How many amps recharge via 12v supply per hour of driving ?

zigdog,
there are a number of factors that will affect the charge rate, the state of all your batteries at the time of starting the charge. e.g. how much load has been on them and how discharged are they? Theoretically, your start battery should charge first (depending on how your dual battery system is set up). Then all other batteries (if connected together).

As a rule of thumb, a days drive (even running the fridge while driving) should see the batteries fully recharged. But as I said, there are a large number of factors that effect this.
regards,

zigdog
This is best way to charge batteries as they charge at car alternators max amperage usely around 100 amps.
from Graham

It will depend on the factors mentioned, the output rating of the alternator, what load you have on it for normal operation of the vehicle, alternator temp and size of cables you are using.

I've measured 50 Amps going into by aux batteries from alternator using digital clamp metre. I suspect the peak in my case might be 60 Amps. But this drops off as battery charges.

Norm C

As already said, there are many factors affecting the charge rate, but probably the main one is the extent of voltage loss in wiring between the main cranking battery and the trailer battery. The only way to really know what current is flowing to the trailer batteries is to measure it. This current is very dependent on voltage and any loss of voltage in the cables (both +12V and ground return) will have a major effect. Unless you have very big cables and good (Anderson for preference) connectors, I doubt that your trailer battery bank will ever see more than 10A.

It isn't generally recognised how important wiring losses can be. In most cases, the alternator will deliver about 14V (cold), less when hot . The battery chemistry defines its voltage to be about 12.6V. The small voltage difference (14 - 12.6 = 1.4V) is what's pushing charge into the battery. We can easily lose a volt in the wiring and this will drop the current into the battery by well over half.

We have a meter mounted on the dashboard monitoring current flow at the batteries, which in our Troopy are mounted behind the seats (much less wiring length and hence losses than in your case). With a half discharged battery we charge at up to about 15 amps initially, dropping to half that as the alternator warms up. (This temperature response is deliberate designed into alternators - it is aimed at optimising conditions for the cranking battery.)

If you'd like to measure the current into/out of your batteries, there are 3 options - 1) clamp meters exist, simply clamp them around the wire to make a rough measurement. 2) Insert a -20 to +20 amp meter into the charging line. Problem here is that the meter can't be viewed while you are on the road.
3) In my opinion very much better - insert in the battery line a very low value resistor (1/1000 ohm is ideal) and measure the voltage drop across it using a cheap digital meter. (each amp will produce 1 millivolt) Suitable resistors and meters are available from Jaycar and similar places. This approach will require a bit of familiarity with the technology, but works very well, as the meter and display can be remote from the point where the measurement is made.

Lastly, chargers are available to increase the charging voltage ( and hence current). You might be interested to read our blog Electricity for Camping where these and many other things are discussed.

To respond directly to your question - I'd expect that you are getting no more than 10 amps into your batteries, and probably average much less than that. Let's say 7 amps average. So each hour you are putting in 7 amphours of charge, which is really not enough for a 200+ Amphour battery bank. You most certainly are not getting the 50 or 60 amps suggested by others. Suggest get an autoelectrician to measure it for you. For the health of your batteries you really need to be able to charge them fully, and I can't see that happening.

Sorry to spread gloom, but wish you the best!

Cheers

John