Best 12v battery replacement? Particularly to use with power inverter?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by wbs887700, Feb 12, 2021.

  1. wbs887700

    wbs887700 Junior Member

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    It appears I need a new 12 volt battery. It is at 13.88 volts with the Prius in ready mode, and 10.47 volts with it turned off. It's still starting the Prius, I don't know how, but it appears to be struggling. But occasionally it does need to be jumped.

    Anyway, I converted my Prius into a camper and I have a 1500 watt pure sine wave inverter connected to the 12-volt battery. I run a 700 watt microwave and a 1000 watt instant pot with the Prius in ready mode and it does just fine.

    What 12 volt battery would be best for this situation? Which one would be the most sturdy when using it for running a power inverter like this?

    Also, what else do I need besides the battery? Do I need a specific kind of ground cable or hardware or any kind of adapter of any kind? And does the battery fit where the last battery went? Does the lid fit on neatly and everything? What modifying needs to be done? I'd appreciate any pictures. I need the lid to fit on properly afterwards.

    This is a video of my Prius camper if you're interested:



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  2. rjparker

    rjparker Senior Member

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    I would use two Toyota agms with the second isolated through an rv isolator of good quality. Someone posted the second battery installation in the extra storage area opposite the primary battery. When in Ready both batteries are in parallel for charging and load sharing. When not in Ready the second battery takes the load, ensuring the primary battery is not discharged.
     
  3. wbs887700

    wbs887700 Junior Member

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    Yeah I don't have the money for that right now. I can only afford one battery right now. I will keep that in mind though. But, of all the prius campers I know of that has a power inverter (I know a lot), I only know of one person that has a second 12 volt battery.
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    #3 wbs887700, Feb 12, 2021
    Last edited: Feb 12, 2021
  4. ChapmanF

    ChapmanF Senior Member

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    Not too surprising; starting the Prius just doesn't take that much. Running the brake pump just before starting takes more. Sometimes if I have an inadvertently discharged battery, I'll be sure to wait for the pump to stop, and a few moments more, before starting the car. In a couple more dire discharges, I got the car started by pulling the brake pump relays first, and putting them back after the car was in READY.

    As long as you are doing this with the car in READY, the power to run the inverter isn't coming from the battery, and so your selection of a battery doesn't especially have to be tailored to it.

    You could check me on that though. I would suggest attaching a voltmeter across the battery terminals themselves, while the car is in READY, and note how low a voltage you see between those two points when the car is READY and the microwave is running, or the Instant Pot is heating. (I assume you do not run the two simultaneously.)

    Those readings would be interesting, if you were to post them here.

    Edit: I first wrote "note the lowest voltage you see", which could suggest using a min/max setting on a meter, but I'm really more interested in how low the steady voltage gets, not so much how low it might go for a mere moment when the magnetron cuts in, etc. It wouldn't surprise if some peaks like that represented a net outflow from the battery, but for a brief enough time to be effectively ignorable.
     
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  5. oldtechaa

    oldtechaa Active Member

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    I believe the max safe continuous current for the DC-DC converter is about 1200W, so as long as you're only running one appliance at a time, you'd be fine and the battery would only smooth out slightly higher spikes, meaning any battery appropriate for the car will work. If you're running both pieces of equipment at a time, not only do you risk blowing the DC-DC converter fuse (or worse), but your battery will only have about an hour to full discharge. Obviously you don't want to get anywhere near full discharge, and your tolerance will decline as the battery gets older, so you probably shouldn't be running both pieces of equipment for more than a couple minutes max.

    Also, your existing battery is still starting the car because it really only has to boot some computers and the HV battery actually does the hard work of starting the engine. My battery has tested as being "bad" for the past 2 years.
     
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  6. ChapmanF

    ChapmanF Senior Member

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    True, a battery may test as too "bad" to start a conventional car for some time and yet still start a Prius most of the time. On the other hand, it still does eventually cross the line from "will start the Prius most of the time" to "won't start the Prius anymore", and a lot of people find it sudden and unexpected when that happens, because they didn't have the same advance warning of slow rrrWrrrr-rrrWrrr-rrrWrrr starts they would have had in a conventional car. It just doesn't start one day, and it's still inconvenient if that was the day you were trying to make a departing flight.

    I find the sound of the brake pump to be something worth listening to when getting in the car. It will do a pretty good impression of a dying wind-up toy if the battery isn't well charged.
     
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  7. sam spade 2

    sam spade 2 Senior Member

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    That is not the battery voltage; it is the charging voltage......which says nothing about the health of the battery itself.

    The best battery in this circumstance would be a "deep cycle, AGM" of a larger size than the OEM one.
    They are often referred to as "marine" batteries.
    BUT that won't conveniently fit where the OEM battery was. And the posts might not match up with the cable ends either.
     
  8. wbs887700

    wbs887700 Junior Member

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    When I run my 700 watt microwave using my 1500 watt inverter (in ready mode) the voltage goes from 13.6-14.1 volts all the way down to 11-11.8 volts roughly for the entire time the microwave is running. It seems too low to me. I'm wondering if I damaged my 12-volt battery because when I bought this Prius five months ago, it was starting just fine all the time. And now the battery is dying and I don't know why. That's largely why I want a really powerful, good battery so the voltage doesn't drop so low.

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  9. wbs887700

    wbs887700 Junior Member

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    Yes I want a deep cycle battery. Because that's what RVs used to power their electronics. Are you saying there's no deep cycle batteries that can fit where the OEM battery fits?
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  10. rjparker

    rjparker Senior Member

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    That's largely why I suggested two batteries with an isolator. No need to reinvent the wheel. You could functionally beta test a high capacity lithium battery with built in battery management to emulate an agm. But a dual battery system is better for rv style use and would cost less. When the car is off you are not discharging the factory 12v battery with your extra loads. I suspect your "everything off" parasitic load is excessive with all the extras such as frig, interior lights, wifi repeaters and computers.
     
  11. ChapmanF

    ChapmanF Senior Member

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    Is this number from a voltmeter built into the inverter, or your own separate meter? If your own meter, where exactly were both probes at the time of the measurement?

    Do you have a kill-a-watt meter or other similar reading of what the microwave actually draws? I believe microwave advertised power (700 watts) tends to be the microwave output of the magnetron.
     
  12. sam spade 2

    sam spade 2 Senior Member

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    Yes, it probably IS.
    BUT......you need to get a handle on the difference between battery voltage and charging voltage.

    The measurement that you quoted indicates that your applied load is TOO MUCH for both the charging inverter AND the battery combined.
    A bigger battery might help some but I would be worried about frying the inverter.......and that would be $$$ to fix.
     
  13. sam spade 2

    sam spade 2 Senior Member

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    Don't know. Probably not.
    Certainly not one that will be an exact replacement.
    The whole idea is to get one that is BIGGER.
     
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