Battery charging assumption - True or False?

Allan,

10 out of 10 for the explanation.

I have to agree with every word. Have you ever thought of writing manuals for these products?

You have left for dead some of the answer's from experts on 12V stuff of late.

Redeye

Yep, nice concise explanation Allan!

WBS,
In the real life situation, the cranking voltage will hit 13.2V within a minute or two of starting. So from that time onwards, the Redarc allows alternator current to charge BOTH batteries. The amount of current flowing to each battery depends on lots of factors, but all else being equal, the battery with the least charge will take a higher current. The current tapers off as the batteries get charged. If the aux battery was half flat, you might find that 30 amps will flow at the start of charging and within an hour might be down to 10 amps, and it will taper down to almost nothing as it approaches full charge.

Think of the Redarc as nothing more than a switch that allows the regulated voltage from the alternator to be applied to the terminals of one or both batteries.

Cheers
Phil

"12.6v Cranking battery - 10.6v Auxilliary 100ah DC battery - Combined Voltage is N0T (numerical number) 11.6v but would be ~11v or possibly less in many cases "

Maybe we need a new thread here on the "two batteries connected together will equalise their charge" myth.

If you connect a 100% charged battery to a 0% charged battery they WILL NOT equalise in charge. You need more than 13 volts to charge a battery - a fully charged battery only puts out 12.8 volts.

Here are the results of an ACTUAL WORST CASE TEST.

I connected a fully charged new AGM 80 Ahr battery to a good fully discharged (to 10 volts) wet-cell starting battery. The MAXIMUM current that flowed for a few SECONDS was 32 amps, but reduced to 7 amps after 1 minute.

The total charge transferred from AGM to Wetcell was only 10.7 ampHours after being connected for 2 HOURS, this will not affect the AGM's starting ability !

Keep in mind this was a worst-case test for charge transfer - a fully charged AGM has an open-circuit voltage of 13.0 volts but a wetcell only has a 12.6 volt open-circuit voltage, so less charge transfer would have occurred. You would get LEAST charge transfer if you had a charged wetcell connected to a discharged AGM.

The voltage on the two connected batteries ended up at 12.7 volts because the charged battery still had 90% capacity. The discharged battery would have charged to less than 10%.

The voltage did NOT drop below 12 volts - if this happens you have a faulty battery.

Mike
you say: "If you connect a 100% charged battery to a 0% charged battery they WILL NOT equalise in charge"

What do you call the "process" where the Voltage in 2 batteries (not connected to a charging device) are connected together, if it's not "equalise" ??

Maîneÿ . . .

"What do you call the "process" where the Voltage in 2 batteries (not connected to a charging device) are connected together, if it's not "equalise" ??"

When you connect ANY two devices together with a low enough resistance, the voltage will be equal.

The voltage will not be the average, if you connect two different batteries together. A fully charged battery has a much lower internal resistance than a discharged battery, which is why its voltage will dominate.

I wrote that the batteries will not equalise in CHARGE. Too many people blindly apply the Voltage-vs-Charge tables - the tables ONLY apply when no current has been flowing in or out of the battery for a long time - they are meaningless when you have two batteries connected together.

Two batteries connected together at the same voltage do not have the same percentage CHARGE - otherwise a battery isolator would instantly charge the Auxiliary.
.

Mike,
thank you,
yes I believe "equalise" and "equal" have the same meaning in the context used :-)

Maîneÿ . . .

"12.6v Cranking battery
10.6v Auxilliary 100ah DC battery
Combined Voltage is N0T (numerical number) 11.6v
but would be ~11v or possibly less in many cases
Yes, different battery combinations will give vastly different "

It's this part of your post that's wrong "would be ~11v or possibly less in many cases " - My real-world tests confirm what theory predicts - that it will be very close to the voltage of the fully charged battery - 12.6 volts.
.

The explanation does not change. It matters not where the secondary battery is located, in the engine bay or in a trailer connected via an Anderson plug, the behaviour of the Redarc isolator is the same.

The only additional considerations of having the secondary battery remote in a trailer is that the cabling from the charging source to the battery is of sufficient capacity to allow effective charging without excessive voltage-drop. If the cable capacity is inadequate then the secondary battery will still charge but the current is limited and hence the charge will take longer to complete.

If you have a really big battery bank (some people have 400 amphours) that's discharged significantly, then you will be reaching the limit of your Alternator to keep up the voltage around 14 volts.

The Auxiliary Batteries may pull the voltage so low that the cause the Battery Isolator to disconnect the Auxiliary Batteries.

Keep in mind that for normal battery installations, current into each battery is determined by the battery - the Alternator attempts to keep the voltage around 14 volts, up until it reaches its maximum current capability.

Interesting proposition Mike, and I follow your logic. But have you actually experienced this behaviour in real-life?

I would have thought that a typical 4WD 100 amp alternator would be capable of maintaing the charge voltage above 12.65 volts (the Redarc drop-out value) even when charging a large bank (400Ah) of significantly discharge batteries.

But then, I have not had the opportunity to observe such situation and may well be wrong.

Hi Mike. Thanks for this.

What happens if your main battery, auxiliary battery and caravan/trailer battery are all different - when the "smallest" battery is charged does that impact the ability to continue charging the other batteries? Should all batteries in a system be of similar capacity for best performance?

Regards

All I have in the charging system for the batteries in my caravan is a relay that closes when the ignition is on. No fancy Redarc or Arrid in the circuit but I do have heavy cabling to the aux. batteries. I have been running this system for about four years now and have had no problems with flat batteries.

. . and an automatic Battery Isolator with all the exotic claims will not put 1% more charge into either battery !!!!
.

"What happens if your main battery, auxiliary battery and caravan/trailer battery are all different - when the "smallest" battery is charged does that impact the ability to continue charging the other batteries?"

A Lead-Acid battery connected to 14 volts will draw current based on its state-of-charge - that's what makes existing car charging systems work where MOST do not monitor battery current or state-of-charge.

As long as the total current draw is within the capability of the Alternator, the voltage will remain around 14 volts - one battery is not affected by the state-of-charge of another battery on the same line.

Once a battery is fully charged, it draws no more current.

By now, WBS (initial post) may be getting somewhat confused!

Rod, think of your batteries as per the following analogy:

Two water tanks of differing size piped together at the bottom and a pump feeding into the interconnecting pipe. As the pump feeds water to the tanks the level in each tank rises equally, the smaller tank does not become filled faster than the larger tank. Both tanks become filled (fully charged) at the same time.

I can explain the reason for this but it may complicate the issue. The analogy is not exact but near enough for the purpose. And of course it assumes batteries of the same type, both flooded lead-acid for example. I don't want to open up argument of what happens with batteries of differing types!

Ray: "All I have in the charging system for the batteries in my caravan is a relay that closes when the ignition is on."

This is fine and simple, however there is some risk of damaging the relay contacts and the wiring through the relay by heavy currents being contributed by the auxiliary battery during the cranking phase. Obviously this has not been a problem yet with you.

There is a crafty way of wiring a relay (not solenoid) to connect the auxiliary battery when the ignition is on but inhibit it during cranking ........... wire one side of the relay coil to the ignition circuit and the other side of the coil to the starter circuit. During cranking both of these circuits are at +12v so the relay has no potential difference across it and does not close. Upon release of the cranking action enough current will flow through the relay coil from the +12v ignition via the relatively low-impedance starting circuit to earth and thus energise the relay to connect the auxiliary battery to charge.

Not exactly conventional relay wiring but it works fine on my son's 4wd.

Incidentally, the reason I said "not solenoid" is because of the lower coil impedance of a solenoid which could partially continually activate the starter circuit. Actually, my son's vehicle uses the relay to then operate a solenoid as a slave.

Pajeros have two Ignition Circuits.

IGNITION1 stays on during starting for Engine power etc.

IGNITION2 stays off during starting, e.g. for AirCon.

Thanks Mike, I learn something new each day!
Then obviously one would connect to "Ignition 2".

Actually, I guess all vehicles with Aircon option would have both Ign. circuits?
Which could explain why Ray has no problem with starting current damage.