Camper Trailer Solar Charging

If the DC DC charger is in the vehicle before the anderson then you will need a regulator for the solar input.
It sounds like he has a simple solenoid from tow vehicle, Nothing mentioned,

If DC DC in the trailer then possibly best to bypass the regulator on the panel" if fitted", as I can't see two regulators in series liking each other. I have seen "duel"ling batteries but duelling regulators is scary too.

If you haven't got a DC-DC charger then I have one listed in the classifieds -
Ctek D250s that might interest you.

That really depends on the setup, many run simple VSR setups without any problems.

As the OP indicates that is how he has been charging his batteries one would assume it works for him also.

Yes.
He may be operating between 70% and 50% SoC and that works after a fashion - and that would be predictable with a charge system with VSR that can't deliver the bulk charge levels of 14.6+ v needed.
He may be relying on instant voltmeter readings that look OK but without the involved battery prep needed are misleading - starting a camp with an apparent 12.8 v but not getting the expected power to run his devices.

An example...

The alternator can produce 14.2 v. There's an 8 m run of 6 B&S to the battery. Assume 20 amps flows. A voltage drop calculator estimates a loss of 0.4 v. So 13.8 v is going into the battery. And there's further loss to the 2nd battery.

14.6 - 14.8 v is needed for bulk charge.

If there's eg 200 Ah capacity in the AGMs and they're at 50%, 100 amps are needed but they're not going in at the pressure recommended.

One for the battery experts: can time substitute for volts? If so, at what rate?

To fully charge a battery you only need to supply it with a higher terminal voltage than the batteries fully charged terminal voltage. Don't believe me ask the battery manufactures or an electro chemical engineer.

Upping the voltage just reduces recharge times, your DCDC charger does not fully charge the battery at 14.4V, I'm assuming it is not a full calcium type. It brings the battery up to around 98%, the finally 2% is done at its float voltage which can be any ware from 13.2V - 13.8V with the most popular non adjustable types.

Your charger does not charge at 14.6V you mention all the time it starts out low and gradually rises. The alternator setup also starts out low and gradually rises as the current decreases, you don't need a lot of volts to charge a battery when its low, only to force charge it when it gets to higher SOC's. As the battery charges its current drops and the terminal will rise with either system.

I have 26mm2 wiring running to the van or there abouts and have no troubles fully charging my batteries and it does it quicker than 20A DCDC charger can from 50% SOC and I have the data to prove that. Show me one DCDC charger manufacturer that provides performance graphs of the DCDC chargers compared to an alternator putting out 14.2V to 14.4V?

If I only ever discharged my batteries to 80% or 90% then a DCDC in the van might be faster as would a 40A DCDC charger but its going to cost a hell of a lot more to just charge the van batteries where my simple VSR is charging both in car aux and van batteries.

There are thousands of users using VSR systems to fully charge their batteries you just need to suitable cabling and have an appropriate charge voltage to start with.

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Yes Sigmund, I think that you have a pretty good example there.

Regarding the question of "time" and "volts" well yes time can substitute for volts but only up to a point.
As you have pointed out, a certain minimum applied voltage is required to achieve a complete charge to a battery, in the order of 14.6 volts for a lead acid battery. To explain in a simple manner, a battery of nominal 12 volts will require an applied voltage at least greater than 12 in order for current to flow into the battery. Furthermore, there is an additional electrochemical effect that can be considered as a "surface charge" of a volt or so which has to be overcome, so the applied charging voltage needs to be greater than the sum of 12v plus the 'surface charge' in order to achieve any charging current.
Once we have achieved sufficient applied voltage then current will flow to charge the battery and that current will be a function of the voltage divided by the battery resistance. Multiply that by time and you are achieving a transfer of energy, so with less current flowing you would need greater time to fully charge the battery. The "rate" you ask about is essentially a simple proportional correlation.

So yes, time can "substitute for volts" in the charge process but only within certain limits.

Alan that is simply not true, go to any manufacturers website, they all indicate you can fully charge their batteries well below 14.6V, generally accepted float charge voltages are 13.5V or there abouts.

Industry float charge their batteries at less than that to maximise battery life.
This has been done to death before, increased voltage just reduces charge times.

As above you only need a voltage higher than the fully charged terminal voltage of the battery and to over come itself discharge to fully charge it.

I love these forums, one minute they argue that car manufactures have lowered the charge voltage to 13.5V to prevent overcharging the battery and maximise its life. Next minute its you need at least 14.6V to fully charge a battery. Car recharging systems have been fully charging batteries from at around 14.4V - 14.4V since batteries were put in cars. These voltages were selected as they give rapid recharge times whilst still maintaining a good service life.

From Optimas website as I had it open at the time:

Recommended charging inforamtion:

Alternator:
13.3 to 15.0 volts, no amperage limit.

Battery Charger:
13.8 to 15.0 volts, 10 amps maximum, approximately for six to twelve hours.

Rapid Recharge:
Maximum voltage 15.6 volts (regulated), no current limit as long as battery temperature remains below 125°F (51.7°C). Charge until current drops below one amp.

Float Charge:
13.2 to 13.8 volts, one amp maximum current, time indefinite (at lower voltage).
Strictly adhere to all limits.

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Leigh, I just knew that you would come back with some nit-picking refutation. What is it with you?

Of course it is "not true". It was a hypothetical value to aid an explanation.
My dissertation began with "To explain in a SIMPLE manner" then I went on to say "in the ORDER of 14.6 volts". The actual voltage is of no consequence, it is merely a nominal value to aid the basic explanation.
I was answering Sigmund's question of "can time substitute for volts?" in a simple manner without nominating precise values. I did not even confound it with talk of "float voltages" and the complexity of the whole charging algorithm.

Ask Sigmund whose response addressed his question with some acumen.

Alan my appologies, I stopped reading at after:

"As you have pointed out, a certain minimum applied voltage is required to achieve a complete charge to a battery, in the order of 14.6 volts for a lead acid battery."

Which as you wrote later is not correct.

Thanks for the expert input gents.

I'd come to this way of thinking after a couple of years of struggling to get my batteries fully charged from a standard setup (spec below). I could drive for 3 days and see the voltmeter read 12.6 on day 2 and 12.8 on day 3 but drop to 12.2 overnight on a draw nowhere near half the 220 Ah max battery capacity.

On one occasion with the rig at home I put the 240 v smart charger on and saw the meter regularly spiking to 15.5 v briefly and dropping back. Since then I learned that's an indication of a seriously discharged battery.

Also since then I learned that a voltmeter reading of the battery is unreliable. I can guarantee they'd never dropped below an indicated 12.2 but the smart charger sensed otherwise.

So this was the rig:
120 amp standard alternator; Redarc smart isolator; 8 m of 6 B&S; 2 x 110 Ah AGMs. When discharged to 11.8 v the system could put 13.7 v and 19 amps into the batteries.

It wouldn't do what I wanted of it over 2 days (19 hours) of driving, even when I'd started with the voltmeter reading 12.8. I had the batteries load tested and they were fine. So whatever the volt x time relationship is it wasn't working for me.

HKB, your rig isn't exactly a standard setup is it, with a 24mm sq wire?

That's what, 3 B&S? So your voltage drop is going to be lower.

My impression is that 6 B&S is generally fitted. Though I note Redarc recommends 4 B&S for van/CT circuits in the 10 m realm.

"So this was the rig:
120 amp standard alternator; Redarc smart isolator; 8 m of 6 B&S; 2 x 110 Ah AGMs. When discharged to 11.8 v the system could put 13.7 v and 19 amps into the batteries."

Depends on the batteries and the cabling, I would not use 6B&S to charge batteries in a camper but many do.

11.8V is around 20%SOC, a 100Ah Marine pro discharged to 40% SOC will initially draw over 60A, 30 minutes later it will be 27.5A@13.47V, an hour later 17.5A@13.77 volts and that's being charged via around 6M of 10mm2 cable.

Again it all depends on your setup, if for example you have 2X100Ah Haze gel batteries putting a DCDC charger bigger than 20A is pointless as they just won't accept that charge rate.

If we go the other extreme and put in LifePo4 then you'll have issues trying to keep the charge current under safe level for the cabling and alternator.