Solid State Battery Isolators

[Pbillington]

/forums/images/graemlins/confused.gif
Can anyone recommend a reliable method of installing two/three batteries in a power boat. The boat is Trophy 2352 sports fishing boat with 5.7L V8 250HP.

Battery installation - 2 No 85Ah connectd via standard battery isolator switch.
Would like to isolate batteries automatically with solid state device to enable instruments to be used whilst at anchor or drifting whilst maintaining at least one battery for engine starting.

Assistance is appreciated

Thanks /forums/images/graemlins/smile.gif

 

[DaveS]

I would suggest the easiest solution is to have the alternator powering the engine start battery directly, with a VSR between the start battery and the domestic(s). With the engine stopped the VSR opens at about 12.7V leaving the start battery almost fully charged. You still need isolator switches to allow disconnection of batteries while the boat is unattended or in an emergency, but conventional switches rather than solid state devices would be fine (and IMHO better).

 

[roly_voya]

The simplest way is to use a standard diode splitter, cheap & reliable. You do get a voltage drop accross them and you need to compensate for this which can be done in two ways. Either by fitting an alternator controller or, if you have a battery sensed alternator (ie one with a remote control box) simply by conecting the sense wire to the house batt terminal. With a bit of work you can convert an alternator to batt sense by taking the back off and extending the field sense wire but you need to be fairly sure how the alternator works and be able to test the result to do this. On a MB this might be worth doing as it would save about £110 for the controller and with lots of motor hours you are not so worried about charge rates but the controller will charge batteries much faster and better, machine sensed alt are only designed the charge the engin batt.

 

[William_H]

As already stated diode isolation is simple but does incure a voltage drop which means the batteries don't get fully charged unless you can get the regulator to sense the battery voltage or can increase the regulated voltage.

The VSR relay type isolation is perhaps the easiest in that there is no volt drop across the contacts.
There is a form of solid state isolation which uses Field Effect Transistors to do the switching of the charger (alternator) to the domestic batteries. It works much like a VSR ie switches to connect the batteries to the charger when the voltage rises to that of charging. It is a bit complex in that the FETs require a higher voltage to turn them on (about 20 volts above -ve) so there is a little power converter to generate that voltage. The FETs when turned on exhibit no voltage drop and very low resistance. They are however a bit more susceptible to excessive charge current (as in a bad a very flat battery).
The device was designed by a electronics magazine and for a while was availble as a kit but a quick check indicates they are not availble now. If you really want I could dig outan article.
But basically VSR is easier simpler and not so hard to build yourself again if you want. good luck olewill

 

[tazzle]

I went with the VSR solution - recently installed one of these clusters from BEP marine (no connection). Works a treat.

 

[halcyon]

Solid state splitter uses mosfets, use multiple to reduce volt drop, still not zero volt, require 10 volt above the voltage you are switching, and a may suffer thermal runaway. Another point, they have flywheel diodes that allow a feedback voltage, so back feed the alternator output from both batteries when in the off state. As charge goes through unit to both batteries, you double your failure points, chances of loosing charge to your engine battery.
Blocking diodes, loose charge due to volt drop, so require mods to charger or another alternator regulator to increase output voltage. So not cheap, or simple due to additional mods, again as charge to the engine battery goes throught unit failure chances are increased, plus further if additional alt control added.
VSR, engine battery is always connected to alternator, so simple installation and max reliability. Downside, requires correct relay specing, undersize can lead to contact welding or burnout, high currant can lead to high contact resistance at the lower normal charge currant. Timed drop out at low volt can stop chatter, but can allow overcurrant in charge cables before drop out.
As with all things, you need to spec correctly and install correctly, and then theres no reason why you cannot get 25 years trouble free use.

Passing thought, following on from the Chinese engine thread around the forums.
Is anyone worried that most charging sytems / items are also made in China ?. A lot of people would not fit a Chinese engine due to possible failure and there safety, would you extend this to electronics.

Brian

 

[Gumpy]

Possibly total overkill for your use but I use this:
http://www.smartgauge.co.uk/smartbank.html

Works 100%

Julian

 

[halcyon]

Nice to see the world's catching up, been making the same system for 25 years now.

Brian

 

[Gumpy]

For me the good thing is when used with a Smartgauge the amount of control you have of connect and disconnect voltages, and whether it charges battery a before adding b in etc.

Julian

 

[Pbillington]

Thanks for the advice. The VSR option sounds like a possibility.

Is there anything available that eliminates volt drop?

 

[DaveS]

If you use a suitably rated VSR with suitably sized cables, wired as I and others advised, then volt drop should be negligible.

 

[halcyon]

A correctly spec'ed relay will drop 0.04 volt at 100 amp.

Brian

 

[Pbillington]

Many Thanks

The Engine is 5.7L V8 250HP Petrol. The alternator is 65Amps - I believe.
How do you know if the relay is the correct spec?
What would be the correct cable sizes?
Appreciate assistance

 

[DaveS]

As I said!

 

[DaveS]

The VSR's current rating should be higher than the alternator's maximum output. A standard size of the rather neat ones that both Raedwald and I use is 100A: that would be very suitable. A slight subtlety here: before you buy, do you ever intend to fit a wind generator, solar panels, or an independent generator? Since you've a mobo, probably no to the first two, but think about the third. If the answer is still no then a single sensed VSR will do fine, the sensed input connected to the start battery. If the answer is either yes or maybe, then you might want to consider the slightly dearer double sensed version, where both batteries are sensed, the auxiliary gen. feeds the domestics, and the VSR closes if either battery's voltage exceeds the cut-in value (13.8V from memory).

Cable sizing is a bit more complicated. The first rule is to keep everything as short as possible. If the total length of all the interconnecting cables is under 2m you can effectively neglect volt drop and just size for current - something like 10 mm2 would be fine for the VCR connections between batteries in your case. (the existing cables to the start battery will be bigger because they have to handle cranking current *.) If length is significant you need to do the sums properly or look up one of the on-line calculators (search previous posts). The importance of proper cable termination cannot be over emphasised: a poor joint can cause more resistance, volt drop, and heating than an under-sized cable. Get a good crimp tool and use lots of vaseline on everything.

* If you want to install an emergency parallelling switch (one of the modules in Raedwall's pic) then the cables to that have to be sized to carry cranking current. Personally, I don't have one of these. If the situation arises, I would use one of my jump leads - which remarkably few boats carry - but that could start a whole new thread...

 

[Pbillington]

Many Thanks indeed. Appreciate the laymans terminology. (Still learning about all the abbreviations used by others!!)

I may use a solar panel to keep one battery charged (topped up) during lay-up on the hard in between trips.
The boat is stored on a trailer in a yard.
But I can connect the solar panel to the main battery via the VSR if necessary.

Thanks again to all contributors.

I'll be off to source the materials.

PRB

 

[DaveS]

Just one point: you wouldn't normally connect the solar panel to the domestic battery/ies "via the VSR" unless you connected it to the starter battery. Now, you could do it that way, but the more normal approach would be to connect it, via a regulator if it is any size, to the domestics battery/ies.

 

[halcyon]

All the VSR we manufacture are bi-directional, you connect the solar panel to the service bank. This allows the engine to charge the service bank, and the service bank to charge the engine battery. If you have a small solar panel, by connecting the engine battery at high voltage, it acts a power dump for surplace solar panel output. Avoiding the need for seperate solar panel regulators. In passing they also include remote link start as standard, not as a seperate item.

Brian

 

[William_H]

Just so there is no confusion. Any volt drop in a VSR or wiring is not a bad thing because it is a resistive volt drop. That is the volt drop is virtually nothing at low currents and a little at higher currents. This simply increases a little the time for fully charging the battery.
ie as the charging current falls the volt drop dissapears.

However the volt drop of a diode is a queer sort of drop in that it is essentially constant regardless of current. (Doesn't follow ohms law)
This means that the .7 volt is dropped no matter how small the current is so the battery being charged through the diode only ever sees .7v less than the charging source which is in itself regulated to 14 volts. So battery is never fully charged. olewill

 

[DaveS]

[ QUOTE ]
All the VSR we manufacture are bi-directional, you connect the solar panel to the service bank. This allows the engine to charge the service bank, and the service bank to charge the engine battery.

[/ QUOTE ]

I agree with all that - the bidirectional version of the BEP unit does just the same, that's why I asked about solar or other generation

[ QUOTE ]
If you have a small solar panel, by connecting the engine battery at high voltage, it acts a power dump for surplace solar panel output. Avoiding the need for seperate solar panel regulators.

[/ QUOTE ]

That's an excellent point - I hadn't thought of that. Presumably there must be a limit size of panel after which this cannot be safely relied on, but maybe too big to be a concern for a normal sized boat?

[ QUOTE ]
In passing they also include remote link start as standard, not as a seperate item.

[/ QUOTE ]

Ah, terminology. I used VSR to mean the device itself, i.e. a relay that opens and closes when particular voltages are measured. I suppose the term could be expanded to include other items within a single package - a bit like referring to a car as a motor - which indeed some people do! /forums/images/graemlins/smile.gif