Battery Protect System - a variation ...


Full Member
In order to protect a battery from excessive discharge some kind of Battery Protection device can be fitted. How often do you hear about someone who has forgotten to turn something off, gone away on a holiday leaving the van on the driveway and come back to a destroyed battery? Yup, that is why they are a good idea!

You can get a ready-made device such as this Victron Battery Protect BP-65 Amp for a reasonable £38. These work by setting a disconnect voltage - so if the battery drops below that value the BP turns off. Other ones are availabke of course

However, something that stops the Voltage sensing Protects working really well is if you use an fairly powerful inverter. When drawing a high current, the battery voltage on a Lead Acid Technology Battery will drop notably. It is quite possible that the voltage will drop temporarily to a low level that is ok for the short term but you would not want to have the battery sitting at generally. What this means is the Voltage setting of a Battery Protection Device could result in it switching off when the inverter runs at full power - which is not the objective of them (and Inverters usually have their own battery protection often and will not start/cuts out if the Voltage drops below a certain level)

To provide some battery protection but get round this voltage issue, I thought I would take some further advantage of a feature of the Victron GX unit (y)

The Venus GX has a couple of relays built into it - one of which is programmable to different functions.

I selected the "Generator Stop/Start" Function for Relay 1
BP - 1 by David, on Flickr

This adds the "Generator Stop/Start" Option in the settings ready for configuration
BP - 2 by David, on Flickr

There are lots of different configurations and parameters that can be used for this option, including Battery Current, Voltage and SOC (State Of Charge)
BP - 3 by David, on Flickr
The one I want to use is the Battery SOC :D

Now this is designed to auto start a Generator via Relay control when the batteries are low and I am using that same principle here for a Battery Protect.
BP - 4 by David, on Flickr
I decided to choose a fairly low SOC of 42% as the 'Start', and then having the 'Stop' at 49%. My batteries are ok at 40% although I would not choose to run them that low purposefully. For some other batteries, a setting of 50% might be more approriate? This - and Battery Protect Devices - are of course a 'last resort' type feature so a low setting can be fine.

So that is how and when the relay goes on and off, but how does a "Generator Stop/Start" work as a Battery Protect System?
Well, instead of the outout of the relay going to a Generator, I am sending it to a 200Amp Hi-Power Relay and reversing the logic of the power :geek:

The Venus GX Relay has three pins - A Com (Common), an NO (Normally Open) and a NC (Normally Closed). When the Relay is off, the Com and NC are connected. When the Relay is on, the Com and NO are connected.
I fed the Com with a +12V signal and connected the NC to the power in of the 200A Power Relay (the Venus GX Relay is just a control relay - it is not used to transfer any significant current within it). This means when the "Generator" is off, the Venus GX Relay is off and so the Power Relay is ON (as it is fed by the NC pin).
However, when the Venus GX Relay turns on when the SOC drops to the set value, the COM gets connected to the NO instead of the NO, and this means the Power Relay looses power and goes off.

The connection from the 12V Circuits (via the 10 way Blade Fusebox) to the Battery goes via this Power relay (Pin 30 and pin 87), so whenever the Relay is energised, the 12V Circuits have power; and when the Relay +12V Control Pin loses its power the Relay disconnects and the 12V Circuits are no longer supplied with power.

Because this relay is controlled via the Venus System, it can be controlled remotely via the Internet :)

This field can be used to manually control the Relay. The description says "Manual Start" but what it effectively does is to disconnect the relay and so disable the 12V Circuits (handy if you have gone away and thought "oh ****, I forgot to turn off ...." :p )
BP - 5 by David, on Flickr
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Full Member
I must admit that I've looked briefly at those Venus GX units, but remembering the price forced myself to look away. I do admire your skills with these toys, but the fear of a problem whilst stuck in the wilds keeps me simple. I just rely on the solar to hold everything up even if I've forgotten to turn something off.


Full Member
Something that is worth bearing in mind - and is a little different to some of the fancy top-end motorhome gadgets - is the Venus system is really just a monitoring and reporting system in the main and not a control system. So if it did go wrong, the most likely result will be everything will still work, just you won't get data :) - but you can still use everything.
I have seen some system that are controlled by a central computer and think just as you are saying - what if it fails in middle of nowhere?? Or even on a regular trip and takes weeks to get fixed? Can put a full stop to a trip away :(

I am a fan of discrete components but with consolidated monitoring.(in my last office job, I ran a dept that needed to consolidate literally hundreds of excel spreadsheets into a few rollup sheets for business costings and then c&p to PowerPoint. That was fun and used to be manual copy and paste all over the place until I automated it with MS VB Macros - changed days of work into a couple of hours!)

The only thing the Venus GX is actually controlling in my set IS this BP system in fact - and in the specific case of this battery protect setup, if the Venus failed, the BP relay would open and 12V systems stop (which is really what you would want as a failsafe) - but ... A jumper between two pins and you would be back in action again :)

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