RV Powerstream P/L replied:

KC and Co
The problem I see is if you divide 20 into 200 and you use the AGM to get the capacity how far do you have to drive to recharge it.

Is the unit ignition activated so that it only works when ignition on and stops when ignition off or does it have a wide acceptance voltage of say 8-15V where it can flatten your battery.

The main benefit of DC/DC charging from an alternator is to use all the available spare amps the alternator can produce to charge the aux battery as high as possible in the shortest run time.

Currently there is only one manufacturer that can achieve the above and that is Sterling Power Products of the UK whether it is by Alternator Smart Regulator, Alternator to Battery Charger or Battery to Battery Charger they are all DC/DC chargers.

If your alternator can produce 50A spare capacity why would you choke this capacity by installing a 20A charger.

The small capacity DC/DC chargers are only better than a correctly cabled cable only system when the resistance of the battery limits the alternator to below the capacity of the charging unit.

Ian

ABR - SIDEWINDER replied:

Hi Kc

From what I can see on the only pdf file I can find is there is no ignition control. Dc20-pj

This would mean you would still need to fit a dual battery isolator and cable capable of carrying 30A continuously over the distance.

These DC2DC chrgers are becoming very popular and is fast becoming one of our best sellers in our 30A unit. (with ignition control).

The problem we are seeing is that many manufacturers are controlling output voltage of the alternator down to as low as 13.5V and this will never allow a battery to fully charge, it will slowly sulphate and loose capacity.

In your particular case you should test your alternator and if it produces 13.8V or more when hot then there is no need for a DC2DC charger as the 200ah AGM will accept closer to 50A and you would increase recharge time by putting a DC2DC charger in the circuit.

The DC2DC charger is a good way to finish off the charge once the batteries are almost fully charged. IE: 80% or more when the alternator is delivering less than the output of the DC2DC charger.

I recommend to all my customers that they fully charge their batteries on a mains charger when they get home from any trips or holidays.

Regards

Derek from ABR

RV Powerstream P/L replied:

The post by Derek says it all except with one exception even if the alternator is down to 13.5V the Sterling units will still do what they are designed to do smart charger the auxilliary battery correctly in the shortest time up to
14.1V 14.4V or 14.8V selectable to suit the batteries and with temperature control of the batteries being charged.
Ian

Peter McG (Member, Melbourne) replied:

I use a Ranox 12v charger and it is terrific.
If you use 50AH a day then you will only need 2+ hours of driving
to fully charge. Check them out: wwwmranox.com.au

Peter

Member - Phil G (SA) replied:

Kc,
I'm not in the business, and agree with Ian and Derek that you would be limiting current to your AGM and initially slowing the recharge.

What vehicle you have (and what voltage the alternator puts out are very important - they need to match your batteries.
For example, if you had a V8 100series, then the voltage might be a bit low - the heat under the bonnet will drop the alternator voltage and the cool AGM in the back doesn't get enough volts and hence not enough current to recharge. In this case an inline charger that upsizes the voltage to 14.4V would be useful.
Alternatively, if you owned a series3 Disco, the voltage might be too high and damage the battery.

I run a similar 3-battery setup to you - twin cranking batteries under the bonnet and 100Ah AGM in the canopy. My vehicle puts out 14.3 - 13.8 volts (depending on temperature), and I use the large 4Ga cable (both pos and earth) to the AGM, so it can easily manage the high current. I roughly measure current by measuring the voltage drop between batteries 1 and 3. And after playing around with a clampmeter, I find that with my wiring, a 0.01V difference equals 1 amp going into the battery. Its a simple way of knowing whats going on.

drivesafe replied:

Hi kcandco, you pretty well hit the nail on the head. If you know your not going to be driving for more than a few hours at a time but look like using a fair bit of power while camped, then your alternator is easily going to replace more of the used battery capacity in the short time available.

These devices will fully charge batteries, but they need the time to do so. The very same thing can be stated about an alternator except that an alternator has one huge advantage over these devices. When you start your drive for the day, the lower the battery state of charge, the higher the current supplied by the alternator.

No matter how you charge your batteries, most people never drive long enough to allow any set to fully charge their batteries and with these devices, they just charge at a set current until the battery is about 85% SoC and then the charging mode charges and usually lower the charge current.

With an alternator, at the very time you need the most charge, when your batteries are at there lowest, an alternator will deliver the highest possible charge current, so you will replace the greatest amount of used capacity in the shortest time and all you need is decent sized cable.

BTW, for these devices to work at full efficiency, you still need decent size cable.

The only device I know of that can come close to what an alternator can do, is the Sterling gear, but at a price.

RV Powerstream P/L replied:

What drivesafe has said is also correct with one exception the Sterling equipment gives the charge to the battery at the recommended charge voltage which is the fastest way to charge a battery so the come close part is with the Sterling leading and the alternator playing catch up as the Sterling has equipment to cater for up to a 300A amp alternator.

Efficiency does come at a price and it depends on you if you are happy to drop to 50%soc and recover to 75% or 80% state of charge and work in that range do not waste your money on other than a correct sized direct cable setup and this is dependednt on how far you travel.

If you want to increase your useable capacity and lenghten the life of your battery use a system that gives you the maximum power in the shortest time with the help of the spare alternator capacity.
Ian

kcandco replied:

Thank you everyone. your answers have really helped. I am overwhelmed with your response. I am pretty sure I now intend to go with charging via the alternator only and do a top up at home or caravan park as available. I measured voltage at the main battery as 14.3v and at the auxiliary battery at 14.2v which is about 1 metre from the main. This was with the engine at normal operating temp after a few minutes drive. Due to the size of the agm battery (65 kilos 55cm x 25cm x 25cm) I have to mount it away from the engine bay and will end up with about 3 metres x 2 = 6metres of cable to do the round trip from the main to agm battery and return. The cable i have at present is DSE W 2302 (630/.12mm) which I believe is approximately 13 square mm in cross section. At the distance of 6m (2 x 3) at 50amp I calculate a voltage drop of .4 volts. I think this is a bit on the high side. Again I would welcome your opinion.

regards Kc

Member - Mike DID replied:

"I am considering setting the new agm battery up in the car or camper trailer and have seen the DC20, PROJECTA 3 Stage Automatic 12V to 12V 20Amp Battery Charger in the auto shop."

Why ? What benefit were you hoping for compared with Alternator-only charging ?

Member - Matt M (ACT) replied:

KC,

Just cast your research net a little wider before coming to your conclusions is all I suggest.

Cheers,

Matt.