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Thread: Is ite better to one large AUX battery or several smaller?

  1. #1
    Jesus Freak antimatter's Avatar
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    Is ite better to one large AUX battery or several smaller?

    Just was wondering what you guys thought.

    I have an isolator with a 17ah SLA battery currently.

    I have 4 brand new smaller SLA batteries that i could replace it with that would effectively double my amp hours.

    I have read a couple places that the more batteries you have the larger the strain on the alternator...i.e. 2 8ah batteries in parralel would be more strain than 1 17ah

    Any thoughts?
    -Jesus- King of Kings Lord of Lords

  2. #2
    Raw Wave
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    One large.
    (A faulty battery wrecks other good parallel batteries. And AGMs can go into thermal runaway. And "n" batteries means n times the failure rate (where failure rate is the individual battery's "self" failure rate).)


    The "more load" is CRAP. AH is AH - it doesn't matter how - end of story.
    And the more load is crap anyhow. I won't repeat my writings elsewhere, but if someone is keen to explain how reducing the size of your main battery reduces alternator "strain"....
    (It doesn't - in fact the opposite, though strain is not the right word. A bigger battery reduces the overall current that an alternator needs to supply.)

    And if someone wishes to justify otherwise, ignore cranking from the battery-size equation.

  3. #3
    Jesus Freak antimatter's Avatar
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    I was hoping someone of your "expertice" would chime in

    So your saying the reasons to opt for one large battery over several smaller ones is because

    a. one faulty battery (in parallel) will cause the others to go bad
    b. more points for failure
    c. AGM what the hooha? (what are AGM's and what is thermal runaway)
    -Jesus- King of Kings Lord of Lords

  4. #4
    Raw Wave
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    Annual General Meeting you ponce! Ooops - mp3car... I'm not dealing with <censored> car club idiots... D'oh!
    (LOL! Apologies, but familiarity and respect breeds... hopefully an understanding of humor, and especially my display of stupid childish (attempts at) humor.)


    AGM = Absorbant Glass Mat. aka Valve Recombination Lead Acid (VRLA) - a subset of SLA = Sealed Lead Acid batteries (another being Gel-Cel) as opposed to flooded or wet cells - the common Lead-Acid battery in vehicles.

    In general, by law, batteries in enclosed confines must be sealed. IE - engine bays can use wet cells, but boots/trunks, passenger compartments, caravans etc must use sealed batteries - unless (the wet cell battery is) in a sealed enclosure which is ventilated to the outside. (Hence the common short-hand "sealed vented enclosure" which confuses many.)


    AGMs typically have half the internal resistance of equivalent capacity wet cells, hence they can supply more current into heavy (low resistance/impedance) loads. IE - half the resistance means they have TWICE the short circuit current.
    [ FYI - because AGMs can supply higher current does not mean they LIKE to do it. This is something many people don't seem to get. Likewise they can absorb much higher charging currents. Except for "internal" installations, they are often used for UPS systems (Uninterruptible Power Supplies), but there their life is of secondary importance - they may be replaced after a mere 5 or 10 discharges. ]

    AGMs can undergo thermal runaway. As temps increase, characteristics change (charge/discharge voltage levels etc) in a non-desirable fashion so their condition gets worse. A collapsing AGM heats as its voltage drops and it accepts more current, further increasing heat and current etc. The good AGMs heat under increased load etc etc. When parallel, the good supply the bad (and reduce voltage). When in series, the good get a higher voltage across them, hence "boil" & heat up.

    AGMs heat because they have no "free" electrolyte to boil off. Wet cells have water/acid to evaporate & "boil" off before they heat in a similar way. (Gel-cells have limited boiling ability.)

    There is lots of stuff involved - the heat (absorbing) capacity of water; that AGMs - once "equivalently" boiled - cannot have their electrolyte replaced, etc. (AGMs have their electrolyte (acid) suspended in gauze or mat that wrap the lead electrodes, hence it is "dry" if cracked open. Hence too why they can be mounted in any direction, though some not vertically usually due to construction limitations.)


    That's the simple stuff. (ha?)


    Paralleling. Hmmm....
    With research, you should establish that if batteries are paralleled, they should be the same type, manufacturing batch, history, temperature etc. IE - IDENTICAL. Otherwise they have slightly different voltages and charge characteristics. (They will be anyhow, but being as "exactly the same as they can be", this differences should be minimised.)
    That pertains to paralleled batteries that are not in use - a higher voltage battery will charge a lower voltage battery, hence discharge & increase internal resistance and the onset of sulfation etc.
    It does NOT apply to batteries in use - ie, being charge or discharged. (It can, but not for typical use & applications.) That is another thing many do not seem to understand.

    So, if one battery differs from the other(s) - or it fails - it discharges the others etc.
    Parallel batteries will "reduce" (voltage) to the lowest battery.
    If the lowest battery has a collapsed cell (ie, it's a 10V battery, not 12V), the others will discharge to 10V (actually the 12.7V batteries will reduce to 10.6V - the fully charged voltage of a 10V (5-cell) battery).
    That 10V (10.6V) is below the lowest acceptable voltage for a 6-cell LA (lead-acid) battery. Normally an LA is full at 12.7V (12.67V in theory) and fully discharged around 11.3-11.6V.
    So at 10.6V, you have truly damaged your good batteries.

    Alas the above is dependent upon how long it takes and how long they stand etc, but I'm giving a simple example. (See f.ex battery life (cycles) versus DOD (Depth Of Discharge) data or graphs. EG - a deep cycle battery lasts 1800+ cycles @20%DOD, 900+ cycles @50%DOD, 500+ cycles @80%DOD (Ref: see here) [ NOTE: It lasts nearly 4x longer if used at a "cranking battery" DODs (20%) instead of deep DOD (80%). Or, increasing DOD from 20% to 50% HALVES it life. ]

    And I used a collapsed cell example because it is so obvious - a 2V drop is obviously a killer (the range of a normal battery being typically between 1V & 1.4V). And that 2V could be 6 partially collapsed cells, or any combination. A similar drop of 1 volt might be as obviously detrimental, but lesser voltage differences still have impacts.


    It is essential to understand that in effect, a good battery does not revive a bad one, the bad battery(s) will compromise the good battery(s). Ever see the "don't mix new batteries with old" etc labels on remote controls etc? That's a battery life consideration as well as damage (by leaking batteries etc).

    Hence why we want to isolate parallel batteries when not in use. That prevents any such battery interaction. (It also allows simple monitoring of each battery - just compare their voltages, preferably after 24 hours (ie, no surface charge) if to ascertain cell condition (though not actual capacity....)


    What if we don't isolate?
    Well, some simple reliability theory....
    Say if we have 4 components each with a 0.01 chance of failure, the overall chance of failure is 4 x 0.01 = 0.04. IE 4x the number of components = 4x the chance of failure.
    But those components might be independent - eg, 4 resistors in series or parallel for a LED. To wit:
    - If one series resistor fails (eg, blows open), it won't damage the other resistors or LED (the circuit opens; no current etc; in this case, there is 4x the chance of the LED extinguishing compared to using one resistor with 0.01 unreliability).
    - If the resistors are in parallel, then one blowing may not damage the others. The LED will receive less current and the other resistors can probably handle the LED. (Oops bad example - the LED current reduces and therefore its voltage, therefore the remaining resistors have a higher voltage and more current, but it's not much extra etc etc. Just trust me...)

    In the above, 4 resistors means 4x the chance (probability) of failure where failure means the LED not being at its intended brightness. (It's off in the series failure, it is dimmer in the parallel failure.)

    The above is simple reliability theory. (And can't be argued. (I hope.))
    But in the case of batteries, whilst using (say) 4 batteries means 4x the probability of not supplying the desired power etc, the problem is that if one battery fails, it compromises (damages) the other batteries. If left long enough, the good batteries will be destroyed.
    Hence in an unmonitored parallel battery system with n batteries, you will be replacing ALL n batteries n times as often. EG - one battery 4 years. 2 batteries every 2 years (4x the cost). 4 batteries every 1 year (16x the cost).
    Hence why many battery manufacturers and suppliers love saying that paralleling batteries is NO PROBLEM. They are simply being honest - it is no problem for them whatsoever.


    With isolation, 4 smaller parallel batteries should be as reliable as 1 big battery of the same capacity.
    In practice, 4 smalls will generally be less reliable because of other "dynamic" issues I have not gone into (different charge/discharge rates etc).
    But sometime 4 smalls may be desirable because if one battery fails, you still have 3 batteries. Sure, each battery might still fail at the same rate as the 1 big one, but if the 1 big one fails, you have none.
    But in general, you try to have one battery that is sized for what you want. If you want redundancy, you have a second battery of the same. That 2nd battery might be an isolated standby and charged separately. Or it may be isolated except when charging so that the main battery generally has half the load & discharge and recharge rate - hence lasting longer than if on its own - but what if both fail at the same time?

    Alas there is no one correct answer - it all depends on what is required. Like UPS - we want a reliable yet intermittent supply - we don't care if they last 6 discharges (not at 2 discharges per year). But for a cranking or audio battery - maybe we want more than a few days life.
    That's an extreme example, but I can't help but consider life time (ie overall cost) in what I implement.
    A UIBI (isolator) costs $10-$20. A voltage sensing isolator may cost $100. Too much? How much do your batteries cost, and how often are they replaced without isolators? Then there is the "woops, aux battery is flat therefore so is the main so I can't start the car!".


    I hope yet again you have enjoyed my short reply. LOL!
    Or rather, probably with a few re-reads, learnt something...
    I know I branched into other theories and examples, but IMO they are an integral part of an overall understanding.
    And yes - I have probably understated things (or does that mean overstated?) by treating things in isolation pertinent to the aspect being described. There is so much more to it, but I am giving the general "practical boil-down" as I see it. (LOL - I even wrote "simple reliability theory" - isn't that anything but simple - especially to the novice, or the rusty like me?)

    I hope to be able to give info that enables people to understand the fallacies of others' writings which are usually misunderstandings of reality - ie, using one "fact" in isolation to other details and circumstances. (Else simply the money makers and the uninformed.) Maybe that's reflected in my reply today in the12volt's new amp in car, need some suggestions which also relates to isolators. (Funny - the subject has been quiet for ages, I now await my #3 thread on the issue!)

  5. #5
    Raw Wave
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    Farck that was long!

  6. #6
    Maximum Bitrate Mickz's Avatar
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    OldSpark, once again a great and accurate reply and also an enjoyable read.

    And no, IMHO your answers are never too long
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  7. #7
    Jesus Freak antimatter's Avatar
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    I really appriciate you taking the time to write that. That would have taken me two hours.

    Again much appriciated
    -Jesus- King of Kings Lord of Lords

  8. #8
    Raw Wave
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    Stop bragging antimatter. I can't help it if I'm a slow typist.
    Ha - it probably did take me 3 hours, but it was interrupted by 2 longish phone calls, hence contributing my MY shock when saw its submitted length.

    I was tempted to delete and resubmit as separate replies - one theme in each.
    But what the heck...
    As usual I expect or want my replies (if worthy) to be copied and improved by others...


    I am curious to see the impact of such education...
    And as usual, if I am wrong - or ambiguous - please communicate that.

    Electricity and batteries - so very complex and confusing, yet so simple in retrospect! (Except the bits that still confuse me.)

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