Preamble this starts on another forum where someone made a statement and I jumped in.
Steve wrote:Using a split charge relay will never allow the leisure battery to get fully charged. For that, you need a Battery to Battery (B2B) charger. Sterling and Durite sell the the best known ones.
Tow Itch wrote:Why won't a split charge relay fully charge a leisure battery? A split charge diode will be poor at charging because of the voltage drop of the diode but this is a split charge relay. I'm quite a big fan of Charles Sterling but his argument against a split charging relay is quite contrary to a lack of charging and is one area where I'm either mistaken or he is a bit disingenuous. [You must be registered and logged in to see this link.]
If we take this last example about using an inverter with flat leisure batteries. I don't know if I've missed something here but surely by the simple means of fusing either or both of the leads from the split charge relay this wouldn't happen. In actuality on a motorhome both sorts of battery would not be charged through the split charge relay. Yes there would be a connection to either the alternator or the starting battery and with the engine running that would be a live link but as stated a simple fuse set at the capacity of the relay would guard against this massive current.Split charge relay
This system is both dated and extremely dangerous and more than likely will make your boat fall short on CE requirements, especially if an inverter is used or a bow thruster. The good side is that it is easy to fit and requires no alterations to the standard engine system, but merely connects the domestic battery bank to the engine battery via a relay, which is energised when the engine starts.
The bad side (and the very dangerous side) is that a relay is prone to vibration faults and over loading. Say, for example, you have a 70 amp relay on your system and a 55 amp alternator, all seams great, but if you fit a 1500 watt inverter which can draw150 amps and one morning the domestic battery is flat. So, you start the engine to charge the domestic batteries, the 70 amp split charger relay will come on line to enable the alternator to charger the domestic battery bank. Then you load your inverter to 150 amps, the 150 amps will not be drawn from the domestic battery because it is flat but be drawn from the engine battery (which is full). That means you will draw 150 amps up the split charge cable and through the 70 amp relay. If you are lucky you will destroy the relay, if you are not so lucky then you will set fire to the cross over cables, hence the dangerous aspect. The system must be suitable for the purpose for which it is installed this is clearly not. Be warned about split charger systems using relays.
You might want to do something if the alternator reduces voltage once warm as many do because of thermostatically controlled regulators.
sparrks wrote:A 60A cartridge fuse to BS 1361 protecting the 70A relay in the previous example would take around 900A to 'blow' instantly (around 0.1s) and could carry a current of 150A for 500s (over 8 mins ) before 'blowing' I can only wonder at the breaking capacities and reliability of cheap auto fuses.
sparrks wrote:A 15A fuse of BS 1361 instant 'blow' current around 100A and a 60A load could take upto 1000s seconds to operate, 16.5 minutes. These figures are for BS 1361 fuses, I haven't found any figures for Auto fuses.
Tow Itch wrote:sparrks wrote:A 15A fuse of BS 1361 instant 'blow' current around 100A and a 60A load could take upto 1000s seconds to operate, 16.5 minutes. These figures are for BS 1361 fuses, I haven't found any figures for Auto fuses.
My first thoughts were "Glad I inserted
andI'm either mistaken or he is a bit disingenuous.
that should give me sufficient room to back pedal." With respect to the stuff written by Charles Sterling. If you do take someones word as a given it's not comfortable when you read a piece by them that you doubt.I don't know if I've missed something here but
Then I was thinking about fast blow fuses and possibly using micro circuit breakers.
Then I thought Knob. It's obvious you're no electrician TI.
Yes you're undoubtedly right sparks but this problem or a similar problem occurs in every single fuse protected circuit. It's the same problem as if you connect up your 16amp EHU through a 13amp plug and try to draw 16amp (which you can do for quite a while) or go bananas and jump in a 32amp commando plug for a hot tub and try to draw about 25amp. A fear is introduced to the reader about a just over 2 times overload but in reality the size of the overload is in proportion to the speed of the break.
The problems vary because of the nature of the equipment used and that some appliances will show a high initial draw but essentially this is a question of how we specify fuses.
What happens to the relay and wiring in this circuit is comparable to components in any other circuit at a point of overload isn't it?
The only thing I know of (or think I know) is that you tend to fuse at one third more than the expected load.
So I think my questions should be:
1) How do we determine the peak demand from the leisure batteries, or is it peak charge offered by the alternator? Then perhaps questions if that charge offered is greater than suitable. Possible case for intelligent charging here.
2) Specing relays. Do we overspec because some are carp(sic) and are never going to handle their marked amperage? Or do we overspec the relay (and the wiring?) because for some reason this situation is unique and fuse to the expected charging amps + 33% rather than to the components used?
3) Basic principles for fusing a circuit. Is there something unique about this case? If not what are general principles. This will be useful for other auto wiring.
I moderate another camping forum and while this isn't relevant to that forum there are a couple of guys who should be able to give an input on this I'll try them.
So peridot and navver are my first thoughts for help. Either forgot or not sure what Jake did but thought it was vaguely "electronic" anyone else help. Come on don't hide your light under a bushel!