Thread: DC-DC charger- which one?

Originally Posted by RobA

We tow an Ultimate and given all modern vehicles have a lazy alternator, hence the invention of DC chargers to compensate for that little design fault, and to overcome that we simply but a RedArc BMS in the Ulti and that resolves the lack of volts coming back from the car via the Anderson plug. If you rely just on volts via Anderson plug camper batteries will not be charged.

I have a Traxide D4-5S kit, which includes the heavy duty cable to the rear complete with Anderson plug that I use for multiple duties; eg running tyre compressor etc. or connection to solar panels (via MPPT) to charge the yellow top or to charge camper batteries while driving or to use a 240V battery charger when at home.

So I thought this was a good setup to charge the 2 x 100A gel sealed batteries on the camper while driving, which are directly connected via a Anderson plug and 50A CB. When camped, I use the plug to connect solar panels (via MPPT). But I have no BMS on the camper, only a 240V charger not used while driving or on trips.

Perhaps assuming I have a good setup was wrong based on RobA's highlighted comment re low volts coming from vehicle?

But I've had a cig plug volt meter reading over 14V when vehicle running at both the front sockets (cranking battery) and rear sockets that come with Traxide kit (aux battery) and I thought this was good voltage for charging the camper batteries while driving, assuming similar volts via Anderson plug? Happy to be corrected if wrong.

Regards,
Scott

PS ... sort of a hijack of OP but just trying to increase my knowledge. I do get a bit lost sometimes with this stuff. The camper is quite new, so no actual experiences with using the car to charge batteries up from a low SoC.

Originally Posted by LRD414

The camper is quite new, so no actual experiences with using the car to charge batteries up from a low SoC.

Hi Scott and you will have no problems with charging low house batteries while driving.

It's not just my isolator that improves charging, your alternator has the unique advantage of adjusting it's current output for different loads.

So when your house batteries are low, your alternator will provide a much higher current, thus enabling your house batteries to charge faster.

This is something no DC/DC device can do.

Originally Posted by drivesafe

So when your house batteries are low, your alternator will provide a much higher current, thus enabling your house batteries to charge faster.

Just wondering, will this higher current output from the alternator also go to the terminals of the main cranking battery?

The voltage will be seen at the cranking batteries terminals but the amperage will go to the lowest SOC batteries

Depends on how you've got the batteries wired up.

ALternators output ramps to suit the load up to the maximum output of the alternator then the voltage drops off and the amps keep going up. this is what most DC-DC chargers cant do, they hit their max out put and stay there. This is where the traxide systems in 95+ percent of cases outperfrom dc-dc systems.

The next paragraph is the really really simple version of direct wired charging for batteries in parallel, the actual flows are a lot more complicated than the simple on/off examples Im using.

The output of the alternator goes to where its needed, if you've got a flat battery and a fully charged one the power flows into the flat battery. If the alternator is hooked up to the fully charged battery and your second battery is hooked up to the fully charged battery then it goes past but not into the fully charged battery. If you have the alternator setup between the 2 batteries then the power doesnt flow towards the charged battery until the flat battery has charged up and both batteries are at the same level of charge. At this point the output of the alternator splits between the 2 batteries equally. Once the batteries are charged then the amps of the alternator drop off to cover just the electrical loads.

If the flat battery is down low enough you'll actually have some amps coming out of the fully charged battery going to the flat battery.

Lets say that your fully charged battery is not the vehicle battery but its the aux battery. lets say the main battery is a little low.

A traxide unit will, within certain limits depending on the unit you get, allow the aux battery to put amps into the vehicle battery. The upshot of this is if you have some sort of charging system hooked up to your aux battery that will then permit you to charge both batteries at once.

I'm not currently aware of a DC-DC charger that will permit that to happen and theres not to many DBS systems that will either.

Originally Posted by SilvaD4

Just wondering, will this higher current output from the alternator also go to the terminals of the main cranking battery?

Hi Silva, and I am not quite sure what you want to know and Booger and Dave have covered it pretty well but I will give you some working examples and these are crudely explained examples.

Let say you have a standard D4, with no form of dual battery system fitted and you have been driving for a few hours, so your cranking battery should be fully charged.

As you are driving along, your D4 will need around 40 amps of power just to meet it's own electrical requirements.

So in this situation, there is a 40 amp draw being applied to the alternator and this current flow is being supplied to the vehicle via the positive ( + ) terminal on the cranking battery.

The Positive ( + ) terminal is being used as a distribution point for the positive power circuit in your D4.

NOTE, because the cranking battery is full, the is no flow of current into the cranking battery and no flow of current coming out of the NEGATIVE ( - ) terminal of the cranking battery but more on this later.

Next situation. You have just started your D4 and as you drive off, the cranking battery draws say 5 amps to replace the energy used while starting your motor.

This time there is a 45 amp draw being applied to the alternator and the 45 amps is supplied to the cranking battery's positive ( + ) terminal.

Again, 40 amps goes on the power the D4's own requirements but 5 amps flows into the cranking battery via the positive ( + ) terminal and 5 amps comes from the cranking battery via the negative terminal.

As the cranking battery charges, it's current demand reduces, so the amps flowing through it reduces.

Now for a dual battery type situations.

You have been camping for a few days and with my isolators, you will have been drawing power from both the cranking battery and the auxiliary battery.

Lets say you have drawn the maximum available capacity from both batteries, with your Cranking battery at 50% SoC and your auxiliary battery at 20% SoC.

This time, once you start driving, there will be a 40 amp draw for the D4's electrics, around as much as an 80 amp draw for the Optima and another 30 amp draw for the cranking battery.

Thats a total of around 150 amps being applied to the alternator.

This time, around 150 amps will be provided by the alternator at the cranking battery's positive terminal. 120 amps will flow passed the terminal and on to the auxiliary battery and the D4's electricals. But 30 will be drawn through the cranking battery. And note there will be an equal 30 amp flow from the cranking battery's negative terminal.

As both batteries charge, the current draw will quickly decrease.

Note, the only current monitoring the D4 can do is the current that flows through the earth lead connected to the cranking battery's negative terminal.

The EARTH RETURN for all other devices is by the body, chassis and the motor itself. Not via the cranking battery's negative lead.

So no other current is monitored or where that current is going to. So claims that there is some sort of special current and/or voltage going to an other battery and the alternator knows about it, is just mythical.

The 25amp charger was recommended over the 40amp option to avoid pumping more amps into the auxiliary than is good for it.

Correct me if I'm wrong Tim but the charge will only supply to demand, not pump out to it's max capacity hoping that whatever's connected will deal with it. Just as you can use a higher amp but same voltage output phone charger than the std one. The phone will only draw whats needed, the excess is just ether. But a lower amp rated charge would be damaged due to the excess draw of the device.

Hi Clubagreenie, this is one of the problems with the marketing they use to flog DC/DC devices. They try to scare people into thinking that they will destroy a battery if they use the alternator as the charging device, falsely claiming that the unlimited current of an alternator will damage the battery.

You got it spot on, the battery itself with govern the amount of current you can charge it with.

NOTE there are a large number of STANDBY type AGMs that have a charge current limit, but most of these types of batteries also suffer from heat found under the bonnet, so they have to be fitted somewhere else.

By fitting these batteries elsewhere, the long cable run becomes a quasi voltage/current regulator and the batteries again self regulate.

We are in the D3/D4/RRS section of the forum and as such, 99% of these vehicles will have an Optima D34 Yellowtop battery as their auxiliary battery.

The Optima D34 Yellowtop battery is a genuine AUTOMOTIVE GRADE AGM, specifically designed to tolerate the harsh conditions found in a engine bay.

These batteries also have NO CHARGE CURRENT LIMIT, and can and will take everything the alternator can produce.

This means if the auxiliary battery was in a very low state, it could easily and SAFELY draw 90+ amps while recharging as you drive.

As these sort of charging currents are only available from an alternator, makes 25 and 40 amp DC/DC devices look like jokes, which in this case they are.