dc-dc chargers

Submitted: Saturday, May 07, 2011 at 23:57
ThreadID: 86129 Views:2648 Replies:7 FollowUps:7
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Hi all,

I am looking at putting a dc-dc charger in the camper but am in a slight quandry. I have 2 x 120ah agm's in the camper and "apparently" they need to be charged at 14.7 volts to get them to 100%. Now if this is correct then why do some manufacturers only have 14.4v/14.6v max output from their chargers. Is this really an issue if they were to be charged at only 14.4v and if not really an issue then will they get to 100% or even close to this.


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Reply By: Battery Value Pty Ltd - Sunday, May 08, 2011 at 07:56

Sunday, May 08, 2011 at 07:56

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

AnswerID: 453593

Follow Up By: Bonz (Vic) - Sunday, May 08, 2011 at 08:21

Sunday, May 08, 2011 at 08:21
Hi Peter, which one of the well known brands are you referring to above? Just about to go down this path myself.
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Follow Up By: Battery Value Pty Ltd - Sunday, May 08, 2011 at 08:35

Sunday, May 08, 2011 at 08:35
Hi Bonz,


cheers, Peter
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Follow Up By: Bonz (Vic) - Sunday, May 08, 2011 at 08:40

Sunday, May 08, 2011 at 08:40
chookers Pete thanx
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Follow Up By: Dust-Devil - Sunday, May 08, 2011 at 13:08

Sunday, May 08, 2011 at 13:08

Watching you on this forum is a 'life experience' in real time.

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Reply By: Ozhumvee - Sunday, May 08, 2011 at 08:18

Sunday, May 08, 2011 at 08:18
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.
AnswerID: 453594

Follow Up By: Roachie.kadina.sa.au - Sunday, May 08, 2011 at 08:44

Sunday, May 08, 2011 at 08:44
G'day Peter,
That is very interesting indeed.
I have had an Arrid Twin Charge DC-DC mounted in the camper trailer for about 6 years. I've never had any issues with it discharging the 2 x 100a/h camper batteries while it's parked up in the shed.....but then again, it's not connected to the vehicle at the time and hence the "donor" batteries are not connected.

On the "new" cruiser I've just set up, I've mounted a Redarc BCDC12 in the cargo area, near the 100a/h AGM battery which it has the job of keeping charged. The supply for this charger comes directly off one of the cruiser's main batteries, via a circuit breaker and a relay. The relay is switched on via the windscreen wiper connection, so the 2 battery banks are electrically isolated when the vehicle is switched off....I can't see how there is any way that the 2 starter/donor batteries to discharge due to the DC-DC charger as the Redarc BCDC12 is located downstream of the relay.

Having said that, I will grab the multimeter and see if there is any apparent voltage present in the cabling between the relay/circuit breaker and the DC-DC charger.

BTW: The list of additions/modifications I've done to the cruiser in the month that I've owned it has grown and is almost complete.......The fun is almost over on that front; now I can get around to just driving it. hahahaha

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Follow Up By: Ozhumvee - Sunday, May 08, 2011 at 09:16

Sunday, May 08, 2011 at 09:16
Yes mine isn't a c...k its a g.l. It certainly does a good job with the battery when travelling . Nothing changed between the fitting of the charge Equaliser and the DCto DC charger except the wiring was upgraded between the starting batteries and the charger.
The charger is sitting across the main battery all the time just not charging the aux. It obviously sucks a bit out of the aux as well as the discharge stops when it is disconnected from the aux.
Yes know all about fitting out a "new" vehicle, spent the last four months full time on the Oka, can see the light at the end of the tunnel now though.
Must remember to remind myself that this is the last time ;-)))
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Reply By: Mick O - Sunday, May 08, 2011 at 09:53

Sunday, May 08, 2011 at 09:53

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

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Reply By: Mike DiD - Sunday, May 08, 2011 at 13:55

Sunday, May 08, 2011 at 13:55
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.
AnswerID: 453615

Reply By: CSeaJay - Sunday, May 08, 2011 at 18:07

Sunday, May 08, 2011 at 18:07

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?
AnswerID: 453638

Reply By: crd patrol - Sunday, May 08, 2011 at 18:50

Sunday, May 08, 2011 at 18:50
Thank you to all that replied it is greatly appreciated. Now I have a little more to think about.


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.


AnswerID: 453645

Reply By: Member - Scrubby (VIC) - Sunday, May 08, 2011 at 18:56

Sunday, May 08, 2011 at 18:56
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 )


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Follow Up By: crd patrol - Monday, May 09, 2011 at 08:43

Monday, May 09, 2011 at 08:43
Cheers Scrubby I will have a look.

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