How to externally adjust alternator voltage

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How to externally adjust alternator voltage

October 25, 2015, 01:32:16 pm

I have been testing my new Delco 40si alternator and the sense circuit is factory set to keep the battery voltage at 14.0 volts which is a little lower than I would like for my wet cell batteries. It is possible with a simple resistor voltage divider to externally adjust any alternator with a sense terminal to produce a higher voltage - no alternator modification is required.

The circuit attached is designed to increase the alternator voltage to 14.5 volts. I built a prototype using some resistors and a project box from my spare parts and have it installed, now the alternator is maintaining a voltage of 14.5 volts at the battery terminal.

The voltage can be adjusted by altering the resistors, if required, for different alternators or battery types. The current draw through the resistor divider is approx 20 milliamps (approx 0.3 watts) so it is not a significant draw on the chassis battery.

The way this works is the alternator will increase the output voltage to produce 14.5 volts at the battery terminal - which is connected to the top of the resistor divider - to obtain the required 14.0 volts at the sense terminal.

Using Ohm's Law:

Current flowing through the resistor divider:
14.5 Volts / 692 ohms = 0.0209 amps

Voltage at the sense terminal:
0.0209 Amps X 670 ohms = 14.0 Volts

Re: How to externally adjust alternator voltage

Reply #1 – October 25, 2015, 02:06:32 pm

For most U300 models with an OEM Leece-Neville alternator, the voltage is adjustable with only a digital voltmeter and screwdriver. If your alternator has a flat plate on the top (see photo below), the following will do the trick:

Voltage Regulator Adjustment

Leece-Neville alternators are equipped with one of two regulators. The Fully Adjustable Regulator has a flat cover plate. The Three Step Regulator has a finned, curved cover plate. Use the following test procedure that is appropriate for you regulator type.

Fully Adjustable Regulator

NOTE: Battery must be at least 95% charged prior to this procedure. Also make sure wire connections and belt tension are OK.

1. Shut off all electrical accessories and run the engine at approximately 1000 RPM.

2. Connect a voltmeter to the alternator outputs.

3. Remove the plastic screw from the regulator (see photo) and insert a small screwdriver in the hole. Engage the screwdriver blade in the slotted adjustment screw inside the regulator.

CAUTION: The adjustment potentiometer screw has high and low stops. DO NOT exert excessive pressure on the screwdriver or you may damage the regulator.

4. Turn the screwdriver clockwise to raise the voltage, counterclockwise to lower the voltage. Set the voltage between 14.0 and 14.2 volts (28.0 to 28.4 volts for 24-volt units).

CAUTION: DO NOT force the screwdriver past the set stops at either end of the adjustment range to avoid regulator damage.

5. Remove the screwdriver and voltmeter and install the plastic screw in the adjustment screw access hole.

Pierce

Re: How to externally adjust alternator voltage

Reply #2 – October 25, 2015, 02:15:30 pm

Pierce:
Excellent info for the LN alternator owners. I wasn't sure if LN alternators were adjustable as my coach didn't have one when I got it.

Re: How to externally adjust alternator voltage

Reply #3 – October 25, 2015, 02:22:30 pm

The whole discussion of "proper" voltage HAS to be taken in context:

Proper voltage for charging a deeply discharged battery (called bulk mode in smart charging) is and should be very different than proper voltage for maintaining full charge in a fully charged battery (called float mode in smart charging).

If the primary function of the engine alternator is charging deeply discharged batteries, then I agree that 14.0 is lower than I would desire.

BUT, read that a BIG BUT, that is very seldom how most use the alternator on motorhomes. So, for example if you were plugged into shore power or used the generator while dry camping before starting the engine (so the batteries are not deeply discharged) the batteries are NOT deeply discharged.

And, even if deeply discharged, after several hours of driving, they are at or near fully charged.

SO, the reality for most alternators is a compromise between rapid charge and proper float voltage. I would sure not want to drive for 6-7 hours knowing that both battery banks are seeing a constant 14.4 VDC. That is excessive for fully charged batteries.

Alternatives-- sure and have first hand experience from the sail boat. High output alternator with SMART REGULATOR. You program the regulator with proper voltages in each stage, how long each stage should last (either time voltage or even amp charge rate) and where the sense wire is connected.

Here is one common example: http://www.xantrex.com/power-products/power-accessories/alternator-regulator.aspx

Is this needed on a motorhome-- probably a waste of money.

What voltage would I choose for the alternator output in a motorhome. Yes a compromise between rapid charging and overcharging after charged. I have no problem with the 14.0 or so that most are set at.

Re: How to externally adjust alternator voltage

Reply #4 – October 25, 2015, 02:46:44 pm

I have just got to ask, seeing as how I used to put electrical tape over the outlets to keep electricity from running out on the floor ang shocking me when I walked across the rug.

A friend said that the "older" alternators (I think he meant my 1989 model) were not constantly outputting 12+ voltage to the battery. They would actually drop off in order to not cook the battery.

Any truth to that?

Re: How to externally adjust alternator voltage

Reply #5 – October 25, 2015, 02:48:58 pm

Being old school , I go with 13.8 VDC for chassis and avoid the issues.
FWIW

Re: How to externally adjust alternator voltage

Reply #6 – October 25, 2015, 04:37:58 pm

Quote from: Dave M – October 25, 2015, 02:48:58 pm

Being old school , I go with 13.8 VDC for chassis and avoid the issues.
FWIW

I'm close, we'll find out this week how the batteries are doing when we go dry camping at Organ Pipe NM. Gives me a chance to run the gen set, as well. One of the best places to take on water down here; we all paid for it!

Re: How to externally adjust alternator voltage

Reply #7 – October 25, 2015, 04:44:43 pm

Norm, my 300 hp cat in my 88 did just that.. drop off after full charge.. batteries were cool.

Nite drifting with lights and perlux fog lites battery was warm

Re: How to externally adjust alternator voltage

Reply #8 – October 25, 2015, 07:08:00 pm

Thanks, John. Was in the coach putting oil of peppermint in prep for storage and turned on the monitor. Engine bat read 13.7 and coach bat read 13.6 and I have a battery minder on the engine battery. I started up the coach and noticed the voltage meter on the dash was reading around 13V whereas it normally would read--while traveling--around 10.5V but the monitor would say 12.4V (during the day) so not sure what was going on.

Re: How to externally adjust alternator voltage

Reply #9 – October 25, 2015, 07:25:04 pm

Best to check voltage at the batteries. The monitor (house battery) and dash gauge (chassis battery) can be WAY off, both because of inaccuracy and also the long wire runs from batteries. So, at this point, draw no conclusions based solely on your dash monitor.

Re: How to externally adjust alternator voltage

Reply #10 – October 25, 2015, 08:43:45 pm

My chassis voltage on the dash gauge and the house voltage in the Audit display both read low. I put in a couple of these wired directly to the batteries:

Battery Testers

I know there are cheaper digital cigarette lighter plug-in meters, but I like the easy visual display and wanted to monitor directly at the batteries. When I start up, I can tell right away if the alternator is working properly.

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How to externally adjust alternator voltage

October 25, 2015, 01:32:16 pm

Re: How to externally adjust alternator voltage

Reply #1 – October 25, 2015, 02:06:32 pm

Re: How to externally adjust alternator voltage

Reply #2 – October 25, 2015, 02:15:30 pm

Re: How to externally adjust alternator voltage

Reply #3 – October 25, 2015, 02:22:30 pm

Re: How to externally adjust alternator voltage

Reply #4 – October 25, 2015, 02:46:44 pm

Re: How to externally adjust alternator voltage

Reply #5 – October 25, 2015, 02:48:58 pm

Re: How to externally adjust alternator voltage

Reply #6 – October 25, 2015, 04:37:58 pm

Re: How to externally adjust alternator voltage

Reply #7 – October 25, 2015, 04:44:43 pm

Re: How to externally adjust alternator voltage

Reply #8 – October 25, 2015, 07:08:00 pm

Re: How to externally adjust alternator voltage

Reply #9 – October 25, 2015, 07:25:04 pm

Re: How to externally adjust alternator voltage

Reply #10 – October 25, 2015, 08:43:45 pm

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