Battery Monitor Shunt - Electrical Expert Please

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TonyS

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I would like to fit a battery monitor and don't want to pay for a Link 10. I like the NASA at under £100 inc shunt. The problem is the shunt is rated at 100A and the boat is fitted with a bow thruster rated at 250A. I think I only use the bow thruster for a maximum of about 3 secs. I have just looked up RS components shunt data and the max permissible overload for BS89/IEC51 class 1 or class 0.5 shunts for 5 secs is 5 times the rated current and for 10 secs is 2 times the rated current. 100mV at 250A is only 25W. Would you agree that it is permissible to use the standard shunt in this application.
 
Hi Tony If you are only concerned about the power rating of the shunt then if you have a shunt as supplied by NASA then it should not be too difficult to make another shunt of the same resistance but much greater power handling capability. The simplest amaterial to use is stainless steel sheet. Make it about 2 inches long between connection bolt holes and about 1inch wide from 18 gauge SS. You then cut or grind the s/s in a wasted shape until you reach the correct resistance.
The resistance is tested by setting up a number of lamps as a load about 25 amps should suffice and measure the millivolt drop across the resistor/shunt. Compare this to what you get with the supplied shunt. You will see from the supplied shunt how big it must be to handle the 100A so go for a design 3 times as big.

That doesn't mean you have solved your problem though. 250 amps on the thruster will produce 2.5 tiimes as much voltage at the analogue to digital converter which means it will probably run out of range of the converter so the current consumption may not be registered.

Bottom line either delete the thruster from the shunt circuit hence from the calculations which will presumably give a cumulative error. however..
If you connect the thruster supply straight to the alternator (which is probably the normal connection anyway) much of the current will be supplied by the alternator (not measured in or out) with a portion coming from the battery. That portion would be error but at least not all.

good luck and let us know how you get on. olewill
 
I wouldn't do that. The voltage seen by the NASA monitor will be 250% greater than its rated input voltage which might cause it to fail the first time it is used. NASA stuff is good so long as you stick to what it's rated at.

However, engine starting currents can be far greater than 250amps so the NASA device must have an intermittent rating greater than 100 amps and recalcitrant diesel engines often need 10 secs or more starting ...

The safest solution might be to have the bow thruster bypass the shunt. In practice, I might want to do that anyway (thus avoiding unwanted additional connections in series with the supply cables to the thruster). Alternatively, you could parallel the shunt with a suitably rated switch, operated only when the bow thruster is used, but that would be too much of an inconvenience for me.
 
On many installations the high current devices such as a bow thruster or anchor winch which are usually only used when the engine is running are connected to the engine start battery circuit, not to the house battery circuit. The battery monitor is usually on the house circuit (unless it is a dual system such as the link 20), and therefore the shunt is only required on the house feed. With the thruster and winches typically being used for less than a minute at a time, the engine alternator will recover the start battery very quickly after those loads are used and if you have a VSR that favours the start battery you will never have a problem flattening it.

Wiring the thruster and winch off the start battery also means you can often share the heavy duty cables required i.e. the same cables can be used to the thruster as to the winch (presuming both are in the bow), and if your engine is centrally mounted and the batteries behind the engine (common in larger boats), the heavy duty cables run forward to the engine, and then off a big buss bar forward again to the winch and thruster, thereby saving cable, weight and expense for those very thick cables (which are probably rated at up to 600 amps to start say a 70HP diesel).
 
[ QUOTE ]
engine starting currents can be far greater than 250amps

[/ QUOTE ] Blimey, you must have a big engine! Most boat diesels are relatively tiny engines with, typically, starters rated around 1hp. Starting current is usually below 100A.
 
Thank you to everyone who replied. I have spoken to NASA this morning for the third time. They say it is OK to use their shunt and it has been used with car starter circuits with no problem. My problem was they initially said yes, then no (300A was too much) then with new info it is now OK. If I do burn the shunt out then thanks William for the design of a DIY shunt. By the way for a bowthruster the requirement is a minimum of 280 AH battery. Our start battery is 80 AH. The alternator only produces 60A max.
 
sunbeam .. i said can be. Engine size, type, and ambient temperature determine what size cranking battery you need. In general, cranking loads are greatest for diesel engines. A 105amphr battery has a typical MCA of 650amps and typical cranking current of 75 to 400amps and more for modern diesels.
 
Cranking current...

The cranking current quoted for a battery is what it's capable of supplying. The actual current is determined by the starter motor. Most boat diesels have starter motors which take less than 100A.

And I don't think anyone's ever called me "sunbeam". Hope you don't talk to the customers that way.
 
Re: Bypass the occasional high current stuff.

Calculation of battery power consumption via a shunt includes a % error. I think working with a % error of 100amps is preferable to a % error of 250 amps, so best eliminate the high amp circuits to begin with.

I found an internet site from a manufacturer that sells a non amp counting battery monitor product. They published some detailed material that basically questions the whole concept of monitoring battery charge via amp hour counting.

There are many factors that cause the accuracy of amp-hour meter predicted battery charge to drift out very significantly when in weekly cruising mode. If this is you objective in fitting an amp hour meter I would not worry about excluding short bursts of high current consumption.

I still intend to fit an amp hour meter on my boat but I expect to periodically reset it to 100% charge after getting a 24 hour mains powered charge in a marina.

Do amp hour meters monitor voltage as well in order to cross check for accumulated amp counting errors? If not then over time as the battery ages I will have to cross check amps used whenever my switch-panel voltmeter touches the amber segment in order maintain an up-to-date picture of the usable capacity of my domestic battery.

Does anyone know of an amp-hour meter that requires few button presses to reset to 100% charge or zero amps used?
 
Re: Bypass the occasional high current stuff.

Amp hour meters are difficult to do accurately. Batteries vary with type, size age etc in their charge efficiencies. The charge will tend to leak away internally over time again depending on those same factors. Charge delivery efficiency will also depend on rate of discharge and temperature. It is possible to use an amp hour meter as a guide with ones own experience. Presumably there may be intelligent designs to allow for some of these things.
 
Look at Link 10...

The Link 10 is one of the best value amphour monitors. It has a very sophisticated program and delivers accurate results. Latest version has a square bezel rather than the round bezel shown.

There's a lot of misinformation about having to reset these, and this is also promoted by makers of alternative equipment. Believe it or not, the truth is that the Link 10 automatically resets itself to zero as long as: a) the batteries have been discharged by at least 10%, and b) the batteries have been fully recharged.

Like many electronic devices, the Link 10 suffers from RTFM problems. It's vital to read the instruction manual carefully if it's going to function properly. People who don't read the manual have problems; those who do think it's a great bit of kit. I've had one for about 8 years and find it very useful.
 
Don't worry about damageing the display unit. The input is designed to take an input voltage of + or - 50mV (repesenting 100 amp charge or discharge).According to Nasa the maximum input is + or- 5 Volt. This is 100 times the normal input.
 
fit the shunt,connected in such a manor that only the Alt and boat load is being recorded, the bow thruster can be straight off the +terminal. simple.
 
Now this is worrying me. I have just got back from the boat having fitted a NASA monitor. On my boat I have 2x100 AH house batteries and a 75 AH starter battery, but they have a common negative with the supply to the boat controlled by a simple 3-way switch on the positive side. There are no diodes etc. I installed the shunt as per the instructions in the negative to the batteries. There is a short length of wire supplied with the unit to connect the shunt to the negative battery terminal, but is it lighter gauge than the battery cables. What will happen when I start the engine? and should I rig this up a different way- advice please!
 
I've just got one of these Nasa BM1's, not fitted yet. I also had wondered about the short neg lead supplied to connect the shunt to the battery. But I think NASA's thinking is that the monitor is designed to watch the service battery only, so they think you will never have starter current flowing through that wire. You have a 1-2-both switch, so have I, so in theory we might one day want to start the engine off the service battery, although in n years of sailing I never have. I might call NASA next week and talk it over with them.
 
I always switch to "both" when I am starting or running the engine and switch to the house batteries when sailing or running on shore power. I always believed that you shouldn't switch whilst the engine was running or you could kill the alternator, so wouldn't want to start on the engine battery and then switch, but maybe I have been doing it wrong for the lastr 15 years!
 
I always understood that good-quality rotary battery switches retain a battery connection when switching between 1-2-both. It's the way I work on our boat and the alternator is still alive and kicking (reaching for piece of wood hastily!).
 
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