Dual battery setup for 2.8 1GD

[dBC]

Inspired by the lack of huge amps in my earlier jumper lead experiment (posted above) I got to wondering whether I could knock something up (that was safer than jumper leads) in order to take it for a drive and see how the alternator would go at charging the camping battery. A quick rumble in the parts bin got me there with: a 40A in-line blade fuse, 1m of 6mm cable, a 70A relay and a switch for controlling the relay. Not exactly suitable for high charging currents, but given the biggest I saw with the jumper leads was 35A it was worth a shot. Unsurprisingly, I've introduced a bigger voltage drop than with the jumper leads... almost 0.3V at 15A, but otherwise the pattern was very similar as with the jumper leads: an initial in-rush of about 35A dropping down to about 15A in less than a minute. What was unknown with the earlier jumper lead experiment is what happens after two minutes, because that was as long as I was prepared to stand there holding unfused jumper leads. With the new set-up and the need to head down south, I was all set to log how well the alternator could do at charging the camping battery.

I'd previously captured a similar charge cycle but with the DC/DC charger, which I've included below for comparison. In both cases the camping battery started the charge cycle from as similar state as I could manage. One was reading 12.22V and the other 12.21V, each after an hour's rest with about 20mA being drawn during the hour. The graphs pretty much speak for themselves. The current into the camping battery during the alternator charge continues to decay away. You can see that the voltage drop (the gap between the pink line and the blue line - caused by my inadequate wiring) reduces as the current drops away. There's no doubt that a properly installed heavy duty VSR will come close to eliminating that drop, but the best you could hope for is that the pink line would sit on top of the blue line giving you an extra 0.3V, which will bring the current up a bit, but nowhere near the 24A being supplied by the charger during that time - as indicated by the pink voltage line in the DC/DC charger plot - it pretty much instantly gets well into the 14Vs in order to maintain the 24A, something you'll never see out of a 2.8L alternator/regulator (unless someone's come up with a tweak?).

The final graph compares the AHs pumped into the battery under each charging method. While it doesn't look too alarming displayed like that, rotate it 90 degrees and you get a feel for how much better the charger is. For example it takes the alternator twice as long to pump in 10AH (1h5m Vs 27m). Both approaches were still adding charge as I arrived at each of my destinations and shut down. It's tempting to equate that decaying current as a sign the battery is getting full, and that's generally true when the voltage is high enough but in this case the battery is a long way from full. The decaying current is really indicating you don't have enough voltage to charge the battery quickly (not helped by the voltage drop in my wiring, but primarily due to the alternator voltage setting).

I'm not sure how big a discharge I'd need to do before the alternator's big-amp advantage kicked in for an extended period, but deeper than I'm prepared to go for the purposes of experimentation, and deeper than I ever go when camping. The car pretty much gets used every day when I'm camping, and I think this test shows I'd need to use it quite a bit longer if I were trying to replenish the battery from the alternator. At any rate, my 6mm experiment wiring wouldn't be up to the job so I suspect this is about as much comparison as I can contribute. A fairly simple test for anyone with a VSR setup in a 2.8L would be to put a clamp ammeter around the feed to the camping battery after you've started up. Unless you've got a very depleted battery, or you've found a way to tweak more volts out of the alternator, I think you'll be lucky to see more than about 15A going in, except maybe for the first minute.

[brogers]

I was working on the car on the weekend so I thought I would post a photo of the second battery spot for the new 2.8 L diesel, as I don't think I have seen one posted yet (?).

I don't know what was there for the old 3L diesels, but as far as I can tell (and others have commented) the factory installed battery tray looks the same design to the starter battery tray.

I couldn't be arsed removing the starting battery, but you can see it is the same spot welded tray, and the 2nd battery spot has the tie-down etc built in.

[ATTACH=CONFIG]24354[/ATTACH][ATTACH=CONFIG]24355[/ATTACH]

This could be a long story...that has been covered many times in the past in various posts...so I'll keep it as short as I can, and hopefully still useful for you and those new to the 2.8 Prado.

In Europe the Prado comes with two start batteries (something to do with cold climate cranking etc). In Australia we have only the one battery standard, and what appears to be an ideal spot for a second battery...but putting a heavier AGM type battery there can be problematic.

I had a 3L Prado (and now have a 2.8 GD Prado) and the difference around the vacant "second battery" location between the two models is that the older 3L prado had an aircon pipe that would have to be moved if you put in a second battery ...(ever so carefully else it could fracture costing lost of money to repair).. whereas the newer Prado has the aircon pipe already located away from contact with a second battery should someone choose to install one ..as you can see in your second image the aircon pipe passes beside and battery tray area...so the location in your second photo is just begging for a second battery to be placed there...and some people have already done so.

However, the issue with the 3L prado with placing a second battery under the bonnet (especially if the battery was anything much larger than a small factory cranking battery) was that some folk ended up with the weight of the battery and movement of the second battery causing the inside of the wheel arch to crack, the call it the inner guard..which proved to be a tricky repair for some. Some people didn't even know they had cracked the guard until it was highlighted on this forum...and the crack would continue to grow.

I think the Prado was designed to handle two smaller "factory" sized start batteries on smooth bitumen roads in Japan, but if you put in an AGM or deep cycle type battery and then use the car off road you run the risk of cracking the inner guard. It looks like they have beefed up the inner guard area somewhat in the new 2.8GD Prado, but I decided not to take the chance of cracking metal with a second battery under the bonnet in my new Prado, and I have placed my large AGM in the rear cargo area...and I promised my mechanic/diesel fitter that I wouldn't put a heavy AGM battery under the bonnet (as he had seen and heard of too many cracked guards.)

...its a shame, as its potentially a useful location under the bonnet begging for a second battery. Some folk have installed a second battery in that area, and if you do, it seems useful to add some strength and support... so some people use a propriety or custom metal battery tray with an additional brace that goes from the battery hold down clamp to the front upper cross member...which stops the battery rocking back and forth and moving about which contributes to the flexing and fracturing of the metal under the battery in the wheel arch area.

...anyway...that's my understanding...there are a few options and a few opinions.

Out of interest, a few of us with 3L prados actually swapped the factory start battery and placed in on the driver side, and placed an AGM or larger deep cycle battery in the spot where the factory battery was...that way you didn't have to move the aircon pipe etc very much...just a CM or so very carfully....and Derek at ABR Sidewinder even produced a second battery wiring harness/kit for that very purpose. I've decided not to move my start battery this time, I've preferred to try and keep things where they belong else mechanics who don't know your car get confused when they work under the bonnet...I have a 33kg Deep cycle AGM battery in the rear cargo area incorporated in a custom drawer system I have made..and it works a treat. Food for thought anyway...and sorry my reply wasn't as short as it should have been.

[LeadWings]

I was working on the car on the weekend so I thought I would post a photo of the second battery spot for the new 2.8 L diesel, as I don't think I have seen one posted yet (?).

I don't know what was there for the old 3L diesels, but as far as I can tell (and others have commented) the factory installed battery tray looks the same design to the starter battery tray.

I couldn't be arsed removing the starting battery, but you can see it is the same spot welded tray, and the 2nd battery spot has the tie-down etc built in.

[120D4D]

Hi Michael, for now, can you hold off with charging either battery just yet and please keep monitoring your cranking battery voltage.

To be able to see if there is a real advantage to the way my isolator might work in your Pardo, can you leave your batteries in the state they are in from just driving.

Will do, I don't have an AC charger handy anyway!

Reference photo Tim:

[drivesafe]

Hi Michael, for now, can you hold off with charging either battery just yet and please keep monitoring your cranking battery voltage.

To be able to see if there is a real advantage to the way my isolator might work in your Pardo, can you leave your batteries in the state they are in from just driving.

[dBC]

For those playing at home, last 7 days cranking battery voltage before being started in the morning:
12.25
12.23
12.30
12.43
12.36
12.32

DC charger disconnected after second day but made no real difference...

If you've ruled out any significant current draw overnight, have you tried putting the battery on a 3-stage charger? If the highest voltage it's ever seen is ~13.8V from the alternator, it might just be in need of a good absorption charge, followed by a good float charge.

[dBC]

I had a chance to do some more experiments with the logger. If you're the sort of camper who manages to run the engine each day for at least a short duration, either as you put the boat in, duck to the shops, or just start the engine to give the battery a bit of a tickle while other campers aren't watching, I was curious to see how a charger would compare with a VSR during that initial couple of minutes of charging. Before each experiment I got the camping battery to roughly the same SOC, a little over 60% by rough estimate. One experiment started with the camping battery at 12.23V and the other at 12.24V, each after an hour's rest with a very light load on (just a few tens of mAs).

I simulated a VSR with a set of jumper leads. The good news is I finally captured some semi-decent voltages out of my alternator.... 13.8V for an extended period. The bad news is I lost a good 0.4V across the jumper leads at peak current (34.54A) although that settled down to about 0.2V drop as the current dropped away. A well-installed heavy duty VSR setup can probably do quite a bit better than that, so my simulation is at best rough. I also fat-fingered my initial connecting of the leads, so you'll see it drops to 0A for 1 second as I get a better purchase. The logger was set to take 4 samples per second, and the interruption took exactly 1 second (4 readings).

The first two graphs show the first few minutes of charging with each (don't be deceived by the different scales on each). The third compares the energy pumped into the battery over the first two minutes (I shifted them on the timebase so that they both counted the start of current flowing as 0:00:00).

[drivesafe]

Thanks Michael, and I have just read it.

Cheers, Tim.

[120D4D]

I am most interested and actually sent you a PM friday. Let me know if you dont have it.

[drivesafe]

OK Michael, would you like to “experiment” with an isolator to see if will not only charge your auxiliary battery, but will help to charge and maintain your cranking battery in a better state than it is now?

You will need to completely bypass the DC/DC device,but beyond that, it should be a straightforward temporary installation.

If you are interested, there is no cost for the isolator, just keep the feedback coming!

[120D4D]

For those playing at home, last 7 days cranking battery voltage before being started in the morning:
12.25
12.23
12.30
12.43
12.36
12.32

DC charger disconnected after second day but made no real difference...

[dBC]

Your graphs are much better annotated than mine - was it done in the supplied software or different software (excel?)?

I'm more a Unix man than a Windows man. Fortunately, some kind soul has posted some open source code that reads the data from the device and outputs it as CSV string here: https://github.com/rsithron/powerup. From there you can suck it into your favourite graphing software which for me is gnumeric and gnuplot (those graphs were done with gnumeric). I had to make some minor tweaks to the makefile and one of the includes in hid.c to get it to build on Ubuntu. There's still one 2s-complement bug with the mAH field that I haven't tracked down yet, mainly because it's so easy to fix it in the mix with gnumeric, but I will eventually fix it properly and submit the patch back via github.

For kicks I put the device on a calibrator to test out their 0.5% accuracy claim on voltage. I supplied a certified 13.690V (with calibration certificates traceable back to god) to both the t-plug and one of the multiple-voltage input pins. The readings were 13.64V and 13.724V respectively, so my t-plug (camping voltage) is under-reading by about 0.3% and my re-purposed voltage input pin (cranking voltage) is over-reading by about 0.25%, but both well within their 0.5% claim. When I get a chance I'll do the same with their current measurement, where they only claim 5% accuracy, but over an extremely wide dynamic range.

I was playing around with the cable from my towball to the camper. I rigged up a larger wire (existing is I am guessing 10 AWG, rigged up am 8 AWG), and as suspected the charging (with no Dc-Dc) went up from 5A to 8-9A. So a larger cable (6 AWG) from the tow ball to the camper battery is this weekends job.

Yes, when you're starting with 14.2 or 14.4V up front a bit of voltage drop is not the end of the world, but when your source voltage is so low (13.6V in my case) every mV of voltage drop hurts big time if you're using it to charge a battery directly.

However what was noticeable when I did the test (car sitting in driveway idling) as that the cranking battery voltage was 13.9V. I noticed that your trace has a very level 13.5 - 13.6 V. When driving I have seen it vary from 13.4 to 13.8V. Wish I knew how it works out what voltage to put out and could influence it......

Yes, that 13.4 to 13.8V is pretty much the outer limits of what I see too. I think I've only ever seen 13.8V very briefly soon after starting up, and only once or twice (that I noticed). I've some anecdotal evidence that it could be temperature related. I was seeing lower voltages on a long roadtrip to fairly warm FNQ a while back, and yesterday when I was capturing that recharge cycle, I stopped a few times, left the engine idling while I popped the bonnet to check the LCD screen and that seemed to bring the cranking voltage higher by 0.1 or so (just judging by my in-cabin voltmeter). I guess it makes sense that they may be deliberately being gentler on the battery as things warm up. Another possibility is raised in reply #218 and #219 above. 120D4D is seeing even lower resting voltages than you, and an impressive 14V when he starts up. So it may be that it checks the voltage before starting the alternator, and sets the initial output voltage accordingly (higher when the battery is lower). That theory would be a fairly easy one to test.

drivesafe has said his 2.8L customers are seeing higher voltages than I am, but I don't think he's specified how much higher.

I think without a way to trick my alternator into outputting more than 13.6V that recharge cycle would have taken a lot longer than 1h7m with an alternator/VSR solution. For the bulk of that charging cycle, the charger's sole objective is to pump 25A into something, and within 30 seconds of starting, the impedance of the camping battery has come up enough that the only way it can do that is to let the voltage increase well past 13.6V. The alternator+regulator's sole objective is to output 13.6V and while it's capable of sourcing a ridiculously high number of amps (~100A?) to do that, it's clear my camping battery isn't going to be drawing 25A at 13.6V. So the alternator/VSR's big-amp advantage is really all over 30 seconds after starting, although I concede the picture would be quite different after a much deeper discharge. I suspect that's what you're seeing with your Lithium batteries as well, but even more so due to your long cable run down to the camper trailer losing you more of those precious volts.

re cranking battery voltage - my voltage at rest seems to be around 12.4 V. I have just come back from a country trip (500 km driving each way) so the battery has had a good long charge, and still 12.4V. I see your resting voltage is around 12.8V, which is what I would expect for a fully charged cranking battery. Anyone else see around 12.4 V as the resting voltage of their cranking battery?

Yes, 120D4D does, and even lower. If you look at those graphs above, my resting voltage drops pretty much immediately to 12.8V once I shutdown, but by morning time it had decayed further down to 12.6V, but yes, I think that does indicate my cranking battery is fully charged by the time I shutdown and there's not much drawing on it overnight, so it's pretty much still fully charged by morning.

[LeadWings]

dBC, glad you like the meter. Definitely gives the ability to understand exactly what is going on, rather than guessing. Your graphs are much better annotated than mine - was it done in the supplied software or different software (excel?)?

I was playing around with the cable from my towball to the camper. I rigged up a larger wire (existing is I am guessing 10 AWG, rigged up am 8 AWG), and as suspected the charging (with no Dc-Dc) went up from 5A to 8-9A. So a larger cable (6 AWG) from the tow ball to the camper battery is this weekends job. I will probably still put in a DC-DC, but I figure as this will be pulling 30A a larger cable is warranted regardless.

However what was noticeable when I did the test (car sitting in driveway idling) as that the cranking battery voltage was 13.9V. I noticed that your trace has a very level 13.5 - 13.6 V. When driving I have seen it vary from 13.4 to 13.8V. Wish I knew how it works out what voltage to put out and could influence it......

re cranking battery voltage - my voltage at rest seems to be around 12.4 V. I have just come back from a country trip (500 km driving each way) so the battery has had a good long charge, and still 12.4V. I see your resting voltage is around 12.8V, which is what I would expect for a fully charged cranking battery. Anyone else see around 12.4 V as the resting voltage of their cranking battery?

[dBC]

I finally got to play around with my recently purchased PowerLog 6S. Thanks for the tip LeadWings, it really is a great bit of kit at a very reasonable price given all it does. Because I don't have any Lithium batteries with per-cell voltage monitoring requirements, I've re-purposed one of those 6 voltage inputs to measure the cranking battery voltage. My setup is a 2.8L Kakadu with a 25A DC/DC charger, an 82AH sealed maintenance free calcium/calcium marine battery as the camping battery, and the stock standard Toyota issued cranking battery.

I captured a typical (for me) overnight camp with the fridge and a fluro light being the main two loads. With the default 2 second sampling interval, you really can pick up a lot of detail. All up I drew out 19.3AH which I replenished with about 1h7m of driving (one short 7m drive followed by ~1hr drive soon after). Some noticeable features include:

. small dips in the cranking voltage (Blue trace) are door openings
. large dips in the same (in the morning) are the starter motor cranking
. you can even see the fridge light come on in the camping current (Yellow trace) around 19:00
. you can also see the fluro lights-out just after 20:30
. and in the morning you can see the phone start to charge just after 7:15

The second graph zooms in on the charging. Noticeable features there are:

. almost immediately after starting, the camping voltage exceeds the cranking voltage (which never exceeds ~13.6V)
. the fridge takes a bite out of the available charging current just after 9;15
. the charger appears to do regular brief back-offs, presumably to suss out the state of the battery

Finally there's an energy balance graph, with the steps representing the fridge cycling, and the steeper decline earlier in the evening being caused by the fluro light.

EDIT: Added a zoomed in pic of the first engine start for the morning. It shows the camping voltage exceeds the cranking voltage (i.e. the charger voltage exceeds the alternator voltage) just 28 seconds after starting up.

[dBC]

How does your DC/DC charger determine when the engine is running? Is it via a signal that is only live when the ignition is on, or does it do a voltage-sense on a signal that is always live? A mate of mine went with the latter, and reckons he was slowly draining his cranking battery through that signal, but I don't know the details of his charger. It might be worth ruling that out in your case though. I'm working on posting some graphs from my recently purchased logger, and you'll see in them that overnight my standard Toyota issued cranking battery decays from about 12.8V down to about 12.6V so I figure it's well and truly fully charged at the end of a drive.

"it rose to 13.95-14.00 within 15 seconds and stayed there for the minute "

That's on a 2.8L right? I can honestly say I've never seen voltages anywhere near that high out of my alternator, but then I don't think I've ever even slightly discharged my cranking battery. You've got me wondering whether it does a quick voltage-sense before starting up, and uses that to determine how high to set the alternator output voltage.