Electric Cooling Fan Soft Start Circuit

I have a 95 suburban and have converted from a mechanical fan to electric. I used a fan from a mid 90's Lincoln MK8. I have a controller rated at 30 amps which is too small for this fan, so I run two additional relays rated at 60A each. (the controller controls the two relays) Both relays split the load of the fan. I read it has a FLA at 100amps during start-up. When the fan comes on, my ammeter in the gauge cluster drops to 10V then comes back to almost 14V in about 2-3 seconds. I can really feel the fan come on. I worry this is hard on my electrical system Without buying a soft start controller for $125+, is there a way to build my own soft start system? The fan has three wires, Ground and two (+)pos. I believe it is single speed. when I hook up either pos wire, the fan rotates at the same speed. would putting a resister on one of the pos wires, and use that to start the fan, then another relay would send full power through the 2nd pos wire a few seconds later work? Thanks Chris Electric cooling fan soft start circuit

You can build your own PWM soft-start circuit. You'd need a >100A MOSFET, but these can often be obtained for a few dollars. I guess if you are using parallel relays (IMO - bad!), the same could be done with FETs if current is a problem... Various circuits are possible, though I'd be tempted to look at the Micrel MIC502 8-pin Fan Control IC. I can't recall all its details other than it was often used for PC fan control; could respond to 2 signals/inputs; and seemed difficult to get... I also recall an initial "hard on" to overcome fan inertia (64 cycles?), but I reckoned that could be quashed (in principle) with extra external circuitry if it wasn't otherwise overridable. (FYI - I was looking at if for use as an up/down dimmer for vehicle courtesy lights etc. One benefit over normal PWM circuits (with logic gates or 555 timers etc) was that at 100% it went into full/liner mode - ie, its output was permanently on - not a 99.9% duty cycle etc. PS - your VOLTMETER drops to 10V etc. I hope no-one uses ammeters these days (although they often use remote or non-resistive loop sensors, they are still essentially useless in vehicles systems - except maybe if the engine is off and you want to estimate battery reserve time). But IMO and generally speaking, the sag should not be too hard on your electrical system unless you have a rewound (or in my experience - a Bosch) alternator or an AGM battery - or if other loads don't like it (eg, audio or high-impedance fuel injectors, but low-Z injectors and the EMS etc should be ok). LOL - my fan is a mere 80W (1600cc engine).

Thanks for the info. I'll get a single 80A relay to replace the dual 60's. I mistakenly called it an ammeter. thanks for the correction. My volt meter drops. I don't have an audio system per se. My upgraded alternator is a 140A mfg'd by Autolite. I upgraded the "Big 3" batt ground, alt to batt, and batt to main fuse box to 4ga and added a ground from alt casing to batt. Would dual batteries help, or be of any benefit? I do not need to run any loads while the truck of off. I'll look into PWM and Micrel. But I get out of my comfort zone with IC's. If it doesn't hurt my system, I may just not fix what ain't broke. LOL did you say your fan is 80W? I wish I could cool with that LOL. Thanks, Chris

Big 3 - ground: and alternator (engine!) to body/chassis, OR battery to body/chassis. If that hasn't been done, DO IT! Fan is probably chassis grounded as is lights and meters (dash) etc... If the fan has an FLA of 100A, an 80A relay may not hold. Considering the start-up current will be greater than 100 FLA (unless they have soft starts or current-limters - which I doubt (then again, when someone said they had 100A -150A fans I was sure they meant Watts!!) ...) Though relays are often conservatively rated and for continuous operation, but then those ratings may be at room temps and need derating under bonnets.... I'd stick to the dual 60A for now. It's just that if one fails, the other burns (or is each one fused for 60A?). And make sure new relays are rated for inductive loads (not just resistive) - eg, horn relays, fan & motor relays etc.    Dual batteries can help in the same way they help audio systems - ie, closer to the load, or simply sharing the load so half the internal (resistance) voltage drop. But audio often uses AGMs (in the boot/trunk close to the load/amp) and AGMs are more susceptible to high-current damage. In the engine bay I'd be wary.... Higher temperatures mean lower battery life, but matched temperatures and batteries mean longer battery life IF hard-connected/paralleled together. By that I mean that if one fails, both fail - unless the failing battery is prevented from wrecking the good one. Hence the desire for battery isolators - which can be a simple relay controlled by the alternator's charge-lamp circuit (see the (LOL!) UIBI - Ultimate Intelligence Battery Isolator). Though seemingly an ideal application for a voltage sensing isolator where the sag might isolate the 2nd battery and hence remove the sag from the system (albeit AFTER the initial sag, and at the expense of NOT sharing the load, but if the 2nd battery handles it or is cheap etc...). But "smart battery isolators" do not operate that way - they must have a delay or delays else they rapidly cycle off-on-off-on as they disconnect and thence reconnect. Besides, what voltage do you set it for - or do you also sense the rate of voltage change?? Ah - reminders of other people's designs and proposals... But dear oh dear, some still survive the market (eg, "smart" battery isolators for systems with charge lamp circuits or charge sensors!). Forgive my ramble, but there is often a lot involved. Not that that may matter - there is no single best answer - it depends on the situation, its placement, and the implementation aims & wallets... [ And on AGMs - especially for audio use - People confuse "being able to supply high current" with "supplying high current & loving it and living a long life despite it". Wet-cel batteries typically have twice the internal resistance (ESR, when fully charged) than AGMs and hence half the short circuit current, but the current difference drops from there - ie, they both supply 100A/1kW just as well but the wet-cell will drop twice as much voltage as the AGM - eg, ~1V as opposed to ~0.5V. But do AGMs react quicker? And according to many peak-SPL competitors, their results are better without caps. So if removing the low-ESR or quicker responding cap does that, wouldn't a higher-ESR wet-cell further improve the results? And since you mentioned Florida (FLA), isn't it the floridaspl forum that reckoned to put audio batteries under the bonnet/hood because they get hotter and hence increase capacity? I loved that one too... increase resistance by increasing temp (why did they spend a fortune on AGM batteries then?!!) Also increase capacity by what - 5%? 10%? 20%? - and reduce the battery life probably 4- to 8-fold (for 20°C & 30°C temperature increase respectively).    Geez - I wonder if it's better spending a few % more on a bigger battery and placing it next to the amp (where it should be!) rather than buying batteries 4 or 8 times as often.... Ah - other people's designs and proposals... (A little knowledge makes the rest of us rich!) ]

Thank you for sharing your knowledge. How blessed am I? Please forgive my ignorance, Engineering Technician on the mechanical side here. I used FLA (full load amps) as start up amps. Is this not the case? I read that the fans pull ~100 amps at start up and then maintain ~38-42 while running. Fan has 8ga ground to frame. Relay coil 14ga ground to frame Here is my Big 3 upgrade,,, unabridged version. 4ga -batt/block (includes 10ga -bat/body) 4ga +batt/fuse block 4ga +alt-batt Added 4ga -alt case/block (same gournd post as current -bat/block) Added 4ga -frame/block Added 8ga -body/block I hope I covered the Big 3 upgrade (did not touch wires to starter) Gonna hold off on dual batt for now, got summer vacations coming up. New Question: Should I run a diode across the fan +/- for when the fan is freewheeling un-powered? How about a SPDT relay where (30) is to +fan, (87a) is ground, and power comes from (87) would this allow the +/- of the fan to be in contact for when the motor becomes a generator while driving? Would this serve to ground out the power generated from the fan under highway driving conditions and still allow normal operation? Thanks Chris

Not bad for an engineer - even if an elec eng... (I'll neither confirm nor deny my elec eng qualifications... if I had any...) Your diode is a good idea - if it handles whatever generating power.... The changeover +12V/gnd relay is often used too. BUT - as I recall, I decided that since the fan only freewheels when it is NOT connected, it is isolated from the system - hence any spikes or generated voltage is irrelevant. So why bother braking it and thereby offering greater wind resistance (hence impaired airflow and even less insignificant impact on fuel economy...) and worrying about generated power (up to tens of Amps?) frying diodes or grounded relays.... [Is a freewheeling fan a problem? Maybe when first connected to power - or will voltages be quenched? I don't see spikes as being a problem - inductance will try to keep the current = 0 as opposed to disconnecting a coil which causes a large voltage spike to try to maintain current flow. Or have I missed (other) reality(s)??] And "full load" ratings (AFAIK) are full-load steady state (and or ie, normal operation). It does not include inrush - that is a "max" rating (or i2t rating for fusing) and transient only. Inrush currents are often many times the equipment rating - eg, light bulbs, PCs, motors.... And ground-loop issues aside, I see no harm in multi-pathing or duplicating engine-chassis grounds. Redundancy is often desirable for that anyhow since that is the most moved connection (excluding the alternator +12V to battery - but that is usually far less damaging if it opens or shorts). And I have never worried about cabling to the alternator chassis. I have never had a situation where the alternator to engine block connection has been that significant (though I used to run the (or a) battery-engine ground to the alternator's belt-tension adjusting bolt). [ As for "absolute ground" arguments, though IMO generally irrelevant, I concluded I wanted my absolute or best ground to be the battery -ve else (and) the starter motor case/ground. Even though the alternator is normally supplying all loads, I considered its ground less important - especially in severe load of fault conditions. If it was that important, I do as Zena do and use floating alternators - hence ground the alternator at the battery. ]

I do not get to carry the distinction "engineer" lest it is succeeded by technician. Motor braking would not be a goal, but I have to wonder of the voltage generated would arc across the contacts within the relay to find ground and thus shorten relay life. (Breaking offering greater wind resistance indeed! good point) I added more grounds at the risk of ground-loop issues on account I was battling ground fault electrolysis. I determined that it was related to grounds. As I recall, the coolant was carrying over .5v which is too much. So, is a diode better than changing to a SPDT relay for grounding voltage generated by the motor? Chris

I'd still question why bother braking? Why not allow it to freespin? (It also acts as an anti-seize.) This reminds me of those that wanted to use the power generated from their free-spinning fan to help charge their system. I pointed out that it would be LESS efficient - ie, the extra drag from the harnessed power would be lost in extra petrol etc - it's more efficient coming from the alternator itself. (In turn that reminds me of the old hydrogen (etc) power bullsh statement... "using the EXCESS power from the alternator" - LOL - the alternator normally never produces excess power; it is all from the engine (petroleum) and used as electrical power.) But if I were to do it, be tempted for the relay since it may be a small incremental cost, and so what if the contacts burn - it won't effect the ON N.O. contacts. It also avoids the problem of sizing and heat-sinking the diode.   (But I doubt that the fan will generate significantly above 12V - and most relay contacts handle 24V - ie, are plated for the >20V threshold and hence handle higher than 30V.) And succession with technician eh? That explains your knowledge and abilities. (They usually don't teach that to engineers!)