How? Charging small 12v batteries from my van (without giving them too much juice)

Thread Starter

cxskate

Joined Sep 29, 2017
3
I am attempting to add a secondary deep cycle AGM battery inside my van to use for power.

So far what I have learned is that hooking the terminals directly to the terminals of the battery in my van would charge my small battery too quickly. Does anyone have any advice or info on some sort of resistor I could purchase so I could charge my small second battery?

I have a 20ah battery that may only want 2 amps of juice...
 

crutschow

Joined Mar 14, 2008
26,727
Do you want the battery to automatically charge when the engine is running and then be disconnected from the vehicle generator when being used for van power?
Do you want the vehicle battery to provide the van power when the motor is running?

If so, you could use a DPDT relay operated from the ignition switch accessory output to perform the switching.

To limit the charge current you could put a small resistor in series with the wire to the vehicle generator.

Below is a circuit to do the above:
RChg would likely be a few tenths of an ohm to limit the charge current. You would have to experimentally determine the proper value to get the desired current.
upload_2017-9-28_23-54-14.png
 

mcgyvr

Joined Oct 15, 2009
5,394
Your post is a bit confusing and lacking detail..
Is this deep cycle AGM battery the "small 12v" batteries you are talking about ? Or are these different 12v batteries that you are trying to charge off the AGM battery?

What type (chemistry) are these other batteries? Lithium/NMH,etc...?
Got specs on this "small" batteries? (manufacturer/datasheet?)
 

Thread Starter

cxskate

Joined Sep 29, 2017
3
Yes the small deep cycle agm battery is the only second battery I'm trying to install in my van and charge without melting it. Is there an easy way to do it?
 

crutschow

Joined Mar 14, 2008
26,727
Is my solution not easy enough?
Do you want to manually connect and disconnect the battery from the vehicle battery for charging?
If so then all you need is a small resistor to limit the charge current.
 

Thread Starter

cxskate

Joined Sep 29, 2017
3
I can't find a 12v resistor anywhere! Could anyone help me find one online even? Yes you think it would be easy.
 

crutschow

Joined Mar 14, 2008
26,727
I can't find a 12v resistor anywhere!
Not surprising.
Resistors are not rated in volts (expect for the maximum they can tolerate) they are rated in ohms resistance.
For your application you would likely need a few tenths of an ohm.
I would buy ten 0.1 ohm, 1 watt resistors.
You then need to experiment to see how much resistance you need (do you have a multimeter to measure the charging current?).
You keep adding them in series until you measure the desired current.
Each added resistor will slightly reduce the charging current.

Note that the engine needs to be running with the main battery fully charged (about 14V charging voltage) when you do this current measurement.
 

Picbuster

Joined Dec 2, 2013
1,026
I am attempting to add a secondary deep cycle AGM battery inside my van to use for power.

So far what I have learned is that hooking the terminals directly to the terminals of the battery in my van would charge my small battery too quickly. Does anyone have any advice or info on some sort of resistor I could purchase so I could charge my small second battery?

I have a 20ah battery that may only want 2 amps of juice...
Look at the batteries spec sheet it will give you all relevant information.
We like to help you but without the spec's we can't.

Picbuster
 
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