Dual battery vsr?

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mb82

223 posts

Master Geek

#239873 8-Aug-2018 20:57

I am looking at installing a dual battery setup with a vsr (voltage sensing relay). So the 'house' battery can be charged by either the current solar setup but also the engines alternator.

I am confused as the house battery is spec'd with a max charge current of 25 amps. How do I ensure no more than 25 amps is going into it through the vsr from the alternator?

Fred99

13684 posts

Uber Geek

#2070415 9-Aug-2018 09:33

This looks like it might do the trick.

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mclean

581 posts

Ultimate Geek

Subscriber

#2070491 9-Aug-2018 10:29

Fred99: This looks like it might do the trick.

It does indeed, nice device. Using one of those there's probably no need for a VSR, unless you want over-discharge protection on the house battery.

MikeAqua

7769 posts

Uber Geek

#2070498 9-Aug-2018 10:43

As you're looking at a VSR then I'll give a recommendation for the BEP clusters. They are compact, robust and simple to install.

I have one on my trailer-boat (two outboards, two starter batteries and one house battery). We put one in my father's launch (two stern drives with starter and house banks).

I find the BEP switches last about 10 years in an exposed, regularly wet location in the boat. I can't comment on the durability of the BEP D-VSR module yet as I've only had it a couple of years. It does do a good job of managing charging of the batteries.

To limit you alternator amps your really just need some sort of regulator.

The unit others have recommended looks like a good solution and allows more managed charging than a simple regulator does.

I'd suggest putting that between the VSR and the battery that can only cope with 25A.

Another thought: Starting the engine will cause the VSR to cut the power supply to the charger, until the starter battery has been recharged. You need to be confident that won't put the charger in an error state from which it doesn't resume charging.

Mike

tripper1000

1604 posts

Uber Geek

#2070562 9-Aug-2018 11:28

25 amps max charge current - ?

It must either be a very small battery or that is the max continuous current. If it is the max continuous current it should have another max rating for short term surges which may mean you don't need any artificial current limiting. Small to moderate sized batteries seldom draw the alternator max current output. Once their voltage comes up a little, current draw tapers off a lot. Also once you put some battery switches, VSR's and cabling between the alternator and battery you seldom see the battery draw max current from the alternator - a few milli-ohms here and there soon add up.

Aredwood

3885 posts

Uber Geek

#2070856 9-Aug-2018 17:22

Simplest way to limit charge current, undersize the wiring a little bit. As charge current varies a lot with even small changes to charge voltage.

Also consider total load on the alternator. As a discharged lead acid battery will draw a lot of current when first connected.

My dual battery system doesn't begin charging the house battery while the glow plugs are still switched on. And I repurposed the aircon recirculation control switch as a charge disable switch. As it always defaults to outside air, and if you switch on recirculation, it switches off by itself after 20 minutes. Bloody annoying not being able to leave the recirculation on. But very useful for charge control of the house battery. If the house battery is badly discharged, I can easily delay charging it while I wait for alternator load to reduce a bit. And it will begin to charge by itself after 20 minutes if I forget to re enable charging. (recirculation control is now via a new switch)

mb82

223 posts

Master Geek

#2070885 9-Aug-2018 19:05

tripper1000:

25 amps max charge current - ?

It must either be a very small battery or that is the max continuous current. If it is the max continuous current it should have another max rating for short term surges which may mean you don't need any artificial current limiting. Small to moderate sized batteries seldom draw the alternator max current output. Once their voltage comes up a little, current draw tapers off a lot. Also once you put some battery switches, VSR's and cabling between the alternator and battery you seldom see the battery draw max current from the alternator - a few milli-ohms here and there soon add up.

That is all I can see on the spec sheet. It is a 12v 120 amp hour agm battery.

MikeAqua

7769 posts

Uber Geek

#2071034 10-Aug-2018 09:21

mb82:

That is all I can see on the spec sheet. It is a 12v 120 amp hour agm battery.

The charge controller Fred recommended has an AGM setting. Wouldn't it regulate current flow, as part of that charging profile?

Mike

mclean

581 posts

Ultimate Geek

Subscriber

#2071053 10-Aug-2018 09:41

With deep cycle batteries there's a risk of overheating the battery during the bulk charge, ie the first part of the charging cycle when most of the charge goes in. Thats why most deep-cycle chargers use current control rather than voltage control during this phase, or have a temperature sensor that you bolt to the battery terminal. The better ones have both. This seems to be less of an issue for starting batteries, I guess because the charge time is much shorter.

Most manufacturers recommended a maximum charging rate of about 0.2C for deep-cycle agm batteries, where C is the ampere-hour rating. For lead-acid it's higher.

The aussie charger that Fred99 suggested seems to manage all this, plus it has built-in MPPT control of the solar input into the house battery.

MikeAqua

7769 posts

Uber Geek

#2071142 10-Aug-2018 10:33

mclean:

Fred99: This looks like it might do the trick.

It does indeed, nice device. Using one of those there's probably no need for a VSR, unless you want over-discharge protection on the house battery.

I don't think that device can replace a VSR. A VSR lets the alternator top up the starter battery, then allows it to charge the house battery.

That charger isn't connected to the starter battery at all, it just controls alternator current to the house battery.

The VSRs I'm familiar with don't prevent over-discharge of the house battery.

Mike

mclean

581 posts

Ultimate Geek

Subscriber

#2071279 10-Aug-2018 13:23

MikeAqua: I don't think that device can replace a VSR. A VSR lets the alternator top up the starter battery, then allows it to charge the house battery.

That charger isn't connected to the starter battery at all, it just controls alternator current to the house battery.

It takes it's power from the starter battery/alternator but only when the voltage is >12.8V. The manufacturer calls it VSR mode, and that's what it seems to be.

tripper1000

1604 posts

Uber Geek

#2071447 10-Aug-2018 16:59

mb82: That is all I can see on the spec sheet. It is a 12v 120 amp hour agm battery.

Ah, AGM's. I've tried them and now I stay away from them. They seldom live up to the hype. They often come with fantastic appearing warranty's until you read the fine print - conditions are usually impossibly stringent.

tripper1000

1604 posts

Uber Geek

#2071452 10-Aug-2018 17:02

Most AGM's require a different charge profile and voltages to regular lead acids, so don't play nicely with alternators. Something like the charge controller linked above is a more technically correct option than a plain old VSR.