AllSpark Battery Slimline Tray - fits all models of AllSpark slimline batteries
$59.00
Need help?
(08) 6205 6868
Date Posted: 0000-00-00 00:00:00
We receive questions almost daily about whether a Voltage Sensitive Relay (VSR) can be used to charge an auxiliary battery or whether a DC to DC charger is required.
To answer this properly, it’s important to outline the 3 general types of alternators used, followed by how a VSR works and then talk about what is required for 100% state of charge when using the alternator as your primary power source, whilst also looking after battery longevity.
It should be clearly understood that most deep cycle batteries (think AGM or LiFePO4 which covers the large percentage of deep cycle batteries used these days in recreational vehicles) will require a final charge voltage at 100% state of charge of 14.4-14.7V. It must slowly increase the terminal voltage from whatever its starting voltage is, up to the finished voltage to give you the best battery longevity and not just force in full voltage all the way through the charging process.
Regulation of current is also an important part of this charging process. As you will see in the below graph, a typical Constant Current (CC)/Constant Voltage (CV) 3 stage charge profile holds a set current rate (sized correctly for the battery bank) whilst slowly and safely increasing terminal voltage. It doesn’t just ram in fixed voltage like an alternator would.
TYPES OF ALTERNATORS
Fixed Voltage Alternators Typically used in all vehicles made prior to around year 2000. These alternators have a constant voltage output of around 14.0-14.2V regardless of battery state of charge or electrical load on the vehicle. The alternator continues to produce over 14.0V regardless of the electrical load. It will put 14.0V into your battery/s regardless of whether the battery is ready to take that voltage level or not.
Temperature Compensating Alternators Started being used post 2000 with the introduction of common rail diesel engines (but not exclusively these) and are very common in Toyota vehicles (Hilux, 70 & 200 series cruisers etc). They generally have a MAX output of about 14.0V when cold, but as the engine and hence alternator heat up, they reduce the voltage output and this can drop as low as 13.0-13.2V meaning you will never get your aux battery charged past about 60-70% state of charge.
Variable Voltage or ECU Controlled Alternators Commonly referred to as “Smart Alternators”, these were introduced around 2010-2011 as a response by some vehicle manufacturers to help meet reduced emissions standards and provide better fuel economy. These alternators are controlled by the ECU to reduce the output from the Alternator once the starting battery voltage is back up to full charge after starting the vehicle. Once that voltage is reached, the output voltage of the alternator is significantly reduced (below 12.7V) and as such is too low to activate a VSR.
NOW, WHAT IS A VSR AND HOW DOES IT WORK?
Firstly, what is a relay? A relay is a switching device that is used to either isolate an electrical circuit or change the operating state from active to inactive. Used extensively in automotive applications for things like Dual battery setups, spotlights and light bars, winches etc and many of your factory fitted electrical devices light indicators, headlights etc. The relay is generally activated by an external switch or timer. Using an electromagnet, the relay turns on and off depending on what the switch is doing.
A Voltage sensitive relay uses an internal switching mechanism based on the voltage signal it is receiving from the starting battery. It is installed between the start battery and an auxiliary battery as a way of automatically connecting or isolating the two batteries based on a factory pre-set voltage range on the VSR. For the majority of VSR’s on the market, it will sit in an isolated state until it reads 13.2V from the start battery for more than 5 seconds (this occurs shortly after turning on the engine) then it completes the circuit. This has the same effect of connecting the two batteries in parallel, allowing the flow of volts and amps between the two batteries. The relay will stay in this active state until such time as the relay reads the voltage of the starting battery has dropped below 12.7V for around 10 secs, then isolates the batteries from each other. It does this as it assumes the engine is now off and that no further power is coming from the alternator. This ensures the loads on the auxiliary battery cannot run down the start battery when the engine isn’t running.
VSR’s were developed in the days before DCDC chargers and when most vehicles had fixed output alternators producing over 14V, plus the majority of auxiliary batteries were also flooded lead acid batteries similar to the start battery chemistry and safe to connect these in parallel assuming similar sizes and chemistry of batteries. Most people these days use AGM or LiFePO4 batteries which need to be charged differently to a flooded battery to correctly charge them and to get the best longevity.
With the introduction of Temperature compensating and Variable Voltage Alternators, using a VSR to charge your battery will never reach a full state of charge in your battery with most only ever receiving about 70-80% and in many instances, especially if the battery is not in the engine bay, won’t provide any increase in charge whatsoever to your battery. This is due to the reduced alternator output controlled by the ECU and/or due to excessive voltage drop over distance to the canopy, rear of wagon or even worse for caravans/campers etc. We see many vehicles trying to use this setup only to find they get less than 12.5V coming out the other end which does nothing to charge the battery by more than about 50%.
SO DO I NEED ONE?
These days with most modern vehicles, VSR’s really should only be used for things like load disconnects, providing a power source to a 3 way fridge running on DC power in a towed vehicle (only when the engine is running) or linking two batteries together of the same size/type for winching purposes.
Any circuit that includes an auxiliary battery that needs to be charged from an alternator source needs a DCDC charger and not a VSR. The only real exception is vehicles with fixed output alternators that provide a constant output voltage over 14.2V and with a similar or same battery to the cranking battery. In this instance a DCDC charger is still recommend for best charging performance and longevity, but you can get away with a VSR only as a low cost dual battery setup, but understand this will affect longevity of the battery as it is constantly force fed by the alternator with no voltage or current regulation. If you have a newer vehicle, run AGM or LiFePO4 batteries or any setup that is outside of the engine bay, then a DCDC charger is required instead of a VSR.
It is really important to understand that a VSR is “NOT” a battery charger. It is simply a switch that allows you to connect two batteries in parallel based on a pre-set voltage range.
Welcome to our website. If you continue to browse and use this website, you are agreeing to comply with and be bound by the following terms and conditions of use, which together with our privacy policy govern Offroad Living’s relationship with you in relation to this website. If you disagree with any part of these terms and conditions, please do not use our website.
The term ‘Offroad Living’ or ‘us’ or ‘we’ refers to the owner of the website whose registered office is Unit 2/29 Prestige Parade, Wangara WA 6065. Our ABN is 61 161 507 106. The term ‘you’ refers to the user or viewer of our website.
The use of this website is subject to the following terms of use:
This privacy policy sets out how we uses and protects any information that you give us when you use this website. We are committed to ensuring that your privacy is protected. Should we ask you to provide certain information by which you can be identified when using this website, then you can be assured that it will only be used in accordance with this privacy statement. We may change this policy from time to time by updating this page. You should check this page from time to time to ensure that you are happy with any changes.
What we collect
We may collect the following information:
What we do with the information we gather
We require this information to understand your needs and provide you with a better service, and in particular for the following reasons:
Security
We are committed to ensuring that your information is secure. In order to prevent unauthorised access or disclosure, we have put in place suitable physical, electronic and managerial procedures to safeguard and secure the information we collect online.
How we use cookies
A cookie is a small file which asks permission to be placed on your computer's hard drive. Once you agree, the file is added and the cookie helps analyse web traffic or lets you know when you visit a particular site. Cookies allow web applications to respond to you as an individual. The web application can tailor its operations to your needs, likes and dislikes by gathering and remembering information about your preferences.
We use traffic log cookies to identify which pages are being used. This helps us analyse data about webpage traffic and improve our website in order to tailor it to customer needs. We only use this information for statistical analysis purposes and then the data is removed from the system. Overall, cookies help us provide you with a better website by enabling us to monitor which pages you find useful and which you do not. A cookie in no way gives us access to your computer or any information about you, other than the data you choose to share with us. You can choose to accept or decline cookies. Most web browsers automatically accept cookies, but you can usually modify your browser setting to decline cookies if you prefer. This may prevent you from taking full advantage of the website.
Links to other websites
Our website may contain links to other websites of interest. However, once you have used these links to leave our site, you should note that we do not have any control over that other website. Therefore, we cannot be responsible for the protection and privacy of any information which you provide whilst visiting such sites and such sites are not governed by this privacy statement. You should exercise caution and look at the privacy statement applicable to the website in question.
Controlling your personal information
You may choose to restrict the collection or use of your personal information in the following ways:
We will not sell, distribute or lease your personal information to third parties unless we have your permission or are required by law to do so. We may use your personal information to send you promotional information about third parties which we think you may find interesting if you tell us that you wish this to happen. If you believe that any information we are holding on you is incorrect or incomplete, please write to or email us as soon as possible at the above address. We will promptly correct any information found to be incorrect.