Boating NZ : FREE TO READ February 2014
is considered a flat battery. Because those variables are relatively small, digital volt meters are much easier to read than their analogue cousins. Determining the value of a tiny deflection on an analogue meter can be difficult. ” Volt meter readings, he adds, can be riddled with pitfalls, and should be interpreted with care. “For an accurate reading the volt meter must – in the first instance – be wired correctly. That means it must be connected to the battery’s positive terminal with a dedicated wire. Many DIYers simply tee-off a positive wire from the switchboard. That’s no good. The reading won’t be accurate because there are all sorts of resistances and voltage drops present in the switchboard – you’ll have a distorted reading. “Furthermore, you should rely on a reading only two to four hours after the battery has been fully charged. The battery needs to stand for a period to allow the surface charge on the plates to dissipate. If you don’t wait, you’ll get a false reading.” AMP METERS As with volt meters, an amp meter provides some easy visual clues about the performance of the boat’s electrical system. It indicates that the charging system is working properly. Depending on the condition of the batteries, the meter might indicate that the batteries are receiving, say, 30 amps – a value that drops off quite quickly as the batteries take more charge. More importantly, when the engine’s shut down, the amp meter reflects the load on the batteries. This is particularly useful if you are worried about the size of the load relative to the capacity of your batteries. By isolating different circuits – fridge/lights/nav instruments/ chartplotter – and monitoring the amp meter to measure the load drawn by each of these, you can manage the electrical system and battery life more easily. Some older- generation analogue meters may have a negative/positive calibration – the needle moves into the negative zone to indicate the amps being consumed. NB: if you have switched off all circuits and the amp meter still shows a load, be nervous. There’s a good chance of a stray current. If so, it will be draining the batteries and may be contributing to electrolysis – a related but completely different can of worms. CRANKING AMPS AND AMP-HOURS Just when you think everything makes sense, volt and amp meters get more complicated. One of the most plaintive cries you’ll hear around marinas is from boaties complaining that their volt meter shows well over 12 volts, but the battery refuses to crank the engine. Why is this? Well, it’s because it isn’t the voltage that starts the engine – it’s the cranking amps. By way of illustration, consider this: you can easily create 12 volts from a handful of torch batteries connected in series (positive to negative) – but they won’t turn over a diesel engine. They also need amps – and plenty of them. You may remember that your start battery carries a CCA rating. It stands for Cold Cranking Amps and indicates the number of amps available, when the battery is in optimum condition, to crank over a cold engine. The cranking amps in the battery provide the required grunt and, depending on the charge/age/ condition of the battery, the CCA value is diminished. Unfortunately, neither a volt meter nor an amp meter measures the available cranking amps in a battery. You’ll need a load tester, and few boaties carry one of those on board. Similarly, neither volt meters nor amp meters measure available amp- hours. You may also remember that your house batteries are – or should be – deep-cycle batteries, designed to take repetitive charge/discharge cycles and their size in reflected in amp-hours. This is the number of hours the battery can deliver a specified amperage. As with a start battery’s CCA rating, so the house battery’s amp- hour rating falls away as it ages – and even more quickly if the battery is maltreated. Neither volt meters nor amp meters nor a combination of the two can sense the level of available amp-hours. Purists will point out that only a hydrometer – an instrument that looks like something from the Spanish Inquisition – can provide a true, accurate sense of a battery’s condition. It is used to test the specific gravity of the electrolyte in each cell of a lead-acid battery. Apart from the fact that few boaties carry a hydrometer, let alone know how to use one, lead-acid batteries are becoming increasingly obsolete in recreational boating. They are being replaced by sealed, maintenance-free AGM and GEL batteries. So where does this leave a boat owner interested in managing and monitoring his electrical system more effectively? Someone interested in learning more than the superficial values provided by a volt meter or an amp meter? The DCSM 600 can display a range of information including tank status BEP's DCSM 600 is a versatile system that does much more than monitor battery health 021470 subscribe online at www.mags4gifts.co.nz/boating-nz 45 subscribe online at www.mags4gifts.co.nz/boating-nz 45 PBoating_volt metres_Feb14.indd 45 21/01/2014 4:39:23 p.m.
FREE TO READ March 2014