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  1. #1
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    Default How long does a dual battery system really last?

    Hi guys, I'm struggling with the dual battery-system lifespan.

    So up until now, on every trip I did (between 3-4 weeks) with a rented and camping equipped Hilux, had a dual-battery system which never gave me any issues. True is that the only thing plugged to the auxiliary battery was a 40l 220v fridge which used to run 24/7, not in freezing mode but just above the minimum.
    I'don't need much more than that other than recharging the cell- or satellite phone sometimes and maybe my camera (but always did that while driving).
    My plan is to tour for 9 weeks. So my question is, assuming that both main and auxiliary batteries are fully charged and assuming that I won't plug the fridge one single time in 9 weeks, and averaging 2 hrs. drive per day (but some days would't drive at all), will it last?

    The reason I'm asking is that I don't really trust 100% the dual battery system. Although the rental company would always say that the main battery does recharge the auxiliary while driving, I've heard that it is not 100% true. The main battery does recharge the auxiliary, but as soon the main is fully recharged it would stop to charge the second. So the Auxiliary would get flat after some time and only provide power for a couple of hours per day (say for the fridge) depending on how many hrs you drove that day. But having just heard, I take that with a pinch of salt...

    If my worries are correct, what should I do? Can I just plan to stop in Maun f.eg. get to a Toyota dealership or workshop and ask them to recharge the second battery?

    Is a battery inverter a solution? But would need to buy it somewhere since the company doesn't rent it and I don't want to change rental company just because it hasn't battery inverters. Or a foldable solar panel perhaps? That wouldn't be tricky to pack in my suitcase (consider that I'm flying from Europe so can't pack an inverter).

    Or did I just post this for nothing because I'm worrying for nothing? I'm sorry, I'm no expert when it comes to car batteries...

    Thank's for any advice.

  2. #2
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    Default Re: How long does a dual battery system really last?

    My understanding is that the alternator charges the secondary battery. There is a solonoid that senses when the crank battery is fully charged (it's a high cycle so charges quickly), then opens a relay which starts the alternator charging the secondary (generally) deep cycle battery. If the secondary battery is not charging then theres a fault in the charge sensor, the relay is not opening to change the secondary battery, or the secondary battery is shot. And then the possibility of cabling and fuses.

    The back up option is a solar panel and solar charger, use that to top up when possible.

    My advice, before you take the car, get them to put a test on the feed to the secondary battery and prove that the alternator is charging it within a reasonable time after starting the vehicle.

    Edit - just read your post again carefully this time, and I can see the concern about some days not driving. I think the battery should last about 12 hours without a charge, assuming a 4.5 amp draw and a 120aH battery, but might not get to full charge after a 2 hour drive. I'd say you need a solar panel and solar charger to boost when you're parked. Talk to your car hire company, then might offer it as an option, or be willing to supply for a price. The kit itself is not expensive, a 100W panel and solar charger can be had for R1,000 - R1,500.
    Last edited by Gungets Tuft; 2021/05/09 at 11:21 PM.

  3. #3
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    Default Re: How long does a dual battery system really last?

    Your aux battery should start charging about 8min after starting the vehicle. If a solenoid dual battery system the aux should charge quite a bit during driving. A DC-DC system charges slower. Get confirmation what type of system is used.
    A battery-220V inverter is not the answer as you have losses converting 12V to 220V. The mentioned solar panel with controller is the answer for the days not driving.
    It is not clear if you are in fact going to use the fridge. If only 2 hours a day the battery should be fine for this and charging sat phone and camera.

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  5. #4
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    Default Re: How long does a dual battery system really last?

    Is the 40l 220v fridge your? or the rental company's?

    Usually these fridge are 12v and 220v.

    As said above, you need a solar setup to sustain the battery when stationary.
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  6. #5
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    Default Re: How long does a dual battery system really last?

    If the battery systems that you have used in the past have lasted 3-4 weeks then I see no reason why it would not last 9 weeks unless you make big changes to the way you operate..

    Does every place that you intend to stay not have 220v power ? I did see you stated to work on the assumption of not plugging in for the whole time, I would check that you may perhaps have the opportunity to do so.
    Then maybe it is an idea to take along a AC-DC charger to do a top up when you can ?

    And as others have said a portable solar setup would be useful.
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  7. #6
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by Nambots View Post
    Hi guys, I'm struggling with the dual battery-system lifespan.
    Yes , is it works in past why not now. But you can set the fridge a bit colder when driving, and back to normal when stationery.

    Else get a solar setup as said, +- 100 watt panel and cheap pwm controller, and connect when stationery with crocodile clamps to the second battery, that will give maybe 6 to 8 hours longer life on the battery.

    We have done many shorter trips with a 40 l engel just running when the vehicle is running, set at -7 or so, but on longer trips that might not be ideal.
    Last edited by JLK; 2021/05/10 at 12:10 PM.
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  9. #7
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    Default Re: How long does a dual battery system really last?

    Thank's for all your advices. On one hand I feel I was probably worrying a bit too much, but on the other I think also that I'll consider a 100w solar panel for good.
    Thank's again.
    Last edited by Nambots; 2021/05/11 at 09:28 PM.

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  11. #8
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    Default Re: How long does a dual battery system really last?

    [QUOTbleNambots;4638347]Thank's for all your advices. On one hand I feel I was probably worrying a bit too much, but on the other I think also that I'll consider a 100w solar panel for good.
    Thank's again.[/QUOTE]

    Remember it's the panel, solar charge controller and some decent cable, crocodile clamps for connection to the battery.

    https://www.sustainable.co.za/solar-...ntrollers.html

    https://www.sustainable.co.za/solar-...ar-panels.html

    4mm cable will carry 10amps over 10-12m

    https://www.sustainable.co.za/solar-...ies/cable.html

    Have fun tinkering, it'll be a distraction the whole trip
    Last edited by Gungets Tuft; 2021/05/11 at 10:07 PM.

  12. #9
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by Gungets Tuft View Post
    [QUOTbleNambots;4638347]Thank's for all your advices. On one hand I feel I was probably worrying a bit too much, but on the other I think also that I'll consider a 100w solar panel for good.
    Thank's again.
    Remember it's the panel, solar charge controller and some decent cable, crocodile clamps for connection to the battery.

    https://www.sustainable.co.za/solar-...ntrollers.html

    https://www.sustainable.co.za/solar-...ar-panels.html

    4mm cable will carry 10amps over 10-12m

    https://www.sustainable.co.za/solar-...ies/cable.html

    Have fun tinkering, it'll be a distraction the whole trip [/QUOTE]


    Thank's again, very useful. I'll take the time to figure out. Cheers

  13. #10
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    Default Re: How long does a dual battery system really last?

    a Well designed (for your application) dual battery system could last indefinitely
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  15. #11
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by ekkekan View Post
    Your aux battery should start charging about 8min after starting the vehicle. If a solenoid dual battery system the aux should charge quite a bit during driving. A DC-DC system charges slower. Get confirmation what type of system is used.
    A battery-220V inverter is not the answer as you have losses converting 12V to 220V. The mentioned solar panel with controller is the answer for the days not driving.
    It is not clear if you are in fact going to use the fridge. If only 2 hours a day the battery should be fine for this and charging sat phone and camera.
    I found this to be the other way round... DC-to-DC charger was WAY faster than solenoid route....
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  17. #12
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by SinWolf View Post
    I found this to be the other way round... DC-to-DC charger was WAY faster than solenoid route....
    Absolutely!

    But it depends on the DC-Dc rating of the charger
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  18. #13
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by SinWolf View Post
    I found this to be the other way round... DC-to-DC charger was WAY faster than solenoid route....
    Me too
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  19. #14
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by SinWolf View Post
    I found this to be the other way round... DC-to-DC charger was WAY faster than solenoid route....
    that would only make sense if:

    1. Your alternator was way underpowered
    2. You had serious losses in your wiring
    3. You had a massive output DC-DC charger.

    Normally the biggest DC-DC charger max out at 30amp, but generally they are between 10-20 amp max output. Either way, no matter how much of the alternators +-100 amps are available, the charger will only put out a max of 20amps into the battery, whilst a solenoid can push the full amperage of the alternator to the battery, minus some losses in the cabling.

    DC-DC advantage is that is can put out higher volts (up to 14,7) which is needed for the final charging stage of some auxiliary batteries (AGMs etc, and Lithium with BMS). but alternator input via solenoid will always provide the fastest bulk charge.

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  21. #15
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    Default Re: How long does a dual battery system really last?

    Real life experience from a few years back:

    We did a just over 3 month camping trip through Southern Africa, mostly without grid power supply.
    When it was available it was only overnight most times.
    However we were travelling most days, being an overlanding type trip, son usually between 2 to 400 Km most days.
    We did stop at a few places for up to a week at a time.

    Our setup was similar to what you describe but we did have a 90 W Solar Panel & controller connected to two 105 Ah batteries while on site.
    While driving the alternator provided charging through the solenoid setup as described.
    (The alternator does NOT stop charging the auxiliary after the crank battery is charged unless you have a blown fuse or the solenoid itself has failed.)

    Running off the system was a National Luna 50 L Weekender camping fridge usually set to between -7 and -3 deg C depending on ambient temperature.
    This ran 24/7 with thermostat control.
    Ambient during the day fluctuated between 20 to 44 deg C, nights usually around 20 to 24 or so.

    In addition we charged a laptop, two tablets, satellite phone, camera as needed but usually while driving.
    We did have a 1 000 W inverter due to a requirement for my CPAP which ran an average of 6 - 7 hours nightly. (At the time 12v was not an option.)

    So most likely a greater load than you would have.

    Under those conditions ....

    After 1.5 months the one battery failed and needed to be replaced (They were not new at the start of the trip, but were fully charged).
    The fuse between the alternator and aux. system had failed, so the batteries had only been getting charge from the solar panel during the time until we realized that there was a problem. (Not sure - maybe 3 or 4 days).

    We continued to use the same system for a few years after that trip for shorter ones but usually with at least some grid power available most of the time.

    The one battery lasted approx. 8 years, the other (Original) another three years.

    There are a LOT of variables there, but maybe that will give you a starting point for your own decisions.

    Enjoy the trip.
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  23. #16
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by Scheepers85 View Post
    that would only make sense if:

    1. Your alternator was way underpowered
    2. You had serious losses in your wiring
    3. You had a massive output DC-DC charger.

    Normally the biggest DC-DC charger max out at 30amp, but generally they are between 10-20 amp max output. Either way, no matter how much of the alternators +-100 amps are available, the charger will only put out a max of 20amps into the battery, whilst a solenoid can push the full amperage of the alternator to the battery, minus some losses in the cabling.

    DC-DC advantage is that is can put out higher volts (up to 14,7) which is needed for the final charging stage of some auxiliary batteries (AGMs etc, and Lithium with BMS). but alternator input via solenoid will always provide the fastest bulk charge.
    I have had both, the solenoid and the Dc to Dc, and with absolute certainty my DC to DC charges way, faster than what the solenoid ever could
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by Scheepers85 View Post
    that would only make sense if:

    1. Your alternator was way underpowered
    2. You had serious losses in your wiring
    3. You had a massive output DC-DC charger.

    Normally the biggest DC-DC charger max out at 30amp, but generally they are between 10-20 amp max output. Either way, no matter how much of the alternators +-100 amps are available, the charger will only put out a max of 20amps into the battery, whilst a solenoid can push the full amperage of the alternator to the battery, minus some losses in the cabling.

    DC-DC advantage is that is can put out higher volts (up to 14,7) which is needed for the final charging stage of some auxiliary batteries (AGMs etc, and Lithium with BMS). but alternator input via solenoid will always provide the fastest bulk charge.
    That's also how I understood it - DC-DC chargers are to compensate for longer distances between alternator and battery and allow smaller diameter cabling to be used. If you could put the same diameter cabling as your car uses between alternator and crank battery all the way to your secondary battery, that would be solution A. But that's seldom possible, so you reduce current one way or another and extend charge time.

    I do have solar input to both my crank and second battery - permanently mounted Brad Harrison plugs - and a spare Victron Blue Solar that I use if I'm stationary for any length of time.
    Last edited by Gungets Tuft; 2021/05/12 at 12:49 PM.

  25. #18
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by Hedgehog View Post
    I have had both, the solenoid and the Dc to Dc, and with absolute certainty my DC to DC charges way, faster than what the solenoid ever could
    what amperage DC-DC system do you have?

    Im not trying to argue, but genuinely interested. I have done a few Dual battery installs and Solar setups ups on the farm, and have a good understanding of the pros and cons of each system. I cannot see how a well set up solenoid system would charge slower than even a 30 amp DC-DC system.

    alternators average between 80 and 110amps, and once the starting amps on the main battery have been replaced, the bulk of that is going to the aux battery, especially if driving in daytime, without lights on, and with windows down and no aircon. So how can a system putting out 20 or 30 amps charge faster than a system putting out 80 or 90 amps?

    like I said, the kicker comes in with the voltage level required in the final charging of an AGM battery.... the solenoid will never get it 100% full, because it will max out at 14.2 volts, while the DC-DC will keep putting out 14.7 volts, no matter what the alternator input voltage is....this is why you need to do a mains charge of your aux batteries when you have a chance..... or use a dc-dc charger over a longer period.

  26. #19
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    Default Re: How long does a dual battery system really last?

    Quote Originally Posted by Scheepers85 View Post
    what amperage DC-DC system do you have?

    Im not trying to argue, but genuinely interested. I have done a few Dual battery installs and Solar setups ups on the farm, and have a good understanding of the pros and cons of each system. I cannot see how a well set up solenoid system would charge slower than even a 30 amp DC-DC system.

    alternators average between 80 and 110amps, and once the starting amps on the main battery have been replaced, the bulk of that is going to the aux battery, especially if driving in daytime, without lights on, and with windows down and no aircon. So how can a system putting out 20 or 30 amps charge faster than a system putting out 80 or 90 amps?

    like I said, the kicker comes in with the voltage level required in the final charging of an AGM battery.... the solenoid will never get it 100% full, because it will max out at 14.2 volts, while the DC-DC will keep putting out 14.7 volts, no matter what the alternator input voltage is....this is why you need to do a mains charge of your aux batteries when you have a chance..... or use a dc-dc charger over a longer period.

    I have a 20amp charger, and I'm not arguing either, just telling you what I have experienced.
    I do not have the knowledge to argue your theory, but I do have the experience of using both products in a variety of vehicles to absolutely state that my DC-Dc charges faster (in other words over shorter distances) than what my solenoid ever did.

    I have heard that it is relative to the speed the alternator turns..

    For example, on a game reserve you drive at low speeds, my solenoid system was useless in this situation, and my battery would discharge to flat while running my 50l fridge, on the second day..

    My Dc to Dc system, has no problem with these situations, at all, and charges no matter how slow I drive
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    With a mild case of "Camping Personality Disorder" or CPD

    I drive a fire engine red Cruiser and tow an army inspired van.

  27. #20
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    Default Re: How long does a dual battery system really last?

    I was advised, for longevity, to not charge a deep cycle lead acid battery with more than 10% of its capacity.

    Also that a deep cycle lead acid battery will only accept so much charge (amps) as it can absorb, no matter how much you throw at it.

    That an alternator does not run (push amps) at its maximum all the time but only supplies as much as needed by the system.

    Is this correct?
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