The 150 series Dual Battery Guide

[mullerwh Member Find all posts View Profile Close mullerwh]

I went with with redtop D34 as a cracking battery...supposedly better for cranking. I run a yellowtop D34 as 2nd battery with an project isolator. Installed 2 years ago and no issues. Will be removing the isolator shortly and installing BCDC1225...just because I have spare as a left over from my trailer

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

Hi mullerwh, and mate, using a DC /DC device, even a 40 amp one, will actually take longer to recharge an low Optima than what your alternator can do.

In your case, with a 25 amp DC/DC device, you will actually take more than twice as long to recharge your D34 Optima if it is low.

Just leave your isolator in place, it will do a much better job.

[mullerwh Member Find all posts View Profile Close mullerwh]

Thanks for the reponse drivesafe. The reason why I'm looking at replacing the isolator is because it seems the Optima yellowtop battery is never fully charged. I rarely use the battery, I only run a fridge of it every few months but drive the vehicle on daily basis for about 30-40 minutes x 2 trips but the battery remains on 12.7v. I'm still trying to confirm if 12.7 is considered to be full. Another thing is the redtop stays 12.6v. I know the projecta has cut out of 12.8v. I'm running a voltage meter in the car onnected to both batteries and during winter time the alternator (without the fuse modification) sits at 13.6 / 13.7v when the engine is running and in my opinion this is sufficient for charging. I do have the fuse from leight but found during winter time that it pushes the voltage to 14.6 which I'm not totally comfortable with.

[Guest Close Guest]

If 12.7V is at rest, then it is pretty much 100%.
Charging at 14.6V is quite normal and not an issue https://www.optimabatteries.com/en-u.../charging-tips

My 120 V6 alternator is at 14.4V when driving.

[dBC Member Find all posts View Profile Close dBC]

If 12.7V is at rest, then it is pretty much 100%.

I found one datasheet that claims the fully-charged open circuit voltage for those should be 13.1V (http://www.federalbatteries.com.au/f..._YellowTop.pdf). But I guess it's possible that the small current drawn by mullerwh's voltmeter is dragging it down a little. mullerwh, do you get different results if you leave your voltmeter disconnected for a week or so, and then check it?

If it's never getting more than about 13.6V it may be that the cells have become unequal. A higher voltage charge will let you push past the cells that are full and get some charge into the ones that aren't (aka boost stage, or equalisation stage). The easiest solution might be to just put your diode back in. What does the yellow top read after a few weeks of running with the diode?

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

Hi again mullerwh, as amts posted, at 12.7v, your battery is well and truely fully charged.

The chart below will give you the SoC for any lead acid battery, while it’s in a vehicle.

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

I found one datasheet that claims the fully-charged open circuit voltage for those should be 13.1V

Hi dBC and open circuit voltage readings are irrelevant in automotive situations, where the battery is still in the vehicle.

For an open circuit voltage reading to be accurate, the battery must be in a No-Load/No-Charge state for at least 24 hours, before you can take the voltage reading.

So short of removing the battery from the vehicle and letting it rest for 24 hours, again, open circuit voltage readings are useless.

[dBC Member Find all posts View Profile Close dBC]

That might be an interesting experiment mullerwh, if you're up for it. Disconnect the voltmeter so there is zero-load on the battery, drive as normal for a week or so (no diode installed) so that you've got it as charged as you're ever likely to, and then disconnect the battery completely for 24 hours so it's neither being charged or discharged. Then see how close it gets to the 13.1V in the datasheet. My guess is it'll still read about 12.7V.

Those VSRs like to share the discharge for quite a bit after you shutdown, so to really test whether you're getting it fully charged you probably want to isolate it once you get home, leave it isolated for 24 hours and then check the voltage.

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

Arguments aside about DCDC vs Alternator charging I know that the last time I checked, my D27F will sit on on 13.1v open circuit after 24 hours.

Yes I run a BCDC1220 and yes I know its slower/less than ideal but that just what I have in my set-up, I also almost never have a heavily discharged Aux to start with though...

[dBC Member Find all posts View Profile Close dBC]

I know that the last time I checked, my D27F will sit on on 13.1v open circuit after 24 hours.

That sounds fully charged to me. Good luck getting it to that level of charge with a 13.4V alternator output.

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

Hi all, I hope your all going well. I've got a recharging question when using a 240V DC charger on a dual battery system.

I've recently got a Ctek MXS 15 amp charger to try resurrect the missus' car battery and top my Prado up whilst I'm at it. With the 2014 GX Prado running a SBi12 isolator based dual battery system (basic lead acid batteries x2), can/should I just connect my main battery to the charger like normal and leave on? Will it charge both main and aux properly?

I understand the isolator will stay on when voltage is above approx 13ish volts, but will the isolator confuse the Ctek, when analysing / float trickle charging? Using recondition mode on the charger will also try analyse two batteries will it not?

Any hints or suggestions are much appreciated!

[dBC Member Find all posts View Profile Close dBC]

If your goal is to charge both, then putting the charger on the cranking battery and letting the SBi12 do it's thing will most likely be ok. Most chargers have 3 basic stages:

1. constant current
2. constant voltage
3. trickle voltage

During 1, it'll put out its spec'd current (15A in your case) until the voltage reaches a setpoint (typically 14.something). As it passes through the low 13s, your SBi12 will close and that 15A will then be shared between both batteries. They'll both then climb towards the setpoint voltage together. When they get there, the charger will then maintain that voltage (stage 2), and each battery will continue to draw what it needs until fully charged. One will probably get there before the other in which case you'll be feeding it 14.something even after it's indicated it's full, but that's no different from what classic alternators do... output 14.4V all day long, and the battery needs to be able to survive that. Eventually they'll both be full and the total current will drop low enough that the charger will move to stage 3 whereby it'll just put out 13.2V forever. From memory my SBi12 didn't used to open until the voltage dropped down to about 12.6V, so both batteries will enjoy the trickle charge until you disconnect.

That's the theory ... there's a chance your charger may do something different so maybe keep an eye on it the first time you try it. If its setpoint is a lot higher than 14.4V then there's a risk you might be overcharging the battery that reaches full first. Some chargers let you set that setpoint by way of telling it the battery type (classic lead acid, AGM, calcium etc). If you're charging two in parallel it's probably safer to go with the lowest setpoint.

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

Hi Ghandi, just connect the charger to your cranking battery and leave it to do what it is designed to do.

As for the settings.

If both of your batteries are wet cell, calcium or otherwise, you can use the reconditioning mode.

As for it being “PROBABLY SAFER” to use the lower setting with AGMs.

If either or both batteries are AGM, under no circumstances should you use the RECONDITION Mode.

If you try to charge AGMs with any voltage over 14.7v, you will destroy an AGM type battery.

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

Thanks everyone, will just leave it on overnight every month or two, to give the batteries a good top up (don't have a booster diode yet to nicely charge both batteries.)

I would also like to recond the cranking battery as it came with the car when I bought it and unsure of its history. Looks like a basic cheap battery, maybe even original OEM?!

Recondition the battery is recommended every year or so with Ctek. Does that sound fair? Doing that and topping up every so often will extend the life and maximise efficency of both batteries?

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

Hi again Ghandi, try disconnecting the auxiliary battery, and you can do this by simply removing the NEGATIVE ( - ) terminal clap on the auxiliary battery.

Then connect the battery charge’s positive lead to the cranking battery’s POSITIVE ( + ) terminal.

Next, connect the battery charger’s negative lead to a good earth point in the engine bay.

Do not connect the battery charger’s negative lead to the cranking battery’s negative ( - ) terminal.

Even though the charger should be off, it is best to get into the habit of never connecting to the cranking battery’s negative ( - ) terminal for charging or jump starting, for safety reasons.

NOTE, you do not have to disconnect any of the cabling connected to the cranking battery, just charge it in place.

The higher RECOND voltage of 15.8v will not effect any of the vehicle’s electronics but also make sure the ignition is turned off AND THE KEY REMOVED.

You need to remove the key because quite a few vehicles now days, power up some circuit while the key is in the ignition but not turned on. Otherwise the charger will be trying to charge the battery and power any circuits that may be live.