Dual Battery Electrical setup options

[rpn Junior Member Find all posts View Profile Close rpn]

And I am not meaning your alternator will charge your battery a few minutes quicker than a DC/DC device can. Your alternator could recharge a Lead Crystal battery in around half the time that DC/DC device could do it and an Optima Yellowtop could be charged in less than 1/3 the time your DC/DC device could charge it.

Whereas, when using a solenoid isolator, your alternator will automatically increase it’s current output to cover the fridge, and will not reduce the charging current going to your battery while the fridge is running. This means no additional during time is needed and you will easily recharge most batteries in a shorter drive time than your DC/DC device could hope to do it in.

It would be interesting to do some tests but the 2.8 manual states that the charging system operates at either at 5A or 15A max in quick charge mode.

[120D4D Avid PP Poster! Find all posts View Profile Close 120D4D]

What do you mean by sharing charge? I have a yellowtop with a vsr and mine sits at 12.6 resting voltage. The optima spec for yellow top says it should be 13.1v - cannot recall ever seeing that

I have a Traxide Isolator (SC80) that allows me to run it like a traditional isolator or shared. Basically the batts stay connected down to 12.0v and then they isolate. Lets me share the load across both batts and works a treat. The prado cranking battery is a hybrid and has a 66ah capacity so I can use some of that capacity.

It would be interesting to do some tests but the 2.8 manual states that the charging system operates at either at 5A or 15A max in quick charge mode.

I wondered about that comment in the book too. I have done some tests at home and can say that my prado charges the aux batt MUCH higher than 15A. It smashes my 20amp DCDC...

[dBC Member Find all posts View Profile Close dBC]

I wondered about that comment in the book too.

I'm not sure anyone has worked it out. The alternator clearly pumps out more than 15A and I can't see any sign of current sensing into the cranker. I can clamp my portable compressor clips straight onto the cranker terminals, switch it on and the volts don't sag at all, meaning the alternator is happy to pump an extra ~17A or so into the battery (it's not actually going into the battery but to the alternator it would look like it is).

It's interesting that it claims to have two speeds though (5A and 15A). I wonder if that's related to the different charging voltages some of us see.

I have done some tests at home and can say that my prado charges the aux batt MUCH higher than 15A. It smashes my 20amp DCDC...

Do you have the capability to measure the charging current going into your aux battery? I'd be interested to know how long it can maintain more than 25A going in. Except when it's seriously depleted, my wet cell camping battery won't accept anything like 25A at 13.5V for even a minute. Your Optima has a reputation for being able to accept high charge currents at low voltages, but it's hard to actually get any measured data on that. Although it's somewhat complicated by the fact that you see a whole lot better than 13.5V.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

The charging system in the 2.8Ltr uses an ECU controlled alternator as you are aware, this alternator is indeed connected to the engine ECU. On referring to the maintenance manual it also refers to error codes available from the engine ECU for fault rectification.

However, the alternator behaves the same as the earlier models, I would hazard a guess and say that although the alternator is connected to the engine ECU and could be controlled by it this does not seem to be the case. The alternator also has a fall back mode whereby it will default to regular charging if it looses communications with the ECU. It appears to me as there is no current sensor that Toyota have made provision for ECU control but in at least the Australian version are not as yet using it.

[rpn Junior Member Find all posts View Profile Close rpn]

On the other hand, DC/DC device are renowned for over charging batteries, particularly when fridge is being powered from the battery being charge by a DC/DC device.

Presumably this would depend on the current being drawn by the fridge etc whilst the engine is running? Assuming a fully charged battery, the DC-DC would be in float mode and would need more than 3.8A (or whatever the trip point is) to trigger a switch to the higher voltage charge mode?

From the DC-DC specs;

Charge Type 3 Stage Bulk *20A at 9-11Vdc, 25A at 11-32Vdc *Constant Current up to:
GEL AGM WET CALCIUM
14.1V 14.4V 14.7V 15.4V

Absorption Constant Voltage until current drop to 3.8A
GEL AGM WET CALCIUM
14.1V 14.4V 14.7V 15.4V

Float Keep Constant Voltage at 13.7V

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Generally it will take less than an amp for the charger to revert to bulk charging, it will vary depending on the manufacturer.

[rpn Junior Member Find all posts View Profile Close rpn]

Generally it will take less than an amp for the charger to revert to bulk charging, it will very depending on the manufacturer.

ok, thanks

[dBC Member Find all posts View Profile Close dBC]

Generally it will take less than an amp for the charger to revert to bulk charging, it will vary depending on the manufacturer.

My Redarc BCDC1225-LV doesn't appear to do that at all. Once it goes into float mode it stays in float mode until you switch the engine off. I know it's gone into float mode in the cabin by seeing the camping voltage drop to 13.2V, and I can confirm that with the LED readout if I pop the bonnet. It'll will maintain 13.2V and source whatever current is required... I often run a ~7A electric kettle and a ~5A fridge, all from the camping battery outlets while driving, The camping battery stays at 13.2V no matter how hard I load it up.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Seems Redarc have made some changes since I last looked at their manuals, I generally steer clear of their gear these days so not up to date it seems:

From the 25A charger manual:

"Float stage maintains 13.3V on the output battery, keeping the battery topped up.
This counteracts the battery's self discharging or loads applied to the battery."

From the above it appears their charger will only revert to boost charging after the battery terminal voltage falls below 13.3V.

Most of the low end chargers simply revert to bulk charge as soon as any load is applied unless they have a "supply" setting. Some higher spec chargers allow you to specify when the charger will return to charge mode.

"The camping battery stays at 13.2V no matter how hard I load it up." As long as you draw less than the charger can supply that is.

[dBC Member Find all posts View Profile Close dBC]

"The camping battery stays at 13.2V no matter how hard I load it up." As long as you draw less than the charger can supply that is.

Yes, true. I don't have anything like 25A worth of loads. I guess I could get close if connected the compressor to it, but I always run that off the cranker with the engine running.

[120D4D Avid PP Poster! Find all posts View Profile Close 120D4D]

I often run a ~7A electric kettle and a ~5A fridge, all from the camping battery outlets while driving

What are you doing boiling a kettle on the move?!

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

It's ok, a 7A kettle will take 91.7 days to boil 1L of water.

[dBC Member Find all posts View Profile Close dBC]

It's ok, a 7A kettle will take 91.7 days to boil 1L of water.

Think you could be out by several orders of magnitude there. Although I just rechecked... at 13.2v it actually draws 8A not 7A, so it's about 105 watts at the float voltage. It only holds 1/2 litre and if I set it up at one rest stop it's usually good to go for a nice cuppa' tea or two at the next rest stop, or the one after if they're really closely located.

I think the physics says it should take about 24 mins to heat it from 25C to 100C.

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

28 mins at 100% efficiency but who is counting?

[dBC Member Find all posts View Profile Close dBC]

28 mins at 100% efficiency but who is counting?

Sounds about right.... certainly a lot closer to 30 minutes than 3 months. It's basically a big resistor submerged in water so there's not a lot of opportunity for inefficiencies. Inefficiencies typically mean heat, but heat is the desired outcome in this case. There'll be a little power lost in the cabling and connectors, but it's negligible. The PowerPoles are good for 30A and the cabling is pretty hefty. Nothing feels like it's getting warm... except for the water. The thermal insulation isn't the greatest, so it loses a bit there too.