Charging third battery via starter battery

Hi rufusking, and you can run current in both directions through a circuit breaker.

Next, the newer the vehicle, the more power the vehicle’s house keeping requires, and this not only applies to LRs, all new vehicles have this “problem”.

If you do a lot of free camping, there are a number of options available for keeping batteries charged.

As for your alternator/motor idea, they are already on the market but getting a small to medium sized inverter generator is a simpler way to go and offers additional uses.

I'm well aware of the unit. The reason I was leaning towards it was the availability of 55A and an option of 14.6V or 14.0, this coupled with the Optima's ability to accept a high charge rate.
I have yet to find an inverter generator that puts out double digit current on "12 Volt" plus I like to keep things simple without a separate charger. I'm more likely to find someone who can fix a stationary engine or alternator than an inverter. Less things to take, less things to go wrong. Bit of a contradiction having a D4.

Hi again rufusking, generator 12v outputs are no good for charging batteries, they are a 12v power supply and as such are not high enough to charge any battery properly.

Although you don’t seem to like the idea, but a small generator and a 60 amp battery charger would do what you wanted and, as mentioned above, be useful for other applications.

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Originally Posted by drivesafe

Hi again Aidan and you have done a lot of work there but it’s not going to give you the info you need.

The only way to get the relevant info is while the vehicle is on the move.

You need some form of data logger, to monitor both the voltage and current at the house battery.

Measuring voltage anywhere, while the motor is idling is not going to give you the operating info you need to make a decision about what will work the best.

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I know what you're saying, but for our purposes I think it will be enough to keep an eye on the voltage at the cigarette outlet (I know I know, not too accurate etc -- but we've established to our satisfaction that it appears to track the alternator voltage "near enough") under typical conditions.

Clearly charging the battery corresponds to another, sometimes significant, load. But in the end, a load is a load, and how it responds to one load should be roughly the same as another. The biggest problem -- we have no idea what the loads are! :thumbsup:

As long as the voltage is never much below 13.8 we should be sweet.

I'll report back after this weekend.

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Originally Posted by drivesafe

The D4 has a 180 amp alternator and you are not getting anywhere near the potential while the motor is idling because the D4’s own electrical requirements are anything up to 50 amps and at idle, you will have very little left for any accessories.

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Thanks for the explanation.

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Originally Posted by drivesafe

Aidan, you need operating info while on the move because the info achieved at idle does not reflect to true ability of any vehicle’s charging capability.

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I can see it now -- screaming down the Hume with me in the load bay taking alligator clips on and off halogen light bulbs! :eek:

(I'm not going to do that by the way)

Like I said, we'll see how far some sloppy observation gets us.

Thanks again for the help

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Originally Posted by Aidan

Like I said, we'll see how far some sloppy observation gets us.

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Hi Aidan, and they were not “sloppy” observations, nor did I intend to infer that they were.

The info you got will be a good starting point but as I posted earlier, sourcing the relevant info is not easy and most people are unaware of what is needed to see what is really happening with a charging system.

Your own observations showed that you were causing a voltage drop, while idling, just by having the A/C on, and this is the problem with taking measurements while the vehicle is idling. You are not seeing what the vehicle can really do.

There is a simple device, that I use regularly, to get basic info, to help evaluate a system, without the need to spend loads of time setting up gear to get more accurate data.

I have a string of Doc wattson Meters. Have a look at the link below.

Doc Wattson Meter - DC Inline

These store enough info for me to be able evaluate a systems potential without too much work.

This will give you an idea of how simple it is to set up to take some On-The-Move measurements using these devices and a few basic purpose made connectors.

I have about a dozen or so of these and while the manufacturer claims they can measure up to 100 amps, 25 amps is about as much as I like to put through them because the thin cable used on them induces a voltage drop in the set up.

If I need to measure higher currents I use two meters connected in parallel.

https://www.aulro.com/afvb/images/im...012/12/927.jpg

https://www.aulro.com/afvb/images/im...016/01/346.jpg

Sorry may seem silly, I have had a few drinks and may be missing the point here. Why not just get the father inlaw to increase revs to 2000rpm while in neutral. Wouldn't this be the same as driving up the Hume.

Hi bcl and you have not missed anything.

With just about any other vehicle, increasing the rpms will increase both the voltage and current output of the alternator.

The problem with increasing the D4’s rpms while the motor is NOT under load, is that the D4’s BMS may increase the current output but may not increase the voltage level.

Increasing the current available but not the voltage will give the wrong info as to whether you need to artificially step the voltage up to get a decent charge voltage for auxiliary/house batteries.

You will only get the actual operating voltage and current level available while the vehicle is on the move.

There are large numbers of people with my isolators in their D4s and they have no problems with charging and maintaining any number of auxiliary and house batteries, with out the need for any additional assistance.

This post form gghaggis gives you an idea of what it takes to get the higher voltage readings.

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Originally Posted by gghaggis

Mine and most of the cars I've installed dual battery systems into generally run quite a few accessories, often permanently wired. I can confirm that under those situations, the charging regime will easily see over 14V

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Originally Posted by drivesafe

Hi Aidan, and they were not “sloppy” observations, nor did I intend to infer that they were.

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I was referring to the "so what is the voltage now" observations we were planning to do on the camping trip.

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Originally Posted by drivesafe

There is a simple device, that I use regularly, to get basic info, to help evaluate a system, without the need to spend loads of time setting up gear to get more accurate data.

I have a string of Doc wattson Meters. Have a look at the link below.

Doc Wattson Meter - DC Inline

These store enough info for me to be able evaluate a systems potential without too much work.

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Thanks for the link. I see it can be used as a "fuel gague" for the battery too, which is nice.

Well first off, the sloppy observations from the weekend. On a 300K drive, including some fairly hilly sections the D4 alternator voltage never went above 13.8V, and more commonly showed 13.7V. Good news is that is also didn't sag when the aircon was turned on. We've decided we need not worry about alternator voltage sag as far as calculating voltage drop goes.

During the camping weekend the aux battery (optima yellow top 55Ah) got as low as 11.9V. When going for a drive at this SOC the alternator never went above 13.8V. That optima must have been pulling some current, but the alternator didn't increase the voltage beyond 13.8V, but did seem to be on the slightly higher value for most of the 4 hour drive home.

Get this. The next morning, pulling out of his driveway, alternator is at 14.1V! He has never seen it that high before.

I give up.

Could it be that the starting battery slowly discharged into the optima after you switched off (considering the low SOC ). On restarting the system sees the part discharged starting battery and increases voltage in response?

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Originally Posted by Aidan

During the camping weekend the aux battery (optima yellow top 55Ah) got as low as 11.9V. When going for a drive at this SOC the alternator never went above 13.8V. That optima must have been pulling some current, but the alternator didn't increase the voltage beyond 13.8V, but did seem to be on the slightly higher value for most of the 4 hour drive home.

Get this. The next morning, pulling out of his driveway, alternator is at 14.1V! He has never seen it that high before.

I give up.

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Hi Aidan, and I’m taking a guess here but when your father-in-law got home, he probably took a while to unpack the D4 and as such, had doors open and there would have been a fair bit of cranking battery capacity used while the doors were open.

As a result, the next morning the D4’s BMS, after monitoring the battery once the motor was stated, set the voltage at a higher level, that the BMS deemed would replace the used capacity as required.

If your father-in-law wants to continue his experiments and get even higher voltage readings, put a small load on the cranking battery while the motor is off, like a compressor fridge.

Leave the load on the battery till the voltage is around 12.2v and then go for a drive.

Don’t be surprised if the readings go up over 15v.

As Gordon posted, this lower cranking battery voltage when the D4’s motor is first started and the way the BMS works to fix the lower voltage by increasing the operating voltage, is the very reason my isolators get the best results for charging auxiliary/house batteries, without the need for any assistance from other devices.