Battery charging - just wondering - Ram Promaster Forum
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post #1 of 60 Old 06-09-2019, 02:04 PM Thread Starter
ubique29
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Battery charging - just wondering

I have searched but can't find the exact answer.


If you have a 180amp alternator charging the stock lead acid battery, then you have a gauge of wire through something like a battery doctor, then to your house batteries (AGM), I am thinking the voltage drop from alternator through it's path to the house battery will be below 14+ volts needed to charge the house battery properly.


Can anyone correct me here or advise on this ?


Thanks


Nigel.
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post #2 of 60 Old 06-09-2019, 02:55 PM
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Not if you have a thick enough cable. Some cars from the factory have the engine/starter battery in the trunk. No issues.

Use an online wire thickness calculator like this one:
http://circuitwizard.bluesea.com/#

(I learnt of this from GaryBIS)

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post #3 of 60 Old 06-09-2019, 03:02 PM Thread Starter
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@travelvanman


Thanks for the reply. If the wire was just running from alternator to battery, then I would have the same concern. My concern really comes from alternator->Engine battery->switch->house battery: by the time the current has done all that traveling and being drawn, is there enough voltage left to charge an AGM.


Hope this makes sense.
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post #4 of 60 Old 06-09-2019, 03:09 PM
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Lots of us here do it all the time. We see 14.2-3V at the house battery after traveling 180A alternator-van battery-isolator-house battery.

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post #5 of 60 Old 06-09-2019, 03:21 PM
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MsNomer is correct. Just make sure your house battery is connected to the alternator with the proper gauge wire from Gary’s (Blue Sea’s) chart.

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post #6 of 60 Old 06-09-2019, 03:22 PM Thread Starter
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Quote:
Originally Posted by MsNomer View Post
Lots of us here do it all the time. We see 14.2-3V at the house battery after traveling 180A alternator-van battery-isolator-house battery.

great news...thank you !
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post #7 of 60 Old 06-09-2019, 03:24 PM Thread Starter
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Originally Posted by keeponvaning View Post
MsNomer is correct. Just make sure your house battery is connected to the alternator with the proper gauge wire from Gary’s (Blue Sea’s) chart.



makes sense, thanks.


I have the 180amp, is that what is used for the wire gauge ?...I have seen some use 80amp fuse, which tells me there is not 180 running down the wire.


Nig.
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post #8 of 60 Old 06-09-2019, 03:57 PM
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The alternator has lots of other stuff to do so you get less AND the battery has internal resistance so the entire circuit’s resistance is not zero so the charge rate is not infinity! In truth a lead acid battery will begin charging at something up to about 60-80 amps but very soon (less than a minute) the rate will drop to much less and in a few minutes it may be 20-40 amps due to the rising resistance in the battery being charged. This is all normal.

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post #9 of 60 Old 06-10-2019, 03:33 AM
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@RDinNHandAZ is correct. You should be concerned about "equalization" current between the batteries and not the current provided by the alternator, which is just 30A for 180A alternator.

When parallel connecting LeadAcid batteries, I recommend sizing the wires according to the max charging current of all batteries combined on each side of the connection (check batteries manufacturers specs) and wiring charts mentioned earlier. For example if you have 2 aux batteries with max charge of 80A, ideally you should size the wiring for 190A (80A*2 + 30A).

Use a dedicated negative wire and not van's body. And don't forget to include the length of both wires (+ & -) unless wiring chart specifically says to use only one direction distance.

Expect to deal with really thick wires...
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post #10 of 60 Old 06-10-2019, 10:06 AM
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You are looking at this wrong. The gauge of wire will cause a voltage drop proportional to the current draw. As the house battery charges, the current draw will go down and so will the voltage drop. As the current drops to basically zero, the voltage drop will go to basically zero, and the batteries will see full alternator voltage. Now, that voltage drop will slow down the charging of the battery - if you have tiny wires, the batteries will see a large drop and will only get a few amps of current to charge, taking a long time to charge - but given enough time with the engine running, they will eventually reach float charge levels.
That said - it's better to use proper sized wire. Look at how much your DC-DC converter can pull, how long the wires need to be, and then look online for an ampacity table for different wire gauges and select beefy enough wires to have less than .5V of drop at the maximum current your converter can pull. As a rough rule of thumb, I'd go not less than 2 gauge (in fact I went 00 gauge). Make sure you have a fusable element as close to the alternator as possible, sized to roughly 1.5 times the maximum current the converter can pull.
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