24v panel 12v battery car setup

I am considering extended car camping and have found an ideal 136W solar panel for this purpose - ideal because it is lightweight, and can be rolled up and stored easily in the car. My intention is to use a jump starter as a battery which contains a dc outlet and possibly an ac outlet (inverter) of approximately 400w. I believe the storage capacity of a decent jumper would be sufficient for my small camping power needs (charging electronics, fan, hotpot, heated throw, nothing more than about 150W), and also serve as an emergency jump / air compressor.

The issue I'm having: 1. the panel is 24v. The car jumpers all charge most efficiently with 110v wall chargers. Most allow you to charge through the (12V) DC port but it takes forever and a day.

So, I think what I'd need to do is sort of bass-ackward: take 24V and put it through a 110 inverter to charge up my jump starter.

I realize the sensible thing in other circumstances would be to take 24V, store it in a 24V battery and then hook it to a 24V-110V inverter, but the compact portability and multifunction of a 12V jumper is hard to ignore.

Anyone have any electrifying ideas about this?? would I need a cc?


  • BB.BB. Super Moderators, Administrators Posts: 32,816 admin
    Re: 24v panel 12v battery car setup

    Get a good quality small MPPT charge controller:

    Morningstar SunSaver 15 Amp MPPT Solar Charge Controller
    MidNite Solar The Kid MPPT 30 amp Solar Charge Controller $285
    MPT-3048 MPPT PV Charge Controller 30A - 12/24/36/48v $350

    The above controllers are all good quality (and not cheap). Anything over 15 amps (good for a ~282 watt maximum array) are overkill at 30 amps... Great if you plan on adding more panels or build out a larger solar system later.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: 24v panel 12v battery car setup

    You can't actually run an inverter directly from a solar panel. No point to having a 24 Volt system and a 12 Volt one; more stuff to lug around.

    As Bill says, get an MPPT charge controller and wire it in. A 136 Watt panel will produce about 8 Amps on a 12 Volt system so the MS 15 MPPT would be all you need. You could even add a second of the same panel, but I'd caution you find out how large the battery is in that portable power unit. I doubt it takes even 8 Amps to charge it.

    And don't be surprised when it turns out not to have as much stored power as you need. "150 Watts" may not sound like much, until you use it for a few hours. 1/3 of the day: 1200 Watt hours, or 100 Amp hours @ 12 Volts (roughly; worse when you factor in losses). There probably isn't 100 Amp hour in that unit. If there were it would weigh like a car battery.
  • AuricTechAuricTech Solar Expert Posts: 140 ✭✭
    Re: 24v panel 12v battery car setup

    The largest jump-start battery pack I've seen (this Peak 900 battery) contains an 18Ah battery. Since the battery pack includes USB and 12VDC outlets, I would imagine that the plates are marginally thicker than those on a regular SLI battery. I still wouldn't want to use more than about 4Ah of its capacity before recharging.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: 24v panel 12v battery car setup

    As a temporary measure a friend, that I am helping set up a 560W PV system for, decided to start out with 280W PV and tried using the built in inverter of his booster pack instead of just buying the proper inverter for his final design size...

    Well the no load draw of that little booster packs inverter really applied a heavy tare load overnight... lucky they were GC2 batts....needless to say he now has a reasonable TSW inverter wired in that has a sleep mode, which that power pack didn't have..

    Also the built in battery pack had 'packed it in after a few hours of late nite reading on day one, mind you it was a few years old...
    Overall I don't feel they are good value for the dollar spent if you want/have to rely on it day after day...


    You might be getting a sweet deal on that 24v panel, but you can lower your system cost a lot if you get a 12v panel(s) so you can use a PWM charge controller...
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • PNjunctionPNjunction Solar Expert Posts: 762 ✭✭✭
    Re: 24v panel 12v battery car setup
    kcm1 wrote: »
    .. I believe the storage capacity of a decent jumper would be sufficient for my small camping power needs (charging electronics, fan, hotpot, heated throw, nothing more than about 150W), and also serve as an emergency jump / air compressor.

    Inside those jumper packs are generic general-purpose agm's. The largest I've seen are the Stanley units, inside of which are 18ah. Most are not user serviceable for battery swapouts, although I did modify mine with a 22ah agm since it was the same size.

    These batteries are limited to 0.3C charge current, which for an 18ah agm means about 5.4A constant charge current maximum. If you were able to open your jumper, you may see this rating. So what we are talking about is an 80W nominal 12v panel, with a small pwm controller, like a Morningstar SunSaver 6A type. Connections can go straight to the battery through the clamps, bypassing the relatively poor cigarette lighter jacks. Of course you have to turn the clamps on first, attach the controller, and the panel last. Above all, you have to be SAFE doing this to prevent an inadvertent short of the clamps.

    The problem is the battery size and what you want to do. Consider that the 18ah battery has a watthour rating of:

    12v * 18Ah = 216wh available. BUT since we don't want to draw more than half of a battery's capacity, we cut that value in half. That means you really only have 108wh to play with on a daily basis.

    So you are going to have to make 108wh last for your nightly duration. A 150 watt load is not going to last very long - much less than an hour. Note that the 400w inverter is a *surge* rating, and not a continuous draw setup. It is also a modified-sine-wave type (MSW) and not a pure-sine-wave (psw) and some things will not work well, run slow, or overheat.

    Tip: depending on how well built / isolated the USB circuitry is, if you overload them, it can take out the logic of the entire jumper pack and render it useless. Most are rated at the standard 500ma standard, so charging modern devices will be real slow, or the usb circuit blows. Better to use an external mobile 12V > 5v usb adapter, like some Anker units plugged into the cigarette lighter jack instead.

    Basically, even with a small system, you still have to very carefully measure your actual loads over time. In this case, will 108wh be enough daily. The other side of the coin is your solar insolation. Do you have 4 hours of solar-insolation where you are when camping? These differ from mere sunrise to sunset hours and need to be looked up online. For instance, let's say you draw your battery to half of it's capacity - now you have to replenish 9ah. Can you do that in time with an 80w panel (near max limit)?

    Let's see:
    9ah / 4.5a solar * 1.78 fudge factoring = 3.56 hours. Let's make that 4. You just might make it provided you do have 4 hours where you are going daily if you discharge that far.

    The jump-starters are fun / ok IF you do all your homework up front first, otherwise you'll be wasting money. Get out the multimeters, measure your dc current draw and total it over time, in addition to measuring actual wattage draw of AC devices with something like a Kill-A-Watt meter. You'll also want to know the real ah capacity of the internal jumper battery pack.

    Most jumper-pack solar installs are just winging it, and it ends back up in the garage on the provided ac charger because in reality the capacity is just too small for what many think they can really handle. :)
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