using a 40AH AGM <span class="highlight">battery</span> as 2nd <span class="highlight">battery</span> in dual <span class="highlight">battery</span> setup

Submitted: Tuesday, Nov 09, 2010 at 23:37
ThreadID: 82401 Views:4551 Replies:3 FollowUps:6
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Hi
I am looking at using a 40ah agm battery as the aux battery in a dual battery system. However battery specs say to limit initial charging current to 12amp. Any suggestions on how this could be done? Will the vehicle alternator automatically reduce the charge provided. vehicle is a 2003 rodeo 3L turbo diesel.

Thanks Kc
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Reply By: Battery Value Pty Ltd - Tuesday, Nov 09, 2010 at 23:55

Tuesday, Nov 09, 2010 at 23:55
16 metres of 4mm auto wire, fused 15A.

You could have purchased a 50Ah pure lead/tin spiral wound AGM battery with 60A+ charging capability. Charges from 0~90% within 45~50 minutes straight off the alternator...

cheers, Peter
AnswerID: 435520

Follow Up By: kcandco - Wednesday, Nov 10, 2010 at 11:53

Wednesday, Nov 10, 2010 at 11:53
Hi Peter

If money were no object a 200ah agm would be going in. I have this setup in my vehicle. This is for a friends car and they are on a tight budget. Battery would be in tailgate, approx 4.5m from dual battery isolater, so therefore 9ish meters of cable. They intend to run 12v tv and led caravan lights at night so estimated absolute max usage 25A per night. I was going to use 7.7 mm diam wire but now wondering if 4mm diam would be more appropriate, (actual core diam given). I would love to lash out a few hundred for a bigger battery!!! They will be driving daily and if 240v available will use that. They are also carrying generator if bush camping and noise is not a problem. If they intended to run a fridge, I am thinking a much larger battery would be needed ie 100AH agm and would then use the 7.7mm cable . Your thoughts appreciated.

Kc
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Follow Up By: kcandco - Wednesday, Nov 10, 2010 at 12:03

Wednesday, Nov 10, 2010 at 12:03
Hi Peter

what is a 50Ah pure lead/tin spiral wound AGM battery with 60A+ charging capability worth?

thanks Kc
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Follow Up By: Battery Value Pty Ltd - Wednesday, Nov 10, 2010 at 14:21

Wednesday, Nov 10, 2010 at 14:21
Hello Kc,

for this small a load I wouldn't even bother putting an aux in.
If the starter battery has an RC rating of around 100 (40~50Ah) or more, then I can't see a problem with that (unless it's old and unreliable).

Since your friends will have occasional access to 240V, it's easy to give the battery some good absorption charging at least once a week using a quality mains powered charger.
Every 4~8 weeks I'd also give it some cell equalisation @ 15.5V for an hour or so.

Next easiest thing to do is to replace the starter battery with a spiral wound pure lead/tin AGM battery.
This battery type can be 90% discharged and cranking's still possible (cranking only requires less than 0.5Ah).
If more capacity is needed, just wire another one up in parallel using B&S2 starter wire.

So what are the expected max charging currents?

John's alreay mentioned the influencing factors.

Here's an example using some hard numbers:

If the battery's ~80% discharged, its o/c voltage will be around 11.9V.
So there's almost 2V between the battery and the alternator (some alternators go higher during the first few minutes, so the voltage difference could approach 3V).
On closing of the isolation switch, there'll be an inrush current through the battery, only limited by its internal resistance, and by the combined wiring resistance. The internal resistance will cause the terminal voltage to go up by around 0.5V (i.e. 12.4V as soon as the charging current is applied). This leaves another 2V which need to be dropped along the wires.
Now you can apply R=E/I, and the wire resistance required works out to be 0.17Ohms.
Now you select a wire which has significant resistance, but can still carry the max current of 12A.
Then you discover this 4 mm auto wire with a resistance of 10.2mOhms/metre, and a current rating of 15A.
Divide 170 by 10.2 and the result is ~16 metres.

So this wire will definitely prevent the alternator from pumping in excess of the max permissible charging current through the battery.
But on the other hand, it'll also make charging a very long process.

It's already been said, either use a battery which can absorb charge at a much higher rate (5~6 times in case of the 50Ah spiral wound), or use a solar panel, or a DC/DC charger.
If selecting spiral wound, you don't have to restrict the alternator current because the alternator itself and the (heavier gauge e.g. B&S6) wiring will be the limiting factors. 25Ah can be absorbed by this battery in just as many minutes.

Hope this is of some help.
There's more information in my profile.

cheers, Peter
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Follow Up By: kcandco - Wednesday, Nov 10, 2010 at 16:29

Wednesday, Nov 10, 2010 at 16:29
Thanks for this info Peter. I am/was concerned limiting the charge current is also going to extend the charge time by too much, I will check out the starter battery and see what RC it is. I appreciate the time taken to give me precise info.

regards

Kc
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Follow Up By: Battery Value Pty Ltd - Wednesday, Nov 10, 2010 at 18:47

Wednesday, Nov 10, 2010 at 18:47
You're welcome Kc,

just thought of another option:

if the starter battery doesn't cut the mustard in terms of cranking due to some prior discharging, then this is most likely due to battery ageing.

A spiral wound AGM can be wired directly in parallel to the starter battery.
This will give high cranking amps, feeling just like a new starter battery.
But for low rates of discharging, the starter battery could still offer some significant Ah.

Basically, this is a way to squeeze more life out of an ageing starter battery by assigning a slightly different task to it - low rate discharging, while the higher rates are being taken care of by the spiral wound AGM.

And when absorption/equalisation charging the two batteries in tandem, the starter might even come back for a bit longer since it's getting to see some decent charging, quite possibly for the first time in its life.

cheers, Peter
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Reply By: Member - John and Val - Wednesday, Nov 10, 2010 at 07:54

Wednesday, Nov 10, 2010 at 07:54
KC,

Depends where the battery is. In practice, unless it's in the engine bay, resistive losses in cabling will limit charging current. The actual current delivered to the battery is heavily dependent on the state of charge of the battery and the alternator output voltage. Both are variable, so it's hard to give a straight answer, but if there's much cable involved I doubt that you'll exceed the 12 amps.

An option I'd consider is a small dc-dc charger. A 10 amp one would keep you safely below 12A and provide optimum charging for the auxilliary battery without costing the earth. If you haven't already purchased the battery, Peter's suggestion is a good one too.

Cheers

John
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AnswerID: 435532

Reply By: Member - Andrew (QLD) - Wednesday, Nov 10, 2010 at 08:35

Wednesday, Nov 10, 2010 at 08:35
I tend to use a similiar setup with a 36Ahr AGM to power the fridge when camping overnight etc. The power is taken off the 6B&S cable run to the anderson plug at the rear of the vehicle via 6mm cable.

Even run down overnight my system does not exceed 12A on my monitoring system. Without knowing all the technical information behind the isolator and the alternator, the current is limited due to the charging voltage of the system, in my case measured as 13.94V. If the charging voltages were up around 14.4V-14.7V then it may be a different story IMO.

I don't believe that the information on the 4mm cable etc is accurate in this case, rather it doesn't take into account the alternator voltages and the effects of some isolators. Happy to be proven wrong. :)

Hope that helps,
Andrew
AnswerID: 435536

Follow Up By: kcandco - Wednesday, Nov 10, 2010 at 12:00

Wednesday, Nov 10, 2010 at 12:00
Hi Andrew

Please refer to my reply to Peter above. Your setup sounds very close to what I have in mind.

Kc
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