Lotus Elan

Raising Alternator Voltage

PostPost by: david.g.chapman » Tue Mar 02, 2010 7:49 pm

I am currently running a Lucas a133 (ex Ford Fiesta) alternator from a scrappy in my early +2. The voltage regulator ran at 14.1V on no load (just the engine running) measured at the starter solenoid, and fell to 13.5V with the lights and ventilation fan on. Regulating quite nicely and with plenty of alternator power in reserve.

The trouble was, I could have done with a higher regulated voltage, as that voltage is at the highest part of the system, and voltages at the lights were just about 12V, and at the cooling fan 11.5V. I would call my wiring adequate, and pretty representative of a standard setup. And we all know about that volt drop from the engine to the battery miles away in the boot.

So I looked into it (some of you may already know about this of course), and there is a way of raising the alternator voltage.

You have to extract the alternator, and open it up. What you are looking for is the voltage regulator, which may be a separate module, or a clearly defined circuit on its own. You have to refer to a diagram of the alternator to be sure, of course. The earth wire from the regulator to the "chassis" or metalwork of the alternator now needs to be identified. If you can't find it go no further, otherwise you need to cut this wire and wire in a power diode to the cut ends, with the cathode (identified by a white line) connected to the "chassis" end.

I used a 600W diode with a forward current of 3A max and a reverse voltage of 200V. Anything similar will do.

The alternator voltage has now been raised by 0.6V, so everything seems to run like a car with a battery in the engine compartment, with a corresponding 0.6V uplift :) .

I will trial it over the next few weeks, and see what effect it has on the electrics. Bulb life will be shortened a bit, but hopefully I wont find out for some time..... The cooling fan will also blow harder :D

NOTE THAT THIS IS NOT A CURE FOR FAULTY WIRING AND EXCESSIVE VOLT DROPS IN THE SYSTEM. PLEASE MAKE SURE YOUR SYSTEM CONNECTIONS AND WIRES ARE OK BEFORE CONTEMPLATING THIS.

Good luck to anyone who wants to try.


Dave Chapman
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PostPost by: gerrym » Wed Mar 03, 2010 3:47 pm

Dave, I would have thought that the charging voltage needs to be pretty carefully matched to the battery technology you are using.. E.g. silver lead acid versus standard lead acid. Otherwise you run the risk of over-charging the battery and causing excessive out-gassing. Is your battery maintenance free (completely sealed) or does it have top-up plugs. At least with the top-up plugs you can keep an eye on the elctrolyte level.

Also, some of the older alternators had a separate sensing wire that carried no current but directly measured battery voltage (hence immune to current flow and voltage drop in the alternator to battery cable, no matter how far away)

Regards
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PostPost by: david.g.chapman » Wed Mar 03, 2010 7:01 pm

Gerry,

I think you are right on both counts.

1. With too much charging voltage across the battery, there will be an increase in outgassing and the electrolyte level will drop much more rapidly then normal. The problem with the Lotus system, though, is the volt drop from the alternator to the battery.

In the Brooklands book for the Elan there is a volt drop test for the positive and negative lines to the battery, to be less than 0.5 and 0.25 V respectively with the headlights on. My car showed a reading of 13.6V at the starter solenoid under these conditions which gives (worst case) 13.6V-(0.5V+0.25V) across the battery, or 12.85V. That's uncomfortably low, and will result in the battery voltage quickly rising and limiting the charging current before that battery is fully charged. Adding 0.6V to everything (which is approximately what happens in theory) gives 13.45V - better.

The highest voltages are when the engine ignition is the only load on the alternator. I now get 14.69V around the engine bay under these conditions which is slightly higher than the recommended max of 14.5V (Brooklands books again). I can live with that, and I will be monitoring things anyway just to check, such as ignition coil temperature in hot weather (I bought a laser guided IR thermometer recently, thanks to this site)!

Those of you with higher regulator voltages near the 14.5V end will not need this mod, anyway. You will get up to 15.2V, and that's too much!

2. Some alternators may have remote sensing at the battery, that's right. To check, you can see if the battery voltage on charge matches the alternator ouput voltage. If they are both in the range 14.1 to 14.5V under no load, all is well. I don't know if the original Lucas ACR series of alternators has this facility - worth checking. I had an ACR15 originally with a puny output of 25A, which is why I changed in the first place.

Dave.
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