Hi Dave, Your ACR doesn't function as you describe; it is just a relay, with no charging intelligence built in except that it will only close when it senses charging (a voltage above a certain level for a certain time) on one or both battery banks. That shouldn't matter too much if both banks are the same battery chemistry and the same age, but they will both "see" the same voltage (minus wiring voltage drops) when the relay is closed, so make sure it's appropriate.
You are heading in the right direction on voltage drops and overcurrent protection (fuses), but voltage is crucial to battery charging and you don't want to lose your boat to an electrical fire for the sake of a couple of fuses... Voltage drop will depend on current (how big is your alternator / battery charger), wire length (total of positive and negative lengths) and wire size - if it's going to be longer than recommended, increase the size of the wire. Blue Sea Systems probably have a calculator on their site or you can search for ABYC voltage drop tables. Under ABYC standards, connections of less than 7" do not need to be fuses, neither do engine cranking circuits, but if you don't fuse other circuits you are asking for insurance / survey trouble if things go wrong... Blue Sea have a great on-battery fuse (MRBF) that you could mount directly on the studs of the ACR without a lot of effort (and now required for your house bank). You are protecting the wires, not the devices with these fuses, preventing short circuit current from causing them to ignite, so if the ACR wires are a different size than the battery cables you need different fuses. https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A I hope that helps! Tim Tim Goodyear Ex-35-3 > On Jul 10, 2016, at 8:17 AM, Syerdave--- via CnC-List <cnc-list@cnc-list.com> > wrote: > > Hello all, > Wondering about the practical vs the theoretical here. > Am upgrading the DC system, adding a Group 24 dedicated start battery, and > connecting the two group 31 in parallel to double the capacity of what will > become the dedicated house bank. Am adding a blue sea switch and ACR. > FYI - an ACR is a device that upon starting the engine essentially 'chooses' > to top up the start battery fully before allowing the the house bank to be > charged. > Anyway, the manual states that the acr should be connected as close as > possible to the battery (to prevent voltage drop from compromising its > function to any degree) and both of its leads fused based on the wire gauge. > All makes sense, especially when the batteries are located At a distance > from the switch or ACR. however.... > The simplest installation is directly adjacent to the switch, not the > battery. The leads to the ACR would be very short and well protected - less > than a foot. (The battery cables themselves are not fused!) In the 33-2, > the distance to battery is still very short - less than eight feet of cable > for sure. > > In this situation, I can only see an infinitesimal difference in voltage drop > by locating the ACR a few feet closer to the battery, really, no meaningful > difference. With a lead length measured in inches the positive wires to the > ACR probably do not need to be fused. (the ground wire IS fused, protecting > against failure of the device itself.). > > Has anyone got any experience with this that would tell me otherwise? > Thanks! > > Dave Syer 33-2 > _______________________________________________ > > This list is supported by the generous donations of our members. If you like > what we do, please help us pay for our costs by donating. All Contributions > are greatly appreciated! _______________________________________________ This list is supported by the generous donations of our members. If you like what we do, please help us pay for our costs by donating. All Contributions are greatly appreciated!
