Installing dc-dc chargers - known problems

1 Installing dc-dc chargers - known problems maximum of about 35-37 amps at juggling horse-power and torque to volts (about 520 or so watts). optimise available power, it juggles Prior to dc-dc charging any voltage volts and amps to optimise charging. drop between that alternator voltage Both expend energy in doing so, but and the battery limited the charging the overall gain in operating efficiency current accordingly. It did that be- makes that well worth while. cause a far from uncommon 5% drop In effect, a dc-dc charger increases in voltage resulted (on full load) in not the voltage from that incoming to volts, but often only volts whatever is required for optimum - and that in terms of available electri- charging. But, if that voltage is lower Collyn's office. cal energy is no longer 520 watts.

2 It is than the alternator voltage, because about 475 watts. That 45 watt differ- some energy is lost as heat (via volt- ence is lost forever - as heat energy age drop) along that cable, it can only The recent trend to dc-dc alternator along that cable. compensate by drawing even more charging was inevitable, not least as current down that cable. This further it resolved so many previous problems Where that battery is in a caravan it is not uncommon for that voltage increases the voltage drop - and thus in one go. It is the approach I recom- loses yet more energy. And that may mend on my website and all recent (under even light loading) to be little over 12 volts. not be available from the alternator. published articles. I am also currently As auto electricians (but rarely their Technical staff at companies like finalising the rewriting of two of my customers) will be aware, it's the in- Redarc and Optima are not dummies.

3 Books - and both will recommend that ternal resistance of the auxiliary bat- They know all this too. They thus this approach be taken regardless of tery, as well as the voltage across it at make it very clear in their installation the age of the vehicle. And explains the time, which limits charging rate. instructions that heavy input cable is why, and how it should be done. Internal resistance may thus limit a still required. Some owners, electrically competent 70 Ah battery to accepting little more But, to a lot of people, including one or not, fit the units themselves, but than 20 amps and, as cable voltage fears, some auto electricians, instal- most have them installed by auto elec- drop is proportional to the current lation manuals seem only for newbies, tricians. Feedback from my book and flowing through it, the voltage drop softies and year one apprentices, all of magazine readers, however indicates will be less.

4 Whom thus have a statistically better that not all are 100% happy with That still limits charging, but nothing chance of getting it right! the results. But rather than blaming the installation, they bad mouth the like to the same extent as (say) charg- Energy lost products. ing a far from uncommon 450 Ah Meanwhile, DIY types in particular The cause may a misunderstanding of battery bank. The latter is more than argue that as vendors say it can cope manufacturers' (correct) statements capable of accepting that theoretically with volts input, then even a close that the units accept a range of input available amps but cannot possi- to volt plus drop does not matter. voltage' (typically as low as 9 volts to bly do so because the available voltage If so installed, that dc-dc charger will 18 volts).

5 Not all installers however is now far too low. indeed be able to charge to some ex- appear to realise the consequences of Heavy cable still required tent. But it will be massively limited reduced input voltage. In practice, the reduced charging as that voltage drop will cause close This is possibly because battery mak- voltage hugely restricted the ability to to a third of the available alternator ers, some auto electricians, and al- charge any but single auxiliary bat- output to be lost as heat. It may still most all DIY installers think in terms teries. It is why users and installers thus charge a small auxiliary battery of amps (and amp-hours). are urged by battery charger makers quite well, but cannot possibly do so (and writer/engineers like me) to use with larger ones. Energy matters!

6 Seriously heavy cable to limit that And that, to a fair extent is what What we are dealing with here now is available electrical energy, watts. voltage drop. And that is the product of amps and Prior to dc-dc conversion, too volts and, if over time, amp-hours light a cable thus precluded fully time volts (watt-hours). charging any auxiliary battery . An alternator under load produces But, as astute readers have (say) volts at a probable 70 amps. worked out, any energy lost as That is a maximum of 980 watts. heat along that cable is no longer That 980 watts is the total electrical available its far end - no matter energy available for running the en- what is connected there. tire vehicle's electrical needs - includ- Juggling volts and amps ing that left over for running fridges A dc-dc charger acts much as and charging auxiliary batteries.

7 Does a torque converter in an That left over' energy is likely to be a automatic car. But instead of An elegant way of housing the readout. Pix: Redarc. feedback from my book buyers and (or whatever is required) that's what edge, my books and articles are the magazine article readers claim is now they seek to buy. only ones that disclose it - and warn of happening. They go to Bunnings or an auto parts the consequences, but as far from all So whilst dc-dc chargers assist to store, and ask for 4 sq mm or 6 sq mm read them, DIY installers may build optimise charging, there is no way cable. But that's not what they are in even greater voltage drop. they can recover energy lost as heat sold. What will be supplied is mm Collyn Rivers along too-light connecting cables. At or 6 mm auto cable. least the minimum size cable specified Auto electricians know that this is must be used.

8 Typically sq mm, and about The writer is an ex motor industry The products work even better con- sq mm respectively. But the typical research engineer, who later switched nected by cable heavier than specified. buyer does not know that. Let alone careers to become a technical maga- that this absurd rating tells you only zine editor and then writer and Auto-cable trap the size hole you can just push it publisher. Many DIY installers do know this. through! His main books in the auto electrical They realize that when the manual says that sq mm or 6 sq mm cable Or they may ask for (say) 50 amp our area are his very popular Mo- cable' - totally un- torhome Electrics, it covers caravans aware that rating' is and camper trailers as well, and is simply, and only, the updated each year. maximum current Also being ordered extensively (even that cable can carry ahead of publication) is his all-new before its insulation and expanded (2nd Edition) Solar melts.

9 That Really Works). This covers solar All of that too is installation in depth. totally obvious to the Also new is as equally fully revised product makers, but and expanded Camper Trailer Book - it is not to the buyers including a great deal on their design of their products, nor and construction. most motoring or RV. journalists. To my Collyn's website is: reasonable knowl- Redarcs's globally selling BMS series. Pix: Redarc.