Dual battery setup for 2.8 1GD

[LeadWings Member Find all posts View Profile Close LeadWings]

Is the low state of charge (2-3Ahr/hr) due to lithium batteries maintaining a near constant voltage while it is being discharged?

Amts, I think you mean "Is the low rate of charge (2-3Ahr/hr) due to lithium batteries maintaining a near constant voltage while it is being charged?" My answer would be I would say not - maintaining a constant voltage when charging would only help you, as opposed to the voltage going up when charging which would give a lower dV between the source and the battery.

The amps to the battery will be a complicated function of the battery voltage, the alternator voltage (also may be a function of amps the alternator is trying to put out, but I suspect the Prado has an alternator capable of 100A so would be negligible in my case) and the voltage drop between the two (also a function of charge current). Lithiums are pretty good at soaking up charge, and these batteries could be charged at up to say 50A no problem.

Looking at the plots the battery terminals were at about 13.2V before charge, that then rose to 13.4V on charge on the trip out and 13.1V rising to 13.3V on charge on the way back. The Prado was putting out 13.7 to 13.8V, so that is only 0.3V to 0.4V difference to push the current along and into the battery. If the Prado could be coaxed to putting out 14.2V, then the greater difference in voltage would result in more amps being pushed to the battery. Maybe double, maybe more. I would not want more than 14.2V to 14.4V, as the higher voltages could overcharge and damage the Lithiums.

Pity is my old tug (VW Passat) put out 14.2V, and I ran a test when towing, but I had not set the logger up properly and it did not record the file. Would have been interesting to see what that setup resulted in.

If anyone in Perth has an older Prado (or any other car) with a boost diode so it outs out 14.2V or above and an Anderson on the hitch we could run a quick test to compare!

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Yes I did mean that

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

So then it may be a DC to DC.

It certainly looks more like the way you may have to go.

[dBC Member Find all posts View Profile Close dBC]

Must say I've been impressed with its functionality, and the insights it can give. Was about $65 delivered from the US, came in 2 weeks. http://www.progressiverc.com/powerlog-6s.html

Thanks. At that price it seems a very useful piece of kit for this sort of analysis. I'll be ordering one and posting some proper graphs, but in the meantime, a while back I had a crack at describing how my charger behaves just by observing the two cabin voltmeters after a shallow-ish discharge (~18 hr overnight stop with fridge and lights) here: http://www.pradopoint.com/showthread...l=1#post553712

The interesting thing for me is how quickly the camping battery voltage exceeds the cranking battery/alternator voltage.... typically just a few minutes, maybe 5 mins max. What that's telling me is that when the camping battery gets to that (unknown*) level of charge, its impedance has come up enough that the only way you can continue to pump 25A into it, is to have the voltage higher than 13.6V. If the alternator were feeding it directly, it could dump massive amounts of amps in and get it to that level of charge much sooner, maybe 1 or 2 minutes instead of 5 but then what? Because the alternator can't increase it's voltage to overcome the increasing battery impedance the current starts dropping away... clearly less than 25A, but how much less I don't know.

For recovering from shallow discharges, I think the VSR solution leaps out of the blocks and jumps ahead by a few minutes, but then hits a wall (limited by 13.6V). I have noticed when recharging after deeper discharges, the camping battery gets stuck on 13.4V for quite a bit longer (limited by the 25A) and that's where the VSR solution really gets well ahead.

(*) it's unknown because you can't deduce anything from the voltage while you're still pumping amps in. But we do know it was a long way from full, by the fact that the charger continued to pump 25 amps in for a long time after that.

[dBC Member Find all posts View Profile Close dBC]

Some further tests suggests that crossover point for me is about 70% SOC, at least for my SMF Calcium/Calcium Marine battery. 70% and above the 25A charger pumps more amps into the camping battery than the alternator can, below 70% the alternator wins out. With the battery at 70% SOC as indicated by the open-circuit voltage after 24 hours isolation compared to the manufacturer's chart, and the charger disabled (output fuse pulled), jumper leads from the cranking battery (13.61V with engine running) to the camping battery were only putting in about 18A at 13.5V (a 0.11V drop across the jump leads) and the current was dropping fairly rapidly as the camping battery climbed towards 13.61V. Meanwhile the charger is happy to pump 25A in all the way up to 15.2V.

You could probably do better with a better choice of battery, but my 2.8L charging voltage is really not up to the job of getting that last 30% into that battery type in a timely fashion. If you regularly discharge well below the 70% mark, than the alternator may still win out overall.

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

Hi Michael, I might just take you up on that offer, but in the meantime, try measuring your CRANKING battery voltage in the morning, before you start your motor.

NOTE, the table I posted up is idea for this sort of testing, as the cranking battery in almost every modern vehicle will have a constant load on it, all be it a VERY small load, but it makes the voltages compered to the table, pretty accurate.

Sorry I missed this and will do in the morning. That said in the past I recall checking and seeing around 12.4(ish) in the cranker which I thought seemed low.

A disclaimer in the meantime - on the weekend I did a run from Melb to Adel (over 2 days) so would sunday's 4 odd hours at highway speeds impact the answer Tim?

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi again Michael, and the fact that you have done some long distance driving will make the measurements more interesting.

If you can take a measurement in the morning and then after a week or so of town driving, take another measurement and see what you get.

I have a new range of isolators and the offer to test one in your vehicle is still on if you want to see if it makes a difference.

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

O.k. Surprisingly I saw 12.25v this morning, it rose to 13.95-14.00 within 15 seconds and stayed there for the minute before I had to head-off.

The only accessories hanging of this battery are brake controller, driving light harness, DC-DC charger.

(DC-DC starts on Bulk but moves through to Float mode within 60 seconds and I saw no voltage fluctuation during this time at all, probably because the Optima Aux was already reading 13.05v before starting it, so no real current draw needed.)

I will report again in a week.

[dBC Member Find all posts View Profile Close 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.

[dBC Member Find all posts View Profile Close 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.

[LeadWings Member Find all posts View Profile Close 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 Member Find all posts View Profile Close 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.

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

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

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