Forum Discussion

LittleBill's avatar
LittleBill
Explorer
Mar 12, 2018

Battery Isolater concerns

there appears to be a new type of isolater which is voltage sensitive.

it allows charging from both sides alternator or converter

Clicky

alot of people talking about matching batterys, but what do people do with these, the minute the converter starts charging over 13.2, the starting battery is now tied in?

seems like quite a mismatch if you say 600AH agm, that gets connected to a 110AH lead acid,

any comments?

10 Replies

  • time2roll's avatar
    time2roll
    Nomad
    Mar 12, 2018
    LittleBill wrote:
    seems like quite a mismatch if you say 600AH agm, that gets connected to a 110AH lead acid,

    any comments?
    So it connects and you have one 710AH battery. Either can only benefit from the converter float charge or the alternator running. Throw 200+ watts solar on there and you have a perfect match.

    Yes flooded and AGM have a bit different resting voltage at which point the relay is already disconnected. No harm will come of it.
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Mar 12, 2018
    A good brand of paralleling solenoid is virtually goof proof. Bi directional if both batteries have dedicated charging sources. I prefer the Blue Seas or Sure Power 200-amp models.

    Forgive me but I fail to understand some of the rationale given in this thread. Please enlighten me. :)
  • scrubjaysnest's avatar
    scrubjaysnest
    Explorer
    Mar 12, 2018
    For the isolation relay between the cranking and coach batteries I added an on/off switch. The isolation relay is operated by a B.I.R.D.
    When on shore power, rarely, I leave the switch off; it's on when ever on solar or running down the highway.
  • road-runner's avatar
    road-runner
    Explorer III
    Mar 12, 2018
    YC 1 wrote:
    That appears to be a simple BIRD. Bi directional relay delay.

    Do a search and you will be reading all day.

    There are several devices such as this one available.
    And the BIRD has been around for at least 10 years. This device might use different technology, but it's certainly not a new type of device.

    Addressing the question of mismatched batteries getting suddenly connected to each other, the potential is there for a problem. In my Fleetwood installation, there's a 150 amp fuse in the interconnection. There have been a few cases I know of where the chassis battery was totally dead, and the fuse blew when the coach battery became suddenly connected to it. In typical operation, the charging source starts bringing up up one battery. When the interconnect kicks in 30 to 60 seconds later, the first battery sometimes discharges into the second battery for a few seconds, but after 30 seconds they are both happily charging from the charging source.
  • LittleBill's avatar
    LittleBill
    Explorer
    Mar 12, 2018
    right, but these are design for boats as well etc. i would think if your using 2 different types, AGM for storage and flooded for starting, then you wouldn't want to combine due to the different voltage set points for each type,

    but every class c/a has a isolator so it can charge, it just seems if it was such a big deal, most wouldn't have it?

    just speculating i guess
  • YC_1's avatar
    YC_1
    Nomad
    Mar 12, 2018
    That appears to be a simple BIRD. Bi directional relay delay.

    Do a search and you will be reading all day.

    There are several devices such as this one available.
  • brulaz's avatar
    brulaz
    Explorer
    Mar 12, 2018
    Use a similar device to feed my truck's inverter.
    It only allows the inverter to run when the truck's running.
    Works perfect for that, as an isolator.

    Finding an ignition sensitive wire to fire a relay in my truck was too big a PITA.

    Dunno about a dual battery setup though.
  • lawrosa's avatar
    lawrosa
    Explorer
    Mar 12, 2018
    When you parallel any 2 healthy batteries (same type and voltage, but different AH's), current will flow from the battery with the higher voltage (or SoC) to the lower due to the difference in voltage and resistance between them. This happens if the batteries are the same capacity or quite different. Current will decrease until the voltage of both is identical. At that point, essentially no more energy will pass between them except the tiny difference in self discharge rates.

    While charging or discharging, the equilibrium between them will be maintained. More current will come from the larger battery but voltage between them will stay constant. Because voltage is related to SoC, they will both have the same SoC.

    Example of a 30 amp load 200AH and 100AH battery in parallel: If 20 amps is flowing out of the 200AH pack, then only 10 amps will flow from the 100AH pack. This is automatically balanced as voltage remains constant.
  • lawrosa's avatar
    lawrosa
    Explorer
    Mar 12, 2018
    Its my understanding that batts will only match voltage with mismatched batts.

    Amps will draw or feed what they need. Least resistance...

    Connecting two banks with different capacities in parallel is technically fine since the batteries will be operating at the same voltage. Charge and discharge current will be shared, based on capacity. It is best if the batteries are of the same type and age. For example, avoid combining a sealed (gel or absorbed glass mat) battery with a flooded (conventional) battery because they have different charging setpoints. Broadly speaking, you can parallel batteries without problems, and the charge controller will look after them. Just make sure you give them plenty of charge. If the system tends to operate at less than a full state of charge, adding new batteries to old will probably just result in the old ones pulling the new ones down and everything getting sulphated.



    If the 100 ah batts are the starting I wouldn't worry. My chevy truck charges the batts at 15 + volts at times.

    And if you read the specs on your starting battery I believe it will say charge at the 14+ volts...

  • landyacht318's avatar
    landyacht318
    Explorer
    Mar 12, 2018
    AGM's are Lead acid.

    I would avoid any voltage sensign relay that is dual sense.

    I'd avoid single sense relays too, especially with solar, as the surface charge would keep both batteries parallelled for a while anywhay after driving, and with solar in daylight the solar will keep them paralleled until sundown, perhaps more.

    My AGM is also full charged resting of 13.06v, and thus if used as an engine battery, on that particular unit, be slightly cycled, unnecessarily.

    There are likely several dozen voltage sensing relays out there by many different manufacturers.

    I'd much rather have a dumb continuous duty solenoid triggered( with optional inline illuminated switch) by the ignition, and preferable triggeded only after engine start, instead of Key to ON.

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