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Charging Question

Tow Itch
Tow Itch
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Join date : 2011-06-20
Location : Leigh Gtr Manchester

Charging Question Empty Charging Question

Post by Tow Itch Fri 21 Feb 2014, 12:35 pm

This isn't strictly relevant to this forum but more related to motorhomes though parts of the basic question do apply. Also we have some people with vans who pull Dandys.

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.]  
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.
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.
Yes/No thoughts?

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
I'm either mistaken or he is a bit disingenuous.
I don't know if I've missed something here but
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.

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!

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Charging Question Empty Re: Charging Question

Post by navver Fri 21 Feb 2014, 10:18 pm

Whoa, a bit heavy for a Friday night. Sorry I don't have my regs book at home any more as I now work in the office but I'll have a go.

Fuses blow simply by overheating and take time. The larger the current the less time they need. Close protection (a cartridge fuse) should blow within 4 hours at 1.5 times the rated current. Breakers typically have a thermal part for overload and a magnetic part for short circuit.

This gives a typical characteristic for fuses of various ratings.

[You must be registered and logged in to see this link.]

Time in seconds is the vertical axis and current on the horizontal. It shows a 60A fuse will take about 1200A to blow in 0.1 seconds and about 6 seconds at 100A.

We protect cables by coordinating the protection to what the cable will withstand. A PVC cable will normally run at 70deg max conductor temperature at full load although I usually choose them not to get any where near that otherwise people start getting worried. The modern XLPE cables run at 90deg.

When a fault occurs, the current will increase and cause the cable to start heating up. PVC cable can rise to a conductor temperature of 145deg before being damaged. So we want the fuse to blow before the cable reaches 145 deg.

The impedance of the circuit (posh word for resistance) limits the fault current. We want enough fault current to blow the fuse within 0.4 seconds for sockets and 5 seconds for lights etc., (to protect people), but we want the fault current limited to below the amount the cables and switchgear and relays, contactors etc will withstand for the duration of the fault (to protect the equipment).

Circuit breakers are good for overloads (a small amount of excess current) but fuses are better for short circuits (a large amount of excess current). We can use a fuse to back up a circuit breaker so the breaker trips on overload but the fuse trips on short circuit. That way the fault withstand current of the breaker may be less than the prospective (max possible) fault current in the circuit.

This coordination is just a small part of what I have to do when designing the electrics for large buildings.

The peak current with the 2 batteries and alernator etc. depends how it is wired. Basically you will have a network with 3 sources of potential(voltage), a load and various resistances. Everthing will have some resistance, some in the alternator, some in each cable, some in the load, some in each battery etc. Ohms law can be used to determine how much will flew in each part of the circuit. Amps = volts divided by resistance(ohms)

The relay will pass charging current from the alternator to the leisure battery, and the invertor will be connected to the leisure battery. If a heavy load such as an invertor is connected to the leisure battery it will draw current from each of the alternator, vehicle battery and leisure battery.

The current from the alternator and vehicle battery will pass through the relay, the current from the leisure battery will not. If the leisure battery is flat, it will contribute very little current to the invertor, if any, and will most likely draw charging current from the alternator and vehicle battery.

So the relay will pass the current flowing to the invertor and the charging current to the leisure battery.

The more current that is drawn, the more the voltage will drop in the vehicle battery and alternator and cables (due to resistance) and this will lower the current drawn by the invertor and leisure battery charging.

If there is a fault, the current will be drawn from the alternator and both batteries. The prospective fault current of a battery wil be the voltage divded by the sum of the resistance of the cables, fault and internal resistance of the battery. The alternator will also have a fault current. As long as each battery and the alternator are individually fused with fuses with a high rupturing current (HRC) such as cartridge fuses, they should blow before cables or equipment become damaged as long as they are all correctly coordinated by the system designer.
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Charging Question Empty Re: Charging Question

Post by peridot Sat 22 Feb 2014, 6:49 pm

Well these posts are certainly in the right section of the forum  Smile 

You are right TI, a split-charge relay arrangement will allow for complete charging of the battery.

Relays are commonly used in high vibration environments so to say that they are prone to vibration faults is wrong. Obviously a poor quality one may not perform correctly, but any poor quality component may fail to fulfil its intended purpose in more extreme environments.

It is also wrong to say that a relay is 'prone' to overloading. Overloading may damage any component, and therefore the circuit design must account for likely circumstances and protect accordingly.

Navver has covered the selection and sizing of fuses comprehensively. I would only add that the issue of fuses being better than circuit breakers for short-circuit protection is even more pertinent where DC circuits are concerned. There are very few safe and reliable DC circuit breakers. Even some from well-known manufacturers should not be touched with the proverbial barge pole, so I would never use the miniature ones offered as alternatives to automotive fuses. When an AC circuit breaker interrups a short-circuit, the resultant arc tends to be extingished as the AC waveform passes through the zero point. With DC, the voltage and current are constant and it is easier for the arc to be sustained with destructive consequences.

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