Robert Wood

I have a NASA Marine BM-2 battery monitor which works very well (and you can actually see the display without having to get really close!). However I have noticed that when on shore power trickle charging, the capacity often sits around 87-90%. The reason for this is that in order to determine what the actual state of charge is, you need to discharge the battery by a small amount and see how the voltage holds up during the discharge. Only then can the monitor decide whether the battery was initially fully charged. Technically the indicated state of charge could go up from say 87% to 99% after a small discharge, although the actual state of charge has dropped from 100% to 99%! When cruising, it frequently shows 100% after a few hours' cruising, not because the charging is more effective, but because the battery bank is repeatedly being charged (engine running) and then discharged (engine stopped, lights on etc), so it can make a reasoned judgement. I would say if the battery is being trickle charged but isn't being discharged by anything, don't worry unless the indicated state of charge is less than 80% - it's the only way the monitors can work. Hope this makes sense!

Just be aware that the reason for a fridge being A+++ may be partly because its got such thick insulation, so you may get less space for food inside than a lower rated one! We run a Neff 240V fridge with no problems whatsoever. The fridge motor draws sufficient power to take the inverter out of standby mode, although the interior light doesn't, and will flash every few seconds if the compressor's off. Our 240V fridge takes an average of 1.75 Amps (measured over 9 hours at night, with no other electrical equipment on). Also be aware that a TV in standby mode may take sufficient power to keep the inverter running, so make sure you switch off at the socket when not using it. I've checked our TV power consumption and it takes around 44W when on (approximately 4.5 Amps at 12V) - this is a modern 32" flat screen LED TV. The one that came with the boat (same screen size) was an earlier model, not LED technology, and took 4 times as much current and really hammered the batteries! Check the rating panel when you buy one. If you have electronic equipment that will run off a 12V external supply, or has an internal 12V power supply, and is not specifically designed to run off a lead-acid battery, you will probably need to stabilise your battery voltage, as 14.4 volts could be too much for the equipment. Bedazzled sell a couple of 12V-12V stabilisers for LED strips, to avoid flickering and overheating, but when I asked they recommended the larger of the two as being more suitable for supplying other electronic equipment, as it provided a more stable output. Their actual words were: I wouldn't recommend the smaller LVDC24W due to its performance under varying loads however the larger LVDC60W is ideal for anything 12v and (obviously) up to 60W. In fact this power supply is a variant of a range designed to supply power to Televisions and Laptop computers and hence is very suitable for anything on 12v

Just a quick follow-up to the discussion about resistors. When I started my engine when it was hot I only needed the throttle to be on tickover, however this wasn't enough revs to fire up the alternator, so I had to slightly increase the throttle to make the warning light go out, and stop the alarm buzzer. I halved the value of the resistor in parallel with the light, and it now fires up at tickover, so no annoying alarm. Be aware, though, that you should determine the wattage of the resistor based on 12V being applied across it, which will happen if you turn the ignition on but don't start the engine. Incidentally I had always assumed that the main (or perhaps secondary) purpose of the resistor is that the alternator will still start up if the bulb has blown.

Previous posts are perfectly correct. I fitted a BM2 over 2 years ago and had to rewire all my battery bank negatives in order to fit the shunt correctly. The negatives for the inverter, main 12V busbar, and the bilge pump all went to different batteries! My main difficulty was how to get all these wires (including 50mmsq & 70mmsq cables) onto one bolt on the shunt.- possibly a short wire to a small busbar would have helped. Incidentally, the small link wire provided with the shunt seemed rather on the thin side, so I replaced mine with 50mmsq cable. The BM2 works absolutely fine and I'm very pleased with it - I can actually see what the batteries are doing when I'm cruising, unlike other makes with such a small display you can barely read it. I was able to position my shunt next to the battery compartment external vent, so it's less likely to overheat if the 200A limit is accidentally exceeded for a short period. Nasa were adamant that it couldn't be used above 200A when asked, and wouldn't commit to an upper "burn out" limit! It would, presumably, just act as a very big fuse, after glowing a lot. By the way, does anyone know if the copper rods are solid or tubes?

My boat has a Sterling DAI-3000C-12xx, dating back to 2008. This is a 3kW sine wave inverter/solar/charger and has a standby mode that, like one of the earlier replies, doesn't recognise small loads. I've used a very simple solution which is to determine by trial and error what is the lowest power light bulb that will force it to stay on (I found that low wattage low energy fluorescent lamps work best, possibly due to a switch-on surge). I now have a small table lamp positioned in the galley, with a 13W low energy lamp. All I have to do is switch this on if I need to use a low powered device. It seems a little bizarre when explaining to guests, but works well. This makes using the 240V ignition on the hob, the 240V oven fan, and the microwave very straightforward. It's not ideal for things like phone chargers - I usually ask guests to wait until the TV is on in the evening! It's a reasonable solution for short term use, however, and I'm even considering concealing the lamp and wiring it in permanently via a light switch, with just a neon indicator to remind me it's on. Before I did this we ended up lighting the hob with a match as it would only power up if the fridge compressor was running or if the TV was on! By the way, the manual for this inverter/charger that came with the boat was rubbish, and for the wrong model - so I couldn't program it. I've now downloaded the correct manual and edited it to improve the English and correct the errors, so if anyone wants want just let me know.