Adjustable regulator on an internally regulated alternator…

Adjustable regulator on an internally regulated alternator…


Why? You ask:

After we put the LiFePO4 batteries on the boat we had to address the three charging systems:

First we changed the solar charger over to a programmable controller.

Second we changed the shore power battery charger over to a programmable charger.

These changes allowed us to set the max charge voltage at around 13.6 to 13.8 volts on these devices following the guidelines for LiFePO4 batteries to charge them to an 80 percent voltage at around 13.6 volts.

So with two of the three charge systems on the boat automated such that we can not mess up the LiFePO4 batteries by overcharging them, we started the search for an alternator and regulator system to allow us to limit the charge voltage to around 13.6 to 13.8 volts as well.

An alternator must be connected to a battery.  A simple alternator regulator senses the voltage in the battery and can stop producing power when a target voltage is met, most regulators are built for automotive applications and lead acid batteries and set the regulation voltage at around 14.2 to 14.5 volts.  There are also fairly expensive “SMART” regulators which are helpful for lead acid batteries but the expense seems a waste for LiFePO4 batteries since you don’t need the four charging stages you get in a “smart” regulator/alternator system for these types of batteries.

With all that in mind we went looking for a simple adjustable regulator for our alternator and found one.  They make internal and external alternator regulators with a potentiometer or variable resistor on board which allows you to set the cut out voltage in the regulator to between 13.6 and 14.5 volts.  Our goal was 13.6 to 13.8 volts for the maximum charge voltage from the alternator into the LiFePO4 batteries.  This is in the range of 80 to 90 percent charge in the batteries and popular belief is that if you do not charge above these levels the life of the batteries can be extended considerably beyond 3000 cycles.

I found a Beta spec.  alternator on ebay, the pulley is a bit smaller and it is a 70 AMP alternator.  The one on S/V Hajime is a 120 AMP OE from Beta Marine.

PC161065 (Copy)

Found an adjustable regulator for Bosch style alternators online:

Dave Barton has external and internal regulator kits, answers emails, ships promptly, what more can you ask?:


After unpacking and opening the new alternator and looking at the old fixed voltage regulator and the new adjustable voltage regulator I decided to solder the (+) lead from the rectifier trio to the new regulator and install it into the 70 AMP alternator.


Today we installed the new 70 amp alternator on s/v Hajime.

Why? You ask would we go from a vroom vroom 120 AMP alternator all the way down to a 70 AMP alternator.  Honestly, over the last 4 years with the Beta 30 in s/v Hajime and with the 120 AMP alternator, regardless of load, we never saw over 45 AMP output from the 120 AMP alternator.  So I guess it’s putt putt for us.

The old alternator had a D+ stud and the new one has a D+ spade connector, but other than that install was straight forward.  We may have to adjust the tachometer to get the RPM reading a little better, it looks to be reading higher than out hand held tachometer tester.


I set the potentiometer as low as possible and in the initial test run it reduced output to around 2 AMPS at a voltage at around 13.35 volts.  I turned the potientiometer about 1/4 turn and it was still reducing 14 AMP draw when I shut down the engine at around 13.45 volts.  We will leave it there and see where we are and if it works as planned?

Alternator was 140 Pound Sterling and the regulator was $25.6 USD.  ANd we needed a spare alternator anyway!


Some alternator diagrams and info:

Click to access Alternator_Secrets.pdf


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2 Responses to Adjustable regulator on an internally regulated alternator…

  1. A&E says:

    Hmmm… Sounds vaguely familiar but in reverse. I’m currently in the midst of swapping out the woefully undersized 60 amp stock alternators on our Yanmar engines for 120 amp Balmars with external regulators and serpentine belt kits. Yes, Cha-Ching!!! But the internally-regulated stock alternators were not doing anywhere near an adequate job of charging our 800 AH AGM house bank, a condition that was made far worse by the builder taking the easy way out and routing charging to the start batteries before going to the house bank. It’s been a lot of work, but it should make everything better. At least we should be able to motor sail for a day with the autopilot and water maker both running without taking down the house batteries as we do now.

  2. svhajime says:

    Jeeeeesh, It is never easy is it! Hope yours (and ours) work out OK!

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