# charging 12v from 12v but limiting current

#### larry gardner

Joined Sep 17, 2008
15
Let me say I have read a lot in the past on the board but have not been on in a while. I have very little knowledge about circuits but find them very interesting. I have some one that can help me with this project locally after I get the information I need here.

I fly electric RC Planes and want to charge 2 (140AH) deep cycle batteries (parallel) in the back of my work van. I do want to limit the current that they are charged at. Some where around 10 to 20 amps. ( not sure what the best charge rate is for deep cycle batteries. I assume lower is better)

The idea is to have the circuit turned on by a auxiliary only while the van is running.

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#### wayneh

Joined Sep 9, 2010
17,498
I do not want to limit the current that they are charged at. Some where around 10 to 20 amps.
I'm confused. I assume you mean you DO want to limit the current? Are these lead-acid batteries?

#### larry gardner

Joined Sep 17, 2008
15
sorry I edited that to say "I do"

The batteries are lead acid batteires

Joined Jul 18, 2013
28,755
I am assuming you want to use the vans alternator to do this?
Your vans alternator regulator peaks out at around 14.5v., the regulator maintains this regardless of load down to very low rpm's.
The alternator current will vary dependent on the condition of the batteries, and any other load that may be on at the time.
Have you considered just placing them in parallel with the Vans batteries?
You could get a clamp on ammeter to do a trial with batteries that are in a fairly flat condition.
Max.

#### wayneh

Joined Sep 9, 2010
17,498
You need to specify the charging current limit that you need.

Have you measured whether there is a problem? I mean, if you just charged the battery off of your system voltage - like jumping another car's battery - does the current exceed the recommended rate for your battery?

Also, can you tolerate a nice long charging time, for instance by limiting current to 5A or 2A or something much lower than the 10-20A? I guess I'm asking if you could live with trickle or float charging instead of bulk (high current) charging.

#### larry gardner

Joined Sep 17, 2008
15
If deep cycle batteries can handle the charge from the vans charging system that would be great. I did not think that they could be charged at high amps. Im worried that after I discharge them at the field that the high charge rates on the drive home will damage the batteries.

A trickle or float charge would be fine. Just wanting something to maintain them.

Joined Jul 18, 2013
28,755
Like I said, I would use a clamp on meter and actually find out what the charging rate is when fully depleted or after an extensive use, the current decreases rapidly as the voltage of the battery comes up.
Max.

#### wayneh

Joined Sep 9, 2010
17,498
I agree - be sure you have a problem before trying to solve it.

Or just put a cheap float charger on them when you get home. I believe those have internal self-protection circuitry so that, if the battery would otherwise draw too much current, the charger protects itself and won't deliver more than the 0.5A or whatever it was built for. This is one I have, got it for \$5 on sale.

#### strantor

Joined Oct 3, 2010
6,811
Im worried that after I discharge them at the field that the high charge rates on the drive home will damage the batteries.
I think your alternator is at equal or greater risk than the batteries.

Joined Jul 18, 2013
28,755
Assuming the Van battery itself is up to par, I don't see the alternator seeing any more load than it would with a normal low battery, I know alternators are rated for around 100amps, which is known to be impossible, at least for any length of time.
But If there is an ammeter or readout in the vehicle, a sign the alternator is being taxed is the voltage will drop to around 13.2v. while the engine is running.
Voltage is normally kept to approx 14.5v max. to prevent overcharging and gassing of the battery.
Max.

#### strantor

Joined Oct 3, 2010
6,811
Assuming the Van battery itself is up to par, I don't see the alternator seeing any more load than it would with a normal low battery,
A normal low battery can kill an alternator all by itself, especially if it's normally low. If he had 2 deep cycle batteries in parallel with his existing battery, and they were low, that would mean (unless there's a diode separating them) that the original battery is also low. So three low batteries, two of them being deep cycle batteries, I'm pretty sure is going to tax the crap out of the alternator.

Joined Jul 18, 2013
28,755
I guess we are back to the old 'Clamp-on' Meter trick?
Max.

#### strantor

Joined Oct 3, 2010
6,811
I guess we are back to the old 'Clamp-on' Meter trick?
Max.
That's probably the best advice we can give without crossing into the dark and forbidden "automotive modification" zone.

Joined Jul 18, 2013
28,755
I'm not sure I see anything being 'modified'?
But that's just me
Max.

#### strantor

Joined Oct 3, 2010
6,811
I'm not sure I see anything being 'modified'?
But that's just me
Max.
I see it the way you see it, but I know how some other people see it, and the decisions about whether or not to close threads generally favor their views. I'm just making predictions based on a long history of observation.

#### larry gardner

Joined Sep 17, 2008
15
Thanks anyway guys this in not going in the direction I wanted.

Joined Jul 18, 2013
28,755
Thanks anyway guys this in not going in the direction I wanted.
In what way?
Max.

#### THE_RB

Joined Feb 11, 2008
5,438
A normal low battery can kill an alternator all by itself, especially if it's normally low. If he had 2 deep cycle batteries in parallel with his existing battery, and they were low, that would mean (unless there's a diode separating them) that the original battery is also low. So three low batteries, two of them being deep cycle batteries, I'm pretty sure is going to tax the crap out of the alternator.
Agreed! But it's not too hard to deal with that.

Just using the right size cable from the alternator (or van battery terminals) to the two deep cycle batteries in the rear of the van will add resistance, and the cable resistance can limit the peak charge current.

Normally the problem is the other way around; vehicles that have a remote battery often have trouble that they cannot get enough charge current into the remote battery because of voltage drops in the cabling.

#### strantor

Joined Oct 3, 2010
6,811
Thanks anyway guys this in not going in the direction I wanted.
if it's a low current trickle charge while driving that you want, consider something like this. But make sure to specify that you'll be plugging it into the cigarette outlet, lest it be considered an automotive modification.

#### LDC3

Joined Apr 27, 2013
924
Just using the right size cable from the alternator (or van battery terminals) to the two deep cycle batteries in the rear of the van will add resistance, and the cable resistance can limit the peak charge current.

Normally the problem is the other way around; vehicles that have a remote battery often have trouble that they cannot get enough charge current into the remote battery because of voltage drops in the cabling.
I saw an advertisement for installing terminals in the trunk (or rear of van) for providing a connection for jump starting to avoid the hazards under the hood. I'm sure those wires would not have a lot of resistance.