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  1. #21
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    Default Re: Connecting a third battery

    Quote Originally Posted by bobhof View Post
    Thanks. I think I will have to run some tests to see how slowly the 2 batteries equalise. The only time the batteries will be charging in parallel is when towing (both charging from the victron 30A DC-DC in the car) or when in camp during the day (both charging via solar from the WRND in the trailer). Most of the time the lithium will be charged in the vehicle from the DC-DC and the AGM will be charged in the trailer from the WRND. I guess I could run an additional wire from the starter battery in the vehicle to the WRND in the trailer so I can charge both batteries with their respective DC-DC chargers when towing. This would definitely be faster. Not sure if the combined 50A draw is perhaps a bit too much for my alternator.

    In the quote above, Fluffy seems to think that it is fine to just hook up 2 batteries in parallel even if they are different chemistries. That was from a while back though, so not sure if lithium was in consideration.


    That post was 2016, before lithium became mainstream in SA, especially in smaller camping systems.
    I'm almost certain what Fluffy meant with different battery technologies in that post was different lead acid sub types as in flooded deep cycle, AGM etc. He didn't refer to lead acid and lithium in that post.
    Last edited by bigboy529; 2022/01/14 at 03:31 PM.
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  2. #22
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    Default Re: Connecting a third battery

    Quote Originally Posted by bobhof View Post
    Thanks. I think I will have to run some tests to see how slowly the 2 batteries equalise. The only time the batteries will be charging in parallel is when towing (both charging from the victron 30A DC-DC in the car) or when in camp during the day (both charging via solar from the WRND in the trailer). Most of the time the lithium will be charged in the vehicle from the DC-DC and the AGM will be charged in the trailer from the WRND. I guess I could run an additional wire from the starter battery in the vehicle to the WRND in the trailer so I can charge both batteries with their respective DC-DC chargers when towing. This would definitely be faster. Not sure if the combined 50A draw is perhaps a bit too much for my alternator.

    In the quote above, Fluffy seems to think that it is fine to just hook up 2 batteries in parallel even if they are different chemistries. That was from a while back though, so not sure if lithium was in consideration.

    mmm - by different chemistries I mean 't within the same family ie lead acids. But do as I say not as I do. I would just sommer add the Lithium to the vehicles aux battery. Honestly, you are really going to battle to damage a discharged Lithium in an 8 hour drive. Trouble starts when you leave a battery with the wrong chemistry and charger for a long time, either on charge or discharge for like DAYS - Often.

    A hungry battery that normally eats well seasoned caviar will be quite happy with some Tinned Sardines now and again.

    Charging while driving is a bit of a funny one. Yes if your system is in the bak of the vehicle and you don't take it out and drive around, then get a good system, but for a van which in any case has a charging system, at best all you get is a possibly better charge for the first night.

    Obviously if you are travelling every day and a new camp spot every night, then a proper DC-DC system in the van is a must.
    Last edited by Fluffy; 2022/01/14 at 04:34 PM.
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  4. #23
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    Default Re: Connecting a third battery

    You are over thinking this and trying to complicate it.

    You already have a DC-DC with battery in the vehicle, you already have a DC-DC with battery in the trailer, keep it that way and keep them separate.

    As suggested earlier in the thread get solar panels to connect to the trailer while standing, look at post 8 again.

    For while driving to your destination you have a cable installed from your vehicles starting battery to a connector at the tow bar where you plug in the trailers charging system. While driving each DC-DC will then charge its own battery.
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  6. #24
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    Default Re: Connecting a third battery

    Quote Originally Posted by bigboy529 View Post
    You are over thinking this and trying to complicate it.

    You already have a DC-DC with battery in the vehicle, you already have a DC-DC with battery in the trailer, keep it that way and keep them separate.

    As suggested earlier in the thread get solar panels to connect to the trailer while standing, look at post 8 again.

    For while driving to your destination you have a cable installed from your vehicles starting battery to a connector at the tow bar where you plug in the trailers charging system. While driving each DC-DC will then charge its own battery.
    Perhaps. Here's what bothers me: I drive enough daily to charge everything and keep my fridge running. No need for solar up till now. My system cost me quite a bit but it works great and I don't run out of power. Now I'm adding more storage (AGM in trailer) and I'm not adding any additional load, but I need to also add a solar panel because I want to leave the fridge in the trailer. It just irks me, but I probably should get over it.

  7. #25
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    Default Re: Connecting a third battery

    Quote Originally Posted by Fluffy View Post
    mmm - by different chemistries I mean 't within the same family ie lead acids. But do as I say not as I do. I would just sommer add the Lithium to the vehicles aux battery. Honestly, you are really going to battle to damage a discharged Lithium in an 8 hour drive. Trouble starts when you leave a battery with the wrong chemistry and charger for a long time, either on charge or discharge for like DAYS - Often.

    A hungry battery that normally eats well seasoned caviar will be quite happy with some Tinned Sardines now and again.

    Charging while driving is a bit of a funny one. Yes if your system is in the bak of the vehicle and you don't take it out and drive around, then get a good system, but for a van which in any case has a charging system, at best all you get is a possibly better charge for the first night.

    Obviously if you are travelling every day and a new camp spot every night, then a proper DC-DC system in the van is a must.
    Thanks. I have a Victron 30a dc-dc in the vehicle charging the bluenova lithium battery. Then I have and AGM and a WRND in the trailer. I stay at one place for 3 days to a week at a time, but drive for a few hours every day. At the moment, without the trailer, the lithium in the car is perfect and has never run out of juice.

    The advice here seems to be to keep the two systems separate. So the trailer battery only charges via the WRND (from the alternator when towing, from a solar panel when parked), and the lithium in the car only charges via the victron when driving. But then I am sitting with a full lithium battery every night yet using the AGM to run the fridge which seems not ideal to me. If I connect the two batteries (ignoring charging for now), then the fridge and everything else will draw from the fuller and higher voltage lithium (which I know will be charged up the following day when I go out driving). Charging aside, is there any reason to not just slap the lithium and AGM in parallel to power everything in the campsite?

    I could move the lithium into the trailer, but I prefer it in the vehicle because it is lighter and it seems to charge really quickly.

  8. #26
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    Default Re: Connecting a third battery

    Quote Originally Posted by bobhof View Post
    Thanks. I have a Victron 30a dc-dc in the vehicle charging the bluenova lithium battery. Then I have and AGM and a WRND in the trailer. I stay at one place for 3 days to a week at a time, but drive for a few hours every day. At the moment, without the trailer, the lithium in the car is perfect and has never run out of juice.

    The advice here seems to be to keep the two systems separate. So the trailer battery only charges via the WRND (from the alternator when towing, from a solar panel when parked), and the lithium in the car only charges via the victron when driving. But then I am sitting with a full lithium battery every night yet using the AGM to run the fridge which seems not ideal to me. If I connect the two batteries (ignoring charging for now), then the fridge and everything else will draw from the fuller and higher voltage lithium (which I know will be charged up the following day when I go out driving). Charging aside, is there any reason to not just slap the lithium and AGM in parallel to power everything in the campsite?

    I could move the lithium into the trailer, but I prefer it in the vehicle because it is lighter and it seems to charge really quickly.
    For me the easiest solution for you is: keep the systems seperate. Get a 120A lithium for the trailer with 160-200W solar input. If your WRND cannot handle litium, get a new controller also. Spend your money wise the first time while thinking of possible future upgrades/expansions.

    I spent some R and $ on my trailer’s power setup the last year, and I finally think that I am done for now (exept maybe for foldable solar panels). I like to keep “what next” in mind when I install certain components so that I do not have to upgrade already purchased units also going forward.

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  10. #27
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    Default Re: Connecting a third battery

    Quote Originally Posted by Bassfisher View Post
    For me the easiest solution for you is: keep the systems seperate. Get a 120A lithium for the trailer with 160-200W solar input. If your WRND cannot handle litium, get a new controller also. Spend your money wise the first time while thinking of possible future upgrades/expansions.

    I spent some R and $ on my trailer’s power setup the last year, and I finally think that I am done for now (exept maybe for foldable solar panels). I like to keep “what next” in mind when I install certain components so that I do not have to upgrade already purchased units also going forward.
    Thanks Bassfisher. I'm afraid I don't want to spend that kind of money yet. I recently upgraded my vehicle's dual battery system (from a deep cycle with manual connection to alternator, to a lithium with a Victron DC-DC and inverter) so I've already spent ~20K on camping power. Hesitant to spend another 20K when what I already have provides enough power for my needs.

  11. #28
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    Default Re: Connecting a third battery

    Video on the benefits of mixing lithium and lead: https://youtu.be/tAuPfgZgXec



    He's talking about a large lead battery bank and adding a lithium to extend the lifespan of the lead batteries, so not exactly applicable in my situation, but interesting nonetheless. One thing I'm curious about is that at 21:30 he talks about the danger of connecting a fully charged lithium to a discharged lead battery and damaging the lithium by pulling too much power from it, but I've been told in this thread that the lithium will just slowly charge the lead battery. Is this because the system the video is referring to has a large lead battery bank, whereas my system has a single lithium and AGM of similar capacities?

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