Practicality of a 24v to 12v DC Charger

Submitted: Friday, Jul 11, 2014 at 15:55
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My Landrover is 24v and to charge the 12v auxiliary battery a 65 amp secondary 12v alternator is fitted to the engine. Works Ok but the alternator is gets a bit of dirt in and is on the water pump belt so if it plays up could impact on engine operation.

I also have a 60amp OEX 24v to 12v smart charger that is not fitted. To simplify charging of the aux battery I would like to remove the alternator and connect up the 24v to 12v charger.

So my question relates to the suitability of a DC charger to charge the battery for shorter trips. Clearly on longer trips the charger will go through its various stages of charging (Bulk, Absorption and Float) but on shorter trips the charger will always be trying to charge the battery in Bulk mode.

Is this an issue - the charger will only be charging when the vehicle ignition is turned on.

Thanks

Garry
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Reply By: Allan B (Member, SunCoast) - Friday, Jul 11, 2014 at 17:01

Friday, Jul 11, 2014 at 17:01
Hi Garry,

It depends I, suppose, on the algorithm of your particular charger, but my pair of Redarc BCDC1220 chargers sense the battery voltage and promptly go to the correct phase.

Accordingly, if the batteries are fully charged and the vehicle has been stopped for a time, on starting the engine the chargers move promptly to the float stage.

Incidentally, I have my circuits arranged so that when the engine is running, the auxiliary loads are transferred from the aux batteries to the alternator so that the aux load is not being carried via the charger even though Redarc tell me that this would not be problem to the charger algorithm. It does mean that the full output of the charger is available to the battery.



Cheers
Allan

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Follow Up By: garrycol - Friday, Jul 11, 2014 at 19:47

Friday, Jul 11, 2014 at 19:47
Thanks Allan - yes the charger should sense the capacity of the aux battery - current an ex UPS 100Ah AGM and when the camper is connected an additional 45Ah AGM via an Andersen plug form the main Aux battery.

Cheers

Garry
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Follow Up By: Allan B (Member, SunCoast) - Saturday, Jul 12, 2014 at 20:17

Saturday, Jul 12, 2014 at 20:17
Gary, you may have some problem fully charging the 45Ah battery in the camper.

The 100Ah in the vehicle (?) will dominate the DC-DC charger. Being closer to the alternator it will receive more current and charge first. Its rising terminal voltage with charge will cause the charger to back-off into the float mode and reduce the voltage on the cable. The trailer battery is already at a disadvantage due to the voltage drop along the cable feeding it and becomes even more so with the reduction of the charger to float mode.

To somewhat overcome this situation it is necessary to use well-sized cable from the charger to the trailer battery. A full remedy is to use a second DC-DC charger positioned close to the trailer battery, but expensive. However, try it as you proposed. It may work OK for you.
Cheers
Allan

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Follow Up By: garrycol - Saturday, Jul 12, 2014 at 20:33

Saturday, Jul 12, 2014 at 20:33
Thanks - all the wiring is in place and has worked for many years - the charger will just see a 145Ah battery rather than a 100Ahr battery with a 45 Ahr attached - it will read the averaged combined voltage and charge accordingly.

Cheers

Garry
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Follow Up By: The Bantam - Saturday, Jul 12, 2014 at 22:21

Saturday, Jul 12, 2014 at 22:21
The charger most certainly will not see a 145 amp battery.

It will see a complex network of two battereies that have different charge characteristics and substantial resistance in series with the battery in the camper.

in factthe cahger wont see a battery at all..it will see voltage and mabe if it is that sort of chager current...it will not be clever enough to see a battery.

it wont read the average combined voltage in the simple way you think.

depending on the type and condition of the batteries, the series resistance involved and the method of sensing the charger uses ..there is a range of things that may occur.

these multistage chargers are known to work better when they have a direct relationsip with one battery or a bank of identical close coupled batteries..

It may work fine.

it may be that the larger battery dominates and the smaller battery with the series resistance suffers and charges poorly.

it may be that the smaller battery continues to draw current after the larger battery is fully charged keeping the dc to dc charger in a higher charge rate..thus over charging the larger battery.

or a range of other posibilities.

persoanlly unless you where going to re-arange the whole shebang, Id leave well enough alone and continue charging direct from the second alternator.

cheers
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Reply By: olcoolone - Friday, Jul 11, 2014 at 17:46

Friday, Jul 11, 2014 at 17:46
If you use a Redarc DCDC 1225 or 1240 it will work fine of your 24v alternator.

Have a look at this diagram.


http://www.redarc.com.au/images/uploads/files/bcdc1240_24v_setup.pdf
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Follow Up By: olcoolone - Friday, Jul 11, 2014 at 17:48

Friday, Jul 11, 2014 at 17:48
BTW..... the Redarc checks state of charge at start up and chargers accordingly.
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Follow Up By: Allan B (Member, SunCoast) - Friday, Jul 11, 2014 at 18:03

Friday, Jul 11, 2014 at 18:03
You gunna front up the $700+ for the Redarc when he already has a suitable charger Richard?
Cheers
Allan

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Follow Up By: garrycol - Friday, Jul 11, 2014 at 19:51

Friday, Jul 11, 2014 at 19:51
Thanks for the diagram - that is how I am intending to wire things up. I am assuming that my OEX functions similar to the Redarc however mine has a much larger capacity and was a lot cheaper.

Cheers

Garry
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Follow Up By: olcoolone - Friday, Jul 11, 2014 at 21:09

Friday, Jul 11, 2014 at 21:09
Garry as far as I know OEX don't make a DC-DC charger like the Redarc.

OEX do sell the ACX7044 that's a 60 amp voltage equaliser so you can charge 12v battery's of a 24v charging system.

The difference is they are linear to the charging voltage divided by 2........ If you have 27v when charging the equaliser will divide that voltage by 2 giving 13.5v. Give or take a few %

This type of device was the forefather of todays Multi stage DC-DC chargers.
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Follow Up By: garrycol - Friday, Jul 11, 2014 at 22:02

Friday, Jul 11, 2014 at 22:02
No this is not a voltage equalizer, I know all about them - is a 60 amp 3 stage smart DC DC charger.

I know OEX make them as it is sitting in front of me :-) in its original box, packaging and instructions.

Stage 1 Bulk Mode
Delivers 60 amps until battery is about 70% charged based on voltage then switches to Absorption mode.

Stage 2 Absorption Mode
The charger maintains 14.7v to the battery for 90 mins until the battery is at 90% then goes into float mode

Stage 3 Float Mode
Voltage is reduced to 13.7v with just enough current to maintain the battery at full charge.
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Follow Up By: olcoolone - Saturday, Jul 12, 2014 at 09:45

Saturday, Jul 12, 2014 at 09:45
If you have the box in front of you how about a model number?

It takes the guess work out of it.
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Follow Up By: garrycol - Saturday, Jul 12, 2014 at 10:00

Saturday, Jul 12, 2014 at 10:00
No guess work at all.

It is an OEX ACX7046
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Follow Up By: Member - Frank P (NSW) - Saturday, Jul 12, 2014 at 10:02

Saturday, Jul 12, 2014 at 10:02
Garry,

Can you find ouot if your OEX senses the battery voltage before it starts and adjusts itself accordingly? It would be desirable if it did.

But if it didn't, I don't think the effect would be any worse than an alternator going to work on a nearly full battery - in fact with the smarts in the charger this is what I think would happen:

- It would start in bulk and apply 60 amps charging current
- If the battery was well charged already the voltage would very quickly climb to the absorption voltage
- It would hold the absorption voltage and if the battery was well charged the current would rapidly fall and the system would soon enter float.

Leigh and others who know about these things, would often-repeated cycles as above applied to a full or nearly full battery have a detrimental effect on battery longevity?

Also Garry, the OEX will probably pull over 70 amps to deliver 60 to the battery. Can your No.1 alternator provide that as well as deal with other normal system loads?

Cheers
FrankP

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Follow Up By: olcoolone - Saturday, Jul 12, 2014 at 10:24

Saturday, Jul 12, 2014 at 10:24
Garry, now I know what it is...... OEX cleared them out ages and ages ago and the part number is no existent. They only do charge equalisers under their own name now.

The charger is a 60amp 24v-12v DC-DC charger and it is made by GSL.

The GSL part number is "REC-1260c"

It should work the same a Redarc DC-DC charger.

Here is the specs for it from GSL....

http://www.gsl.com.au/tl_files/pdf-automotive-products/REC-C%20Manual.pdf

60 amps is getting a bit big for one or two batteries and as Frank had said it will place a big load on the alternator.
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Follow Up By: garrycol - Saturday, Jul 12, 2014 at 11:26

Saturday, Jul 12, 2014 at 11:26
Thanks for that information - the 24v alternator will have no issues providing the required max of 60amps (will normally be much less) and supply the rest of the vehicle as well.

Yes the charger does sense voltage in the battery and adjusts charging voltage and amps supplied depending charging stage.

Cheers

garry
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Follow Up By: Member - Frank P (NSW) - Saturday, Jul 12, 2014 at 11:45

Saturday, Jul 12, 2014 at 11:45
Garry,

I note in the link olcoolone provided it says "Before starting the charge cycle the battery voltages are evaluated to
detect faulty batteries" and

"The [output] LED will remain off when:-
B) The battery has been detected to be faulty. Check 12V with a meter to establish if the battery is with in the range of 7V to 13V."
(Quote has been edited slightly for brevity)

Are you sure yours goes beyond just detecting a faulty battery and actually adjusts the charging parameters at start-up?

If you are, then in my opinion go ahead and make the change.

If not then I would seek further advice about the effect of frequent application of a full charge cycle to an already charged battery.

Cheers
FrankP

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Follow Up By: garrycol - Saturday, Jul 12, 2014 at 12:18

Saturday, Jul 12, 2014 at 12:18
Thanks Frank - if the battery is as low as 7v then I am not going anywhere anyway as I have converted my ignition to 12v electronic ignition and it runs off the aux battery (have a small backup 24v to 12v converter to run the ignition directly if this happened).

Any way - I can see the status of charging from the LEDs on the charger which will be mounted on top of the 24v battery box which is in the cabin behind the passenger seat so can easily be seen from the drivers seat.

Cheers
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Reply By: The Bantam - Saturday, Jul 12, 2014 at 18:49

Saturday, Jul 12, 2014 at 18:49
lets not over complicate the issue or start looking for problems that probably wont occur.

Even if the dc to dc charger fires up at whatever the bulk voltage is...it does not necessarily follow that 60 amps flows.

Just because a given charging device is rated at 60 amps..it does not necessarily follow that 60 amps flows..at any time.

If the battery is fully or nearly fully charged very little charge current will flow...probably of the order of 5 amps.

The charger should then at worst switch to absorbtion mode..still in the order of 5 amps flowing...maybe less

IF the battery is not fully charged ( whatever that is or how you assess that)..lets say partly discharged..none of this will be a problem.

most travelers have the problem of not driving for long enough to replenish what was used over night.

There are multistange battery charges out there that will not get out of bulk chagting mode the whole holiday...because the batteries have not been fully charged due to insufficient run time/

I don't think the DC to DC charger will present a problem from the point of the multistage function.

HOWever.....we need to look at what the maximum initial charge rate is of the battery in question.

Many 100Ah AGM batteries have a maximum initial charge rate of the order of 20 amps.
The battery in question comming out of a UPS....hmm...likley

if the DC to Dc charger is capable of delivering 60 amps..and the battery has a maximum initial charge rate in the order of 20 amps...there is a problem right there.
AND
there will be the very same problem if the battery is being charged directly by an alternator.

THIS is the problem that you should be concerned about, and the reason many people have premature failures in AGM batteries.

GO to the manufacturers site..do not pass go, do not collect $200 and check the specs for the battery in question...or perhaps it is printed on the side of the battery.

cheers


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Follow Up By: The Bantam - Saturday, Jul 12, 2014 at 18:52

Saturday, Jul 12, 2014 at 18:52
Oh just another thaught.

Every single vehicle I have owned that was not a 4wd had the alternator on the same belt as the water pump.....why is this a problem.

cheers
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Follow Up By: Allan B (Member, SunCoast) - Saturday, Jul 12, 2014 at 20:02

Saturday, Jul 12, 2014 at 20:02
The Bantam is right on all points (except on the spelling of thOught).

In regard to the charger output on startup with a fully charged battery, it will output a relatively low current for a short time before moving to a float output. All this is determined by the voltage of the battery at the time.

As an example, as I referred to in the first reply above, my Troopy has 2 aux batteries, each charged from two BCDC chargers. If the vehicle has been standing for a few minutes or 24 hours, with the batteries fully charged, both chargers will input only about 5A for a minute or two before dropping to less than 1A float. And this or lower is where it stays until some charge is drained from the batteries.

I have seen expressions that the chargers will force boost or bulk rates to the detriment of the batteries..... not so. The chargers are smarter than the commentators. The movement of the charger through the stages is achieved by sensing the battery voltage, hence a fully charged battery will exhibit a voltage that governs the charger stages. The same principle applies to a battery that is not fully charged..... the charger senses the battery voltage and delivers the appropriate current. It really is quite a simple and reliable procedure.

I would certainly endorse Bantam's point about a charge source maximum current output, be it alternator or charger. This will be significant when the battery SOC is low hence high charge current. If this charge current exceeds the recommended battery charge current then the battery may be damaged. But of course, the rating of the charge source is not necessarily the current that will normally flow, but it may do so if charging a very discharged battery. It all depends on the relative voltages. High alternator ratings allow for supplying loads such as lights etc whilst also having enough capacity to charge the battery. DC-DC chargers however are supplying only the connected battery load and do not benefit from being sized larger than the battery charge rating. My two chargers are each 20A maximum so are limited to my battery specs.



Cheers
Allan

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Follow Up By: garrycol - Saturday, Jul 12, 2014 at 20:41

Saturday, Jul 12, 2014 at 20:41
Thanks for the thoughts bit is not an issue - remember this aux battery is already installed and is currently charged by a dedicated 65amp alternator so at anything above 2000rprm it is getting a full 65 amps - equivalent of bulk charge all the time.

The battery has no issues and after its life backing up computers has spent four years being my aux battery.

Cheers

Garry

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Follow Up By: The Bantam - Saturday, Jul 12, 2014 at 22:09

Saturday, Jul 12, 2014 at 22:09
yeh ya realy dont get it do ya.

1. the alternator at 2000 rpm engine revs will probaly be capable of outputting about 75% of its rated output..so down arround 55 amps.
If its typical of diesel 4wds, 2000 rpm will be arround 80Kmh

2. as we have said the battery may not draw the full capacity of the charge source ever....more particularly it wont draw ist maximum unless it is at a low state of charge.

SO....depending on how the battery is used and the vehicle is driven...you may have been lucky to not have maximum alternator output and a low state of charge at the same time.

but if you go away on a trip where you camp overnight..and mabe have a higher drain that in the past resulting in a los SOC and then get straight on the highway every morning...you may have a number of high current events.
events that would not occur if you did not have a high charge capacity and a low SOC at the same time.

NOW..if you have a DC to DC charger, that outputs a consistent higher charge voltage.....you may have a consistent higher voltage and charge capacity at low RPM.

Check the specs on the battery.

One thing that may save you is that sealed lead acid batteries AGM included when they get some age under them tend to have lower charge acceptance...this my reduce your charge current overall.

BUT..if you put a new battery in the same situation...you may find it does not last long.

serilusly..check the specs on the battery.

cheers
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Follow Up By: garrycol - Saturday, Jul 12, 2014 at 22:22

Saturday, Jul 12, 2014 at 22:22
Not a diesel, 2000rpm is about 45kph so alt is generating most of its amps most of the time.

Not a lot different to the charger except the charger will throttle back after it reads voltage around 70% of max.

Thanks for your comments

Garry
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