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Thread: 2nd Battery not preforming like I expected.

  1. #11
    Low Bitrate
    Join Date
    Dec 2011
    I really recommend that you pick up a current sensor ("fuse tester") - I got one at Harbor Freight for under $15:

    All you do is plug this little guy in place of your blade fuse, put the fuse into the tester, and then start up the PC. This will give you a readout of how many amps the PC is using, and you can use it to determine how long you should be able to run off of battery power. It might help you diagnose any problems with your setup - maybe it's pulling way more amperage than you are expecting? You should also use a multimeter and determine what voltage the battery is operating at, and see if it cuts off a specific voltage.

  2. #12
    Maximum Bitrate
    Join Date
    May 2012
    Grandville, MI
    Quote Originally Posted by OldSpark View Post
    redheadrod, surely you jest...

    Although battery differences are why you do not keep them paralleled (nor mix in series), those difference mean little during charging. And when being loaded, paralleling will increase capacity unless one is faulty or at a different capacity (percentage-wise, not AH-wise).
    This link explains why you want to use similar batteries way better than I could...

    Two batteries in parallel halves the internal resistance. Or rather, the internal resistance is the same but are combined the same as 2 parallel resistances (which is always LESS than either of the individual resistances).
    I find this confusing...
    I went through training with Metra a few years back and we spent some time with Batteries...
    I just looked some and I am finding that you are correct but we had a demonstration that would make you believe otherwise. Something other than internal resistance must have been at work with the demonstration we had.

    It was a demonstration on why it can be a bad idea to use two batteries in parallel. The over all capacity is more but not 2x more as you might think.
    They pushed that you really wanted to isolate the batteries from each other at any time other than charging.

    We had 2 partially charged batteries of the same type and model and hooked them in parallel to a car. The car would not start.. Would not even crank over.
    When you used just one of the batteries the car would crank over slowly but would start. This happened with both batteries.
    So apparently it must not have been an internal resistance issue as I have thought for a few years now but something else entirely.

    I have also seen reference to this when jumping a car as well.

    I am unaware of any alternator that uses diodes to charge 2 batteries. That requires 2 rotors and stators - ie, 2 alternators.
    Alternators output up to 14.4V (steady-state) for 12V lead acids. [ 14.2V is a common set point. The older 13.8V spec has long been discarded. ] Alternators don't care what loads they power - it could be several paralleled 12V batteries - provided the alternator is loaded within its specs.
    I HAVE one here... It is made by Wrangler. It is 130 AMP Hot/Cold rated GM compatible alternator with external Voltage Regulator. It has TWO outputs for batteries.
    if you are using it with just one battery you MUST hook it to both posts with another wire. It is in storage so I could probably take a picture of it but I purchased it used and I believe the web site for the manufacturer is but I am not 100% certain since I do not see this alternator offered anymore.

    Who the heck uses diode isolators? Even in the old days, clever people used relays for battery isolation. (Which battery's voltage should the alternator sense?)
    These days MOSFETs might be used instead of diodes, though I would still ask why (when a relay can be used...)?
    I had one of these and I may still have it around somewhere. They had huge heat sinks on them and for proper use you had to tweak the voltage on your alternator higher to overcome the drop in voltage. It looked much like an audio amplifier. The large Diodes were potted inside the large heat sink and you had 3 connections. The common or Feed and then one each for each battery. Because I couldn't get my alternator voltage up I ended up replacing this with a constant on RV solenoid.
    Relays are a bad idea unless you can get a very large Relay. RV constant use Solenoids are similar to starter solenoids but designed to be on constantly with large current capacities and for use in a dual battery system.

    If you do a Google search for "Battery Isolator" you will find MANY of the diode style of isolator. It is very cheap and requires no extra hardware to run and won't wear out.
    Probably used much more in Marine use than anything else.

    Your other points seem subjective else without any definition (eg "quality", or what outlasts a car battery in a car {and why would it be preferred}?).
    Most of the companies I remember selling batteries made with "Ultra Pure" copper plates and similar designs don't appear to exist anymore. (US AMPS used to make some awesome batteries)
    It appears that to get the type of batteries I was talking about you need either an Odyssey or Kinetik type battery but I am not as familiar with these as the older style batteries.

    I had a long talk with one of the engineers for the Wrangler Alternator I had and he explained why the Optima Battery is a very BAD battery for car audio. He was the one that mentioned the US AMP battery.

    What he told me is that the Optima Battery was designed for Off Road use mainly and for some racing applications. If used as the main battery in a car it will destroy your alternator eventually. Reason why is because as it discharges the design of it falsely requires higher and higher amperage to charge it. He told me a near dead battery shows to an alternator as a nearly 400 amp load and if you COULD charge it at that rate you would blow it up. Otherwise at this rate you will put a tremendous load on your alternator. He told me he could take a near dead optima battery and recharge it without issue but required a current limited charger which is not cheap. I went through 5 red top batteries and 2 alternators before I talked to this guy and it all made sense so I dumped the Optimas and got a US AMP battery. Doesn't matter now but that battery worked as well as the Optima battery, was half the size and I had NO issues with it even after 5 years using it as my main battery. I never had dimming headlights while feeding over 500 watts to my Subs or the other 200 watts to the other speakers.

  3. #13
    Raw Wave
    Join Date
    Nov 2009
    That Battery University stuff does not refer to rechargeable batteries. However the issues about being matched when still connected by unused (or loaded) are the same, hence why we isolate parallel batteries when NOT being charged. If users need to parallel them for extra capacity or lower monoblock stress, then fine -but they should otherwise remain isolated (except if charging).
    When charging, only the voltage and type of battery needs to be matched (ie, 12V, lead-acid), their capacity, brand, and history & age does not matter. (That's in with consideration of the usual norms - eg, don't connect a faulty battery; follow precautions or recommendations if paralleling discharged and charged batteries; temperatures are similar; etc.)

    2 batteries in series ADDs the internal resistances, but in parallel it's the normal "parallel resistance".
    And surprise surprise, a battery is often modeled as a VERY large cap, and parallel battery capacities are added just as capacitance is.
    EG - two paralleled 12V 100AH batteries each with 10mΩ internal resistance is equivalent to a 12V 200AH with 5mΩ internal resistance. Same voltage, double the fault/short current, etc.
    (fyi - the "surprise" is not mocking - it is merely intended at my surprise that the common "model" works in parallel mode. However in series...)

    Again, yes, isolate paralleled batteries when NOT charging.
    The reason is that they will not be the same, and whichever is "lower" will drag down the other. (The "higher" does not "hold up" the lower, the lower discharges the higher.)
    Though matching or luck may minimise the differences, eventually one fails and kills the other; or thermal runaway (mainly in AGMs) occurs, etc.

    Simple reliability consideration:
    If one battery lasts 10 years (ie, 10 year lifespan or MTBF) and you have 2 of them, you replace 2 batteries in 10 years.
    Connect them in parallel, yo replace four batteries in 10 years. (ie, one fails after 5 years and kills the other so you replace both. The same again in 5 years.)
    But the above is VERY simplified and far from reality. The MTBF is the MTBF for a single battery in that environment with flat distribution. The increased failure rate due to interactive failure is ignored. ETC.
    The main point is that unlike the typical reliability of interconnected components, interconnected batteries interact with each other and a faulty battery will cause earlier failure in the others (including series connected batteries).

    Diode isolators. Alas I find they are far more expensive than relays. (Relays being "electrical isolators" and include solenoids {yes, I understand THAT ambiguity!}, contactors, etc.)
    But my main reason against them - WHICH battery do you connect the alternator's Sense terminal or tweak the alternator voltage to?
    If it's a manual tweak (not using the alternator's Sense), what current is passing thru that diode? Is the diode drop 0.6V @ 1A, 0.7V @ 5A, 1.0V @ 20A, or 1.5V @ 40A? etc. IE - undercharge the battery under load (and supply a mere 13.3V to loads?), or cook the at low loads?
    And is the sense wire is used, and the current to the other battery & load is different, then does it cook or undercharge?
    MOSFETs are better, but do they have as low a contact voltage as a relay? (And withstand vehicle surges?)

    Hmmm - copper plates in bateries. You mean lead?
    Apparently Optima started using recycled or non-pure lead which may have been one of their alleged downfalls.
    But I haven't been recommending Optima, and on this site, "Optima Jim" who was their eMarketing Manager (or whatever) for a while did little to counter what I had heard. Then again, he reckoned paralleling Optima batteries had no effect on their reliability...
    Their construction is supposedly "suited" to the rough stuff. Alas I merely drive road cars and broke wheel rims and shock-absorbers etc because I speed up on rough dirt roads, but my batteries have all been fine (though the wets may spill acid when upside down).
    I did run when I dropped my 13 year old Yuasa 38AH UPS AGM battery (UXH38-12) from higher than 1m and its corner hit the concrete, but it didn't explode and it's still in my ute as the main cranker.
    Not that AGMs should be used for cranking, but my old wet cranker failed 2 to 3 years ago and I had the UXH38 as an emergency jumper.
    For an AGM I would have chosen a Deka here in Australia, or a Kinetik or Deka in the USA. Though now I reckon I'd use a 100AH Yuasa UXH100-12 (it's cheaper than my UXH38-12)

    Not that Optima's "rugged" design should impact its reliability. I think some people like them despite reality - one guy I know loves them despite 2 that failed within 3 months, and he only gets 3-5 years from the rest. (My circles have been getting 6-8 years life from batteries 20-25% the price of Optimas.)
    And I won't comment on batteries that are generically suited to BOTH cranking and deep-cycle.

    As to alternators...
    If they are anything like the Bosch alternators I recently dealt with, I'm not surprised they blew.
    But IMO alternators should be self limiting. The only "failure" I have had with my Jap alternators was relatively recently after one hour of load torture (unbogging my vehicle using a secondary UXH30 feeding a small winch and my cranking UXH30 cranking the engine in gear (with splugs in place but IGN disabled) and an occasional engine run for recharging & 14.4V winching whereby 2 minutes before freedom, the alternator failed. Subsequent autopsying revealed the alternator was fine EXCEPT that the regulator's sensing diode's solder-joint had melted due to the alternator's heat. But that is far from a normal situation - 1 hour with little air flow and extreme charging surges & demand (a 75A alternator, and one UXH38 takes 40-45A immediately after a mere cranking!).
    BTW - re you reply in another thread, down here a rating is expected to be under normal operating conditions. Down here, a 90A rated alternator will be 90A at normal operating temperature, not when cold.

    Re the combination of alts and batteries to remove audio headlamp flicker...
    Just decrease the wire gauge to the audio. (Ha ha - ie, there are many solutions depending on the aim.)

    And as to being subjective... I probably meant vague or lacking definition. Like what is an "Ultra high quality battery"? My $80 batteries are obviously better quality than the local $500 Optimas because mine last 8 years.
    "Quality" is defined by the user, and varies with the application. Things like reliability, price, performance etc can be better defined or explained. But quality might be crap reliability, great performance, and price whatever that performance requires. Not that that's what I meant to refer to either...

    But AFAIAConcerned, we are saying the same or similar things. Apart from Optima being crap (), paralleled batteries whilst charging is ok.
    And thus things like "You do NOT want to use a Ultra high quality battery with a standard car battery" are meaningless or incorrect if referring to batteries that are isolated when not charging.
    Oh, I didn't finish the "2 parallel batteries aren't quite 2x capacity". No, not if they are not equal - ie, one drags down the other. However, if discharged at the same rate as one battery was, the combined capacity will be more than 2x...

    Funny how this makes me think if a line I often quote "Life's a poo. The you die."
    And I think batteries account for much of that poo.
    (I use poo because I don't think that word gets censored. But feel free to SHIfT in whatever word you feel appropriate. And with apologies to any reader that I may have just offended.)

  4. #14
    Join Date
    Sep 2011
    I'm running a dual-battery set-up with 2 Group 31 Optima Blue top's in parallel off of a White-Rodgers 12V 586 Series Solenoid (think High-Power Relay). I wired the coil on the solenoid to a 3-way switch so that in it's normal position, the solenoid claps shut when the engine is on (passes the accessory 12V's to the coil) and disconnects when the engine is off (or not in Accessory). Then, I can throw the switch to disconnect the 12V to the solenoid at all times, thus NEVER having the batteries connected, or into the 3rd position which puts 12V from the secondary battery into the solenoid coil, clapping it shut so that if my main battery dies, I can throw a switch and jump myself with my secondary battery.
    The nice part about running it this way is that if I have the vehicle off, I can get about 3-5 hours out of my PC, depending on what I'm doing with it and still not touch my front battery.

    My load is a 3930K CPU, 1GB Video Card, 3 SSD's, all off of an Opus 360 Pwr Supp, a 1000W monoblock pushing about 300W to a much too small 8", 650W amp pushing the 4 components, 1W wifi card, some misc. USB devices, and an Inelmatic XF1000 Monitor.

    Isolating the batteries, as stated several times above, is definitely the way to go, both for safety and health of the batteries, as well as convenience in configurability (new word?). I didn't see if your secondary battery was a starter or deep cycle but I'm thinking that there may be an issue with mixing the 2 in a non-isolated set-up. There are also MOSFET style isolators that will allow you to always have both connected to the charging circuit but never to each other (popular in RV's).

  5. #15
    Raw Wave
    Join Date
    Nov 2009
    IMO relays are better than MOSFETs. And cheaper.

    Diode isolators - forget them. That's ancient technology which is large, expensive, and has problems since the batteries will be at different voltages.

    Sizzlehappens is correct about mixing unmatched batteries when not being charged. Even keeping matched batteries connected (long term) when not being charged has problems - eg, if one fails it will fail the other battery.

    IMO the best system is an alternator controlled relay - the charge light circuit energises a relay when the alternator is charging. (I have dubbed this method the "UIBI" or Ultimate Intelligence Battery Isolator. That name is to counter the so-called "Smart" or "Intelligent" voltage-controlled battery isolators. Their only smarts is in their marketing hype when not applied to systems that do not have a charge-lamp circuit (ie, an alternator/regulator D+ or L circuit).)

    UIBI eg:

    By including a diode between the D+ or L and the relay #86, other switching as described by shizzlehappens can be added - eg, a push button or switch bypass to manually connect the batteries; each switch or +12V source with its own diode (to prevent each +12V source powering the other sources and circuits).

  6. #16
    Low Bitrate
    Join Date
    Dec 2011
    Regarding the relay - how do you know whether to get an 80A or 200A or whatever? Same goes for the fuses you use between the batteries. I'm a bit mystified - I would imagine that (essentially) shorting the two batteries together, you would only be limited by the internal resistances of the batteries, so you could potentially have a thousand amps running between them.

    In my setup I used an 80A isolator relay hooked up to the ACC/ON line, and used 60A fuses on each battery. It's been a week now and I have had no issues, but I think that if I would let my aux battery drain enough, it might pop a fuse.

    Anyone have any input?

  7. #17
    Raw Wave
    Join Date
    Nov 2009
    Not that you'll have 1,000 Amps between typical 12V batteries - a 70A AGM might be 3,000 to 6,000A but into a dead short - but you have asked the $64 question.

    Whilst easy to calculate the required fuse and relay capacity for other loads, how the heck do you allow for peak recharging surges?

    I have a 140A relay with copious cable and a 50A self-resetting circuit breaker at each end.
    I know the 2nd 30AH AGM can take 45A after some flattening (ie, a few unsuccessful cranks), but I haven't measured it after severe discharge (ie, down 20% or 50% etc), though I suspect it may peak at about 45A anyway.
    But I reckon even a 30A relay with 30A breakers would probably handle the typical 45A recharge.

    The problem with fuses is that you don't know that they have popped due to some pathetic/annoying short-term high initial recharge peak. Hence why I fitted self-resetting CBs instead (~$8 for 50A and under), but could have rewired my dash or other voltmeter or fitted some sort of alarm instead.

    Remember that flatter batteries have higher internal resistance which limits the current.
    And the high initial current quickly drops (unless limited by the source/alternator). My 45A drops reasonably quickly to under 10A with 30 or 60 seconds. IOW a 30A fuse (& relay etc) will probably not blow (though it might fail quicker...).

  8. #18
    Raw Wave
    Join Date
    Nov 2009
    CyberBill - Re the "thousand Amps" running between 2 batteries, I thought I'd do a sample calculation since it is a common question or opinion from many dual battery users.

    Using data for the Odyssey Batteries PC-2250 since it's one of the biggest common AGM/VRLA batteries available.
    Its capacity is 126AH @C20 or 114AH @ C10 and its short circuit current is 5,000A. It weighs 39kg (86lb).
    Its internal resistances is 2.1R (ie, 2.1mΩ = 0.0021R and fully charged OC (Open Circuit) voltage is 12.84V.

    Assume two PC2250s are connected together. One is fully charged at 12.84V, the other at its published "end voltage" of 10.2V.
    Hence 2 x 0.0021R = 0.0042R across a voltage of (12.84 - 10.2V = ) 2.64V.
    Hence the current I = V/R = 2.64V/.0042 = 628A. That is in "the thousand Amp" ballpark...

    BUT!! - it will never be that high.
    Why? Various reasons...
    - the 10.2V "end voltage" is under load. It is not its Open Circuit voltage.
    - internal resistance increases as the battery discharges, hence the flattish PC2250 will be much higher than 2.1mR.
    We are also ignoring connection and wiring resistances.

    If we use the PC2500's published OC voltage of 11.7V at 10% SOC (State Of Charge) - ie, 10% capacity remaining; 90% discharged noting that Odyssey claim a max of 80% discharge, and many will recommend no more than 50% discharge (to triple the life span etc), then...
    I = (12.84-11.7)V/4.2mR = 1.04V/0.0042R = 247A. (Under half of the previous 628A)

    That ~250A is equivalent to what many starter motors take - eg, 4 cylinder Jap cars with 60AH or smaller batteries, so conduction-wise, it shouldn't be a problem.

    As to the internal resistance that the 10% SOC PC2250 battery has, I don't know.
    I don't recall seeing published figures. (I suspect because then audio buffs etc that insist on low internal resistances would then realise how pointless the "low" internal resistance of an AGM is if it's discharged - especially if it's those buffs that recommend bigger batteries BEFORE getting a bigger copious alternator, or even before doing the Big-3!)
    But let's say it trebles at 10% SOC compared to 100% SOC (which I don't think is unreasonable, though I can only recite vague memories, and noting the limited charge current of some very flat batteries), then the former ~250A will halve to ~125A (ie, total resistance is now [2.1 + (3x2.1)] = 8.4mR which is double the previous 4.2mR. Double the resistance means half the current (at the same voltage).

    If the 2nd PC2250 is only 20% discharged (ie, 80% SOC - the normal limit for cranking batteries), its OC = 12.58V.
    Hence I = (12.84-12.58)/.0042 = .26/.0042 = 62A. (Assuming both have fully-charged internal resistances).

    Using 2 exceptionally low internal resistance batteries and ignoring inter-connection resistances and assuming fully-charged internal resistances (which are the lowest possible - ie, the highest possible current flow), then:
    - up to 250A may flow between a full and a 90% discharged battery;
    - up to ~60A may flow between a full and a 20% discharged battery.
    The real currents for these 0.0021Ohm internal resistance batteries will be lower.

    Most typical AGM car batteries might have internal resistances double those above (ie, halve the above currents), and wet cells usually have about double the internal resistance of AGMs of equivalent capacity (so have the current again).
    Remembering too that internal resistance increases with discharge state, and temperature, and the age of the battery.
    And internal resistance increases for smaller capacity batteries.
    And then there is the resistance of the terminals and interconnections and relay etc.

    Then there is the charge current... EG - 14.4V into a 12.8V battery at 0.0021R = (14.4-12.8)/.0021 = 1.6/.0021 = 760A.
    But that's another story!
    Last edited by OldSpark; 09-22-2012 at 07:15 PM.

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