charging "van battery' from alternator

I think the simple answer is 'Yes' provided:

* You have the right gauge wire all the way to the battery to minimise voltage drop
* You drive for long enough

AGMs like a higher charging voltage than the typical 13.8V from older alternators. Have you measured your existing charging voltage at the existing battery ?

13.8 will still charge, but it will take longer than say 14.2, 14.4V than some of the newer vehicle alternators put out.

People shell out money for devices that boost the voltage higher (eg 0.5 to 1v) to charge their batteries faster and to allow for voltage drop from long and thin wiring.

Nolbrin, you are likely to get some divergent views here. In my experience, you can get near enough to full charge from the set up you propose, but you MUST use very heavy cable for the full run to reduce the voltage drop (which is a factor of current flow, cable length and cable thickness. I've been doing it for years, first in CT, now in van.

I have previously used 25mm sq double insulated welding cable via 75 Amp Anderson Plugs for this purpose, with great results. I currently use 12mm sq cable via 50 Amp Anderson Plugs and it is fine.

Ensure you put a fuse or circuit breaker as near as possible to each end of the cable, connected to battery terminal if possible. My connection is simply from the terminals of the AGM deep cycle in the engine bay to the terminals of the batteries in the van (via fuses and Anderson Plugs). This means whenever the van is plugged to the vehicle, the batteries are in parallel.

Norm C

If you really want to do it properly than get a DC to DC charger installed in the van and run a heavy wired circuit from the vehicle battery to the van using anderson plugs.
Also do this via a relay in the vehicle that turns the power off to the van when the ign. is off.

As above, the voltage across the battery terminals need to be from ~13.8V to 14.4V.

If the rear battery is very discharged, it may take a high charging current initially and therefore have a larger alternator-battery voltage drop (since voltage drop V = IR = current times cable resistance), but as the battery charges, the voltage drop reduces etc etc.

Be wary of statements like "dc-dc converters maintain (eg) 14.4V at the battery.... hence charging faster".
This assumes there is no current limiting. IE - some converters can only handle 8A whereas the battery may accept much more.
In such cases, the battery will recharge faster without the dc-dc converter....
When the "naked cable recharge current matches that of the converter, the converter charges faster, but it may not have time to catch up to the naked cable recharging.

I consider dc-dc converters to be useful only where voltages are so low at the far (battery) end that boosting is required for charging. If "speed" were the issue, I suspect spending less money on extra cabling to be more beneficial. (Many swap front/rear batteries else move the battery and alternator closer together....)

Yet again, the best solution depends on your circumstances etc.

Now I'm no expert but my understanding is a DC to DC charger will out perform standard alternator charging in 99% of applications.

Most deep cycle batteries need a charge of 14.5 to 14.8 volts depending on brand and charge specs...... 0.2 to 0.5 volts can make all the difference from having a battery that is only charged 70% to a battery that is charged to 100%.

Remember a battery at 12.7v can be nearly fully charged and at 12.2v around 50% discharged.

Batteries are one big chemistry set, treat then right and you have no problems, treat them wrong and????.

Have a look at "Arrhenius equation" regarding chemical reactions

An automotive charging system was never designed to charge aux batteries let alone charge a starting battery to 100% SOC

Another good thing about DC to DC charging in automotive applications is being able to limit current to an acceptable level, in an ideal environment a battery can accept and survive a higher voltage and supply current but this is with correct temp monitoring.

Battery cable size is a different thing all together, you have to work out what maximum charge current or load current is and use the highest figure plus as rule of thumb 20%.....then fuse the cable accordingly.

Minimum battery cable size for low charge and draw should be 6B&S but preferable 3B&S.

Going bigger is no real advantage as it is harder to install unless you require high current supply.

For example-: if your going to have a maximum charge current of say 25 amps and a draw current of 15 amps max 6B&S cable is fine as it can handle 25amps + 20% = 30 amps comfortable over long distances, so fitting 3B&S cable that has a much higher supply rating is not going to make no difference at all.

6B&S cable is 14mm2 and 3B&S cable is 25mm2

But 99% of the work we do we will always use 3B&S cable when doing due battery installs....why so people can use the aux battery to jump start or winch and we don't always know what the customer is going to do in 12 month time.

Interesting that we have people using cable sizes to current limit battery charging, AND statements about how effective dc-dc converters are.
Yet none mention the circumstances such as alternator voltage. battery voltage, cable resistance or current, charge times, SOC etc.

For the non-expert it does indeed seem complex.
For the experienced it tends to be straightforward provided the conditions are known. As I said above, there is no single or best answer that applies in general.


For example, at the moment I am using a 38AH AGM UPS battery as my only battery. My reduction starter motor draws about 140A and the beast starts within a few seconds.
The typical initial charging current is 40A dropping to ~10A after 30 seconds.

That initial 40A is FIVE TIMES what the Waeco converter can supply (AFAIK that has an 8A limit) so I am charging 5x faster WITHOUT the converter. It wouldn't be after about 1 minute that the Waeco could charge quicker, but since the battery voltage is 14.4V anyhow, it won't.
Similarly for my rear battery depending on what I'm using - whether the same AGM or larger flooded batteries.


Are there really 99 others that offset my and other's non-gains with converters? I doubt it - certainly no-one that has reasonable driving periods, but maybe for those that are stationary with undersized (long) cables and under performing alternators. (Solar users - NEVER!)


I can only imagine that a long discharge would have a greater and longer "high" current charging period. (My battery is rated for 1.60V/cell end voltage at 140A for 1 minute.)

So in my case, I have no interest spending money on something that reduces my rate of charge.
As to current limited charging - that's a different issue (PWM FETS etc).

How is current limited charging set on your dc-dc converters?
And what is the highest rated converter you guys are using?
I think the Waeco 8A converter costs ~$120 but am unaware of its adjustable voltage nor current....


FYI - the alternator is a 3-wire 70A Hitachi LR170 from a boxer-Subaru. It does initially charge at ~14.7V or higher, but drops to 14.4V within a minute.
I've only had this alternator for ~1 week & I'm still getting the feel for it.
It is the first alternator I have had that exceeds 14.4V (excluding incorrectly wired/sensing alternators, or those boosted with sensing dividers or diodes).

If you want to charge the battery to full capacity correctly directly cable system will never work regardless of what size cable you use. Direct cable charging with cable is a technical impossibility

If you want a proper system that works, use 6B&S cable to the caravan and a Ctek D250S DC to DC charger mounted close to the battery.
The Ctek D250S can also be used as solar regulator if you want to add solar panel.

Without getting too technical, I have a 2.7ltr petrol injected Hilux. Setup is normal cranking battery, S4V starter solonoid that brings in 4 batteries in both the rear of the ute and the pop top which totals 400amps house storeage.
I have all 6mm cables with Anderson plugs and additional outlets. 150watt Solar panel to plug in when stopped.
I have not had any problems with this setup at all, as I kept it simple and don't worry about things like a couple of volts lost here and there.
All I have found is that I loose charging power on dull days with the solar panels.... so simple, I am buying another 150watt panel to aid charging.
The charging whilst driving has never gave any problems.