First time dipping my toe in the water of this part of the forum, and into dual battery systems in general. I'm going to be putting a dual battery system in my daily driver FJ60 in the coming months and I'm starting the process of designing the system and acquiring parts. I have a background in electronics repair and design, but it took me a bit to wrap my head around the lingo like DC-DC chargers, solenoids/isolators, MPPT, and a dozen other acronyms. I have a fair grasp on it now but I'm still not an expert. This build will be simple and budget friendly, and will probably undergo a major revision a few years down the road. The enemy of good is perfection, and while I'm a little OCD, I'm not going to let dreams of the perfect system stand in the way of installing a well-planned simple system now.
My question pertains specifically to incorporating a winch. I'm going to run two lead acid batteries (until the house battery dies and then maybe switch to a different type) with an isolator between them, and will put a fuse on on the cable from each battery to the isolator. It's safer that way and best practice. I may go with resettable breakers, but for now let's assume it'll be fuses. See this stolen photo:
Now, when I go to use my winch, I'll increase the engine RPMs a bit, but I'd also like to run the batteries in parallel for this. They should already be in parallel anyway since the solenoid is energized and connecting the path between batteries. Here's the issue, however ... I'd like to have a fuse rated around 150A (derated to around 100-125A at engine bay temps), which is above my alternator output but below the rating of the 1/0 cable I'll be using. Plenty good to quickly catch a dead short if the insulation gets cut and the cable contacts the body. Let's say I'm stuck in some mud and in addition to the weight of the vehicle (4800lbs) there's additional resistance. My winch will supposedly draw 435A when it's at max capacity of 9500lb. According to the time constant graph on the Littelfuse datasheet, even at 300A of current draw and 60*F, the 150A fuses will blow in 1 second. Or one of them will anyway.
With my current single (starter) battery setup, the winch is wired directly to the battery with no fuses. I have an accurate voltage gauge in the cab that I watch and I monitor the temps of the winch and the cable while pulling. So how do I solve this issue with dual batteries? In practice are people just not making their winches work hard enough to blow the fuse(s)? Do I need second, fully manual isolator - essentially a big switch - in parallel to the first isolator so that I can bypass the fuses? Really, it would be taking the current draw on the house battery and splitting it in two down each cable, which would probably give the 150 fuse just enough headroom to not blow under most circumstances. So there would be current draw on the house main run, the house bypass run, starter battery, and the alternator - it might divvy up the load enough. That margin is a little thin for my liking though. Certainly this has been figured out, right?
Other things... I was initially leaning towards a Redarc SBI12D dual sensing isolator. The maximum current you can get on one of those is 200A. That also may not work for a winch. A friend recommended the BEP 720 marine isolator, also Aussie-made. It's also dual voltage sensing and has nearly identical characteristics to the Redarc unit, except it can handle 500A max. The house battery is going to be running some interior LED lighting, chargers, GMRS radio, and a fridge. Pretty basic. I may add an MPPT and a solar panel down the road. Monitoring will be done with two simple voltmeters in the cab, one off each battery. I'm going to add a manual override switch in the cab - both isolators are capable of that - in case the starter battery is dead, so I can start off the house battery. I'm interested in hearing about different "self jump" systems though, especially if I eventually move to a deep cycle AGM battery for the house. That is a must for us though, probably the #1 reason I'm going to all this trouble. A dead starter battery after a few days in the backcountry is a bad situation to be in. We've been there and luckily got some help and were able to get moving.
So, I wrote y'all a novel-length explanation. What do you think about my winch question?
My question pertains specifically to incorporating a winch. I'm going to run two lead acid batteries (until the house battery dies and then maybe switch to a different type) with an isolator between them, and will put a fuse on on the cable from each battery to the isolator. It's safer that way and best practice. I may go with resettable breakers, but for now let's assume it'll be fuses. See this stolen photo:
Now, when I go to use my winch, I'll increase the engine RPMs a bit, but I'd also like to run the batteries in parallel for this. They should already be in parallel anyway since the solenoid is energized and connecting the path between batteries. Here's the issue, however ... I'd like to have a fuse rated around 150A (derated to around 100-125A at engine bay temps), which is above my alternator output but below the rating of the 1/0 cable I'll be using. Plenty good to quickly catch a dead short if the insulation gets cut and the cable contacts the body. Let's say I'm stuck in some mud and in addition to the weight of the vehicle (4800lbs) there's additional resistance. My winch will supposedly draw 435A when it's at max capacity of 9500lb. According to the time constant graph on the Littelfuse datasheet, even at 300A of current draw and 60*F, the 150A fuses will blow in 1 second. Or one of them will anyway.
With my current single (starter) battery setup, the winch is wired directly to the battery with no fuses. I have an accurate voltage gauge in the cab that I watch and I monitor the temps of the winch and the cable while pulling. So how do I solve this issue with dual batteries? In practice are people just not making their winches work hard enough to blow the fuse(s)? Do I need second, fully manual isolator - essentially a big switch - in parallel to the first isolator so that I can bypass the fuses? Really, it would be taking the current draw on the house battery and splitting it in two down each cable, which would probably give the 150 fuse just enough headroom to not blow under most circumstances. So there would be current draw on the house main run, the house bypass run, starter battery, and the alternator - it might divvy up the load enough. That margin is a little thin for my liking though. Certainly this has been figured out, right?
Other things... I was initially leaning towards a Redarc SBI12D dual sensing isolator. The maximum current you can get on one of those is 200A. That also may not work for a winch. A friend recommended the BEP 720 marine isolator, also Aussie-made. It's also dual voltage sensing and has nearly identical characteristics to the Redarc unit, except it can handle 500A max. The house battery is going to be running some interior LED lighting, chargers, GMRS radio, and a fridge. Pretty basic. I may add an MPPT and a solar panel down the road. Monitoring will be done with two simple voltmeters in the cab, one off each battery. I'm going to add a manual override switch in the cab - both isolators are capable of that - in case the starter battery is dead, so I can start off the house battery. I'm interested in hearing about different "self jump" systems though, especially if I eventually move to a deep cycle AGM battery for the house. That is a must for us though, probably the #1 reason I'm going to all this trouble. A dead starter battery after a few days in the backcountry is a bad situation to be in. We've been there and luckily got some help and were able to get moving.
So, I wrote y'all a novel-length explanation. What do you think about my winch question?