Microsoft Word - Q and A on Alternators DC to DC and Scotty…

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Safiery Pty Ltd ABN: 87 624 588 807 30/75 Waterway Drive, Coomera QLD 4209 (07) 210 22 55 3 Proprietary Information not to be published or copied without written approval from Safiery Pty Ltd. double 70mm2 for 140mm2 joint size for 0.3V drop. What Fuse Size should be used? A) The fuse is there purely to protect the cable from melting and/or catching fire. It will NOT protect the electronics from power spikes or adverse voltage levels (most people think the fuse protects everything). a. As a guide we divide the fuse current rating by 7 to give the maximum fuse size. So a Killa-wasp 80A with a 125A fuse needs a 125/7 = 18mm2 What if the alternator to battery fuse melts over time? A) A melted, deformed fuse indicates sustained power close to the fuse limit but generally not over the limit. Advice would be a bigger alternator or reduce the power load on the DC to DC (only possible with Scotty) to reduce the current. However, as Scotty is tuned to deliver maximum power at road speed, and it is during this time that the fused circuit will be at maximum current, it would reduce the benefits of Scotty fast charging and as a solar backup. MARINE APPLICATIONS What is the situation with outboard engines? A) Outboard engine alternators have a big alternator output power ratio. At idle the alternator power produced is very low. a. Fixed DC to DC has to be sized so it won’t bog down the outboard EFI at idle and kill the engine. Field results are that this is a very small number. Possibly with twin outboards 20-25A at idle. As a result this is the max size with most outboards. b. Variable DC to DC (Scotty): Scotty can deliver 25A at idle but 125A at full power. Scotty will not kill the engine at idle. What is the impact of cabling size from Alternator to DC to DC? A) It is common to pick up a roll of cable and read on the roll “maximum current”. This number is the current limit before the cable will melt and catch fire. It is totally irrelevant to power loss over distance. a. One reason small power DC to DC’s are promoted by 4WD shops is that they can install with smaller cables and without the need for lug and mega fuse fitting precision. For 50A over 5m, 50mm2 cables has a 2.3% voltage loss. That’s only 18W power loss. Install a 16mm2 cable (what is commonly done), the power loss is double at 36W but the charging voltage has dropped by 0.6V and the power transfer will most likely only get the battery to float voltage at a slower rate. b. Now step up to big power: 200A target. A 120mm2 cable will give 0.3V drop which is good for charging. Installing only a 50mm2 cable will give a 0.8V drop and the system will not charge properly. At Safiery our guide is at 3.5m, a single 70mm2 will give 0.3V drop. At 6m we recommend

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