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Battery interconnects for SLA battery bank?

Discussion in 'Electronics & Electrics' started by aXis, Jul 13, 2018.

  1. merlin13

    merlin13 Member

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    Egad, Bwain - looks nice.

    Off topic but flip side of that concept, as he's not fuse-isolating every used one he's got there (emphasising the words used and SLAs in this overall equation), one of those goes down and he loses capacity as the associated batteries fight with and against the dead/dying one anyway, yes/no?... :)

    Original thought behind my suggest was offload the little SLas for $5, maybe $10 each possibly? Times that by 25, contribute towards a newer gruntier battery or two, ideally deep cycle.

    But OP is wired up and wired in, he's happy, I'm happy, you'd better be happy...
     
  2. cvidler

    cvidler Member

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    Bigger batteries are always the better choice (space/weight not being an issue),

    less risk of failure (not so many eggs in one basket)
    better lifetime (the same load will present less impact to the battery in terms of C, both charge and discharge, reducing internal temperature, outgassing etc.)

    of the UPS' I mentioned above, the three APC ones, two had the standard compliment of 8 7AH batteries, I'm always replacing one or two when a power outage occurs and they discharge (I don't run them right down to flat either, as soon as an outage starts the attached servers shutdown then the UPS turns off), the one with the 4x 26AH batteries, hasn't need any new batteries for 2 years now, as such, I've just bought 8x new 26AH batteries to convert the other two UPS over.

    On topic, they are getting wired up with 8GA cable, and using the factory style single fuse (100A - which imho seemed way over specced for the 12GA factory wire)
     
  3. Privatteer

    Privatteer Member

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    Baintech 12 WAY is a similar type I've used at work. Not the cheapest option think they are about $50 a pop.

    I had a UPS built from 7.2Ah batteries since I got them constantly free on one job. Used the insulated spades and a earth bar from a switchboard. Positive was on insulators of course.

    Edit:
    Since you mentioned you can do PCB's it is possible to get PCB spade terminals ie ebay
     
    Last edited: Jul 24, 2018
  4. mtma

    mtma Member

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    The power distribution panel used in the First robotics competition came to mind but I don't ever recall it being this complicated. Probably spotted an older one from another season!
     
  5. rainwulf

    rainwulf Member

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    They are SLA, they don't need to be stored in any particular direction.
     
  6. caspian

    caspian Member

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    indeed, I have (quite literally) a couple of hundred thousand of them installed on end.
     
  7. OP
    OP
    aXis

    aXis Member

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    Fingers crossed, all of my parts have arrived today and I'll start routing PCBs for the power distribution units.

    Each PCB will have 6 x mini blade fuses - 1 per battery (5A) and then 1 for the whole bank (20A). Cables for the batteries will be under 40cm of either 0.75mm2 or 1mm2, and I can make them equal length if need be.
    The PCB will have 30A screw terminals going to dual Anderson powerpole connectors set up for a parallel daisy chain. I'll connect the main positive/negative to different ends of the chain to keep the cable path symmetrical for all banks. Probably a final in-line fuse to protect the daisy chain cabling.


    Click to view full size!
     
  8. merlin13

    merlin13 Member

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    No LED indicator of any sort to show you a blown fuse/problematic battery tree, or you're also looking at fitting some sort of battery measure/monitor external to these boards?
     
  9. OP
    OP
    aXis

    aXis Member

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    Good point on the fuse blow indication, it will take up a fair bit more space but is doable. I'm not sure how helpful they would be though - unlike regular loads most of the batteries will have similar voltages so there will be little voltage drop to drive an LED. You could see if a battery developed a significant internal short or several dead cells, but a bad fuse on a healthy battery wouldn't show up.

    To be more robust I'd need an op-amp comparing each battery voltage to the main output - a small voltage difference could then drive an indicator output. A couple of quad op-amps could do it.

    I will have external monitoring of the combined battery bank voltages and currents, but nothing individual.
     
    Last edited: Aug 8, 2018
  10. merlin13

    merlin13 Member

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    Without going into individual voltage comparators yadda yadda yadda and blowing this all out to be bigger than Ben Hur (I'll ignore anyone who sez Arduino...), have seen a Rough-as-Arseholes LED fuse monitor from the Mr Gumby School of Electrical Engineering that was simply a Widget consisting of a Zener and small limiting resistor/LED combo wired in directly across the fuse.

    - when the fuse is intact, zero volts on the Widget, ergo no light indication.

    - if/when the fuse blows, some sort of volts across the Widget (clamped by the Zener so as not to blow the LED), suitable high efficiency/low milliamp LED lights up - Aha! Must Be Thaaaaat One...

    In theory a sick battery "should" have low enough volts across it to trip the Widget when the fuse blows, else there shouldn't have been enough current draw to pop a 5 amp fuse anyway, Yes/No/Anyone?

    Obviously would only work if the fuse actually blows and there's still some sort of adequate voltage drop across the fuse terminals, and it might be awkward fiddling the Widgets to watch both each 5 amp and the combined 20 amp fuses as in your intended setup, but this only is a Rough-as-Arseholes suggestion.

    And personally I might be inclined to forget about the 20 amp fuse if you've got each battery already lurking behind it's own dedicated 5 amp one, unless you're engineering for a catastrophic battery bank failure. In which case the 5's should go Pbfflt! well before the 20 anyway.
     
  11. cvidler

    cvidler Member

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    you can buy fuses with inbuilt LED indicators, no point trying to build one.
     
  12. OP
    OP
    aXis

    aXis Member

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    Yes, but that's about the only condition it will work.

    The 20A fuse is to protect the upstream cabling - otherwise an upstream fault could supply 5 x the battery fuse rating. 5 x 5A isn't so bad, but if I switch to 5 x 10A it's not so great.

    I've got a PCB router and all the parts, building my own LED indicators would be heaps cheaper than special fuses. I already bought 120 mini blade fuses on ebay for about $6.

    That said I'll probably skip it and just do periodic battery/fuse checks.
     
  13. Phido

    Phido Member

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    I am looking at doing something similar with my battery bank.

    I got, for free over 24 x 12v 100AHr batteries. Sure I want one big battery, but if you try to price 1000 or 2000 Ahr batteries recently.. Free.

    I run a 24v system so two in series, each going to a 15 amp fuse in a big arse fuse holder (mine is 12 inputs).

    The fuse holder has a led to indicate blown condition. I am also wiring voltage monitor for each bank and a current meter as well. Each bank is connected to a switch so it can be easily isolated and tested. There is also a bulk current meter and shunt and overall voltage monitor and emergency disconnect.

    This then attaches to my main bus bar, to the 3000w sine inverter, to the 3500w 24v solar array via a charge controller and fuses, to the 24v lighting (which is then seperately fused) etc etc.

    On ebay you can get DC volt meters for a $1 each. Ammeters are much the same. Believe me, you want decent monitoring. Trying to problem solve paralell setups without it is impossible. I found this out
     
  14. cvidler

    cvidler Member

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    Keep in mind fault current on batteries (and large arrays of smaller batteries of course) that big can be many kA, you want fuses that can break that big of a current flow, your regular blade fuses will not do the job and just turn into an arc lamp instead.
     
  15. Phido

    Phido Member

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    That is what I worry about. Hence I have some very large current fuses going into and out of the bank. I think if you went over 24v that would likely be a bigger issue. I also have circuit breakers I am thinking about using, but I don't think they would be any better (although they are rated for much higher voltages). I am also thinking about fusing both sides, perhaps with something more heavy duty.

    Proper monitoring also reduces the risk. Eventually I would like it all managed by a controller with relays that manage the banks and has a wear algorithim and ties into all the monitoring.

    The battery bank is also external to the premises. Enased and packaged, in an underground filled block bunker.

    At least these are full sized blade fuses, not those micro things.

    I was very interested how Tesla fuses their rather huge packs. Thousands of tiny fuses at both ends.
     
  16. cvidler

    cvidler Member

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    You need to check the specs for whatever fuse/breaker you use and check what maximum fault current they can handle, and match (exceed) that of the battery bank short circuit current.
     
  17. merlin13

    merlin13 Member

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    15 amp inline fuse for 100 amp/hr batteries?? Bit brave methinks - to paraphrase a popular quote, "You're Gonna Need A Bigger Fuse". Or better yet, breaker.

    Unless your charging circuit can't supply more than 15 amps, because flat 100 amp/hr batteries will initially want to eat more than 15 to start with...

    Or are you actually set up to trickle-charge them?
     
  18. cvidler

    cvidler Member

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    Considering a typical SLA battery has an internal resistance of a fraction of a milliOhm, the fault current can easily exceed 10,000A.

    12V (nominal) 0.5mOhm (middle of the range for a SLA) = 24kA

    I love the stupid shit this guy does, so we don't have to.
    5000A melting big metal things
     
  19. Phido

    Phido Member

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    They are in parallel. So 15 amp x 12 connections(of 2 in series 12v 100 ahr) = 180 Amps in or out current max. However I don't like the idea of 180 amps coming off the tiny bolt at the end. So I have a 50 amp circuit breaker from that. I might bump that up to 100Amps, (24v x 100amp = 2400w, but the inverter is rated to 3500w surge) so I can surge start my inverter with 2400w short loads at night (the solar panels are more than able to make up for it during the day 60+ amps continuous).

    Each battery should only ever delivery or charge ~7-10 amps peak.

    Ideally I would have every bank with a thermocouple and volt/amp monitoring and relayed all with a microcontroller.

    The problem with 12v batteries is if you have a cell short out, then you get over charging, night drain etc. That is a bigger an more realistic problem than hot slagging everything.
     
  20. OP
    OP
    aXis

    aXis Member

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    Latest progress update:

    Got a couple of distribution boards routed and wired up, three more to go. Took a few weeks to re-order better full size blade fuses, the mini ones just didnt sit well enough to be reliable. Also made up some MDF battery trays with handles, they turned out quite nice.


    Click to view full size!
     
    CQGLHyperion likes this.

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