Wednesday, April 10, 2013

Sentry Battery Charger

My old Guest battery charger boiled my batteries awhile back, resulting in me having to replace three one-and-a-half year old batteries.  I was not amused.  I paid the yard to replace them, saving my back the effort of hauling out and carrying in two big 150 pound 8D house batteries, plus the little Group 24 for the generator.  While doing so, the mechanic asked why I wasn't using the Sentry battery charger instead of the guest?

"That's the best battery charger you can buy right thar.  It cayunt boil yer batteries 'cause it shuts itsef raght off." he said, in his deep southern drawl, which I'm trying to cleverly capture by carefully misspelling words.

"It doesn't work."

"Why don't ya fixit?"

"Because it's 33 years old.  Who would fix it?"

"I kneow a guy who kin fixit." he said, and I struggled to spell words in a southern drawl as I write this, without insulting southerners because I love southern accents.  "Thet's a fifteen hunnert dollar charger right thar.  I'd git it fixed."

So I did, and long story short, it didn't work.  It would just cycle on and off, on and off, continually.  I had the Sentry repair man over to look at it a few times.  He'd scratch his head, test a few things, and declared that it was working perfectly.  I must have a short somewhere.

We all reach a point that, even when we know we're right, we know the fight is futile.  I decided to fix it myself.  After all, I used to build computers.  How hard can this be?  Especially when I have Google as my assistant.

So yesterday I hopped on Al Gore's interwebs and googled "Sentry battery charger C140 3NL manual".    Surprisingly, I got no hits.  Well, let's broaden the search.  I tried "Sentry C140 manual".  Nothing.  I mean, c'mon.  Everything ever published is on the internet, all the way back to Guttenberg's Bible.    No one has taken a few hours out of his life to copy, convert to PDF, and post a Sentry C140 manual?  What is this world coming to?

From the computer business, I  learned that the more things change, the more they stay the same.  I googled "Sentry battery charger manual" and got a few hits.   I clicked on a couple.  They were Sentry manuals for new chargers.   I scrolled down to "calibration" and found that the new ones have two adjustments called "drop out" and "pull in".  They were simple rheostat looking dials called potentiometers.   When the battery voltage drops to 12.8 volts, the "pull in" dial was triggered and turned on the charger.  When the battery voltage hits 13.9, the "drop out" setting turns the charger off.  Simple.

So I sketched the manual's drawing on my screen, carefully labeling each dial, and hied myself to the engine room.  I checked the controlling battery's voltage (the charger monitors the voltage of only one battery, called the controlling battery) and it was 12.5 volts.  Perfect.  I removed the charger's cover.

Lots of wires, but only one circuit board.  There, at the bottom, were the potentiometers, which you can see in the pic below.

I turned on the charger and turned the dials to their extreme settings, lowest pick up and highest drop out.   The charger kicked on.   That's not supposed to happen.  Something's wrong.  I turned the dials the opposite direction and the charger turned off.  As it turned out, my dials were the reverse of the new models.   The left was the pick up dial and the right the drop out.

That yellow cylindrical thing, by the way, holds the magic smoke.  If you do anything wrong, it warns you by releasing the smoke.  If that happens, you need to either get a new board, or replace the yellow smoke cylinder.

The manual has all kinds of technical mumbo jumbo in it about how to calibrate the charger.  I'm much more pragmatic than that.  I have the pick up dial set all the way down, and I'm just monitoring the battery voltage.  When it drops down to 12.8 volts or so, I'll turn the pick up dial very slowly until the charger kicks on.  I'll then monitor it until the battery reads 13.9 volts, and then slowly turn the drop out dial until the charger turns off.

At 13.9 volts, the battery will "settle" to 13 volts or so and be fully charged.  Easy peazy.

I'm retired.  I have all the time in the world.


  1. I'm sure you'll figure it out....

    I've always tried to repair the old stuff and not replace.... "They" Don't build them like that no more....

  2. You want to stop over and fix my 32 volt Sentry charger ?
    It is doing the same thing.
    I fixed mine by putting a 24 hour timer in the 120v ac line and have it turn on the charger for 45 min. a day.
    It works. :))

    BTW My charger is not easy to get to.

    Bill Kelleher

  3. Good on you.
    I've got that same model charger on my Hatteras and I'll give those potentiometers a go.

    A link to any manuals you've stumbled upon would earn you a spot in heaven.

    Great work!!

  4. Hi Dave,
    Are you still using your Sentry charger? I'm refitting my sailboat for a live-aboard and considering using my C140-3NSL Thanks, Jeff

    1. Hi Jeff. No. I sold Drift Away a few years ago. But I hope this old blog is useful to you.