Smart charger - some advice please

[CharlesM]

Hello All

One on the final essential tasks before going playing, and maybe heading south to Trinidad, is to install a smart charger/regulator.

I have a 24 volt system and 2 x 24 volt alternators.

Question - can I connect the 2 alternators to the smart charger? I had a look at the docs on those available here (St Martin) and they dont say anything about this. I don't really understand how these things work, but imagine it tells the alternator what output is required based on the battery (bank)'s status. Is the signal sent by the regulator good/strong enough to take 2 leads and feed to the 2 alternators instead of only one?

Also, I did some searching here, and see people talking about VSR's. Can anyone enlighten me as to what a VSR is?

Cheers
Charles

 

[Guest]

An alternator regulator alters the voltage/current through the rotor coils to make the stator (outer) coil produce the correct voltage. If you have two alternators they will not be identical and so a single regulator can NOT work 2 machines.

Also beware that most car alternators were never designed for boat usage. Boosting the charge currents can cause major overheating in both the alternator and the wiring. Make sure the system is up to the task and get some local help. Even boat alternators like the ones sold in the main US marine store can overheat. One unit got so hot the bearings seized. Which left the poor owner without an engine as the fan belt could not be used.

 

[oldsaltoz]

G'day Charles,

We Have a smart charger on board, twin engines, only one alternator is connected (85 amp on ours) this charges the start battery for that engine and the 6 house batteries.

The second alternator charges only it's own start battery (70 amp). We also have a change over switch to start either motor from either battery in case one goes flat but have yet to install a change over switch for the Smart charger in case of alternator failure.

As the Smart system has a range of options its a good idea to have a marine electrician install the system and set up the variables.

Ours has been running for 6 years now and never a problem. I'm a two hour flight or 12 hour drive from the boat so it's important that the system is reliable.

Avagoodweekend......

 

[markdj]

You need 2 smart regulators and then either 2 splitter diodes or 2 Voltage Sensitive Relays to split the charge into your domestic and engine batteries.

 

[john_morris_uk]

You need to give a little more information for an informed response to your question.

I don't know your boat that you mention in your bio. How many engines do you have, and how many batterys? In other words, do you have separate batterys for starting and domestic batterys for your power requirements when no engine(s) are running. Have you got two engines and two alternator or one engine and two alternators? What sort of a boat is it, power or sail? (I presume its a sailing boat, but charging requirements are different with a power boat.)

Also, 'Smart Charger' is a little ambiguous. Do you mean a Smart Charger that is connected to shore power (when available) or a generator when shore power isn't? Or does 'Smart Charger' mean one of the various devices that are available to alter the charging characteristics of an alternator?

A 'VSR' is a 'Voltage Sensitive Relay' - used to connect two banks of batteries together when the sytsem is charging. The idea is that it 'drops out' and disconnects when the system isn't charging - and therefore stops your domestic loads discharging the engine start battery.

 

[Bejasus]

see HERE for a couple of twin alternator set ups. Also check out the FAQ's for answers to your questions.

 

[Guest]

I would definitely not take the advice of Adverc. Ignoring the difference between identical alternators. Two engines will never run the same speed and 2 pulley systems are never the same ratios. So in the end, for a given input there will be a different output voltage on each of the alternators. Simply connecting the outputs together would not work. Advercs solution seems to use the inefficiencies of the diode splitters to put back some matching. If one alternator produces more voltage then its diode will be conducting more current. The internal resistance of the diode will mean more voltage is dropped across the diode. Hence this will help to even the loads. I can easily show this to work well by using the worst diodes available on a big heat sink and nice long wires. But the idea is to charge batteries not make heat.

This is not a solution. Using Diode splitters with the sense wire on the high usage battery, in a boat application, will mean that the voltage drop to the house battery is bigger. If the sense is on that diode output then the starter battery is seeing excessive charge voltage as its diode has very little current. In a car a lot of power is drawn from the starter battery circuit. However in a boat there is normally only the starter motor connected. It never really gets used.

If you use systems like this then the starter battery will be abused. The only way to charge reliably and efficiently is to match the boat battery banks to the alternators. If you want an automatic system and can cope with the complexity then add relays to sense the alternator activity and control the isolation of the battery banks. Otherwise just remember to switch over to the other bank to even out the usage.

Starter batteries and automatic relay systems are OK. They force all the batteries to see the same voltage and so they can neither be over or undercharged if the regulator is working. If I had a catamaran, like they described, I would definitely use a separate systems in each hull. The more things that are connected together the more chances of a complete failure.