Charging 12v batteries on hook-up

Discussion in 'Mech Tech' started by alpha981, Sep 18, 2017.

  1. I have a 12v fully automated charger to wire in so when the van is plugged in on a site to 240, it can charge the 12v battery(s)

    My question, would it be viable to either:

    a) set it up so both batteries (leaisure and main) can be charged together? I'm not sure if the charger would be able to understand, or even have the capacity to charge two at once. Also, this seems to me to mean having connections permantly open between both batteries, therefore negating the split charge system.

    b) wire it so the charger is switchable between each battery.

    c) get ANOTHER charger and wire one in per battery (seems like a bit overkill!)

    In theory it's only the leisure that should need charging, but I'm yet to rewire the van to pull ALL the lights /radio etc across to the leisure - at present its a mix between what accesory uses which battery!

    Any thoughts...?
     
  2. I think if you have a split charge system in place and the 12v automated charger attached it would do both batteries, mine tops up the main battery then when that is fully charged it flicks across and does the leisure until that is at capacity
    Carries on like that so only need the connection to the main one in my setup
     
  3. bluerustybucket

    bluerustybucket Supporter

    B. on a toggle switch maybe ?
     
  4. In short no to charging two different batteries at the same time. The easiest option is B to put a changeover switch in the output of the charger so you can switch the charger to either battery.
     
    snotty likes this.
  5. I've a 12volt smart charger plugged in though the otherwise redundant towbar socket. And my 2 batteries can be switched via a "off/1/2/both" switch. I never charge them both together, just periodically switch batteries.
     

  6. I've got one ready to wire in but have also mused over the best way to do this.

    I've got all my accessories driven off the leisure, so (when i get around to it) i'm going to plumb the c-tek into the leisure. There is no way theoretically i can run down the main battery * when camping, and if i did, then i can jump start off the leisure - i keep a set of cables in the van for this reason.

    Edit: * ok yes there is actually, the eberspacher takes power for its spark plug off the main battery. maybe i should switch this over to the leisure.. although tbh it only gets used for very brief periods as it's too hot :)
     
  7. Don't the intelligent split charge relays charge both ways?
     
    paul2590 likes this.
  8. I took a positive feed from both batteries into the van and under the rock and roll bed. Thats where my split charge relay is. My split charge is of the simle type utilising the alternator as the trigger. By looping off the feeds from the batteries to a three position switch i can decide which will feed my leisure circuit. Either van, leisure or neither. Last option seems odd but i have my charger wired to the leisure circuit so it provides power when using neither battery and charges the battry which is connected when i need to do so.

    This suits the way i use the van but possibly an issue in doing this might be that if the battery you wish to charge is flat and you are using a lot of current in the van then it may take longer to charge. This hasnt been an issue for me as frankly you dont use as much as youd expect unless using an absorption fridge on 12 volt which is never the best idea and youd need a decent charger to achieve due to the current draw.
     
  9. mikedjames

    mikedjames Supporter

    A voltage sensing relay should switch both ways so if either battery is well charged the other will start to charge. So battery charger to leisure battery , alternator to starter and a VSR between will automatically make the connection when it is correct to do so. Fit and forget with no fiddling with switches.
     
  10. Some do like the Durite type, but most such as the Tec 3m or Smartcom, Combimaster switch an internal relay which isolates the leisure battery from the sensing circuit.

    Which is what you don't want to be charging miss matched size/state of charged batteries, otherwise the smaller/fuller charged battery can be overcharged. If a integral relay type voltage sensing relay is fitted best to use a double pole switch to also switch off the earth to the voltage sensing relay, isolating it so it won't switch on when you charge the starter battery.
     
    paradox and snotty like this.
  11. Not big on battery stuff tbh but isnt this the case anyway when charging from the alternator! If so then whats the answer. Would Matching battery resistances be of benefit?
     
  12. Yes but an alternator is a constant voltage charger with the voltage set to fully charge the battery just below its gassing voltage. Normally set at 13.8V-14V for the older lead antimony batteries these vans came with. Newer lead calcium/silver batteries can tolerate higher voltages before gassing in which cases the voltage can be up to 14.2V some up to 14.4V.
    Smart chargers take the voltage above the gassing level for a period to equalize the cell voltages then switch to a lower float charge voltage, hence you often have different charging programs for different types of batteries. If you have two miss matched batteries, one battery could be above gassing level for a prolonged period while the other is under charge.
     
    mcswiggs, Cossair, snotty and 2 others like this.
  13. Good explanation. many thanks. Manual switching would seem to be a safe bet then.
     
  14. Sproggy4830

    Sproggy4830 Supporter

    I am having a hot start issue so bought some jump leads just in case. First time I opened them was when I needed them . Only then did I discover how good quality they were for the money but also how flaming short they were.
    So as Ed China says, top tip , make sure you check the length of them before buying and storing under the seat
     
    rob.e likes this.
  15. mikedjames

    mikedjames Supporter

    Dont disconnect the ground on the VSR. It may half switch on get hot and break.

    When you are charging either battery the VSR switches on at a point below fully charged voltage .
    At that point the voltage drops on both batteries to a lower voltage but the VSR has hysteresis so holds the connection. The voltage on the pair of batteries will now be high enough to hold charge on the better charged battery and charge the less charged battery. Eventually the voltage rises as the second battery charges until it reaches a voltage where both are charging at a rate determined by their on charge voltage. The voltage needed to overcharge a small battery should be the same as a big battery.
    So if they are both similar chemistry you can not overcharge the small battery if the charger itself backs off charge current to protect against overcharging.

    What you cant do is mix AGM and normal wet cell batteries for instance.
     
  16. So a durtie is ok? Better be, I just ordered one!!

    Sent from my Nexus 6 using Tapatalk
     
    theBusmonkey likes this.
  17. theBusmonkey

    theBusmonkey Sponsor

    Dont worry ;).
    We've had the same Durite VSR connected to the starter for years and both batteries are kept in good order. It allows excess charge from the solar panels and the Ctek to trickle into the starter when the leisure is full of beans.

    In addition to what's already been said regarding the OP original question....
    I've always said not to mix battery chemistry and for the reasons Bigherb states above, it's best to stick to the wet cell type for both. Mixing chemisty will work, but you are shortening the life of expensive kit by not balancing the ability of the alternator/charger to the different charging requirements of mis-matched batteries.
    Ours wet cel/ FLA's have removable caps for servicing as both the solar charge controller and the Ctek MXS10 will push charging voltage up to 14.4 under the right conditions.
    Every 2 to 3 months I check the electrolyte level and top up as required. It's a ball ache but I get to give all the connections the once over at the same time:thumbsup:

    I haven't had to connect to 230v hook-up for the last 3 months and we've had the fridge running constantly during this time coz we use it as a supplement to the weedy boat fridge.

    So the combination of decent, serviceable wet cell batteries, 190w of solar controlled by a quality MPPT controller, a Durite VSR, Ctek 10 amp for hook-up an alternator in good condition plus correctly sized and fused cabling will give all the power you need.

    Don't scrimp on cable size. Look at the cable that runs from the starter motor to the + on the starter battery. It's massive! Volt drop is your enemy on DC so use the various calculators available on line if you are unsure of the correct cable gauge to use for battery to battery connections and for running cable to appliances/sockets. For example, the cable I used for the VSR was 25mm sq. Probably over spec but I had it! Fuse to protect the cable, so any fuses must be rated below the safe amp carrying capacity of the cable. It's good practice to select a fuse just above the maximum current draw of the appliance. In my instal the VSR cable can handle about 170 amps but I've fused it at 50amps with midi strip fuses either side.
    :thumbsup:
     
    mcswiggs, Iain McAvoy and mikedjames like this.
  18. Thanks once again for all your input - I think I'm going to opt for a switchable charging system to run the LB most of the time, with the option to switch across if required.
     
  19. A durtie...what :eek:?
     
  20. Durite relay

    Sent from my Nexus 6 using Tapatalk
     

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