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Use vehicle 12volt to charge secondary 12volt system

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FlatButt

New Member
Hi everyone.

I am wanting to install a small 12volt SLA battery into my Van.
The 12v SLA will be used to power a camera that is set to activate via movement.
I don't want to run it from the starting battery on the van but I do need the van's charging system to keep it charged.
Is this even possible and if it is what components would I need and what would the circuit be.

The reason is that I am wheelchair dependent and there has bee several attempts at breaking into my van. Thank goodness there is no seat for the sod's to park their butt on and drive away.

I need the camera to use the secondary 12v SLA so that the starting battery is always ready to be used,

Cheers and thanks in advance for your ideas, help, suggestions etc.
 

Tony Stewart

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Most Helpful Member
If the battery is drained and fairly new, the charge current might exceed 10A and dissipate 10 Watts if small and get hot. So a 30A Silicon diode is best. with cathode case for grounding. Cost 3-4$ from digikey/mouser.

So this is a consideration for using a heatsink or mounting the cathide (bar) to the body sheet metal as a heat sink such as a thread mount diode with a pin for Anode , then Vbat (-) to Anode and Vbat(+) to a high current fused source such as cigarette lighter outlet. This way the Vbat will see main battery voltage minus 0.6 to 1V during charging and current when ignition is off (if switched) otherwise if not vehicle battery rests < 13V to 12.5V and diode drop will prevent any if drain if full charged and Vbat voltage will always be 0.7V less than main bat, which is perfect for SLA. and main voltage @ 14.2 with alternator charging.

Verify Vbat with a voltmeter with ignition off and on. for low battery and charged battery. No switch is required.

I chose the diode on ground side purely for thermal reasons to connect Cathode to ground. In theory it can be in series on either side of the SLA.

I chose the cigarette lighter for power source to SLA + , because it is fused (25~30A) and convenient.

If you need explicit instructions, ask and someone can offer.
 
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MikeMl

Well-Known Member
Most Helpful Member
Hi everyone.

I am wanting to install a small 12volt SLA battery into my Van....
Have you measured the voltage across the van's starting battery, especially after it just having been driven a few miles, with the engine running at fast idle? Use a DMM; do not rely on the van's voltmeter if it has one.

Make of van?

Capacity of the SLA in Ah?
 

Tony Stewart

Well-Known Member
Most Helpful Member
large SLA's might be 19Ah, 12V 50mΩ when new.

often n*Ah*ESR remains fairly constant for n cells.
So as battery Ah reduces with aging, ESR also rises and is a multiple of 6 cells in series.
But all it takes is the weakest cell to break the whole string.

Assuming Vbat drops to 8Vmin or 0% SoC for gel-cell, and alternator too high at 14.5V ,
Gel cell ESR rises rapidly below 10% SoC which limits surge current.
But as SoC rises above 10% Vbat might be 11V (EST.) Wh

(14.5-11) *0.05Ω= 70A

So a fresh new battery not charged might blow the fuse, so we might consider a 30A
PTC cheap in series, Aka Polyfuse, unless we get the SLA part number and make to know better
 

dr pepper

Well-Known Member
Most Helpful Member
If your not into electronics a marine chandlers might be your answer, house boats have a secondary charging system for the house battery, which maintains priority to the engine battery charge wise.
Adverc and sterling do them.
There are cheapo ways of doing it like using power diodes, I wouldnt reccomend that as if the aux battery is well down or faulty the charging system will charge it at maximum capacity which wont do the alternator any favours, and eventually will kill the engine battery too, plus diodes introduce a voltage drop.

http://sterling-power.com/products/volt-drop-alternator-splitting-system-pro-split-r
 

ericgibbs

Well-Known Member
Most Helpful Member
I am wanting to install a small 12volt SLA battery into my Van.
hi Flat,
One option is to use one of these converters.
Connect to the converter input to the nominal +12V of your main battery output wiring, via a fuse and a blocking diode and the converter output to the 12v SLA battery, via a fuse.

Set the converter Vout to 13.8v , the 'float' charge voltage for a 12V SLA.

E
 

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tcmtech

Banned
Most Helpful Member
The simplest way would be to just wire in an auxiliary relay that comes on whenever the ignition is on and put a properly sized self resetting circuit breaker on it.

If it's based on a cargo type box van model that was built in the last 20 years there is a good chance it already has that option designed in for remote power to a trailer through the factory wiring harnesses. If so adding the relay and fuse or circuit breaker is plug and play level electronics and tieing your battery into it is just a matter of finding the wiring harness end at the rear bumper that would have went to the trailer power socket and tapping off the larger power feed wire.
 

rfranzk

Member
If you use a relay it is important to use one rated for continuous duty otherwise the windings will overheat and fail. Campers and RV's have been using the vehicles charging system to maintain an external battery for years now. A google search of "Battery Isolators" will give you a variety of choices.

This one looks interesting. http://www.powerstream.com/battery-isolator.htm
 

dr pepper

Well-Known Member
Most Helpful Member
With a relay if one battery is discharged 100's of amps can flow for a short time when the relay contacts close, so yes using this method you need a beefy relay.
 

FlatButt

New Member
Thanks to All of you that have answered my question.
I have purchased a couple of solar cells capable of producing a charging current and have successfully hooked them up to charge the SLA battery.
A fried suggested this would be an easy way to do things and a way to do it without risk.
So thanks again for all of your valued suggestions and help.
 

Tony Stewart

Well-Known Member
Most Helpful Member
Another easy way is to use a 50A diode to drop V alternator=~14.2 to 13.8V to 13.5V
http://www.digikey.com/product-detail/en/microsemi-corporation/APT60S20BG/APT60S20BG-ND/1494822


Since cathode is on back. if you clamp to frame ground for a heatsink and electrical connection then you only connect Anode to Vbat- and then Vbat+ to car Vbat+
900mV @ 60A
upload_2016-10-30_19-38-33.png


Another way is simply put a little 2W festoon 12V light bulb or even a 3V filament XMAS light in place of fuse, then if it is ON , you can see the battery being charged and it turns off with the ignition using any switched fuse outlet. Then a small diode can be used to prevent over charge and the ignition panel prevents any possible drain. from the main battery.
 
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dr pepper

Well-Known Member
Most Helpful Member
At least you know if theres no connection to the vehicles electrics nothin bads gonna happen to them.
 
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