12V Backup Battery charger

[Stefan Schulz (Profession]

Hi there,
I am pondering the following question:
The environment is a boat with outboard motor (build in alternator to
recharge the battery). The device of interest is the VHF radio.
In order to be able to keep the power to the VHF at all times but not
drain the main battery I would like to get a sealed lead acid (say 12v
6Ah) to be charged whenever there is a charge going into the main battery.
The small one will power the VHF only. Of course there must not be a
'back flow' into the main battery therefore draining the little one.

Is this doable? Or would you suggest to keep the two systems totally
separate and use a solar charger instead?

Thanks for your input
Stefan

 

[ChrisGibboGibson]

Stefan Schulz wrote:

Hi there,
I am pondering the following question:
The environment is a boat with outboard motor (build in alternator to
recharge the battery). The device of interest is the VHF radio.
In order to be able to keep the power to the VHF at all times but not
drain the main battery I would like to get a sealed lead acid (say 12v
6Ah) to be charged whenever there is a charge going into the main battery.
The small one will power the VHF only. Of course there must not be a
'back flow' into the main battery therefore draining the little one.

Is this doable? Or would you suggest to keep the two systems totally
separate and use a solar charger instead?

Yes it's "doable"

Lots of different ways.

Here are just a few.

1. Use a relay to parallel the two batteries whenever the ignition is on.
2. Use a relay energised from the internal alternator to parallel both the
batteries whenever the engine is running.
3. Use a voltage sensitive relay that senses when the main battery is above a
certain threshold (usually about 13.3 volts) then energises itself.
4. Use a schottky diode from the main battery to the VHF battery.

All these items are available from boat and RV stores.

Or you could roll your own.

Gibbo

 

[Tweetldee]

"Stefan Schulz (Professional Web Solutions)" <stefan@pws.co.nz> wrote in
message news:clvmtd$ul9$1@lust.ihug.co.nz...

Hi there,
I am pondering the following question:
The environment is a boat with outboard motor (build in alternator to
recharge the battery). The device of interest is the VHF radio.
In order to be able to keep the power to the VHF at all times but not
drain the main battery I would like to get a sealed lead acid (say 12v
6Ah) to be charged whenever there is a charge going into the main battery.
The small one will power the VHF only. Of course there must not be a 'back
flow' into the main battery therefore draining the little one.

Is this doable? Or would you suggest to keep the two systems totally
separate and use a solar charger instead?

Thanks for your input
Stefan

The simplest solution is to buy a couple 100V/25A rectifiers. Schottky
rectifiers will offer the lowest voltage drop, but are a bit more expensive.
Install one from the alternator output to the main battery, the other from
the alternator output to the SLA battery. Be sure to mount the rectifiers
on a heat sink, and use an insulator on each rectifier to keep them isolated
from each other, and from ground.

--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

 

[Pooh Bear]

"Stefan Schulz (Professional Web Solutions)" wrote:

Hi there,
I am pondering the following question:
The environment is a boat with outboard motor (build in alternator to
recharge the battery). The device of interest is the VHF radio.
In order to be able to keep the power to the VHF at all times but not
drain the main battery I would like to get a sealed lead acid (say 12v
6Ah) to be charged whenever there is a charge going into the main battery.
The small one will power the VHF only. Of course there must not be a
'back flow' into the main battery therefore draining the little one.

Is this doable? Or would you suggest to keep the two systems totally
separate and use a solar charger instead?

You can stop the 'back flow' with a diode. Diodes have a forward voltage drop
that means your little battery will run at a slightly lower voltage.

If you use a Schottky type diode - the volt drop is quite small - 0.3 - 0.4
volts - so may be entirely practical. Indeed - the smaller battery is
effectively in a 'float type' application and doesn't need to see the full
13.2 odd Volts that the main battery needs on charge.

There are other smarter ways of eliminating the volt drop using a mosfet - but
it starts getting complicated. I think a schottky diode will do just fine.


Graham