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Multi Bank Batt Charging


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I thought this might interest some. Many "modern" boats, or boats that have been updated use VSRs to join batt banks. A VSR is a voltage sensitive relay.

The basic setup is that the alt charges the start battery, and when that reaches a set voltage (say 13.2v) the VSR engages and joins the start batt to the house, thereby charging both.

 

This is usually a good setup, and automated. Many old boats had issues caused by improper use of the old 1-2-both switches, and a VSR automates this so that there is no manual input required.

 

However, VSRs are not perfect. One issue is that if you have a large load (microwave, capstan winch, electric winches etc) connected to your house bank, and use a dual sense VSR (engages with over set voltage from either bank), then when the large load is on, the voltage may drop sufficiently to disengage the VSR, even if, for example, the Alt is charging. This means the whole load will come from the batts, even if the Alt is going.

 

Some of the later engines (and some better older ones) come with a voltage sense cable to the batts, and will adjust for any losses in the cables, diode splitters etc. If you have one of these and a large load like above, a diode splitter block might be a better bet.

 

A diode splitter block is a device with one input, and 2 or more outputs. Current can only pass in one direction, so they can't discharge your batts. Their downside is that they drop about 0.6v between the input and the outputs. In the case of the alts above, no issue, the alternator will just raise it's voltage to compensate automatically. Most external regulated alts can also be manually adjusted.

 

With the diode splitter installed, in the large load situation above, the alt will run at max output to supply what it can, and the balance will come from the batts. :-) 

 

Some consider the diode splitter block old tech, but in some installs, IMO it's a better solution than VSRs. I use one in Island Time (I have a microwave, draws about 120 amps), and with a 115a alt, it means there is little draw when using  the microwave, provided the motor is going - which it is for microwave use.

 

I do use a VSR to charge the radio/emergency bank though, horses for courses.

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Two extra points to add.
For many boats, the House bank and start bank are usually two very different capacities. Thus the loads are very different and the banks will draw different charge currents. This can result in a multiple number of charge problems.
The VSR is fine when you have a small boat that has two of the same batteries for start and for house. Usually a small powerboat that uses a second battery to run Stereo, fish finder etc etc and the engine start is for that only. When the two batteries reach a similar charge, the loads are close to equal and both will then charge equally till full.

Re the Splitter, there are two types. A simple Diode splitter which tends to be cheaper. These have a natural Voltage drop there needs to be a small modification done to the ALT to increase the charge Voltage to compensate for the loss.
The better splitters to use are FET splitters. These have no loss and thus deliver the full amount of charge. If you have a charge controller on the ALT, then proper charge to full capacity will take place.

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For me the best system has two seperate alternators, an engine alternator and a higher capacity house alternator. I like the redundancy. In an emergency the engine and house system can be put in parallel. I realise not all installations have room for two alternators but for my money if you can then two seperate electrical systems : an engine system and a house system are the way to go.

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another option is one of these

http://www.balmar.net/products/digital-duo-charge/

I have 3 battery banks and 2 alternators. The house bank has a separate large frame alternator.

The regular engine alternator is set up to feed the "winch" battery (anchor and halyard winches). This Duo Charger then charges the start battery from the winch battery.

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The Duocharge is basically a fancy VSR.

 

As Wheels said, if your battery banks require different voltages, then different charge sources are required. Its a sound reason to stick with one type of batts, to reduce complication. 

 

I referred to diode splitter blocks generically, yes the FET based ones are the ones to select if getting a new one.

 

I've no issue with dual alts, seen often on bluewater boats, much less so on local ones.

 

It can cost a lot to add another alternator, - often needing brackets fabricated, and good, hot rated alts, with good external regulators are expensive. Most people don't do that unless they live aboard or plan long cruises.

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The VSR then is designed around connecting the Alt to start battery?

 

I preferred the Alt connected to the house and an ACR to start batt. system.

Three separate switch so you can use any source with any load and combine if you want.

I can see the problem with VSR & big load with windlass connected to house.

 

I like the two alt scenario, especially as surely current will be halved to same bank?

(As long as both alts Vset output is the same)

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The Duocharge is basically a fancy VSR.

 

As Wheels said, if your battery banks require different voltages, then different charge sources are required. Its a sound reason to stick with one type of batts, to reduce complication. 

 

I referred to diode splitter blocks generically, yes the FET based ones are the ones to select if getting a new one.

 

I've no issue with dual alts, seen often on bluewater boats, much less so on local ones.

 

It can cost a lot to add another alternator, - often needing brackets fabricated, and good, hot rated alts, with good external regulators are expensive. Most people don't do that unless they live aboard or plan long cruises.

The Duo Charge is a bit more than a fancy VSR, you could say it is a combination of a VSR and a Blocking Diode as it does not allow a reverse flow of current from the recipient battery, important in my case to keep the start battery fully charged.

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