The advantages of a VSR are that you get the potential full advantages of a dual battery system mentioned above, namely, that a) if the car is off and you're powering accessories, you won't drain your cranking battery, and b) if you need (in a pinch) to use your secondary battery to crank the car, you can override with the click of a switch. Circuit diagram for installing a VSR into a car Thus, your secondary battery starts charging. When the VSR detects that the car is on, it says "Hey the car's on! OK let's connect the two batteries". The VSR is triggered by detecting when the alternator is running and charging the car battery. This lets you run those accessories without drawing too much current through a single wire.Ī VSR is a relay that separates the car's main cranking battery and the secondary accessories battery. In cars and motorcycles, you use a relay on something like the accessories wire (that's only live when the bike is on) to power other high-current accessories, like extra lights, a fridge, or heated grips. This is suitable for under-bonnet set-ups where cables are short and batteries are the same kind.Ī VSR is a high-current relay that has simpler wiring than most relays.Ī relay (for those who don't know) is a switch that's triggered by voltage. The next best dual-battery system is one with a VSR. Won't work with latest cars (~2015 onwards) with smart alternators.Prevents you from draining starter battery.Dual Battery Systems with a VSR (Voltage Sensitive Relay)
So a dual battery system in parallel will work and won't cause any huge problems, but it doesn't cost much to go to the next best solution: a VSR. If your starter battery fails, you can perhaps use current from your accessories battery to either start the car, or to charge your cranking battery. You can (maybe) use your second battery to start the car.If you have another battery to power your current-hungry accessories, you'll be able to run your battery flat and still be able to drive home. Secondly, if you connect two in parallel, you lose out on the two main benefits of dual battery systems: This means that if you connect two batteries in parallel, you'll never get full potential out of your system. Batteries are at the same condition level.A short distance difference of just one metre usually results in a few tenths of a volt difference with the current that we normally see in 12V systems. The voltage that arrives at each battery is the same.If not, then you may not charge one to its fullest level. The batteries have the same capacity and chemistry.This works, but only with a precarious set of assumptions: It's tempting to think "Ah, I'll just connect the batteries in parallel!" Won't work with latest cars with smart alternators.Gets 80% out of your accessory battery doesn't charge it too fast.Engine bay only, lead acid batteries only.Dual Battery Systems - Direct Connections Let's go through the pros and cons of these in turn. These are expensive but have multiple benefits. This works better, and isn't very expensive. Voltage Sensitive Relay/Isolator(a.k.a.There are three main ways you can connect two (or more) batteries in a dual- or multi-battery system You build a dual-battery system into a car or truck for mainly one reason: to keep your accessories (fridge, chargers, epic sound system) on one battery, and your vehicle system (starting and operating critical electrics) on the other.
In building our 4x4 adventure vehicle system, I learned a lot about the way people build dual battery systems in cars and trucks.
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