Direct charging small 12 volt battery

[Quigs]

With spring on the way down here in OZ, the grass is growing and my ride-on mower has a flat battery.

I could charge it by using an auto battery charger through the inverter and get crap efficiency, or I could tap into the 1000 ah main battery bank (tap 12 volts, that is)and apply this direct to the small battery.

Is this action wise?  Or am I risking loud noises and magic smoke?

Advice will be appreciated.

Quigs

[drdongle]

A DC to DC converter would be a place to start, though you didn't say what voltage your main battery bank was.

[Norm]

How large is your riding mower?

Most 'red necks' just jump start their riding

mowers from their car battery....and the next

time they have to mow,if the battery on the mower

has run down they jump start it again....

   One customer I had bought new belts ...blades

etc. but put off buying a new battery for 5 years!

                ( :>) Norm.

[MaxFlow]

Although not charging it with your main bank, how about useing a small solar panel secured to the hood, with all the solar you probably get down under, it seems to me it would be a more simple solution?

For days that your not cutting, pull it out from the garage and fuel it up.

You might want to check out http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=44768

[ghurd]

I have decided those are 'NOT' weather proof.

G-

[MaxFlow]

Just tape over the panel a piece of uv protected vinyl?

[ghurd]

The instructions said "a bead of silicone around the panel if used outdoors".

Didn't seem to help a whole lot.

I killed 8 or 10 of them. Hehe. Oops!

I have decided the VW panels, like on ebay, are a lot better quality and can be sealed easier and better.  And can be cheaper per watt. And make more than the 170ma they are rated at too.

G-

[wpowokal]

Quigs, tapping any battery bank will cause an inballance in the batteries which is undesirable.

Use the battery charger off the inverter while there is power comming in, then you will bypass the battery inefficiency.

If you have solar panels you could tap one panel at 12v, with obvious reduction of amps to your batteries when charging the mower, but no efficiency loss.

allan down under

[Nando]

Quigs:

If your small battery is 12 volts and you can connect it to a 12 volts section of your battery bank, you can charge it that way, remember that it will charge to the voltage that the battery has ( 12 to 14 volts).

Make sure that you keep the proper polarity -- + to + and - to -.

Regards

Nando

[Quigs]

Thanks for the comments. I suppose my underlying question was 'If I connect a very large 12 volt battery to a very small 12 volt battery, with correct polarity, will the large one charge the small one without risk?'

Quigs

[ghurd]

Yes. But only if its done right. 12V to 12V.

The basic problem.

A large, full battery bank and a small low battery.

Too much potential for excessive amps.

Direct connection I would consider very bad.

Simple fix.

A high watt resistor in line.  AND A FUSE!

Slower charging

Maybe a 2 to 4 ohm, 25 watt resistor.

Maybe a 8 or 12 ohm, 15 watt resistor.

Cheap on the surplus market. US$0.10~$2

Search "sandstone resistor".

The resistor limits the amps. Saves the battery, and who knows what else.

More efficient than an inverter and battery charger. Very efficient at very low charging amps.

I sure would not connect a 24 or 48V main to a 12V lawn mower battery without a 100 ohm resistor.  And I would check the situation often.

BTW, Some 12V supply chargers for 6V battery powered devices use this concept, but much higher resistance values.

That's why they say 'don't let it charge more than 1 hour', or something like that.

With 12V to 6V, the 6V battery will eventually cook.

Do not forget the fuse!

Maybe 5A?

Because you... me... somebody... is going to connect it backwards sooner or later.

Use a fuse.

G-