Slave charging units

[john0740]

I'm looking at options for battery charging as part of a boat refurbishment project. Idea is to have an alternator with controller charging house (230Ah) and engine (40Ah) batteries.
Three options
(1) Use of split charge diodes
(2) Use of split VSR
(3) Use of a slave charger such as a Xantrex EchoCharge or a Balmar DuoCharge.

I am favouring option (3) but this seems to be an uncommon solution. I would welcome comments re pros and cons.
Thanks
8F

 

[rogerdog]

VSR -simple - low cost - no voltage drop - works ! but sorry have not any experience of the charger route.

 

[VicS]

Snag with diodes is the voltage drop which requires the alternator to be battery sensed to overcome it. Although there are electronic splitters that solve the problem eg X-Split and some other dodges as well.

VSRs avoid the voltage drop problem.

Read the website of someone who makes diode splitters to find all the reasons they are better than a VSR. Read the website of someone who makes VSRs to find all the reasons they are better than diodes. .... Spin a coin to choose between them!

Quite a number of other systems to look at.

Sterling, including the alternator to battery chargers (if you want to avoid alternator modifications)

Mastervolt

Adverc

Driftgate

To name but a few.
Depends if you want to merely divide the charging between two battery banks or whether you want to enter the realms of alternator boosting and sophisticated control systems

 

[pvb]

Welcome to the forums!

The main reason people fit slave chargers is to limit the maximum charge voltage applied to the starter battery, so as to prevent it being over-charged. I think the key consideration is whether or not you have a "smart" regulator (Adverc, Sterling, etc) controlling your alternator. If you have, a slave charger will limit the maximum voltage delivered to the starter battery. The EchoCharge simply limits it to 14.4v max, whilst the DuoCharge is more sophisticated and has 4 different preset programmes for different types of battery (eg gel, AGM, etc).

If you don't have a smart regulator and have no intention of fitting one in the future, then a simple VSR would be your best choice.

I suppose, to take this a stage further, it might be worth considering what to do if you think you might want to fit a smart regulator soon. In that case, rather than buying a regulator and a slave charger, you might consider the Sterling Alternator-to-Battery Charger. This electronically boosts the charging voltage to achieve maximum charging efficiency, and also has a lower-voltage unboosted output for the starter battery to ensure this doesn't get overcharged. The Alternator-to-Battery Charger has several settings for different types of battery and will also provide a true float voltage - this is something that ordinary smart regulators cannot do.

 

[halcyon]

Back in1970 when I started designing systems, I set out a number of design objectives, the main one was

The alternator should always be permanently connected to the the engine battery. So that any failure of the charging system would not effect the engine battery chargeing, that is except for alternator failure.

So when thinking about the above three options, follow the charge paths, and list the number of points that can fail , the number of additional connections etc to charge both engine and service batteries.

Then think are you happy with the odds of a system failure?

Brian

 

[William_H]

I am not familiar with the "slave charger" I assumed it was like the so called alternator to battery smart charger.
These chargers might seem ideal. They allow the start battery and alternator to run at the usual 14v regulated but take power from the battery /alternator and boost it up to a voltage sufficient to enable a smart charge regime to charge the house battery. ie something close to 15volts until near full charge is achieved when volts drop down to a suitable voltage for long term maintenance. The control circuits are similar to a smart regulator for the alternator or a smart battery charger from the mains.

The problem with the boosting of the voltage is the loss of efficiency. So to get 40 amps into your house battery you will probably need to suck 50 amps from your alternator. The advantage is that the engine battery just jogs along with 14 volts on it not being heavily charged.

If you use a VSR then there is no loss of efficiency. If you use a smart regulator the higher voltage comes from a higher field current in the alternator so all the current from the alternator is effectively put into the batteries. (except for any other additional load)

If you use diodes there is some loss of efficiency due to volt drop across the diodes which can be up to 1 volt at high current. At 40 amps that is 40watts wasted (in heat) which must be produced by upping the alternator voltage.

So for me I would go with VSR and if you want, a smart regulator on the alternator. The engine battery should be OK if well charged being in parallel with the house batteries via the VSR. And you still have that simplicity Halcyon referred to. good luck olewill

 

[john0740]

Thanks for the feedback. Putting split-charge diodes to one side for the moment, I see the advantage in both (2) and (3) being that the lead from the alternator output goes directly to the house battery, so connections are minimised. Is there any point putting an isolator or fuse between the alternator and house battery?

I think that the main advantage of (3) over (2) is that the slave charger prevents the engine battery ever discharging into the house battery. A VSR would not provide for this.

 

[pvb]

[ QUOTE ]
I think that the main advantage of (3) over (2) is that the slave charger prevents the engine battery ever discharging into the house battery. A VSR would not provide for this.

[/ QUOTE ]Actually, that's the whole idea of the VSR. Generally, when a VSR is fitted, the alternator output is wired to the start battery. When the engine is running, the VSR detects the higher voltage and closes, connecting the charge to the domestic batteries as well. When the engine stops, the VSR detects the lower voltage and opens, thus isolating the start battery.