dc-dc chargers

Hello,

over the years, I've found 14.4V is as good as gold for absorption stage charging.
There is a relationship between charging voltage and depth of discharge.
If you discharge a battery deeper, you need to apply a higher charging voltage.
Time is also a factor, but if the voltage is barely raised above the float level, it'll take an enormous amount of time to fully charge the battery.
Another reason why absorption charging at 14.8V sometimes is recommended, it's a way of equalising the battery cells.
This has something to do with de-sulphating the negative plates, an attempt to make all 6 cells behave the same.
Again, this should only become necessary if the battery gets discharged regularly below say 40% SOC.

Absorption charging at 14.4V, and more so at 14.8V should be done in a very controlled way, to limit the associated loss of electrolyte. The whole concept of three stage charging is a tradeoff really, between loss of electrolyte, and sulphation.
If you don't get the balance right, battery life goes downhill faster due to either effect taking precedence.

And finally, all DC/DC chargers I've come across so far tend to overcharge the battery especially if there's a concurrent load connected. One of the better known brands, applies the absorption voltage for up to 12hrs, which is almost 10 times the recommended amount. This causes significant loss of electrolyte in the medium term, or short term if done on a daily basis.
And what makes things worse, some DC/DC chargers don't have temperature compensation.
As a result, the battery will gas substantially on warm/hot days, due to the double whammy of high temperature and vastly extended absorption times.

Hope this explains things.

cheers, Peter

I've also found that the 24vDC to 12v DC charger that I have in the Humvee which is charging a single 12v AGM to run the 12v accessories will slowly flatten both the 24v batteries and the 12v battery even if turned off (it is wired through the ignition) if the Humvee isn't used for a few weeks. The only way to stop it discharging all the batteries is to disconnect them all.
Never had the problem with the previous system which was a Charge Equaliser feeding the 12v battery. It expired after about 5 years so updated to the later technology.
I've spoken to other DC to DC charger users and it seems to be a common problem.
I suspect that even though the charger is "off" that it still monitors voltage and the small residual drain is enough over a few weeks to flatten the batteries. This was confirmed by the manufacturer.
If the vehicle is regularly used it isn't a problem but if left for more than a week or so then they all need to be charged before it can be used.
Ain't technology grand ;-))
Something to think about if your 4wd/motorhome/camper is left sitting for any period of time, you may need to connect either a solar panel or mains charger to keep everything topped up.

Crd,

I've been using the Redarc DC-DC and BMS systems for 18 months now and am very happy with the way they are managing my various batteries. I have a 110 Hybrid as a vehicle axillary and two 140 A/H AGM's in the rear pod.

The latest iteration of their DC-DC, the 12-40 also has an inbuilt MPPT solar controller built in giving a dual facility. Might be worth checking out.

Redarc Review DC-DC 12-20 and BMS



Cheers Mick

Three Stage Charging is the fastest way to charge Lead Acid batteries without shortening their life - it definitely isn't the only way to charge batteries to 100%.

Using a lower-than-recommended Cyclic voltage just lengthens the charge time.

I've regularly fully charged batteries using 13.8 volts.

CTek

There I said it. Don't know what all this secrecy is about - just watched Bourne Identity did you?

Please adavise in straight English - are you saying the CTek DC-DC
damages batteries? I have one and need to know?

Thank you to all that replied it is greatly appreciated. Now I have a little more to think about.

Peter,

I checked voltages at the batteries this afternoon and this is what I found...

I have 14.03v at the start battery.

14.02v at the anderson plug at the back of the patrol (this goes to the camper through a redarc isolator and circuit breaker from under the bonnet). I haven't had a chance to hook up the camper and check the voltages of these batteries. I will do this tomorrow.

14.02v at the aux battery (under the bonnet, through another redarc isloator and circuit breaker) and 13.86v at the 2nd aux battery in the back of the patrol.

All of these figures are with the engine at idle.

I was thinking of mounting a dc-dc charger in the patrol (as well as the camper) to charge the batteries "properly" but do I really need it with these voltages.

crd

G`day crd,
There is a selection of these chargers is available from RV Powerstream, I chose the Stirling DCDC 1250 to recharge my 200 ah RV Gel battery because I sometimes run my 80 lt fridge on deep freeze full time and the 50 amps available from this charger shortens the engine run time.
I think it is the only 12 volt DC_DC that offers 50 amps. ( I may be wrong but I know of no other )

Regards.

Scrubby.