![]() ![]() Absorbed Glass Matt (AGM) Valve Regulated Lead-Acid (VRLA) deep-cycle batteries are usually interchangeable with with wet low maintenance batteries. Battery selection and sizingįirstly, different battery chemistries require different charging voltages. It seems there are no large surge currents. I’ve installed 100 A mega fuses (the smallest available?) and haven’t had one blow yet. The fuse rating should be close to the maximum alternator current, even a bit less, and certainly no greater than the maximum current rating of any components and cables in the system. Fuse sizingĪ blown fuse is better than something melting and catching fire. Over-sized cables and relays are really only for very high-current accessories (e.g. The actual surge current will be much less because of internal resistance in the batteries. Through 10 m of 6 AWG cable (round trip), I estimated a surge current of 166 A. In the worst case I may have a completely flat aux battery (10.5 V under load) and a fully charged starting battery (12.65 V). High surge currents are possible when connecting a fully charged battery to a flat battery. A bigger relay will only cost more and not perform better. My relay is rated for 100 A constant current. The relay should be rated higher than the maximum alternator current. Fridge and Solar recommended 6 AWG cabling for all installations, which may be inadequate for long cable runs and/or high output alternators. ![]() I used 6 AWG cable as per the table above. The length of the cable run from the main to aux batteries is about five metres. The current ratings are only really important for the smallest cables (see realistic charging currents below). Read maximum cable length in metres at the intersection of alternator current and cable size. Selecting the minimum cable gauge depends on the length of the cable run from the main to aux battery: Alternator maximum current, Aĭual battery cable sizing table, calculated for a 0.5 V drop (in one line) at maximum alternator current. Fat cables are expensive and more difficult to install however. Larger diameter cables deliver higher currents with lower voltage losses. The cable on the left is protected by ‘pond tubing!’. Note the battery terminal boots and switch. Switch for the battery isolator ground (optional).A best practise is to remove both mega fuses before doing any mechanical work in the vicinity of the isolator. For safety, the terminals on the isolator should be insulated, the same as the starting battery. The VSR terminals are at the same potential as the positive terminals on your batteries and the system is grounded to the chassis. I have also found heater hose and ‘pond tubing’ useful for protecting cables. Split corrugated tubing to protect the cables.Extra ring terminals for the heavy gauge cables.I bought some cheap red car audio cable on ebay and taped the ends black for negative. Car audio cables can be cheaper but they have softer insulation. Large gauge battery cables can be very expensive. A simple component but prices vary considerably. I reckon you can buy all the components separately and save. I did not use the three battery terminals in the Projecta kit.There were insufficient terminal lugs to install the fuses.No fuses and fuse holders were supplied.4 m of red battery cable was supplied.That was not a good purchase as several items were not adequate, not included or not required: Dual-sense VSRs monitor both batteries and will allow charging the main battery from the aux side. Single-sense VSRs monitor only the main battery. This allows starting the engine with the aux battery if the main is flat. Some VSRs can be operated with a manual override, else a manual switch can be connected in parallel with the VSR. A VSR should always keep the starting battery charged. Switch the engine off and the VSR will disconnect the aux battery. A VSR will connect the aux battery when charging voltage is sensed at the main battery. VSRs are popular for several good reasons: high current capacity, automatic connect/disconnect and simple installation. Wiring diagram for dual-battery system with Voltage Sensitive Relay isolator.
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