.
    I found eBay item 220821428259, which accepts from 12 to 34v in, delivers 12v at up to 6 Amps ( i.e. would handle 2 or 3 of your batteries in parallel ), cuts off charging when the batteries reach 13.7v, and costs 23 Quid with free shipping. However, you'd have to clarify with the Seller whether the "automatic voltage recognition" would allow it to be set for 12v output when the input is around 24v.
     
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