Redarc in Vehicle Battery Charger

Hi Phil,

Yes, I'm aware of those diodes that trick the alternator into raising the voltage. I'm not a fan of them. I figure that a very smart team of engineers spent a lot of time putting together a complex system of interconnected computers, systems and sensors in my car. I don't feel comfortable with dicking around with the power supply to all those. Although many people use those diodes, I'd prefer to adapt to what has been provided rather than change it to suit my needs.

Re the DC-DC charger's maximum amps - yes I am aware of that too, and that alternators can generally provide more if they have the capacity. However, not all batteries should be charged at high rates. That consideration is not a practical proposition for a crank battery, but it is for an after-market auxiliary. My under-bonnet auxiliary is a commonly used 110AH maintenance-free wet cell. Wet cell, not AGM, because they tolerate the heat of the engine bay better. The battery manufacturer's spec for max charging current is 10% of C20 capacity, in my case 10% of 110AH, = 11 amps. I have set my programmable DC-DC charger to that. A battery that needs 11 amps and gets 20 from a non-adjustable DC-DC charger will probably be ok, but if it got 100 from a high capacity alternator it might not fare so well. I think that checking for a manufacturer's limit on charging current is often overlooked.

Also often overlooked is temperature compensation. For best performance and life of aux batteries, charging should be temperature compensated, and should stop at battery temps higher than 50 degC. (In my vehicle in hot weather that is an inconvenience, but I care for my battery and make suitable arrangements.) A good DC-DC charger will account for those often overlooked factors, whereas your average vehicle alternator system won't.

My comments about alternators only charging to about 80% were based on the low alternator output provided by some vehicles. Maybe I didn't make myself clear enough on that - if so, apologies. Those systems that have outputs around 14.4V may provide a full charge providing, as you say, that the cabling is good enough and the charging system holds the voltage at that level long enough to put a decent charge into the aux battery. But if the alternator is only generating 13.2 (or 13.9 if you have your booster diode) and the battery requires 14.2 or 14.4V, or in my case with a calcium battery, 15.6V, then it is impossible for the battery to get its absorption charge and as a consequence it will have a reduced life. In day to day use the battery will appear to be fully charged, but it won't be and will suffer in the long term as a result.

From above ... "and the charging system holds the voltage at that level long enough to put a decent charge into the aux battery". The output of the vehicle's charging system is controlled by the regulator looking at the crank battery. If your crank battery is fully charged the alternator output will be down. If your auxiliary battery is well discharged it will not be getting what it needs to properly recharge it because of the reduced output from the charging system commanded by the fully charged crank battery.

A DC-DC charger addresses all those points.

For those who view their aux battery as a relatively cheap consumable to be replaced every 18 months or so, or for those who regularly hook up to 240, then perhaps all this is a bit over the top. But for those who like to stay away from the grid, rely more on vehicle charging and want long battery life as they tour this country, I don't think it is.

Cheers

Quote "Also are you aware that the DC/DC charger is restricted to its maximum amps usually 10 or 20 while a 120amp alternator will start off by charging much quicker ( as much as the battery will absorb and this can be up to 120amps for say an Optima), so that the batteries will be charged in fewer hours driving just using the alternator."

You are making it sound like these devices are rare in high current models. Redarc has models up to 40 A, Stirling have them larger than many alternators will supply their maximum rated outputs many others have models over 20 A as well. Add to that, C-tek have a 20 A model that can be combined with a box that will pass al of the alternators spare output up to the time the charge current drops below 20 A.

You mention high recombination batteries like Optima. The big problem with direct alternator charging is that the most popular battery amongst caravanners is the low recombination batteries like Fulriver. These require a higher voltage to charge them to any significant capacity than the modern alternator puts out.

Thanks Phil,

"To each their own I guess"

Yes, in any discussion about 12V history shows that's about what it comes down to.

"but I have to argue with QuoteThe output of the vehicle's charging system is controlled by the regulator looking at the crank battery.Unquote.
That is true for about 2 minutes after start , as 2 batteries linked by a decent cable will equalise and the regulator will see only one big battery.
My ABR Sidewinder electronic relay keeps the batteries linked down to 12.4 volts and I can assure you that both batteries have the same volt level ie SOC until 12.4 volts is reached and the second battery disconnects.

So the alternator will see one big battery as it is charging as soon as the voltage sensitive relay connects them."

My concern with that is that if you have batteries of different chemistries, neither is getting its optimal charge. That is why I favour the DC-DC charger - each battery gets its optimal charge.

"I have used plain solonoids controlled by ignition, electronic votage sensitive relays and on my last Range Rover had 2 completely separate alternators and charging systems. The battery life was 3plus years for an Optima Yellow top on a solonoid and it was still OK but leaked acid from the negative terminal, 5 years for an Absorbed Power AGM running on a separate alternator, and 31/2 years for a "cheapy" Chinese AGM on the same system. All were under bonnet on the Range Rover and all outlived a Supercharge Allrounder conventional, and a Century conventional."

A mate of mine is an electronics engineer. He tested the charging system on a number of vehicles while developing a DC-DC charger (no names, no companies, no pack drill, sorry. And for the record, no particular interest, other than I agree with the concept). He found the Defender/Disco/Rangie charging systems to have a far more sophisticated algorithm than any other vehicle tested. That LR algorithm, in addition to your second alternator, maybe gave a similar result to that of a DC-DC charger - close to ideal charging profiles - and hence your good battery life without a DC-DC charger. But for those of us with plain vanilla charging systems ....

"BTW my understanding re the Prado is that the lower voltage is simply to save petrol particularly on the urban test cycle so Toyota can advertise better economy. I am yet to be convinced of the usefulness of stuff like this in the outback, just like auto turn off at lights. Great in Sydney but no use in the bush."

Couldn't agree more, but we who own one (or similar) have to deal with it.

Cheers