BoatBanter.com - Hummmmmmmmmm (feed back loop ) ....

"b393capt" wrote in
oups.com:

When I plug the TV into 12 VDC (via an 12vdc to 18vdc step up
transformer), I get a light humming sound in the TV, picture is still
great.

Bad house battery cell.....The charger is pulsating its high resistance,
causing a pulse at the AC line frequency on the DC power supply of
everything aboard....sorry.

It could also be corroded contacts between the charger and the battery
post. I hope it is for you.

Chargers don't put out DC. They put out pulses of DC at the AC line
freq. Superchargers with computers put out pulses with measuring times
in between pulses to see how they are doing. That'll really drive
everything crazy if there's a resistance from corrosion or a bad battery
cell with low specific gravity caused by cell sulphation. Cell
resistance from the low gravity is effectively in series with the cell's
"battery". So, when you shove a 10A pulse through a bad cell, the
resistance in series drops some of the drive voltage, causing it to add
to the cell voltage during the pulse. If the gravity is really low, the
pulse can go over 20 volts on some chargers.

The series resistance is also easy to spot in DC cabin lighting, either
type. When you turn on another load, without the charger on, the lights
dim that are already on. When the bilge pump cycles, the lights dim, for
instance.

Finding which one of these problems is quite easy. Bypass the
connections the house has by putting a DC voltmeter (prefer a DVM)
directly across the battery posts (not the clamps on the battery posts).
Load the battery down without the charger running by turning on your big
loads while monitoring the meter. Good batteries hardly drop at all, a
few tenths of a volt. Batteries with poor cells drop more because the
series resistance inside the cells drops the voltage when you pull heavy
current through it.

Are all the cells full of DISTILLED water? I regularly am asked to
troubleshoot problems like this. Way too many times I open the caps on
the batteries, especially batteries some idiot or idiot boat company has
made really hard to get to, and find the batteries way low on electrolyte
because noone ever topped them off. The guy at the boat shop said they
were "maintenance free", to get them to buy them. Bull****. No such
battery exists. Some just make it difficult or impossible to top off the
lost electrolyte, like gelcells and AGM overpriced crap.

If you must add water, and the water is below the plate tops, give the
batteries a week to recover from the abuse before trying the testing
again. Sometimes they do...sometimes it's gone too long and they are
sulphated, the bottom of the plates eaten away.

Many "hum problems" are cured with DISTILLED water....(c;