Let's try replacing the 12v battery with a much smaller, lighter one

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Myself248, Jul 26, 2020.

  1. Myself248

    Myself248 Junior Member

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    2012-2020 is a pretty long, good run for a lead-acid battery. I'm not sad that it died.

    But I was just offended by the $200+ pricetag of the OEM replacement. I looked at several other threads about alternatives, including Group 25 mobility battery which is somewhat cheaper, but still, it felt like too much. 45AH is big enough to crank the actual engine! All this thing has to do is run some computers until the DC-DC starts up, and maybe the headlights for a minute or two, why is it so big in the first place?

    Also, I had places to go and the old battery wasn't getting me there, so I grabbed the first thing that was handy: a 12V, 7AH "alarm system battery", rescued from an old Belkin UPS. It's smaller than a pound-cake and will happily start the Prius! Ran on that for a couple days actually, while formulating the rest of this wacky scheme.

    If a 7AH battery will do the job in a pinch, I'm willing to gamble that I can replace it long-term with the ubiquitous 18AH battery familiar to FIRST FRC competitors, APC UPS owners of a certain age, PPPRS racing competitors, and countless others. It's so small it fits laying down.

    Also, it's $35.

    Here it is, wedged into place with some closed-cell foam just to prove out the idea:
    battbox-foam.jpg

    So cozy! I took better measurements of the area and mounting holes. Here's an early sketch to see how much clearance I had to the right side (up in the image) and front (left in the image):
    battbox-clearance.jpg
    (Ignore the extra holes; they're an artifact of how I had to fold the paper to get my holepunch in there to punch the ones I cared about.)

    Distilled that into this drawing, which is no longer trying to use every available millimeter, but rather has the 165x180mm battery outline around it. (And some notches so you can get your hands under it for removal someday.)
    battbox-sketch1.png
    And I know, the battery isn't quite centered, and the notches may not be symmetrical. It works, it's good enough!

    There's a bit of a hump towards the left side of the space (I finally oriented this one so up is front), so I ended up making a two-layer bottom to the box, observe the inset of the bottom layer:
    battbox-crosssection.png

    That outward-bent wing is intended to mount the circuit breaker which you saw stuck to the battery with Dual-Lock in the foam image. (I removed the 100A fusible link, which is intended to protect the white B+ wire that heads up to the underhood fusebox, and put a 50A self-resetting thermal breaker in its place, more appropriate for the smaller battery.)

    Here's the whole thing as designed:
    battbox-wholebox.png

    The parts are made out of 1/4"-thick ABS plastic, heat-bent into shape. It's what was in the scrap box. And as a result, I slightly mismeasured the scrap and where I intended to have notches for a tiedown strap, there wasn't enough plastic for that. Ah well. Here's the top piece being bent:
    battbox-heatbend.jpg

    No photos of the bottom piece, but the two layers are held together with flathead screws and brass inserts. McMaster 97595A251 and 93365A160.

    Here's the whole thing all bolted down and strapped in. I left the breaker mounted with dual-lock for now; it feels plenty secure. I put the strap the other way from my original intent, threaded between the layers of ABS, and it seems copacetic. The battery is resting on some foam in the bottom of the box and then the strap squishes it down to compress the foam, so it doesn't budge.
    battbox-strappedin.jpg

    So far, so good! I'll probably be eating my words come winter, but I'm willing to take a gamble.
     
    #1 Myself248, Jul 26, 2020
    Last edited: Jul 26, 2020
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  2. Grit

    Grit Senior Member

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    Great ingenuity, hope it holds up thru winter season for many years (y)
     
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  3. TMR-JWAP

    TMR-JWAP Senior Member

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    Because, obviously the engineers at Toyota are just plain stupid.

    Or perhaps....

    They take into account the random fool who leaves the interior light on...
    or the guy who leaves the car unused for a month while the SKS is searching for a key
    or the guys who leave the radio on while working on the car while not in ready mode
    or the guys who just leave the door open while working on the car while not in ready mode
    or 10 years worth of use and degradation that still leaves enough AH to keep the car working
    or a hundred other design considerations I'm sure they take into account.

    Hell, a couple 6v lantern batteries would work just fine....until they don't. Then a 200 dollar battery is going to look pretty cheap compared to a 200 dollar tow + a battery and wasted time. Or the hassle involved if a wife or daughter happens to be in the car when it craps out.
     
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  4. ASRDogman

    ASRDogman Senior Member

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    How much labor did that cost you??
    $25 a year for that battery is what it cost.
    Though the car/computers don't care where the 12volts come from, it's still risky.
    And they only last a few years. I've replaced many ups batteries. Unless the car battery,
    they die without warning. Then you are stranded, again. Every year or two, or sooner because you
    are using older used ones.
    You're EXPENSIVE computers will not like that. When they demand that voltage and amperage, and it's
    lower, they could burn up trying to draw the power.
    It's a good idea, in a pinch. But that is something you do UNTIL you can get the correct part.

    But, it's your car....
     
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  5. bisco

    bisco cookie crumbler

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    well done!(y)
     
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  6. Myself248

    Myself248 Junior Member

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    Thank you for pointing this out! Yes, that's actually been on my mind this whole time. I've left my lights on (accidentally discovered the sequence that defeats the auto-off function, why doesn't it ding when you do that?), and they handily depleted the battery overnight. Even the big stock one. Being big did not help it.

    But having a second battery, which remains disconnected most of the time, is a great way to recover from situations like that! If you can find a way to keep it charged, that is.

    Presently, I have a little jump-starter pack that lives in the seat-back pocket. I forget to charge it, and it's sort of a PITA to use, especially in foul weather.

    The right way to do it is to equip the vehicle with a proper dual-battery system. With a smart relay or ideal-diode to make sure they both charge when charging is available, but isolate it so an idiot load doesn't kill the reserve battery. I've put these systems into countless vehicles at work, and long contemplated having one in my personal ride.

    Of course, the stock battery doesn't leave enough room around it to fit much of a reserve battery in there, much less all the wiring kibble to make it work, plus knuckle-room around all the terminals. It could be done, but it would probably have to live in the cargo tray, and that's just more awkward than I'd like.

    Buuuuut with the dinky little 18AH, it's a whole different game! When I was checking to see if everything would fit, I did toss two 18AH batteries in there side by side, and there's plenty of room for activities.

    I've been doodling around with that double mount in CAD, but I'm not gonna pull the trigger on it until I've got a few months on this battery and I can make some assessment of how it's doing. But if it all pans out, I'm looking at 2x 18AH batteries ($35+35), one Victron Cyrix-ct ($44), another 50A breaker ($7), a pushbutton and miscellaneous lugs and wiring (scrap but let's say $10), and I'll have a proper dual-battery self-recovery setup for still less than the cost of replacing the original battery that left me dead in the first place.

    (And the chapter beyond that may involve a mobile radio install, but that's yet unwritten.)

    Unexpected sexism, but okay.

    I don't pay myself for leisure time, so that's a big ol' zero, actually the Fusion 360 practice will likely pay for itself back at the day-job. (And yes, I believe part of the definition of a shadetree is that wrenchin' on the car is considered leisure.) How much did you invoice for postin' that reply? We're here for fun, dude, this doesn't go on a timesheet.

    [citation needed]

    As I understand it, SAE J1211 doesn't require that modules (in a nominally-12-volt system) operate below 10 volts or above 18, but it does require that they survive being connected to a lower voltage indefinitely. There's a related ISO standard and I'm sure Toyota's suppliers are held to one and/or both. If you have evidence to the contrary, that's concerning indeed and I'd love to see it.

    Anyway, yeah, I'll be keeping an eye out for weird behavior. So far so good, but winter will be interesting!
     
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  7. NorCal PiP

    NorCal PiP New Member

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    $184.93 4th of July weekend for oem battery internet order pick up same day at a local Magnussens Toyota dealer in high rent Palo Alto CA a few miles from home. This to replace the original battery that lasted eight years. Not looking to re-invent the wheel given the astonishing performance of the oem battery. They got that thing right. imho (Current price is 190.43)
     
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  8. ASRDogman

    ASRDogman Senior Member

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    Dude? I'm NOT a cowboy...
    And the correct way to do it is to NOT do any of those things that kill the battery.
    Unfortunately, people don't do that, as you know.

    Rigging a ups battery with a switch that's easy to flip when/if something like this happens,
    is better than being stranded. IF, you can train others to do it.

    I guess your time is not valuable, but mine is. And yes, I do INVOICE my time on Prius Chat.
    I send them a bill every month, and they pay it.

     
    #8 ASRDogman, Jul 27, 2020
    Last edited: Jul 27, 2020
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  9. Mendel Leisk

    Mendel Leisk Senior Member

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    I go down these sorts of rabbit holes every so often, but less often now. Too much conjecture and loose ends in the initial premise.

    What you've got in there now is essentially a knock-off version of the type of battery that comes with a JNC660 jump pack. It's "ok", but doesn't have the legs of the OEM spec battery.
     
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  10. C Wagner

    C Wagner Member

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    Hi Myself248 and others. How did the experiment with the two (or one) dinky little 18AH AGM batteries go? I have an unknown age OEM battery failing now in a 16 year old Gen II car. I am running a desulfator on the OEM battery in case that helps (takes months usually) but also intrigued by the lighter, healthier battery. My MPG isn't great at the moment. I rebuilt the HV battery pack and Dr Prius gave it 102% so I don't expect the mileage is due to that battery. I do think it's due to the ICE coming on all the time to add a view volts to the 12.0 sick auxiliary battery. A smaller cheaper AGM would hold its voltage at 12.6 or wherever it should be a lot better, as long as the SKS I have won't take too much from the little battery.
     
    #10 C Wagner, Jan 18, 2021
    Last edited: Jan 18, 2021
  11. Grit

    Grit Senior Member

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    You rebuilt the hv pack before troubleshooting the 12v battery? o_O
     
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  12. C Wagner

    C Wagner Member

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    #12 C Wagner, Jan 18, 2021
    Last edited: Jan 18, 2021
  13. CR94

    CR94 Senior Member

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    I've been using a $65 35AH U1 AGM for nearly 6 years now. It works perfectly, for one-third the price. I shimmed the mounting bracket to make it fit, and adapted the battery to hook up to the original vent tube. Considering its age, I now also have a similar replacement ready. Bought the second one from Harbor Freight, using one of their 30%-off coupon.
     
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  14. C Wagner

    C Wagner Member

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    Bravo! Thanks for the info. This is why we love the forums! A tiny question - I have "SKS" (the key doesn't have to be in the slot to work). Do you? PLease say yes. I've heard some complain that 'you really need a large battery if you have SKS)...

    Carl
     
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  15. C Wagner

    C Wagner Member

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    Another question: The 35AH U1 batteries has the flat tabs with holes in them. The Prius and all car expects "SAE posts" How did you convert the battery's tab to posts? Also, let us know how SKS works with this battery. I calculate that the battery is about 22% smaller in capacity than the OEM 45AH battery. Thanks!
     
  16. Mendel Leisk

    Mendel Leisk Senior Member

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    By the time you finish cobbling changes to the hold-down bracket and cables, simpler to just use correct spec battery? Maybe even cheaper lol: and is this going to last as long?
     
  17. C Wagner

    C Wagner Member

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    CR94 writes that he's used a 35AH U1 SLA battery for 6 plus years with no issues. If the MPG stays high--engine doesn't have to come on often to recharge the aux battery and it works okay with the SKS keyless option--what's not to like? $65, some adapters from tab to SAE (I hope CR94 will enlighten us) and a shim or something to make sure the smaller battery is properly secured. The OEM battery is 3-4x the price.

    BTW, my Toyota OEM aux battery registered 11.1v before I went to engage the READY mode - temperature was 10F this AM! The Prius engine with 166K, God love it, started right up, since it doesn't use the aux battery to start - and doesn't even have a traditional starter.

    Of course I hope that my experiments with desulfation result in a healthy-again aux battery, but I'm thinking the 35AH $65 battery looks pretty good.

    I'm eventually hoping to run 800W of inverted AC devices off the thing, as in a power outage. It would be interesting to know what the max constant current draw from CR94's smaller aux battery is.
     
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  18. PriusII&C

    PriusII&C Member

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    While you wait for @CR94's response, there is a data point here for your reference Fitting mobility 12volt AGM battery. | Page 21 | PriusChat (@milkman44 post#412).
     
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  19. C Wagner

    C Wagner Member

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    Thanks. That page (and Page 22) were helpful. I saw that someone was using the 35AH battery with SKS, I think. Also, a picture on the next page, shows a potentially good way to connect tabbed batteries to the SAE/JiS Prius connectors for pos and negative-- a large (conductive!) screw is put from the prius terminals through to the battery tab and fixed with a nut. I don't think that's as ideal as finding a good tab->SAE post adapter, but if the screw connection can carry enough current, it might be okay.

    In my case I was thinking of using the prius to power an AC Inverter. I know that the safe draw (before the Inverter fuse blows!) is 100A or lower. I'd therefore prefer not to use the screw-to-tab solution, but to find tab-to-SAE post adapters, because they'll have a lot more surface area, I think.

    EDIT: This is what I think we need to convert U1 tabs to SAE posts: I've ordered a pair. Flag to Post Battery Terminal Adapters | L to Round Terminal
     
    #19 C Wagner, Feb 3, 2021
    Last edited: Feb 3, 2021
  20. Grit

    Grit Senior Member

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    o_O
     
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