Running a car without an alternator

[Greg_D]

I have a new hill climb car and i am changing the engine on it for something a little fruitier but this leaves alternator placement severely compromised by, well, everything.

the plan is to use the car on track as well as up the hills but i was wondering how long a fully charged gel type battery would last running spark for a 4 cylinder car (ignore starting duties, a separate battery pack would be piggybacked for that duty)

i have no idea, in my mind it could be 2 minutes or 2 days or anywhere inbetween.....

[The Black Flash]

I've done 10 miles in a road car with the alternator disconnected, (it was probably more but I don't know when it failed). So you'd think a 2 minute blast up a hill would be ok.
What about charging it up between runs though?

[bumrar]

I accidentally left my alternator unplugged after doing some fettling. I got from Preston to Oulton park (roughly 60 miles-ish), parked up for the day and got home again. Things started going a bit 'dim' coming into Preston. Thats in a Lotus 7 type kit car with a nearly new, fully charged gel cell battery

[Prof Prolapse]

Will you not be absolutely murdering your battery by letting it run, presumably, so low?

[Greg_D]

possibly....i don't know, that's why i'm asking about the pro's and con's

someone technical will no doubt be along in a minute and quote the amperage of coil packs vs charge in a battery etc etc etc and why or otherwise it is a brilliant idea...

[Toltec]

It is going to depend on the capacity on your battery and the load of course. On a road car providing you do not turn on any ancillaries probably a few hours.

Why don't you put a, suitably high range, meter on the main feed and check what current is pulled at different loads. If you allow for 50% or less capacity use you will probably be fine.

[Huff]

I've done it when the rectifier failed in my R1-engined Fury on a day-out. AA man's clamp-on ammeter showed a static draw of about 3A - well , its mostly just bike electrics. Fitted with a charged 38Ah car battery it drove home (80miles / 2hrs) just fine, and restarts during/afterwards were no problem.

However:

Running without the alt means the system voltage is lower than normal and sinks in use. Figure on 12.6v to start with, dropping off very quickly if you go for a small battery. This will affect everything from fuel pressure to injector duration, and it is noticable. Not a great idea on a competition car..? (NB the Fury even idled noticably better at 13.8v than at 12.something!).

[jeff m2]

Prof Prolapse said:

Will you not be absolutely murdering your battery by letting it run, presumably, so low?

A regular one yes, but a Deep Cycle is designed to be able to be discharged.

Switch to diesel, get a big deep cycle battery you'll be ok for a couple of months

[alfa pint]

My old MG B lasted about 2 hours on a fully charged battery and a knackered alternator. I think the radio ran the battery down more than the ignition system!

[mmm-five]

Also consider if the regs for your class/competition have any specification regarding being able to restart under your own power.

We (marshals) don't mind turning you around and letting you roll down the hill, but we very much don't like having to push uphill - especially if it's a 'road' type car (rather than a Gould/Pilbeam/etc.)

[Dr Interceptor]

You would need to isolate the second battery to prevent it discharging, so you always have battery power to spin the engine up.

I would imagine on a small engine with no ancilliaries you could get a couple of hours running out of a standard battery.

[anonymous-user]

Do the math!

work out (or better still measure) the current consumption in amps when running (it will peak at high rpm becuase both injector duty and coil dwell will be at their highest)

Look up the battery capacity in Wh. pick a mean voltage (may as well be 12v) Power = Volts x Amps, and calc how long the car could run of that battery (in reality, you want to probably allow a factor of 100% to cope with varriations in battery temp, current draw, and state of charge etc)

eg, say you pull 10A at 12v, thats 120W. at typical meduim sized car battery is ~1000Wh, so that would run your car for 8.3hrs (assuming that A) you can use a 100% depth of discharge (DOD) which you can't, and B) you car would still work at 0v (which it won't)

In reality, such a battery would likely run you car for an easy 2-3 hrs before the voltage fell below critical values (usually around 8V in terms of ECU operation etc, but as mentioned before, you will need to ensure your injector dead time and coil pack dwell times are corrected for battery voltage (easy task to do by simply using a typical adjustable bench powersupply to varry the system voltage as you measure coil charging time and exhaust lambda)

[Toltec]

Building on some of the above -

If the battery capacity is adequate then a dc/dc converter could possibly be used to maintain the running voltage independent of the battery output. It would have to be something robust enough to withstand peak current demands, electrical noise and the environment though.

[NHK244V]

banger racers run without alts and they usually manage 20 mins pluss before swapping batterys (average 2 races) BUT that's on a carb car, leccy fuels pump will use way more power as will the ECU ?
Thare's no hard fast answer here you need to measure the power consumption of the particlar engine first.