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Charging a 12v battery from...a 12v battery?


By Anonymous Hero, Section Homebrewed Electricity
Posted on Fri Jun 27, 2003 at 10:51:13 PM MST
I want to charge 1 12v battery from another...

I want to install a 20AH SLA battery under the seat of my truck to power things so I don't have to worry about the main battery going dead. I want it to charge up from the truck...and then float once it's charged. Basically...something like this: http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=42292 only with 12v input. How can I do this?
Charging a 12v battery from...a 12v battery? | 10 comments (10 topical)

Re: Charging a 12v battery from...a 12v battery? (none / 0) (#1)
by Andrew on Fri Jun 27, 2003 at 11:26:41 PM MST

Simply hook it up to your engines charging circuit.
It should keep it topped off.
-Andrew


CAUTION! Very Dangerous Advice (none / 0) (#3)
by TomW on Sat Jun 28, 2003 at 06:56:19 AM MST

You can damage your batteries, yourself or your wiring doing a direct connection from an SLA to an automotive charging ckt.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it


[ Parent ]



Re: Charging a 12v battery from...a 12v battery? (none / 0) (#4)
by jimu on Sat Jun 28, 2003 at 07:01:49 AM MST

I agree with Tom..

Never connect a SLA directly to an automotive system charging system.

The way I have charged sla batts from the car, is to use a 12 volt to 18 volt DC laptop
auto adapter , which in turns runs a small 1 amp regulator circuit using a cheap lm317
chip set to about 14 to 14.4 volts.. This gives you 2 things.. Battery isolation, and nice slow charge current.

JimU

[ Parent ]



Re: Charging a 12v battery from...a 12v battery? (none / 0) (#2)
by Demetri on Sat Jun 28, 2003 at 12:03:37 AM MST

Check out a battery isolator, $25-$30 at any auto parts store. Wal-mart probably has them, probably Harbor Freight too, if you catch them at the right time. Best way to use a second battery without a separate alternator, lets you pull power from one battery without draining the other. Hope that helps. Sorry if I seem short or if I'm explaining things you know, been a longggggggggg day. Off to bed with me, night all.

Demetri
Uncommon sense required.



Re: Charging a 12v battery from...a 12v battery? (none / 0) (#9)
by RobD on Sat Jun 28, 2003 at 07:48:51 PM MST

Hi, Just one point. You can't charge from one battery to another without the the charging battery discharging somewhat. RobD

[ Parent ]


Re: Charging a 12v battery from...a 12v battery? (none / 0) (#5)
by Demetri on Sat Jun 28, 2003 at 10:13:51 AM MST

Whoa, I missed the SLA part. Yeah, forget what I said. Demetri P.S. Why an SLA?
Uncommon sense required.


Re: Charging a 12v battery from...a 12v battery? (none / 0) (#6)
by troy on Sat Jun 28, 2003 at 10:33:46 AM MST

Yup, SLA makes all the difference.  If you just hook it up direct to the vehicle's charging system, you will periodically get an overcharge condition, which will damage SLAs.  You will also get a charge rate that overwhelms the smaller SLA, thus the need for some type of charge regulation.

The big advantage of course is that it's sealed, so if it tips over, no big deal.

Good luck and have fun!

troy

[ Parent ]



Re: Charging a 12v battery from...a 12v battery? (none / 0) (#7)
by Andrew on Sat Jun 28, 2003 at 02:52:11 PM MST

Whoa! Its really that bad?
I'm sorry, I thought that the charging circuit from the car would be current and voltage regulated. If not what kind of hell are you putting the starter battery through. I like the idea of using a power supply powered from dc 12v, It shouldn't be that hard to hook it up in your car anyways.

-Andrew

[ Parent ]



Just a for instance.. (none / 0) (#8)
by TomW on Sat Jun 28, 2003 at 03:42:55 PM MST

Andrew;

To put a different angle on this consider the following:

Your car has an alternator that more than likely can produce 30 amps minimum and possibly up to 100 amps.

Then we have the battery. By design auto batteries are capable of spitting out several hundred amps to start the car for short periods of time.

So now you take your partially discharged SLA [or any battery] and connect it in parallel with this system that is quite capable and happy to push a lot of current into this lower voltage load.

Even if the engine is off and its just the battery you could see your little SLA being fed extremely high current its not capable of absorbing safely.

The vehicle battery and vehicle charging system will dump every amp it can to bring the SLA up to voltage and thats the real safety problem.

It would be extremely hard on the average SLA to get charged in this manner, also.

Just qualifying what I have said so you know why not just that its not good.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it


[ Parent ]



Re: Charging a 12v battery from...a 12v battery? (none / 0) (#10)
by scoraigwind on Sun Jun 29, 2003 at 05:37:01 AM MST

You can hook the battery direct to the vehicle system (through a 5A fuse) and it will be charged at a regulated voltage, but the voltage is a little high for SLA and the current might be too high as well.  Connect it through a blocking diode and this will pull the charging voltage down to about 13.8 which is safe.  You might also consider putting in a small resistance if the battery gets hot or the fuse blows due to excessively high current.  0.5 ohms, 20 watts or similar would keep the current down to a reasonable value.  Wire the fuse, diode and resistor in series on the positive lead from battery plus to battery plus.

The diode also prevents the battery from discharging into the vehicle system when parked and using power.
Hugh Piggott http://www.scoraigwind.co.uk



Charging a 12v battery from...a 12v battery? | 10 comments (10 topical)
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