Magazine, Tools & Home Improvement

Charging Booster or Isolating Relay? The Decision Support

As in our solar contribution briefly mentioned, we have supplemented our external power supply by a charge booster. But is a B2B loader a sensible investment, or is it not enough to have a disconnect relay and the sole charge via the alternator? We get to the bottom of the question and show when a charge booster can be useful and under what conditions a disconnect relay should suffice.

Why do you need a disconnect relay?

By default, campers are equipped with two different batteries — one for the start of the engine and the other for the supply of our consumers under construction. The alternator in the vehicle supplies the vehicle’s consumers while driving charges the starter battery for the next engine start and also the body battery.

As soon as the alternator generator stops supplying power, the isolating relay comes into play. It interrupts the connection between the two power stores.

Without a relay, both batteries would be connected even after stopping the engine. If we consume power during setup, we will also draw juice from our startup battery. In the worst case, it is sucked so empty that starting the engine is no longer possible.

A disconnect relay is standard in every motorhome. This prevents unwanted discharging of the starter battery.

The problem of the alternator current

Ever since the Best AGM Battery or gel batteries have made their way into our mobile homes, it is difficult to charge these battery systems fully. As mentioned in my battery contribution, you need appropriate charging curves (IUoU) to achieve a full charge or to prevent overcharging. The fewest alternators have these. So often after about 80% state of charge is over.

Another issue can be long lines. Thus, the starter battery is usually near the generator; the living room battery much further away. If the cable cross-section is too small, voltage drops occur which also reduce the charging current.

With newer vehicles, the whole thing goes one step further. To achieve the prescribed exhaust gas values, in more modern diesel vehicles, the charge is cut after a particular time or after a full cost of the starter battery. In the end, the array is just as empty after a day’s driving as before.

Advantage of a B2B loader

By using a charging booster, the restrictions above are eliminated. This results in the following benefits:

  • Full charge thanks to IUoU characteristic curve (for longer journeys).
  • Compensation of voltage drops if the cable cross-section is too small.
  • Bypassing the alternator shutdown on new vehicles.
  • The link between 24 volt and 12 volt systems (depending on the choice of the charging booster).
  • Better load throughput even on short distances.
  • A B2B loader also acts as a disconnect relay.

But the advantages of a B2B loader do not say anything about whether you need it. Instead, it depends on one’s travel behavior. If you are rarely free and mainly on campsites, the purchase is at best a “nice-to-have” in my eyes. Even those who already own a power generator or a fuel cell in the motorhome can do without an additional charge booster in most cases.

Generators or boosters

Also, with this question, it depends again on the travel or standing behavior. If you want to load in the stand, a generator is the better choice. It works more effectively and costs less than the camper’s diesel engine. The same applies to a fuel cell and of course shore power. The charge of standing with a charge booster should be the last option to get the ability. Where also a rattling “Moppel” at the camping neighbors often gnawed something on the nervous costume. We only throw in our little 2-stroke generator when we stand alone. The booster is switched on exclusively while driving.

Which charging booster did we install?

As our truck has a 24 volt DC alternator, the cable length to the body battery is almost 5 meters and we are mainly free, we decided very fast to install a charging booster. The choice fell on a Farron B2B charger with a maximum charging current of 40A.

So far, the device is running well. It reaches its full capacity when properly discharged and works the charging curve properly. If the batteries are only slightly released, then the device has its problems. Thus, the charge booster only switches to the main charge when the battery takes at least 10 A, or the battery voltage falls below 12.5 volts. Especially in combination with a solar system, this can lead to problems. But as our batteries are usually discharged much more in-depth, this problem rarely occurs.

When is the purchase of a charging booster worthwhile?

I am mostly free and would like to charge my batteries effectively, even during shorter trips. Solar is not available.
I provide myself with solar power and would like to have an additional charging option in bad weather (while driving).
I want to load my 12 V system under construction through a 24 V alternator.

When is a disconnect relay enough?

It should be used mainly land electricity at Stell- or campsites.
I own an old vehicle with sufficient cable cross-section and a “normal” wet battery.
When power is low, I use a power generator/fuel cell

Tips for installation

As always, you should only lend a hand if you know what you are doing. Charging Booster work with high currents, what the correct dimensioning of cable cross-section requires. The following points are to be considered among other things.

  • The charge booster should be mounted as close as possible to the body battery to keep the cable length as short as possible.
  • The cable cross sections must be designed for the maximum currents.
  • The cables must be protected with the appropriate fuses.

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