Updated 7 May 2017
Dc-dc charging charges boat, cabins, camper trailer, caravan and motorhome batteries faster and deeper. Collyn Rivers explains how and why.
Pic: original source unknown.
Applying a voltage across a battery causes it to charge. That voltage, however, must be higher than the battery already has. The higher that voltage difference the quicker a battery consequently charges. Alternators charged at a more or less constant voltage until 2000 – and a few still do.
As a battery charges its voltage rises. But if charge voltage is constant, the difference between that and the charging battery falls. Batteries so-charged rarely exceed 80%. As a result, many in caravans never exceed 65%. Moreover they take hours for even that.
Dc-dc charging – how starter batteries are charged
A vehicle’s starter motor is designed to work with 70%-80% charged batteries. The energy required to start cold engines is tiny. It’s less than 2% of battery capacity and typically replaced within two minutes. It’s cheap, rugged and simple. Fine generally for starter battery charging – but not RV use.
Dc-dc alternator charging overcomes these problems. Working fast, deeply, yet safely, it accepts whatever voltage available. Next, it converts it to that optimally required. It bulk charges at whatever current the battery accepts. Such units thus constantly increases charge voltage as battery voltage rises. This enables charging fully, deeply and rapidly.
Such technology, used for telephone exchange batteries for decades, was later adapted for vehicles. See Battery charging and battery chargers.
Dc-dc charging truly scores with batteries distanced from the alternator. Voltage drop prejudices charging, and fridge operation. Heavy cable is necessary, but a dc-dc unit close to the battery ensures voltage. It also extends battery life, and can transform three-way fridges.
Dc-dc charging – voltage sensing relays
These systems require a VSR (voltage sensing relay). The VSR senses starter battery voltage. It directs charge to that battery for two to three minutes after starting. Then it allows auxiliary battery charging once the starter’s exceeds 13.6 volts. The VSR also isolates the starter battery if its voltage drops below 12.6 volts. Some dc-dc chargers have VSR functionality inbuilt.
Until 2000, vehicle alternators put out 14.2-14.4 volts. Some post-2000 alternators are temperature controlled. These charge at 14.1-14.2 volts when cold. They reduce to about 13.2 volts once warm. They became common from 2009, and more so post-2013. Both types (that never drop below 12.7 volts when driving) work well from dc-dc alternator chargers.
A third type of alternator: Variable Voltage – was introduced around 2013. These are controlled by the engine’s central computer unit. They vary voltage from 15.4 volts to 12.3 volts. Some drop voltage to zero.
Bc-dc charging – regenerative braking
These alternators are commonly used for regenerative braking. They increase alternator voltage, thereby force charging the starter battery. That battery is (here) normally 80% charged. Braking boosts this to 100%. This provides all electrical energy required from that extra charge. During this, alternator output is zero, or too low for charging. It causes the VSR to ongoingly open, thus precluding auxiliary charging for minutes each time.
This necessitates a modified form of dc-dc charging. It is often known as bc-dc. With this, the charger senses starter battery voltage. I cover these units in Variable voltage alternator problems with caravans
The article also shows how to know your alternator’s type.
Bc-dc units are made by companies including Redarc and Sterling Products. Some companies now produce all such products as bc-dc. Those for use with Variable Voltage Alternators are of Low Voltage form. The above Link explains.
Installation is generally similar to that below, but there is no voltage sensing relay. A signal lead is taken from the ignition switch. Makers give full details.
Installing dc-dc charging
Dc-dc charging ensures a battery is alternator-charged safely, deeply and fast. To do so the unit must be close to the main energy draw load and battery. That load is typically a fridge or fridge freezer. It’s where the dc-dc unit and battery need locating.
Whilst dc-dc charging optimises charging, you still need adequate cable. This typically requires 10 mm² (ideally 13.5 mm²) from source to charging unit. Makers explain this – but it is not always stressed.
Most such units also have inbuilt protection to ensure starter battery charge priority. Recharging generally takes only two to three minutes. If inbuilt protection is not provided (excepting for bc-dc units) a VSR must be used.
Some dc-dc charging units have an inbuilt solar regulator, and/or mains battery charger. The sketch below shows a typical installation.
Installation may vary from brand to brand and type to type. The unit shown will charge an auxiliary battery from a 12 volt alternator. It does so at up to 40 amps.
How to install a typical dc-dc or bcdc charger. The function safeguarding starter battery voltage is inbuilt. Pic: Redarc.
Programming dc-dc charging
Different battery types require different voltage/current settings. Dc-dc chargers have programs for lead-acid, gel cell, AGM batteries etc. No programming is needed for alternator voltage. The systems accept whatever it is.
Lithium (LiFePO4) batteries have different needs. Redarc consequently has units specifically for this purpose. It stresses they be used only with LiFePO4 batteries they recommend.
The specialised Redarc LFP series of dc-dc battery chargers for (specific) LiFePO4 batteries. Pic: courtesy of Redarc.
Issues with dc-dc charging
Basic dc-dc alternator charger does not work well (or at all) with variable voltage alternators. Equally important, doing so may damage the auxiliary battery/s. See above re bc-dc charging.
Initial charging of a deeply discharged battery is generally limited to a basic dc-dc charger’s capacity: not the alternator’s. Dc-dc chargers under 20 amps thus usually take longer to charge close to flat batteries to half charge. Thereon, charging is hugely faster. The CTEK Smartpass dc-dc charger overcomes this by using alternator charging alone until dc-dc charging usefully takes over.
Pic: CTEK ‘Smartpass’ dc-dc charger. Pic: courtesy of CTEK.
This is of less issue with 30-50 amp units. With these, charging limitations are mostly alternator output, and (for some batteries) the maximum they’ll absorb. This is a lesser issue with gel cells and AGMs – and none with LiFePO4s.
If you like this article you’ll truly benefit from my books, moreover are technically accurate, yet in down to earth English. Batteries and battery charging is covered in depth in Caravan & Motorhome Electrics. Solar or RVs is covered likewise in Solar that Really Works! Solar for larger homes and properties is covered in Solar Success. The Camper Trailer Book andCaravan & Motorhome Book cover innumerable issues in depth. Click for my Bio.
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