Thread: DC-DC charger- which one?

Thanks to everyone for their comments.. certainly some interesting debate about which system works best, and making me have a think about the true cause of the low voltage problems, before jumping down the path of getting a DC-DC setup.

So, I have done a bit more diagnosis and have found:
a) prior to charging the traxide isolator was flashing with main battery at 11.9v
b) following repeated charging cycles from my CTEK 5.0 multistage charger, I'm seeing the max voltage get up to 12.8v, though fairly rapidly (about 1-2hrs) deteriorate to 12.4v once removed from charge (and the vehicle just sitting their). Note at this stage I have not deconnected the Traxide DBS so this voltage is across both batteries.
c) the alternator is cranking out a healthy 14.2V when the vehicle is running.

Based on this analysis, my feeling is that either:
1. there is something drawing current from the vehicle (or some poor connections somewhere)
2. the cranking battery needs replacing

So, I'm thinking of further testing the system by charging both batteries up to 12.8v again, then disconnecting the batteries manually and the DBS and watching how each battery voltage changes.. well, drops.. over time.

I guess another question, is what sort of voltage should I be expecting from normal operating conditions across the main battery? It was replaced under warranty about 1.5 - 2years ago

Hi mijango, from what you have posted, my guess is that it's your cranking battery.

BUT.

There can be a number of reasons for your battery voltage dropping and at the now 12.4V, your battery is at 80% SoC.

The maximum charging voltage you posted, 12.8v, while charging, is not correct for a fully charged battery while being charged.

You should see at least 13.5v, before disconnecting the charger.

At 12.8v, your battery was still being charged, and this can take some time, many hours.

One of the primary causes of low battery states, is the shopping trolley syndrome. Lots of short drives.

These short drive not only stop the battery from being fully charged but they progressively cause the battery's condition to diminish.

The initial problem is that the battery is hard to fully charge and ultimately, it will die an early death.

The charging you are doing can help, but if you have the time, try fully charging the battery then leave it on the charger, in float mode, for 8 to 10 hours.

Then disconnect the charger ( just turn it off will suffice ) and let the battery settle for around 12 hour and then repeat the charge/float cycle.

Do this as many times as is convenient to your needs and then see if the battery has improved.

NOTE, your can use the charge/float conditioning of the battery even when you need to drive the vehicle every day. Just charge/float every night.

The charge cycle charges the battery capacity that can be charged, but the float cycle helps to reverse sulfation of the battery's plates, which in turn over a period of time, actually increases the battery's chargeable capacity.

The conditioning of the battery is actually more beneficial than just charging it.

BTW, you do not need any special form of charger. Any three stage charger, dear or cheap, will condition a battery when used as described.

After you have carried out a few charge/float cycles, try removing the negative terminal from both batteries and let them sit over night.

Measure the voltage in the morning, but don't be surprised if the Optima has a higher voltage reading. This is normal

Thanks for the response Tim.
In terms of charging, I probably should have clarified that I have been conducting full 24hr charge cycles (usually 12-14h charging, followed by letting 10-12h rest and repeat) as you advise (think I actually read this in one of the previous posts). Furthermore, each time I have taken off the charger, it has been in float mode.
Still though, the main battery is only reaching 12.8v, not the 13.4v you mention.
Re your comment that you'd expect the Yellow Top battery to retain a higher voltage than the cranking battery after charging, is this because you (And I) expect the main battery is in a poorer state of health, or down to the characteristic of the battery? Or - to put it another way - if I did the same test with 2 spanking new batteries (1 yellow top; 1 land rover std main) would they both hold charge at the same (or very similar) rates?

Hi again mijango, under normal use and testing, you would not disconnect the negative terminals of either battery and as such, the chart I posted up earlier in this thread would give you a fairly accurate indication of the two batteries state of charge.

But while the batteries are connected to the vehicle, they will have a load on them, even if it is a very small load, it is still a load and the chart is for loaded voltage measurements.

By disconnecting the each battery's negative lead and leaving the batteries overnight before testing, you are now testing batteries in an OPEN CIRCUIT situation.

if your cranking battery is still fully charged, it's voltage should be somewhere around 12.7 - 12.75v. This is the same voltage for most lead acid batteries in a fully charged OPEN CIRCUIT state

Whereas the Optima should has a fully charged OPEN CIRCUIT voltage of 13.1v.

This is why you should get different voltage readings.

I meant to mention that after getting the OPEN CIRCUIT voltage reading for your cranking battery, you then need to reconnect the negative lead and take another reading with your ignition on, but not with the motor running.

If your cranking battery is good, you should see 12.7v in the OPEN CIRCUIT state, and about 12.4v and no lower with the ignition on.

Alls well with your battery if the above is what you get.

If the OPEN CIRCUIT voltage is less than 12.4v, before you put a load on the battery, you more than likely have a battery on it's last legs, and when you put a load on it, it will drop much lower.

The reason for disconnecting the batteries, is so if there is something in your D4 draw power with the ignition off, it will have no effect on the disconnected batteries.

So this test should rule your cranking battery in or out as the problem.

With all the charging you have already done, I suspect it will still be a dud cranking battery.

Originally Posted by mijango

Any thoughts or advice much appreciated..

Hi mijango,

Today I bumped into this website (see link below) and upon reading I thought it would of help to those on this thread as well as the broader LR community.

So I posted it on D3 /D4 / RRS as a new thread. However it got moved to the technical forum because is was not D3 /D4 / RRS specific.
So I have re-posted it here just for your info.

The info on this site was only recently updated (3rd May) so it may have new info for those who have "been there done that".

Design Guide for 12V Systems

Most of what he says makes sense I do see he does not use DC DC chargers himself seem to agree with most of what has been said in this and other threads about the same topic

After being involved with 4WD's with duel battery systems for more than 35 years, I had always used bulk chargers.

Up until about 8 years ago I got tired of short life cycles with deep cycle batteries.

A client, a battery developer, told me the reason for my battery failures was due to uncontrolled amperage going into my deep cycle batteries, in particular agm's. I did swap to orbitals for a couple of years, but for my type of camping and touring they simply did not hold on as long as a larger capacity agm (for the same size).

My batteries were dying prematurely due to them sucking more current into them, then they were designer to draw...not so for the orbitals which could have current smashed in heavily.

I have had no battery failures since using my 20amp redarc, and use another redarc in my camper (actually last 3 campers).

I will never be swayed to go back to bulk chargers.

Brett...

If your saying your alternators were providing unregulated amperage? then they had a problem with the regulator, a properly working alt will not provide unregulated voltage or amperage.