Thread: DC-DC charger- which one?

Originally Posted by manic

Sure I get it. I have a voltage meter on the dash and I can see the alternator is often knocked down to ~13v so I figured a DC-DC at nearer 15v might be quicker at topping up batteries and the alternator quicker at restoring bulk.

I'm considering the following set up.

Starter Battery
Deep Cycle Battery (AUX)

All Auxilary sockets, fridge, lights etc run off a 100AMP relay that gets power from the starter/alternator when the car is running and switches to the Aux battery when starter voltage drops below ~12.4v

A DC-DC charger is the only device connected to the AUX bat when the engine is running.

This set up will see all devices powered by the alternator when engine running, and the starter battery will get charged quickly (priority) from the alternators bulk amperage. The AUX will get a regulated DC-DC charge without having to provide any power.

On short stops the starter battery is likely to provide the required power, on longer stops the AUX takes over with solar support.

On a 90 with a small fridge and LED lighting I think this set up might see the AUX battery kept at a high/better state of charge most of the time. Especially when the vehicle is not being used for camping.

Yep, an excellent set-up. I do my motorhome that way, but I would suggest that you switch your loads to the house battery at 12.7 volts. You really always should have your cranking battrey at peak charge in case of freezing cold morning starts!

Originally Posted by Silenceisgolden

Obtain a 14.5 volt regulated supply, connect it to a battery and leave it there.... call me back when the battery stops drawing current.

There, you just said it - when the battery stops drawing current. Not the alternator forcing more in than the battery is drawing. You're whole argument is moot as if your theory was correct, please explain how the cranking battery lives happily for years being charged at these voltages?

It's a point that's been mentioned a dozen times here but you conveniently ignore.

Originally Posted by Tombie

This paragraph alone should have people questioning the validity of his data..

Quote:
Today, even low-priced cars have more computing power than had early space craft. Their vehicle electronics are rugged enough, but typically require 13.8 volts to run, and some only 13.5 volts ? far too low for charging. And they require that voltage to be stable. Further, adding an auxiliary battery bank may not necessarily cause problems, but is all but certain to be blamed if things go wrong. It may also invalidate the warranty.


Hi Tombie and I wouldn't give that "journalist" any credence what so ever, and that quote of his you posted up is really ironic.

I have had plenty arguments with him over the years and he is by far one of the most disingenuous individuals I have ever come across.

About 5 years ago, I was on a caravan forum explaining to a Toyota owner how the variable voltage worked on Toyotas and how it differed from how Land Rovers system worked.

Then, totally uninvited he posts up that there is no such thing as alternator working voltages below 14v, they never have and it was all B/S.

Now thats from a bloke who is a self-proclaimed expert in this field.

Originally Posted by Silenceisgolden

Other lead-acid batteries are designed for cranking the engine which requires a very high current but only for short duration. These batteries can be damaged by even infrequent discharging.

What a load of crap, while dedicated deep cycle batteries should not be used as starting batteries except in an emergency, there is absolutely no reason why you can not use a cranking battery as a deep cycle battery.

Before any form of deep cycle battery was available for RV use, everybody used cranking batteries as there auxiliary/house batteries and they had no problems.

The primary difference between a dedicated deep cycle battery and a cranking battery when used in cycling applications, is that you get more cycles from a dedicated deep cycle battery than you do from a cranking battery.

But this is why deep cycle batteries are dearer than cranking batteries. But the amount of energy you get from each type of battery is about dollar for dollar the same.

Try getting the correct info before lecture others on a subject you quite obviously don't understand.

Ohhh I love this topic.... we should do it every few yearsjust for fun JStill no colourful language or abuse of people from me... sticking on topic J

First to Tim's point about a low revving alternatorproducing less power. I think you have a point on a DC/DC charger increasingits current draw at a time of low alternator output. I had not considered that(see I can admit when someone makes me think of something else). However I don'tthink I have seen that scenario in my D4. When just after starting to drive andthen idling (while the main battery is still being recharged) I still see ahigh voltage (well above 13.4v) so I don't think the D4 alternator suffers fromreduced voltage due to an idle situation. Its capacity and design is such thatI don't see this problem. I will look out for that though now that you havementioned it.



Tim you mentioned:
?If you discharge a battery down to 60% SoC or 12.2v as you claimis happening, then it will take quite a few hours of continuos driving with analternator voltage of 14.4v, to recharge that battery?.



I know you know that an open circuit voltageand a battery voltage under load are very different. Yes I get a 12.2v voltageunder load (from the car drawing power). This does not indicate a 60% state ofcharge. The car is achieving fuel savings (and fuel savings I am very happywith!) by at times restricting alternator 'drag? on the engine. At these timesthe battery supplements the cars electronics. Short of stopping the car,disconnecting the battery and waiting a few hours before taking the crankingbattery voltage.... i don't know to what SOC... However I trust that the LandRover engineers have figured that out and that it is perfectly fine.

Re Different battery types and chargingifferent chemistry types require different charge rates and voltage rates tomatch their chemistry. The top voltage ranges from 14.2 up to 15.1. All theevidence/opinions on this forum suggests that no matter what types of batteriesand capacities, just hook them up to an alternator and at the end of any lengthof cable as long as its thick enough and you should be OK. You may never get afull battery but you should get by. My preference is to hook each (car andcaravan) to a device that is set to the specific charging requirements of thebattery and its SOC and let it look after it. This will get the battery asclose to fully charged as you can.



Detecting current draw by the car. There isa big shunt (a device for measuring current) on the negative battery terminal.It measures what is being drawn by the car and makes adjustments. I know thisbecause I incorrectly connected my first DC/DC charger directly to the batterynegative post. I had real problems. The car ran for hours on 12.3V. No a goodstate. As soon as I move the negative to connection to the earth position theshunt could then detect the current being drawn by the DC/DC charger and thecar compensated accordingly. That would suggest that the D4 can detect current being drawn by any device properly connected.



More soon.

Good grief....

A DC-DC charger to charge a battery that the alternator will charge anyway... Placing an additional current load above that of the batteries requirement.

Then a switching mechanism (relay) to switch items drawing current (fridge etc) from one power source to the other. That should spike nicely!

All so the battery is charged? Which it will be doing anyway whilst the vehicle is running regardless of the accessories attached (how do you think your main battery works)!

The accessories will "skim" excess power from the same wire...

Biggest enemy is voltage drop in the circuits...

If your alternator is putting out 13v at full steam it's likely that system draw is very low at that time and is being regulated.

Switch on the fridge etc and see if it climbs..

Also, where are you measuring the voltage? And what are you using to measure?

No matter how powerful the alternator, output is limited according to system demands.

The batteries are largely redundant to the system when the vehicle is running....

Voltage = Flow to make something flow (current) you raise the voltage above the object (battery) you are trying to "flow" into (within its functional design limits)

I am a bit confused about the low voltages the D4 alternator puts out and how this can charge my second battery properly.

Tim (or Tombie) if my second battery was at 50% S of C, what voltage could/should I expect to go into my second AGM battery (with decent cabling) ? Assuming I had the software fix if this was needed.

Brett...

Originally Posted by drivesafe

What a load of crap, while dedicated deep cycle batteries should not be used as starting batteries except in an emergency, there is absolutely no reason why you can not use a cranking battery as a deep cycle battery.

Before any form of deep cycle battery was available for RV use, everybody used cranking batteries as there auxiliary/house batteries and they had no problems.

The primary difference between a dedicated deep cycle battery and a cranking battery when used in cycling applications, is that you get more cycles from a dedicated deep cycle battery than you do from a cranking battery.

But this is why deep cycle batteries are dearer than cranking batteries. But the amount of energy you get from each type of battery is about dollar for dollar the same.

Try getting the correct info before lecture others on a subject you quite obviously don't understand.

Yep, always used cranking batteries as house batteries up until recently as I got them for nothing. Lasted fine for years, with a lot more than 'infrequent discharging'.