Dual battery setup for 2.8 1GD

[LeighW]

While I agree with what you wrote I think the above highlighted needs a bit of a correction.

The alternator will only go to a voltage that it can reach with its maximum current and the load resistance characteristics. It will function much the same as a DCDC charger with a current rating the same as the alternator.

True, the voltage is likely to be higher than that which you describe for the DCDC device because it most likely has a higher current capability than the DCDC unit, but it is still a current limited supply. If it could supply an infinite current yes it would go instantly to 14.4V or whatever its maximum setpoint voltage is. (I note that you did state "to around 14.4V") As it is not an infinite current device the voltage will settle somewhere between its maximum setpoint voltage and the battery voltage as determined by the total circuit resistance.

If the batteries are very flat and there is appropriate sized cabling between them the alternator voltage would be very close to that of the battery determined by the batteries internal resistance at that state of charge.

Cheers
Prone

An alternator acts as a constant voltage source, it will maintain its "float" voltage till such time that it is unable to, ie overloaded, note the rated voltage at maximum output will be lower than the designed float voltage ie around 13.5V. Interestingly if overloaded the alternators current output tends to rise quicker than the voltage drops due to the negative temperature coefficients of the rectifiers and windings.

The prado's alternator has sufficient reserve current to force a 100Ah battery that has been discharged to around 50% SOC to its float voltage with seconds of starting the car, if you have more than one aux it will take a little longer depending on how you have configured your wiring.

In my setup I have a 100Ah aux in car, and two 100Ah batteries in the Vista, within 30 seconds or so of starting the car the cranking battery is at 14.5V, all batteries are interconnected with a double run of 6B&S cable, the Vista's batteries are in parallel configuration.

A typical battery will draw around 60 amps when first connected, this will drop to around 40 amps after 10 minutes or so and be around 30 amps after 25 minutes.

The Prado alternator has a reserve capacity of around 70 amps with all accessories turned off.

[LeighW]

So I wonder if it might just be best, in the new 2.8 GD Prado (given that at this stage we don't have access to a voltage booster diode for that vehicle) to use a VSR to do most of the aux battery charging, and use solar when stopped, and then just connect to 240 volts in a camp ground or when at home (something like a cteck 240v charger) to do an occasional "full charge" condition and charge to both main and aux battery..perhaps?

My issue with most VSR's is that they seem to be set to isolate the start battery as soon as you shut down the car engine, when in fact you could probably use say 10-20% of the start battery charge before it needs to be isolated..I think?

Thoughts dear gents?

One of drivesafe's isolators will allow you to do just that.

[maulbeagle]

Yes 12.7V is the voltage at which the SBI12 isolates the batteries.
Mine seems to have a bit of tolerance though, I think I needs to drop a bit lower than that (around 12.5V from memory) before it actually isolates.
After a decent drive, my main battery seems to hold this voltage for a fair while. I can't say how long, maybe i'll check when I get home from work (a 25 minute drive).

[brogers]

Thanks maulbeagle,
Can I just double check with folk then please, doesn't the SB112 isolate the main start battery when the main start battery hits 12.7 volts...(is that right?)...so does that mean that whilst the Sb112 doesn't isolate the start battery straight away when you turn off the engine (as I believe some VSRs do), it does however isolate the start battery pretty quickly after shutting down the engine, thus not allowing you to use some of its state of charge on say a fridge?....I really don't know, but I would have thought it would be slightly better if the start battery was isolated at a slightly lower voltage than 12.7v....no where near flat of course, but a bit lower than nearly fully charged?

Thoughts/comments please?

[Prone]

Hally,

Your DCDC charger does not start charging at 14.4V, it may only be 12.6V for instance if the battery is low and then gradually rise as the battery charges, it won't get to 14.4V till the battery reaches a SOC of around 80%. The alternator on the other hand will instantly go to around 14.4V then gradually taper down to 13.8V or there abouts as the engine bay warms.

While I agree with what you wrote I think the above highlighted needs a bit of a correction.

The alternator will only go to a voltage that it can reach with its maximum current and the load resistance characteristics. It will function much the same as a DCDC charger with a current rating the same as the alternator.

True, the voltage is likely to be higher than that which you describe for the DCDC device because it most likely has a higher current capability than the DCDC unit, but it is still a current limited supply. If it could supply an infinite current yes it would go instantly to 14.4V or whatever its maximum setpoint voltage is. (I note that you did state "to around 14.4V") As it is not an infinite current device the voltage will settle somewhere between its maximum setpoint voltage and the battery voltage as determined by the total circuit resistance.

If the batteries are very flat and there is appropriate sized cabling between them the alternator voltage would be very close to that of the battery determined by the batteries internal resistance at that state of charge.

Cheers
Prone

[maulbeagle]

I use the Redarc SBI12 and it definitely doesn't isolate the batteries when the engine is stopped. It waits until the voltage drops to the specified level, allowing both batteries to be used for a while.

[brogers]

So I wonder if it might just be best, in the new 2.8 GD Prado (given that at this stage we don't have access to a voltage booster diode for that vehicle) to use a VSR to do most of the aux battery charging, and use solar when stopped, and then just connect to 240 volts in a camp ground or when at home (something like a cteck 240v charger) to do an occasional "full charge" condition and charge to both main and aux battery..perhaps?

My issue with most VSR's is that they seem to be set to isolate the start battery as soon as you shut down the car engine, when in fact you could probably use say 10-20% of the start battery charge before it needs to be isolated..I think?

Thoughts dear gents?

[drivesafe]

Hi again Hally, and I think you need to do a bit of research as to what constitutes a fully charged battery.

Leigh has covered the charging possibilities pretty well, but you need to know the difference between a fully charged battery and a charging voltage.

A fully charged battery will have a SETTLED voltage of 12.7v. It is irrelevant as too whether the CHARGING voltage is 13.5v or 14.7v.

If the battery is fully charged, it is fully charged!

The one point of leigh’s post that I will reiterate is that while a low charging voltage will take longer to charge the battery from the 80% - 90% level to a fully charged state, the fact that an alternator will get a low battery to that 80% - 90% level MUCH SOONER than a DC/DC device can, means the low ( or higher ) voltage at the final charge stage is of little consequence to the total charge cycle.

[LeighW]

Hally,

You ran a fridge off one aux for a week?

"So yes the dc-dc will always charge at a higher rate thats what there supposed to do"

The fact that it is a 20A charger does not mean it will always be charging at 20A,
an alternator can supply a lot more than the 20 amps your DCDC charger is limited to
when the battery is low.

Even with a lower voltage than the DCDC chargers 14.4V MAX voltage the alternator may be able to replace the "bulk" charge quicker than the DCDC charger, the higher voltage only becomes beneficial when the battery gets above 80%-90% SOC. Your DCDC charger does not start charging at 14.4V, it may only be 12.6V for instance if the battery is low and then gradually rise as the battery charges, it won't get to 14.4V till the battery reaches a SOC of around 80%. The alternator on the other hand will instantly go to around 14.4V then gradually taper down to 13.8V or there abouts as the engine bay warms. If you only discharge your batteries to 80% SOC then the DCDC may be quicker, but if your like most and discharge them to 50% SOC or lower the alternator will replace the bulk charge quicker even at 13.7V.

In the case of a DCDC charger versus a standard alternator or or boosted one in an "in car"
situation the alternator will be much faster. In a trailer setup there won't be much
difference between the alternator and DCDC charger to "Full" charge and if you have more than one 100Ah battery the alternator again will be much faster than a 20A charger given adequate cabling is installed.

I know of quite a few who have removed DCDC chargers as they found the were getting
much better performance from their old VSR setups.

Depending on how you use the setup, even with the low charge voltage of the 2.8 some if not most would be better off sticking with a VSR setup.

[maulbeagle]

Here we go ...

So yes the dc-dc will always charge at a higher rate thats what there supposed to do

Grab the popcorn !

[Hally]

Hi Hally and sorry mate but there is no real info in your post, other than your battery was fully charged, that would indicate your DC/DC device did a better job than an alternator or if you had done the same thing without a DC/DC device, would your alternator also got your battery fully charged as well?

To get a proper comparison, first off you would need to know how low the battery was when you started your drive.

How long was your drive.

And then do the same thing with just your alternator as the charging source.

And even after a drive with an alternator voltage of 13.5v, if this is the voltage at your auxiliary battery, it to could indicate the battery is fully charged.

Ok then of course Batterys full when left (who would take flat batterys camping)

We were gone for 2 weeks
After a 1 week we changed spots and aux was at 11.9 trailer was 12.3 and start at 12.3 after 4 hr drive both aux and trailer were 14.4 when pulled in to next spot.
The alt was 13.5/13.7 while driving. Being that its a 20 amp dc-dc with both batterys down you can safely say it was pulling 20amp from start battery.
Of course if you used a normal relay then they would be full at 13.7 (because thats all the alt would put in, or would they because what about the amps going in? Remember volts can be a surface charge)

So yes the dc-dc will always charge at a higher rate thats what there supposed to do

[drivesafe]

Over Christmas we did a 1600k Hoilday towing a camper. The hole time driving volts did not go below 13.5 this was measured through my scan gauge.
I do have duals but i run a 20amp redarc dc-dc and all my auxiliary stuff pulls out of secondary ie hid spots, light bar on rhino rack, fridge, camper trailer wiring, redarc tow pro, work lights/rev. I have a volt meter on my secondary and it would charge to 14.5 obviously because of the dc-dc. IMO I think the inverter is the better way to go in this day in age they are small neat and easy to fit

Hi Hally and sorry mate but there is no real info in your post, other than your battery was fully charged, that would indicate your DC/DC device did a better job than an alternator or if you had done the same thing without a DC/DC device, would your alternator also got your battery fully charged as well?

To get a proper comparison, first off you would need to know how low the battery was when you started your drive.

How long was your drive.

And then do the same thing with just your alternator as the charging source.

And even after a drive with an alternator voltage of 13.5v, if this is the voltage at your auxiliary battery, it to could indicate the battery is fully charged.

[brogers]

Over Christmas we did a 1600k Hoilday towing a camper. The hole time driving volts did not go below 13.5 this was measured through my scan gauge.
I do have duals but i run a 20amp redarc dc-dc and all my auxiliary stuff pulls out of secondary ie hid spots, light bar on rhino rack, fridge, camper trailer wiring, redarc tow pro, work lights/rev. I have a volt meter on my secondary and it would charge to 14.5 obviously because of the dc-dc. IMO I think the inverter is the better way to go in this day in age they are small neat and easy to fit

Hi Hally...what do you mean " I think the inverter is the better way to go"...is that something different to a DC-DC charger?

[paddyb]

Fitted an InterVolt duel battery system and tested on a camping trip down south for a week. I had a Waeco CF50 in the back and a camper trailer hooked up with a 120ah AGM battery, solar charging when off the Prado. The battery I fitted is a Bosch DC31 S4, (330L x 172W x 234H, info for lski) no room to spare. I had one issue with the auxilary battery dropping below 12V when someone bumped the control on the Waeco above the 3 setting. Other than that, the voltage stayed above 12.5v, measured intermittently and keeping an eye on the InterVolt Alert voltage. Each day we went off on side trips, so this would have topped up the charge on auxilary battery. I deliberately hooked the Waeco up and dropped the auxilary battery voltage below 12V before we drove for 4 hours.I have kept the Intervolt settings on default, which are:
Combined Voltage 13.7
Isolate Voltage 12.8
Alert Voltage 12.0
Hope this info helps someone.

[InnerCityBoy]

I note you have the Century N70 as your aux batt which I believe is around 305mm long? Do you have much room left within the aux battery provision? Reason I ask is I'd like to fit my 120ah agm in there which comes in at around 330mm...

Don't have a measure handy but there are a couple of finger-widths left ... you would likely end up with your battery hard up against the radiator hose though.